ALBERT

Description: This study calibrated a gas transport model that uses multicomponent approaches, i.e., the dusty gas model (DGM) and Stefan–Maxwell (SM) equations, to soil gas concentration depth profiles to determine aerobic biodegradation rates of hydrocarbons in unsaturated soils. I found that viscous advection induced by multicomponent gas transport in porous systems and Knudsen diffusion may be neglected for soil systems with intrinsic permeabilities 〉10 –13 m 2 , and intrinsic permeability of low-permeability layers, such as cemented limestone or concrete pavement on the ground surface, calibrated by the DGM model ranged from 10 –15 to 10 –18 m 2 . The SM model may be applicable for gas transport in soil systems with low-permeability layers if low obstruction factor values are assigned for these layers. The obstruction factor defined in the DGM and SM equations accounts for impedance to gas diffusion in unsaturated soils and is similar to tortuosity for Fick’s law of diffusion. Results further showed that flux rates of hydrocarbon vapors from sources in unsaturated soils need to be measured or assessed before models are fitted to hydrocarbon vapor depth profiles. If the flux rates are not assessed, and if the obstruction factor and intrinsic permeability in the low-permeability layers are of uncertain magnitudes, models may produce nonunique results for biodegradation rates of hydrocarbons. Thus, I propose a two-step procedure for determining aerobic biodegradation rates of hydrocarbons.

Description: A combined heat-pulse and sensible heat balance method can be used to determine evaporation using temperature measurements and thermal property estimations. The objective of this study was to investigate the applicability of the combined heat-pulse and sensible heat balance method by comparing laboratory experimental data to both an analytical and a multiphase heat and mass transfer model. A bench-scale laboratory experiment was performed to measure soil thermal and hydraulic properties at fine spatial and temporal resolutions. Comparisons of experimental and numerical results confirmed the applicability of the heat-pulse and sensible heat balance methods to determine evaporation rates. Results showed close agreement with experimental water loss measurements. This study demonstrated the ability and versatility of using the heat-pulse and sensible heat balance methods in numerical heat and mass transfer models to determine evaporation rates. Calculated soil thermal properties were in 93.4 and 97.5% agreement with experimental results for water content values 〉0.05. Deviations were observed at low water contents due to sensor sensitivity. The calculated evaporation rates yielded cumulative water losses that were 96.8 and 97.7% in agreement with experimentally measured weight loss data. Late Stage 1 evaporation was overestimated due to observed temperature rises by two of the heat-pulse probes. Despite this, the combined heat-pulse and sensible heat balance method is a powerful tool that can be used to determine evaporation rates in situ.

Description: Land conversion in the tropics from primary forest to agricultural land has altered soil hydrologic processes. Woody vegetation is known to increase infiltration rates and saturated hydraulic conductivity ( K S ) in primary forests compared with agricultural land, but it is less clear if this relationship holds for a gradient of woody vegetation. In addition, the mechanisms for the effect of woody vegetation on K S have yet to be fully examined. To quantify the effect of woody vegetation structure on vadose zone hydrology, we estimated K S in 15 plots across a dry tropical riparian vegetation gradient in Nicaragua, taking into account covariates such as soil properties and livestock impact. Using single linear regression, we found that leaf area index (LAI) had the greatest correlation coefficient of 0.331 to K S , followed by hoofprint density (0.291) and clay content (0.291). Furthermore, the relationship between LAI and K S was greater for finer soils than for coarser soils. We found that a forest soil had eight times more preferential flow paths than a pasture soil, and most of these were root-initiated flow paths, suggesting a possible mechanism for the positive correlation between LAI and K S . We show that the K S predictions with a pedotransfer function could be improved by incorporating LAI. Our findings support the importance of preserving woody vegetation in key areas on the landscape to maintain hydrologic functions of tropical soils and ecosystems.

Description: Laboratory and numerical studies were conducted to investigate the transport and release of Escherichia coli D21g in preferential flow systems with artificial macropores under different ionic strength (IS) conditions. Macropores were created by embedding coarse sand lenses in a fine sand matrix and altering the length, continuity, and vertical position of the lens. The length of an artificial macropore proved to have a great impact on the preferential transport of E. coli D21g, especially under high-IS conditions. A discontinuous macropore (interrupted by fine sand) was found to have less preferential transport of E. coli D21g than a continuous macropore of the same length that was open to either the top or bottom boundary. At low IS, more extensive transport in the preferential path and earlier arrival time were observed for E. coli D21g than Br – as a result of size exclusion. Two release pulses (one from the preferential path and the other from the matrix) were observed following a reduction of the solution IS for flow systems with macropores that were open to either the top or bottom boundary, whereas three pulses (two from the preferential path and another from the matrix) were observed for systems with discontinuous macropores. Numerical simulations of E. coli D21g under both constant and transient solution chemistry conditions had very high agreement with the experiment data, except for their capability to predict some subtle differences in transport between the various lens configurations.

Description: We coupled a radiative transfer model and a soil hydrologic model (HYDRUS 1D) with an optimization routine to derive soil hydraulic parameters, surface roughness, and soil moisture of a tilled bare soil plot using measured brightness temperatures at 1.4 GHz (L-band), rainfall, and potential soil evaporation. The robustness of the approach was evaluated using five 28-d data sets representing different meteorological conditions. We considered two soil hydraulic property models: the unimodal Mualem–van Genuchten and the bimodal model of Durner. Microwave radiative transfer was modeled by three different approaches: the Fresnel equation with depth-averaged dielectric permittivity of either 2- or 5-cm-thick surface layers and a coherent radiative transfer model (CRTM) that accounts for vertical gradients in dielectric permittivity. Brightness temperatures simulated by the CRTM and the 2-cm-layer Fresnel model fitted well to the measured ones. L-band brightness temperatures are therefore related to the dielectric permittivity and soil moisture in a 2-cm-thick surface layer. The surface roughness parameter that was derived from brightness temperatures using inverse modeling was similar to direct estimates from laser profiler measurements. The laboratory-derived water retention curve was bimodal and could be retrieved consistently for the different periods from brightness temperatures using inverse modeling. A unimodal soil hydraulic property function underestimated the hydraulic conductivity near saturation. Surface soil moisture contents simulated using retrieved soil hydraulic parameters were compared with in situ measurements. Depth-specific calibration relations were essential to derive soil moisture from near-surface installed sensors.

Description: Water repellency (WR) might affect the spatial and temporal dynamics of a wetting front during infiltration and redistribution in a way that is difficult to predict with standard approaches. Therefore, the objectives of this study were to simulate the wetting plume geometry with a three-dimensional numerical model and to test whether electrical resistivity tomography (ERT) is able to illustrate the geometry under highly dynamic conditions. At our study site under agricultural use (Gleyic Podzol, groundwater affected), persistent WR in the subsoil to the 120-cm depth was responsible for a conical plume geometry observed after ponded tracer application with Brilliant Blue (BB) and bromide. The process was invasively observed with hydraulic sensors. At the same time, ERT was used to monitor a second ponded infiltration event under equal boundary conditions at the same site. Numerical simulation of the process showed that hysteresis in the water retention curve is needed to describe the specific infiltration plume geometry correctly. The main wetting function was derived from scaling the main drying curve with measured contact angle data. A comparison of wetting front arrival times among the hydraulic model, sensors, and independent ERT observations indicates an overall good agreement and shows the usefulness of ERT measurements under highly dynamic in situ conditions. Our results confirm the need to include strong hysteresis effects scaled with independent contact angle data when simulating infiltration dynamics in a water repellent soil to avoid an underestimation of the wetting front arrival.

Description: The occurrence of thermally driven convective air flow within waste rock or natural soil profiles has been well established; however, the potential impact that convective air flow may have on water storage within reclamation soil covers has not been previously explored. We conducted a numerical modeling study to evaluate the effect that convective air flow may have on stored water within a soil reclamation cover placed over a coke stockpile at an oil sands mine in Alberta, Canada. Coke is a carbon, sand-like byproduct of heavy oil processing. Two-dimensional simulations of thermally driven convective air flow were conducted for two different field sites based on available field data. The elevated temperature within the coke stockpile resulted in the development of strong convective air flow cells that drew in drier atmospheric air over the lower slope positions while releasing it across the upper slope and plateau areas of the cover. The magnitude of the gas flux and the intensity of the convection within the cell were a function of the air permeability of the coke and cover material, the depth of the coke, and the slope of the stockpile. It was estimated that convective air movement through the cover could produce as much as 1 to 2 mm/d of enhanced drying of the cover in lower slope positions. Field observations of water content distributions within the cover provided corroboration that the cover has undergone enhanced drying at lower slope positions.

Description: Eminent environmental challenges such as remediation of contaminated sites, the establishment and maintenance of nuclear waste repositories, or the design of surface landfill covers all require accurate quantification of the soil water characteristic (SWC) at low water contents. Furthermore, several essential but difficult-to-measure soil properties, including clay content and specific surface area, are intimately related to water vapor sorption. Until recently, it was a major challenge to measure detailed water vapor sorption isotherms accurately within a reasonable time frame. This priority communication illustrates potential applications of a new, fully automated, and rapid Vapor Sorption Analyzer (VSA) to pertinent issues in vadose zone research. Detailed vapor sorption isotherms for 25 variably textured soils were measured with the VSA within 1 to 3 d. Links between generated isotherms and pesticide volatilization, toxic organic vapor sorption kinetics, and soil water repellency are illustrated. Several methods to quantify hysteresis effects and to derive soil clay content and specific surface area from VSA-measured isotherms are presented. Besides above mentioned applications, potential relationships to percolation threshold for solute diffusion in unsaturated soil and to soil cation exchange capacity are discussed to stimulate new and much-needed vadose zone research.

Description: In arid regions, groundwater resources are prone to depletion due to excessive water use and little recharge potential. Especially in sand dune areas, groundwater recharge is highly dependent on vadose zone properties and corresponding water fluxes. Nevertheless, vadose zone water fluxes under arid conditions are hard to determine owing to, among other reasons, deep vadose zones with generally low fluxes and only sporadic high infiltration events. In this study, we present an inverse model of infiltration experiments accounting for variable saturated nonisothermal water fluxes to estimate effective hydraulic and thermal parameters of dune sands. A subsequent scenario modeling links the results of the inverse model with projections of a global climate model until 2100. The scenario modeling clearly showed the high dependency of groundwater recharge on precipitation amounts and intensities, whereas temperature increases are only of minor importance for deep infiltration. However, simulated precipitation rates are still affected by high uncertainties in the response to the hydrological input data of the climate model. Thus, higher certainty in the prediction of precipitation pattern is a major future goal for climate modeling to constrain future groundwater management strategies in arid regions.

Description: Conglomeratic alluvial sediments (sand–gravel–cobbles) are common in fluvial, periglacial, and tectonically active regions but have received little attention with respect to unsaturated flow, specifically moisture–tension–conductivity relationships, due to difficulty in making measurements in the field or laboratory and lack of agricultural value. We used a field-scale infiltration experiment, a one-dimensional layered forward model, and parameter estimation modeling to examine in situ flow behavior between residual and partial saturation in a four-layer system under steady infiltration (0.84 cm h –1 ) for 19 h. Prior information from ground-penetrating radar, grain-size distributions from core samples, and long-term tension () and moisture () monitoring were used to define geologic structure, simulate test behavior, and provide initial parameter estimates. Vertically distributed measurements of ( t ) and ( t ) from the experiment were matched using four parameters ( s , α, n , and K s ) of the van Genuchten–Mualem (VGM) relationships for each material layer and a Metropolis–Hastings (MH) search with multiple, independent-chain runs to 10 6 samples each. Scale reduction factors indicated convergence of independent chains for 11 of 16 parameters. Final distributions of individual parameters varied from normal to nearly uniform distributions, and some parameter pairs showed high cross-correlation ( R 2 〉 0.9). Results showed that (i) VGM relationships can be applied to these coarse, conglomeratic soils to characterize unsaturated flow behavior across the natural range of partial saturation, (ii) even under high sustained infiltration rates, these coarse conglomeratic soils remain well drained, despite relatively low porosity and significant cobble fraction, and (iii) high uncertainty and nonconvergence of MH chains does not lead to significant misfit of the observed data. These findings imply that a significant cobble fraction does not markedly reduce infiltration at low saturation levels that develop under natural recharge conditions.

Description: Many phenomena of practical importance, such as water movement in soils and the recovery of oil and gas from subterranean reservoirs, involve capillary pressure hysteresis in natural porous materials. Experimental capillary pressure–saturation relationships provide important information on pore systems and multiphase displacements. Data, at various levels of detail, are available for strongly wetted soils and sphere packs, but very few detailed hysteresis data have been reported for consolidated porous media, such as rocks, at equivalent wetting conditions. A new porous plate apparatus and procedure have been used to give extensive capillary pressure hysteresis relationships within a few days, a significant improvement on previous methods. Air–brine data are presented for three types of consolidated sandstones. Rate of outflow and inflow were recorded for each data point. Imbibition scanning curves have been obtained starting from the primary and secondary drainage curves. Drainage scanning curves that branch from imbibition curves starting at different levels of initial water saturation have also been measured. The data met a stringent test of quality in that all hysteresis loops exhibited closure as predicted by theory and as usually reported for unconsolidated media. The overall form of the hysteresis curves is related to fluid distributions, percolation effects, and trapping. Initial wetting phase (WP) saturation versus residual nonwetting phase (NWP) relationships provide detailed information on the extent of trapping for imbibition starting at different initial saturations, a parameter of particular interest in carbon dioxide sequestration. Features of the dynamic outflow–inflow data provided indication of the differences in flow rates for drainage and imbibition.

Description: Visualization of the root system architecture of plants is possible using a combination of magnetic resonance imaging of water mobility and tractography. Diffusion tensor imaging (DTI), a magnetic resonance application, was used to provide a three-dimensional map of water mobility inside a pot containing soil and roots. Tractography generates channels that constitute pathways of facilitated water movement, representing the roots, calculated from water diffusion properties obtained from DTI experiments. Examples of pea ( Pisum sativum L.) and corn [ Zea mays L. var. indentata (Sturtev.) L.H. Bailey] root growth are provided.

Description: Iron and sulfur cycling is an important control on contaminant fate and transport. We present the effects of soil structure, specifically the presence of a soil layer, on linked biogeochemical and hydrological processes involving Fe and S cycling in the vadose zone using repacked soil columns. Synthetic rainwater was applied to a homogenized medium-grained sand column, a homogenized organic-rich loam column, and a sand-over-loam layered column. Water samples were voltammetrically analyzed for total sulfide, Fe 2+ , and aqueous FeS clusters (FeS aq ) using a mercury drop electrode. Aqueous FeS clusters were observed in the loam and layered columns, with the greatest concentrations of FeS aq occurring at the sand–loam interface in the layered column. Redox potential (Eh) measurements showed rapid cycling of redox conditions at the interface (hours to days) suggesting that FeS aq formation was favored under conditions of disequilibrium. However, the relative stability and aqueous nature of FeS aq allowed for it to be transported, which has broader implications for chemical fate and transport in the vadose zone. The co-occurrence of FeS aq with iron-rich soil aggregates near the sand-loam interface in the layered column suggest that FeS aq may act as a precursor to the formation of aggregates that, in our column, subsequently caused an order of magnitude decrease in hydraulic conductivity. These findings suggest that FeS aq should be considered when predicting the transport of contaminants, particularly metals that may associate with FeS aq , in systems with prominent iron and sulfur cycling.

Description: A field experiment was performed in a grassland at the NOAA-CREST–Soil Moisture Advanced Radiometric Testbed (CREST-SMART) facility, which includes a mobile L-band dual-polarized radiometer with an in situ soil temperature and soil moisture observation network, located near Millbrook, NY. During the day-long field campaign, intensive spatiotemporal measurements of L-band brightness temperatures, surface temperature, soil moisture, and soil temperature at 3-, 7-, and 12-cm depths were collected during three passes at 0830, 1130, and 1430 h. During the second and third passes, half of the field was irrigated. Soil roughness and water content of the short grass that remained after mowing the study area were measured. The Tau-Omega radiative transfer model was used to assess the performance of the soil moisture retrieval using measured soil temperatures at different depths. In addition, the collected microwave observations at the three different times of the day were used to assess the impact of the diurnal variation of soil temperature on the performance of soil moisture retrieval. Obtained results showed that the root mean square error (RMSE) decreased throughout the day to reach 0.03 m 3 /m 3 for the afternoon pass when 12-cm soil temperature values were used in the radiative transfer model. In addition, during the three different passes, the lowest RMSE was consistently obtained when the 12-cm soil temperature was used, which suggests that, for this investigated site, soil temperature at the 12-cm depth can be a surrogate for soil effective temperature when L-band microwave temperatures are used. In terms of diurnal variability, observations from the afternoon pass led to the highest agreement between observed and retrieved soil moisture values.

Description: Seedlings are rejected every year in forest nurseries because of insufficient root development and root plug cohesion. Adequate substrate bulk density and aeration properties are of critical importance for root growth and water and nutrient uptake. The quality norms for nursery substrates, based in part on the ratio of coarse to fine particle sizes, have recently been questioned with respect to substrate performance. We conducted experiments in 2008 and 2009 under commercial nursery conditions to determine the effects of substrate physicochemical properties on seedling growth and nutrition. Seedlings were grown in 13 different substrates composed of coarse and fine peat particles (threshold size: 0.5 mm), perlite, and vermiculite, covering a broad range of physical properties with adequate available water and various coarse/fine particle ratios and aeration properties. Substrate properties had significant effects on growth parameters in both years. Seedling growth was affected by water stress in 2008 and low gas diffusivity in 2009. This study emphasizes the importance of management on substrate performance, indicating that short periods of moderate water stress (matric potential lower than –10 kPa) may hinder seedling growth. It also suggests that when irrigated substrates are maintained within the 0 to –5 kPa matric potential range, gas diffusivity should not be lower than 0.003 to 0.005 cm 2 s cm –2 s –1 , in agreement with the modeled O 2 profiles in root plugs. Hence, when designing and using substrates at the nursery scale, more attention should be paid to aeration properties after potting and to the management rather than to the coarse/fine particle ratio.

Description: A theoretical framework is presented for modeling the chemomechanical behavior of multiphase porous media, in general, and unsaturated soils, in particular, which can address skeletal deformation, fluid flow, heat conduction, solute diffusion, chemical reaction, and phase transition in a consistent and systematic way. A general expression is derived for the electrochemical potential of a fluid species with explicitly accounting for the effects of osmosis, capillarity, and adsorption. The equilibrium behavior of porous media is investigated, and the composition of pore water pressure is identified. Explicit formulations are developed for the effective stress and intergranular stress, with consideration of physicochemical effects. It is shown that the negative water pressure measured by a conventional transducer can be significantly different than the true pore water pressure. It is also theoretically revealed that, other than the soil water characteristic function, a new pressure (or potential) function accounting for the physicochemical effects is generally required in analyzing the coupled chemomechanical processes in unsaturated soils. The new theory is capable of effectively explaining many salient phenomena occurring in water-saturated porous media with a degree of saturation varying from an extremely low value to 100%, including Donnan osmosis, capillary fringe, air entry value, initial hydraulic head during seepage, and pressure solution. The new theory can be used to analyze the multiple coupled physical and chemical processes in the vadose zone.

Description: Commercial dielectric soil water sensors may improve irrigation management by providing continuous field soil water information. The accuracy of these sensors may be compromised, however, when low quality, high electrical conductivity irrigation water is used. The magnitude of this effect is expected to vary among sensors depending on the sensor measurement frequency ( f ), decreasing with increasing f , thus providing a potential criterion for sensor selection. We examined the effect of varying solution electrical conductivity ( w ) on the sensor-measured dielectric permittivity ( s ) of the following three soil water sensors: the WET ( f = 20 MHz), 5TE ( f = 70 MHz), and ML2 ( f = 100 MHz). We also evaluated the efficacy of using a simple, cost effective, two-point calibration technique (CAL) to adjust for differences among soils. We found that s varied dramatically, in some cases more than 15 dielectric units, among sensors in different soils under uniform soil water and w conditions. The relative s response of the WET and ML2 sensors was consistent with their respective f , and the 5TE response was not. Thus, the WET s was almost always greater than the ML2, while the 5TE was generally less than the ML2. In all cases, the ML2 s could be described with a single, commonly used calibration equation. We also found that varying w from 1.2 to 6 dS m –1 had little effect on s , indicating that a single calibration may work for that range of irrigation water quality. Finally, we found that the CAL procedure, as evaluated with the RMSE, was effective for all soils and sensors.

Description: Diurnal water table fluctuations (DWTFs), normally observed in shallow unconfined aquifers, are commonly used to estimate groundwater evapotranspiration (ET g ) by applying the soil water balance pioneered by W.N. White in 1932. A key element of White-based methods is the drainable porosity (or specific yield), a soil water storage parameter that significantly depends on both the vadose-zone soil moisture fluxes and water table (WT) elevation. However, it has traditionally been treated as either a constant or a function of the WT under hydrostatic soil moisture conditions. Recent research has shown that, at any given shallow WT position, vadose zone fluxes cause the drainable porosity to behave hysteretically, hence requiring estimation of two distinct parameters, drainable ( d ) and fillable porosity ( f ), during ET g estimation from DWTFs. We present a White-based method to estimate ET g that implements separate d and f parameters and hence accounts for both WT elevation and unsaturated zone moisture fluxes. The modified method not only improves the ET g estimates but also extend the applicability of the White method to periods during rainfall, unlike most previous implementations, which have been limited to omitting such periods. The modified method is demonstrated here for estimation of ET g from two crop fields in Northeast Florida for two growing seasons, each approximately 50 d, in 2010 and 2011, and the ET g estimates are compared with the standard Penman–Monteith method. The modified method significantly improved ET g estimates compared with the hydrostatic d based method, reducing the root mean square error by 94 and 96% at hourly and daily resolutions, respectively.

Description: Application of broiler ( Gallus gallus ) litter (BL) to pasturelands in karst regions like the Ozark Highlands can potentially reduce water quality due to leaching of BL-derived nutrients and trace metals. The objective of this study was to determine long-term linear trends in drainage and soil leachate water quality under natural precipitation from a silt-loam soil amended annually with BL at three application rates (0 [control], 5.6 [low], and 11.2 [high] Mg BL ha –1 ]. Automated equilibrium tension lysimeters were used to continuously monitor and collect leachate from an undisturbed soil profile with a history of litter applications under forage management at a depth of 0.9 m for the 8-yr period from May 2003 through April 2011. Average annual flow-weighted mean (FWM) concentrations and loads of NH 4 –N, As, Mn, and Ni decreased linearly ( P 〈 0.05), while Cu and Se increased ( P 〈 0.05) linearly during the 8 yr. Nearly all water quality parameters measured were unaffected ( P 〉 0.05) by BL rate alone. Continued annual additions of BL linearly increased ( P 〈 0.05) the average annual FWM leachate Na concentrations relative to the unamended control. Results indicated that pasturelands with a history of BL application may continue to release BL-derived metals, such as As and Se, at concentrations harmful to health regardless of current management practice long after litter application has ceased. Land application of nutrient- and trace-metal-containing animal wastes in regions with underlying karst features needs to be carefully managed to minimize subsoil leaching losses.

Description: Soil ice content impacts winter vadose zone hydrology. It may be possible to estimate changes in soil ice content with a sensible heat balance (SHB) method, using measurements from heat pulse (HP) sensors. Feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties in partially frozen soils. The objectives of this study were (i) to examine the SHB method for determining in situ ice content, and (ii) to evaluate the required accuracy of HP sensors for use in the SHB method. Heat pulse sensors were installed in a bare field to measure soil temperatures and thermal properties during freezing and thawing events. In situ soil ice contents were determined at 60-min intervals with SHB theory. Sensitivity of the SHB method to temperature, heat capacity, thermal conductivity, and time step size was analyzed based on numerically produced soil freezing and thawing events. The in situ ice contents determined with the SHB method were sometimes unrealistically large or even negative. Thermal conductivity accuracy and time step size were the key factors contributing to SHB errors, while temperature and heat capacity accuracy had less influence. Ice content estimated with a 15-min SHB time step was more accurate than that estimated with a 60-min time step. Sensitivity analysis indicated that measurement errors in soil temperature and thermal conductivity should be less than ±0.05°C and ±20%, respectively, but the error in the soil heat capacity could vary by ±50%. Thus, improving the accuracy of thermal conductivity measurements and using short time steps are required to accurately estimate soil ice contents with the SHB method.

Description: Impedance probes are popular electromagnetic soil moisture monitoring devices used for a variety of applications but require site-specific calibrations to provide accurate measurements. Several calibration techniques have been reported in the literature, although laboratory-based procedures involving wet-up (via upward or downward infiltration) and dry-down are commonly performed for permanently installed sensors. Wet-up calibrations can be completed substantially faster (〈1 d) than dry-down calibrations (1–2 wk), but it is uncertain which technique is preferable to provide the most accurate calibration. The objective of this study was to compare the results obtained from laboratory-based infiltration wet-up and dry-down calibrations of the Stevens Hydra Probe soil moisture sensor. Soil samples for this study were obtained from agricultural sites in Saskatchewan, Canada, at depths of 5, 20, and 50 cm across a variety of textural compositions. Results demonstrate that utilizing either infiltration wet-up (according to the procedure in this study) or dry-down procedures provides accuracies of 〈0.061 m 3 m –3 root mean square error (RMSE), which was superior to manufacturer calibration accuracy across all samples. However, superior calibration accuracies (i.e., the lowest RMSE) were achieved using the dry-down procedure across all soil samples, resulting in a lower RMSE of 0.01 to 0.04 m 3 m –3 (at 95% confidence). A significant correlation ( r value = 0.61, p 〈 0.05) exists between the differences in infiltration wet-up and dry-down calibration RMSEs and clay content. This suggests that the difference between the two procedures tested in this study is more significant in finer textured soils. The findings of this study indicate that the dry-down procedure produced the lowest RMSE and is therefore the preferred calibration procedure, particularly for finer textured soils.

Description: Soil moisture is a key variable in most environmental processes, and the cosmic-ray neutron probe (CRNP) fills a niche for intermediate-scale soil moisture measurements. In this study, the CRNP estimated soil moisture was compared with a soil moisture measurement network including 108 time domain transmissometry (TDT) probes. We also used a Hydrus-1D numerical model of measured soil moisture at targeted locations by inversely fitting soil hydraulic parameters used to simulate soil moisture in the near surface (0.03 and 0.05 m) during the growing seasons of 2011 and 2012. Both simulated and TDT-measured soil moisture were used in constructing the depth-weighted mean areal soil moisture for comparison with the CRNP estimates. The results showed that near-surface soil moisture estimated by the numerical simulation improved the correlation between the sensor network and the CRNP estimation, especially during rainfall events. The CRNP estimates of soil moisture exhibited a dry bias under relatively wet conditions at the beginning of the snow-free period because of the almost binary spatial distribution of soil moisture. Using a combination of soil moisture measurements and near-surface simulations, the CRNP output was recalibrated to capture the wetter conditions, resulting in a RMSE (0.011 m 3 /m 3 ) of less than half the original calibration RMSE (0.025 m 3 /m 3 ). The calibration was validated using CRNP data from the 2013 growing season against independent soil moisture values.

Description: The globally used herbicide glyphosate [ N -(phosphonomethyl)glycine] and its most frequently detected metabolite, aminomethylphosphonic acid (AMPA), were studied in a unique 12-yr field-scale monitoring program. The leaching of glyphosate, AMPA, and soil particles was studied in a shallow drainage system beneath a 1.26- ha field. Five annual glyphosate applications were applied with different autumn application dates. Solute mass flux from the drain system following the five glyphosate applications were compared to determine how different factors affect the leaching of glyphosate, AMPA, and particles. Glyphosate and AMPA leaching were highly event driven, controlled by the time and intensity of the first rainfall event after glyphosate application. A high similarity in cumulative drainage and leached pesticide masses with time suggests near-constant drainage and leaching rates. There was no clear relationship between particle-facilitated transport and the transport of glyphosate or AMPA. However, soil particles, glyphosate, and AMPA all showed distinct, simultaneous concentration curves, indicating common dominant transport mechanisms. Also, soil-water content at the time of application and the level of the groundwater table relative to the drain depth exerted clear controls on detection of solutes in the drainage water. To summarize our findings, we present a leaching risk chart to illustrate the dependence of glyphosate, AMPA, and soil particle leaching based on rainfall intensity and the timing of rainfall events after glyphosate application.

Description: Underground nuclear explosions (UNEs) produce radionuclide gases that may seep to the surface over weeks to months. The objective of this research was to quantify the impact of uncertainties in hydrologic parameters (fracture aperture, matrix permeability, porosity, and saturation) and season of detonation on the timing of gas breakthrough. Numerical sensitivity analyses were performed, with barometric pumping providing the primary driving force for gas migration, for the case of a 1 kt UNE at 400-m depth of burial. Gas arrival time was most affected by matrix permeability and fracture aperture. Gases having higher diffusivity were more sensitive to uncertainty in the rock properties. The effect of seasonality in the barometric pressure forcing was found to be important, with detonations in March the least likely to be detectable based on barometric data for Rainier Mesa, Nevada. Monte Carlo realizations were performed with all four parameters varying simultaneously to determine their interrelated effects. The Monte Carlo method was also used to predict the window of opportunity for 133 Xe detection from a 1 kt UNE at Rainier Mesa, with and without matching the model to SF 6 and 3 He data from the 1993 Non-Proliferation Experiment. Results from the data-blind Monte Carlo simulations were similar but were biased toward earlier arrival time and less likely to show detectable 133 Xe. The estimated timing of gas arrival may be used to deploy personnel and equipment to the site of a suspected UNE, if allowed under the terms of the Comprehensive Nuclear Test-Ban Treaty.

Description: There is a current and expanding need to measure surface infiltration rate parameters for stormwater infiltration practices used to mitigate the detrimental effects of land development activities on watershed hydrology. We have developed a falling-head soil surface infiltrometer, termed the modified Philip–Dunne (MPD) infiltrometer, that is inexpensive to construct, easy to use, and requires a minimal water volume per test. Because of these characteristics, many MPD devices can be deployed simultaneously to obtain infiltration rate data at multiple locations within a given infiltration practice. Green–Ampt theory was used to derive the expressions needed for analyzing the falling-head data to solve for the field-saturated hydraulic conductivity ( K fs ) and Green–Ampt wetting-front suction (). The accuracy of the analysis was determined using numerical experiments in which falling-head data were generated from a computational solution of the axisymmetric form of the three-dimensional Richards’ equation for homogeneous and isotropic porous media with specified input parameters. The falling-head data were then analyzed using a quasi-analytical procedure, and the resulting values of K fs and were compared with the input values. The accuracy of K fs and derived from data acquired using the MPD device was then assessed using physical experiments involving three large barrels packed with different types of sand. The K fs values obtained for the media in the barrels using an MPD infiltrometer were, on average, 82% of the values obtained from whole barrel falling-head tests. The resulting uncertainty in K fs values from the MPD infiltrometer is considered to be small compared with the orders of magnitude of variability commonly observed for K fs values in the field.

Description: The Sullivan Mine No. 1 Shaft waste rock dump was built on a natural slope and covered by till. The outflow of O 2 –deficient gas through a leachate drainage pipe in an enclosure at the base of the dump resulted in four fatalities. A numerical model was developed to understand the mechanism controlling gas flow, which was found to be the relative buoyancy of the gas phase within the dump compared with atmospheric air. Changes in atmospheric air density are caused by atmospheric temperature variations, whereas dump gas-phase density is relatively constant due to a steady internal dump temperature. When the air temperature is lower than the internal dump temperature, atmospheric air density is higher than the dump gas density, inducing upward dump gas flow and air entry into the drainage pipe. Downward dump gas flow occurs and exits the drainage pipe when high atmospheric temperature leads to an air density lower than the dump gas density. A gas flow behavior similar to what was observed at the No. 1 Shaft dump could occur in other covered dumps.

Description: Drains are special internal or boundary conditions in numerical simulation models. Instead of approximating them by a hole surrounded by a very dense grid of finite elements, the single node or single element approach based on the theory of Vimoke and Taylor proposed in 1962 offers a good alternative. Several authors have suggested that the Vimoke and Taylor constant to adapt the hydraulic conductivity of the element representing the drain should be changed by a certain factor. However, different correction factors have been given. Here this correction factor is derived for a control volume finite element numerical simulation model for different ratios of the size of the control volume representing the drain and the effective drain radius. It is shown that this relationship is dependent on the way the hydraulic conductivity is averaged at the interfaces between the neighboring control volumes. The relationships were obtained by optimization against an analytical solution for a steady-state, saturated situation. By applying the additional correction to a hypothetical transient situation for four soil types it was shown that the application of the additional correction factor resulted in 3 to 13% lower drain discharges compared to uncorrected simulations. Consequently, higher groundwater levels of on average 2 to 4 cm were obtained when applying the additional correction. For situations where exact predictions of drain discharge are needed, typically when solute transport is considered, it is advised to make use of the additional correction. Model specific correction factors may be required.

Description: The UTCHEM flood simulator was used to develop a numerical model to simulate colloidal silica transport through sand columns. Most existing numerical models for colloidal silica modeling include the gelation process, in which the viscosity gradually becomes orders of magnitude greater than the initial grout viscosity. However, in field grouting applications of shallow, loose, cohesionless deposits, injection at high viscosities may be limited due to allowable pressure limitations. In these cases, the injection is planned to be completed just before the gelling reaction begins. Thus, modeling the gelation process may not be necessary. The UTCHEM simulator accounts for fluids with varying densities and viscosities, making it a useful tool for simulating colloidal silica injection in cases where gelation does not need to be modeled explicitly. The model was validated using laboratory data from five column tests in which loose sand was treated with colloidal silica grout and one column test in which sand was treated with sodium silicate. The numerical model accurately represented the physical experiments. The numerical model provides a validated tool that can be used to design and optimize stabilizer delivery for laboratory and field applications in which gelation does not need to be modeled explicitly.

Description: This study evaluated three algorithms of the iterative ensemble Kalman filter (EnKF). They are Confirming EnKF, Restart EnKF, and modified Restart EnKF developed to resolve the inconsistency problem (i.e., updated model parameters and state variables do not follow the Richards equation) in vadose zone data assimilation due to model nonlinearity. While Confirming and Restart EnKF were adapted from literature, modified Restart EnKF was developed in this study to reduce computational costs by calculating only the mean simulation, not all the ensemble realizations, from time t = 0. A total of 11 cases were designed to investigate the performance of EnKF, Confirming EnKF, Restart EnKF, and modified Restart EnKF with different types and spatial configurations of observations (pressure head and water content) and different values of observation error variance, initial guess of ensemble mean and variance, ensemble size, and damping factor. The numerical study showed that Confirming EnKF produced considerable inconsistency for the nonlinear unsaturated flow problem, which differs from the apparent consensus opinion that Confirming EnKF can resolve the inconsistency problem. In contrast, Restart EnKF and its modification can resolve the inconsistency problem. Restart EnKF and its modification outperformed EnKF and Confirming EnKF in the various cases considered in this study. It ws also found that combining different types of observations can achieve better assimilation results, which is useful for monitoring network design.

Description: Better methods of estimating soil thermal conductivity and diffusivity from temperature measurements taken in practical situations where the thermal properties vary spatially will aid both soil science and engineering. In recently published work, I presented a method for estimating these properties when they vary vertically and the system may not be in the periodic steady state. This study extended that analysis to two and three dimensions. Using numerical solutions of the heat equation for conduction in inhomogeneous solids, synthetic temperatures were generated on a fine spatial grid and used to verify that the methods were accurate for daily periods, even in the absence of the periodic steady state. Errors due to increasing the spacing of the generated temperatures and to ignoring lateral variations were investigated. Temperatures on coarser grids led to errors that ranged up to a maximum of 20% at spacings of 0.25 m laterally and 0.1 m vertically when thermal conductivity varied fourfold in one lateral direction and ninefold vertically. In contrast, one-dimensional analyses on a fine grid with 0.01-m spacing produced errors of up to 54% that decreased to 7% when conductivity changed laterally by a factor of 1.2 instead of four. This error could be reduced by inclusion of a few lateral temperatures. Analysis of synthetic data that included a sharp spatial transition in thermal properties showed that the method would still be applicable where abrupt changes of soil material or water content occur.

Description: Few studies have reported on the sorptive interactions of oxytetracycline (OTC), a veterinary antibiotic frequently detected in wastewater, surface water, and soils, with common Fe oxide minerals. The primary objective of this study was to investigate the sorption behavior of OTC on magnetite (Fe 3 O 4 ) as a function of different solution properties such as pH, ionic strength, and initial OTC concentration. Results indicated that sorption of OTC by magnetite was rapid and complete within 24 h. The effect of ionic strength on sorption was negligible at low OTC surface coverage. The sorption envelope indicated that with increasing pH, OTC sorption on Fe 3 O 4 decreased. Surface complexation modeling of magnetite–OTC interactions suggested the interactions of divalent anionic species of OTC with the magnetite surface. Four possible surface complexation reactions were postulated to explain the pH dependence of OTC sorption, indicating a rather complex potential sorption mechanism. Our results suggest that magnetite has the capability of immobilizing OTC in the environment, which may be of use to scientists in predicting the fate of OTC in Fe-oxide-rich soil.

Description: The gradient method is widely used in conjunction with Fick’s Law to estimate emissions of soil CO 2 . This requires accurate estimation of the soil gas diffusion coefficient ( D P ) and the CO 2 concentration gradient, typically from measured water content and CO 2 concentration. Shallow application of water via precipitation or irrigation causes a temporary reduction in the near-surface soil gas diffusion coefficient, which compromises gradient-based flux estimates when undetected by a water content sensor. Our objectives were to analyze the effects of soil surface wetting and temperature on CO 2 concentration and efflux using laboratory and field measurements and to compare four widely used models for estimating D P . A laboratory test was conducted to determine the effects of water application on the soil CO 2 concentration and the CO 2 efflux calculated with the gradient method. The D P parameter was very sensitive to the soil water content and was most reliably calculated using a power function model in a Millville silt loam field soil, while both power function and SAPHIR models were similarly reliable in a Kidman fine sandy loam laboratory test. A single CO 2 sensor at a depth of 5 cm with water content monitored at 2.5 cm provided reasonable estimates of the soil CO 2 efflux validated with an automated chamber. We found that under most conditions, the CO 2 concentration gradient in the soil profile is a reasonable estimator of CO 2 flux when measurements of the soil water content and known porosity values are used to estimate the gas diffusion coefficient. However, shallow wetting events require improved monitoring of spatial and temporal changes near the surface or appropriate modeling of hydrodynamics there.

Description: Because root water uptake is a considerable component in the soil water balance, a lot of attention has been paid to defining and applying root water uptake models. These models can be grouped into empirical vs. mechanistic root uptake models. Intermodel comparisons are valuable in understanding the different concepts used. Such a comparison is sometimes difficult because the level of information required by the models is different, for example, information on the root length density distribution. Here a three-dimensional root length density distribution function is introduced that makes it possible to compare two empirical uptake models with a more mechanistic uptake model. Adding a compensation component to the more empirical model results in prediction of root water uptake distributions in the root zone similar to those predicted by the more complex model.

Description: A new constitutive relationship for phase partitioning of water in frozen soils is proposed. This relationship extends to unsaturated conditions established relationships for gas-free conditions by smoothing a thermodynamically derived relationship to eliminate a jump discontinuity at the freezing temperature. This relationship is shown to compare well with experimental data on unfrozen water content as a function of temperature for different total water content values. Using this new relationship, a modified nonisothermal Richards equation is solved for flow in freezing soil. The results based on this modified Richards equation are shown to compare well with data from two different column experiments.

Description: The COsmic-ray Soil Moisture Observing System (COSMOS) rover may be useful for validating satellite-based estimates of near-surface soil moisture, but the accuracy with which the rover can measure 0- to 5-cm soil moisture has not been previously determined. Our objectives were to calibrate and validate a COSMOS rover for mapping 0- to 5-cm soil moisture at spatial scales suitable for evaluating satellite-based soil moisture estimates. Region-specific calibrations for the COSMOS rover were developed using field-average soil moisture measured using impedance probes and the oven-drying method on volumetric samples. The resulting calibrations were applied to map soil moisture on two dates in a 16- by 10-km region around the Marena, Oklahoma, In Situ Sensor Testbed (MOISST) in north central Oklahoma and one date in a 34- by 14-km region in the Little Washita River watershed in southwestern Oklahoma. The mapped soil moisture patterns were consistent with the regional spatial variability in surface soil texture and with soil wetting by an intervening rainfall. The rover measured field-average soil moisture with a root mean squared difference (RMSD) of 0.03 cm 3 cm –3 relative to impedance probes. Likewise, the regional-average 0- to 5-cm soil moisture determined by the rover was within ±0.03 cm 3 cm –3 of the best available independent estimates for each region. These results demonstrate that a COSMOS rover can be used effectively for 0- to 5-cm soil moisture mapping and for determining the average soil moisture at spatial scales suitable for satellite calibration and validation.

Description: Groundwater recharge is primarily influenced by land use and climate. Nevertheless, it is the flow and transport processes that take place in the vadose zone that ultimately control the quantity and quality of groundwater replenishment. Vadose zone monitoring systems (VMSs) were implemented under agricultural fields. The VMSs provided continuous information on both the temporal variation in water content and the chemical composition of the sediment pore water at multiple depths in the deep vadose zone (~20 m). Models for vertical unsaturated flow and chloride transport were calibrated to the transient data. The calibrated models were then used to investigate the temporal characteristics of groundwater recharge coupled with chloride fluxes under a crop field and a grapefruit orchard. Examination of the transient data obtained under the two sites provided insight into the percolation processes and solute mobility in the deep vadose zone. Model simulations resulted in average recharge fluxes of 131 mm yr –1 under the orchard and 199 mm yr –1 under the crop field, even though average annual water input into the irrigated orchard was 2.6 times higher than that into the crop field (rainfed). The model also predicted a 44% drop in average recharge, as well as significant chloride accumulation in the vadose zone as a consequence of a reduction of 19% in rain following climate change scenarios in the area.

Description: Land management of agricultural crops can have impacts on soil hydrology. The objective of our research was to evaluate subsurface drain flow and soil water storage dynamics due to land management practices of select annual- and perennial-based biofuel cropping systems. The cropping systems were continuous corn ( Zea mays L.; harvested for both grain and ~50% of the corn stover) with and without a winter cereal rye ( Secale cereale L. ssp. cereale ) cover crop, mixed prairies (harvested annually for aboveground biomass) with and without N fertilization, and corn–soybean [ Glycine max (L.) Merr.] rotations harvested only for grain. Subsurface drainage flows and soil water content profiles were continuously monitored when soils were unfrozen during 2010 through 2012. Cropping systems were evaluated based on cumulative drainage and drainage event peak flows, time lags, and total durations. Cropping system influence on soil water storage did affect subsurface drainage flow characteristics and cumulative drainage. Prairies or the use of a winter cereal rye cover crop had greater lag times by 127 to 179%, lower peak flow intensities by 23 to 36%, and lower cumulative drainage by 37 to 46% than either corn–soybean rotations or continuous corn without a cover crop. The lower cumulative drainage was attributed to greater evapotranspiration rates and lower stored soil water that resulted in a decrease in peak flow intensities and increased time lags of both peak flow and drainage initiation. Low antecedent soil moisture resulted in low peak flows and cumulative drainage. Based on these findings, prairie systems or the use of a cover crop may aid in mitigating flood frequency in subsurface-drained landscapes.

Description: Evapotranspiration (ET) was quantified for two rangeland vegetation types, aspen and sagebrush-grassland, over an 8-yr study period by comparing the following approaches for estimating ET: eddy covariance systems (EC, available for only 6 yr); soil water storage loss measured by time domain reflectometry (TDR) and neutron probe; and model simulation. The research site, the Upper Sheep Creek catchment in the Reynolds Creek Experimental Watershed, is part of a study on the effects of prescribed fire and vegetation removal. Estimates of seasonal ET for the aspen using EC with the turbulent fluxes adjusted to force energy balance closure, a 180-cm TDR soil profile, two 225-cm neutron access tubes, and the Simultaneous Heat and Water (SHAW) model agreed well with each other. If the two neutron probes were averaged, the RMSD of all approaches over the 6 yr was within 8% of the average. For the sagebrush-grassland, a 120-cm TDR profile underestimated seasonal ET for all years except the year immediately following the prescribed fire, when rooting depth likely had not recovered. A 195-cm neutron probe access tube located within 30 m of the stream underestimated ET for every year except when there was no streamflow, suggesting that lateral flow may have biased the results for this tube. A comparison of the other methods (EC flux adjusted to force energy balance closure, soil water loss measured from a 225-cm neutron access tube, and SHAW model simulation) agreed within 3% during the 6 yr with EC measurements at that site.

Description: A method for determining soil hydraulic parameters based on periodic point source solutions of the linearized Richards equation is proposed. Closed-form solutions were derived for buried and surface point sources with a sinusoidally varying flux. These solutions describe the dependence of the matric flux potential (MFP) amplitude and phase shift on the distance from the point sources, frequency of source flux alternations, and soil hydraulic parameters. The Fourier series representation of square waves was used to extend the point source solutions for sinusoidally varying flux to square-wave cyclic inputs of water with a 50% duty cycle—a preferable operating mode for field experiments. Close to the sources, the amplitudes and shape of MFP waves from a step input were more pronounced than those from a sinusoidal input, but the difference diminished as distance increased. The proposed method involves recording the pressure head variations with a continuously reading tensiometer installed below a pressure-compensating emitter controlled by an irrigation computer. Among 16 cyclic step-input irrigation tests, one series involved pressure-head measurements at a single depth and varied water-pulse durations; the second involved pressure-head measurements at various depths with a fixed water-pulse duration. The soil saturated hydraulic conductivity, the coefficient α of Gardner’s conductivity model, and the coefficient k of the linearized unsteady-flow equation were estimated by matching the periodic solution for cyclic step inputs to the measured pressure-head data with a two-step inversion procedure: an amplitude-shift (AS) estimation procedure followed by the Levenberg–Marquardt (LM) optimization algorithm. The AS procedure was effective, and the LM optimization yielded only minor improvements. Our main findings are: (i) the periodic solution for step inputs yields reasonably good predictions of pressure-head variations measured at various depths below an emitter for different periods of water application; and (ii) the proposed point-source method seems to yield consistent and reliable estimates of the three soil hydraulic parameters.

Description: Solute diffusion in variably saturated porous media plays an important role in the transport of contaminants and dissolved chemicals in the vadose zone. Various empirical and semiphysical models have been proposed to describe the saturation dependence of solute diffusion. In this study, we applied concepts from percolation and effective medium theories to develop a theoretical framework to model solute diffusion. Percolation theory, which has been successfully applied to describe saturation-dependent air permeability, electrical conductivity, and gas diffusion in porous media, provides a universal scaling, a power law in the water content (less a threshold value) with an exponent of 2.0. To evaluate our universal scaling, we used a database including 106 experiments (766 data points) from published studies and found good agreement between theory and measurement.

Description: Implementation of the dual-probe heat-pulse (DPHP) approach for measurement of volumetric heat capacity ( C ) and water content () with distributed temperature sensing heated fiber optic (FO) systems presents an unprecedented opportunity for environmental monitoring (e.g., simultaneous measurement at thousands of points). We applied uniform heat pulses along a FO cable and monitored the thermal response at adjacent cables. We tested the DPHP method in the laboratory using multiple FO cables at a range of spacings. The amplitude and phase shift in the heat signal with distance was found to be a function of the soil volumetric heat capacity. Estimations of C at a range of moisture contents ( = 0.09– 0.34 m 3 m –3 ) suggest the feasibility of measurement via responsiveness to the changes in , although we observed error with decreasing soil water contents (up to 26% at = 0.09 m 3 m –3 ). Optimization will require further models to account for the finite radius and thermal influence of the FO cables. Although the results indicate that the method shows great promise, further study is needed to quantify the effects of soil type, cable spacing, and jacket configurations on accuracy.

Description: In highly stratified soils as on the Tibetan Plateau, uncertainty associated with a vertical profile of soil and hydraulic properties largely restricts the performance of Soil Vegetation Atmosphere Transfer (SVAT) model. In lieu of commonly used pedotransfer functions (PTFs) or artificial neural networks (ANNs), soil hydraulic properties in this study were inverted from an Ensemble Kalman filter (EnKF) analysis of Synthetic Aperture Radar (SAR) surface soil moisture. The calibrated SVAT scheme using inverted soil hydraulic variables C 1 and geq was better matched with in situ field measurements than the uncalibrated SVAT scheme using soil maps–based PTFs on a local point scale. It was shown that the inverse calibration of two soil hydraulic variables solved the forecast bias (underestimation) in surface soil moisture due to the assumption of vertical homogeneity and the site-specificity of empirical PTFs. Additionally, at a SAR spatial scale, the calibrated SVAT scheme appropriately captured a high vertical gradient between surface and subsurface soil moisture, while the uncalibrated SVAT scheme could not. This suggests that it is possible to infer the SVAT soil hydraulic variables that are the main error source in SVAT scheme from the SAR soil moisture data assimilation analysis.

Description: Nitrous oxide is a greenhouse gas and contributes to stratospheric ozone depletion. Soil physical conditions may influence N 2 O reduction and subsequent N 2 O emissions. We studied how soil water-filled pore space (WFPS) and soil bulk density ( b ) affect N 2 O reduction and surface fluxes. Columns were repacked with soil and arranged in a factorial design at three levels of WFPS (60, 75, and 90%) and three levels of soil b (0.94, 1.00, 1.07 Mg m –3 ). Over 19 d, 15 N-enriched N 2 O was introduced at the base of the soil columns and N 2 O fluxes were measured. Relative gas diffusivities ( D p / D o ) were also calculated. Soil b and WFPS interacted to affect the recovery of N 2 O- 15 N and the antecedent inorganic-N contribution to surface fluxes. Reduction rates of N 2 O- 15 N ranged from 0.15 to 0.47 mg N 2 O-N g –1 soil d –1 . Calculated D p / D o values correlated ( P 〈 0.01) with soil NH 4 + –N ( r = –0.73), NO 3 – –N ( r = 0.93), cumulative N 2 O-N flux ( r = 0.76), and N 2 O-N 15 N enrichment ( r = 0.80) and were affected by a soil WFPS x soil b interaction. Soil N transformations and the net surface N 2 O flux is dependent on the soil’s D p / D o , and WFPS alone does not suffice to discriminate between N 2 O emission sources. Consequently, the soil surface N 2 O flux may be comprised of N 2 O originating from deeper soil layers transported upward and/or from production in the topsoil.

Description: In a series of mesocosm experiments, with barley ( Hordeum vulgare L.) grown on podzolic soil material, we have investigated inorganic carbon cycling through the gaseous and liquid phases and how it is affected by different soil amendments. The mesocosm amendments comprised the addition of 0, 9.6, or 21.2 kg m –2 of crushed concrete waste (CCW) or 1 kg lime m –2 . The CCW and lime treatments increased the dissolved inorganic carbon (DIC) percolation flux by about 150 and 100%, respectively, compared to the controls. However, concurrent increases in the CO 2 efflux to the atmosphere (ER) were more than one order of magnitude higher than increases in the DIC percolation flux. Analysis of soil solutions, coupled reactive-transport modeling studies, and a decrease in soil carbonate contents over the experiment altogether suggested that the increased ER from amended mesocosms was derived from the carbonate contained in the amendments, which, hence, mostly escaped to the atmosphere. Our results are important in the context of climate change due to the widespread application of lime to acidic soils. The CCW amendment had no adverse effects on plant growth and groundwater quality.

Description: Surface nuclear magnetic resonance (SNMR) is a geophysical technique for water exploration in the shallow subsurface. To investigate the vadose zone, it is necessary to increase the method sensitivity; therefore we developed a new approach for SNMR experiments. Adapted from laboratory applications, an artificial static magnetic prepolarizing (Px) field is superimposed on the Earth’s magnetic field to enhance the equilibrium magnetization of the pore water and thus to increase the SNMR sounding signals. We numerically investigated the basic feasibility of this approach by modeling the relevant static and oscillating magnetic fields and resulting SNMR responses. Configuration parameters, e.g., current intensities, loop sizes, and layouts, were optimized by numerical simulations. The most feasible field layouts comprised two Px loops symmetrically arrayed with respect to the transmitter/receiver (Tx/Rx) antennae or a single Px loop with its radius approximately three times larger than the respective Tx/Rx loops, thus yielding a 10-fold increase of the recorded signals compared with conventional SNMR. Eventually, we simulated synthetic sounding curves obtained from a water body layer at various depths and thicknesses, concluding that in the shallowest subsurface, the use of two loops to enhance the signal is more effective than a single antenna, but the results deteriorate for deeper water layers.

Description: Accurate information on the dry end (matric potential less than –1500 kPa) of soil water retention curves (SWRCs) is crucial for studying water vapor transport and evaporation in soils. The objectives of this study were to assess the potential of the Oswin model for describing the water adsorption curves of soils and to predict SWRCs at the dry end using the hygroscopic water content at a relative humidity of 50% ( RH50 ). The Oswin model yielded satisfactory fits to dry-end SWRCs for soils dominated by both 2:1 and 1:1 clay minerals. Compared with the Oswin model, the Campbell and Shiozawa model combined with the Kelvin equation (CS-K) produced better fits to dry-end SWRCs of soils dominated by 2:1 clays but provided poor fits for soils dominated by 1:1 clays. The shape parameter α of the Oswin model was dependent on clay mineral type, and approximate values of 0.29 and 0.57 were obtained for soils dominated by 2:1 and 1:1 clays, respectively. Comparison of the Oswin model combined with the Kelvin equation, with water potential estimated from RH50 (Oswin-K RH50 ), CS model combined with the Arthur equation (CS-A), and CS-K model, with water potential obtained from RH50 (CS-K RH50 ) indicated that for soils dominated by 2:1 clay minerals, the predictive ability of the Oswin-K RH50 model was comparable to the CS-K RH50 model in which RH50 was the input parameter but performed better than the CS-A model where clay content was the input parameter. The Oswin-K RH50 model also has the potential for predicting dry-end SWRCs of soils dominated by 1:1 clays.

Description: The averaging behavior of two capacitance water content sensors in nonuniform wetted sand samples was assessed and compared with the averaging behavior of a time domain reflectometry (TDR) water content sensor in identical samples. Four different nonuniform wetting situations were assessed. In one of the four experiments, the orientation of the capacitance sensor was altered, while in another the sensitivity along the length of the sensor was tested. For one of the tested sensor types, the overestimation of the volumetric water content (VWC) was 0.034 compared with the value determined by drying, while the corresponding TDR value deviated 0.009. The conclusion is that in a nonuniform wetted soil profile, the capacitance sensors tested weigh the wet parts more heavily than the dry parts, resulting in an overestimation of the average VWC. Therefore we conclude that the capacitance sensors tested are less suitable for applications in situations where both nonuniform wetted profiles are encountered and accurate VWC measurements are required.

Description: A method to simulate freeze–thaw and permafrost conditions on a large peat-soil column, housed in a biome, was developed. The design limits ambient temperature interference and maintains one-dimensional freezing and thawing. An air circulation system, in a cavity surrounding the active layer, allows manipulation of the lateral temperature boundary by actively maintaining an air temperature matching the average temperature of the soil column. Replicating realistic thermal boundary conditions enabled field-scale rates of active-layer thaw. Radial temperature gradients were small and temperature profiles mimicked those for similar field conditions. The design allows complete control of key hydrologic processes related to heat and water movement in permafrost terrains without scaling requirement; and presents a path forward for the large-scale experimental study of frozen ground processes. Because subarctic ecosystems are very vulnerable to climate and anthropogenic disturbances, the ability to simulate perturbations to natural systems in the laboratory is particularly important.

Description: Root system architecture and associated root–soil interactions exhibit large changes over time. Nondestructive methods for the quantification of root systems and their temporal development are needed to improve our understanding of root activity in natural soils. X-ray computed tomography (X-ray CT) was used to visualize and quantify growth of a single Vicia faba L. root system during a drying period. The plant was grown under controlled conditions in a sandy soil mixture and imaged every second day. Minkowski functionals and Euclidean distance transform were used to quantify root architectural traits. We were able to image the root system with water content decreasing from 29.6 to 6.75%. Root length was slightly underestimated compared with destructive measurements. Based on repeated measurements over time it was possible to quantify the dynamics of root growth and the demography of roots along soil depth. Measurement of Euclidean distances from any point within the soil to the nearest root surface yielded a frequency distribution of travel distances for water and nutrients towards roots. Our results demonstrate that a meaningful quantitative characterization of root systems and their temporal dynamics is possible.

Description: A sophisticated dual-permeability flow model of the T-tunnel complex, Rainier Mesa, Nevada National Security Site, was developed and calibrated to facilitate predictions of radionuclide transport from underground nuclear tests within a perched zone of saturation downward through a series of variably saturated, sparsely fractured and faulted Tertiary tuff units to a regional flow system. The hydrogeologic complexity of the site necessitated a multistage calibration effort to capture the dominant flow characteristics of the site including: laterally and vertically extensive saturation in the Tertiary volcanics, pressure heads consistent with water level measurements, variably saturated flow conditions involving a thin unsaturated zone situated between two saturated zones, unsaturated Paleozoic carbonates located immediately adjacent to saturated Tertiary volcanic units, and fracture saturations congruent with field observations that range from fully saturated to dry. The incorporation of discontinuous fault networks into the dual-permeability model played a key role in reproducing the salient hydrologic features of the site. Successful model reproduction of observed cumulative tunnel discharge volume and tunnel discharge rates before tunnel sealing served as a novel transient test of the ability of the discontinuous fault networks to realistically honor field-scale fault network connectivity and permeability. The excellent reproduction of field observations by the numerical model, and the uniqueness of the primary parameters used to calibrate the model, demonstrate the advantage of incorporating discontinuous fault networks over equivalent fracture properties in sparsely fractured, dual-permeability media.

Description: As a remedial approach, vacuum-induced pore-water extraction offers the possibility of contaminant and water removal from the vadose zone, which may be beneficial in reducing the flux of vadose zone contaminants to groundwater. Vadose zone water extraction is being considered at the Hanford Site in Washington State as a means to remove technetium-99 contamination from low permeability sediments with relatively high water contents. A series of intermediate-scale laboratory experiments have been conducted to improve the fundamental understanding and to recognize the limitations of the technique. Column experiments were designed to investigate the relations between imposed suctions, water saturations, and water extraction. Flow cell experiments were conducted to investigate the effects of high-permeability layers and near-well compaction on pore-water extraction efficiency. Results show that water extraction from unsaturated systems can be achieved in low permeability sediments, provided that the initial water saturations are relatively high. The presence of a high-permeability layer decreased the yield, and compaction near the well screen had a limited effect on overall performance. In all experiments, large pressure gradients were observed near the extraction screen. Minimum requirements for water extraction include an imposed suction larger than the initial sediment capillary pressure in combination with a fully saturated seepage-face boundary. A numerical multiphase simulator with a coupled seepage-face boundary condition was used to simulate the experiments. Reasonable matches were obtained between measured and simulated results for both water extraction and capillary pressures, suggesting that numerical simulations may be used as a design tool for field-scale applications of pore-water extraction.

Description: One issue in understanding the performance of pedotransfer functions (PTFs) is knowing the soil properties that contribute most to PTF errors. Classification trees provide a means for identifying these properties. The objective of this study was to use a classification tree to identify patterns in PTF residuals. The analysis was applied to PTFs developed by Vereecken and coworkers in 1989 to estimate water contents at –10 and at –1500 kPa. Errors below and above certain threshold values were defined as acceptable and unacceptable, respectively. The threshold acceptability values were assumed to be 0.04 and 0.01 cm 3 cm –3 at –10 and –1500 kPa, respectively. The sensitivity of the trees to these values was studied by reducing them by 10%. Tentative predictors of the trees were sand, silt, and clay contents, bulk density (BD), soil organic C (OC) content, and horizon position (HP) in the soil profile (topsoil or subsoil). At –10 kPa, unacceptable data sets had very low clay contents or low BDs, given the poor representation of soil structure in the PTFs. At –1500 kPa, the unacceptable data sets had high clay and low OC contents with a HP of subsoil, or had high clay and high OC. Results were found to be moderately sensitive to small variations in the threshold. The least accurate estimates occurred for conditions in which the attributes used in the PTF were probably insufficient to unambiguously determine soil water retention data. Our study showed that classification trees can be helpful in identifying most optimal predictors for a PTF.

Description: This study presents a new method for deriving the breakthrough curve (BTC) of Br – tracer by measurements in the laboratory. In this method, the breakthrough is recorded by an ion selective electrode placed in a flow cell into which a peristaltic pump redirects part of the effluent. Special care was taken to make the measurements insensitive to variations in ionic strength in the effluent by mixing in the ionic strength adjuster. To evaluate this newly proposed method, BTCs of Br – and 2 H were measured for two saturated columns during steady-state flow under ponding boundary conditions. A concentration pulse was generated by the newly developed infiltration head with two independent sets of injection capillaries, which dosed water or tracer solution into the ponding water while keeping the ponding height constant. In addition to the continuous automatic method, discrete samples of effluent were analyzed for Br – by ion chromatography and for 2 H using laser spectroscopy. Experiments were conducted on packed sand and on an undisturbed sample of coarse sandy loam soil. In both cases, the results showed good agreement between BTCs of Br – and 2 H measured using the proposed method and those derived using traditional methods on effluent samples collected during the same experiment. Additional experiments showed good reproducibility of the BTC measurement during replicated experiments.

Description: Three assays were conducted to evaluate the reliability of lysimeters in detecting concentrations of Cu, Pb, Sb, and Zn in interstitial water of the vadose zone. Two box lysimeters (BLs) made from polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF) and three tension lysimeters (TLs) of different types were used. Assays were conducted with two concentrations of dissolved metals, undiluted and diluted in a 1:10 ratio, and at three different pHs. The assays were made to evaluate the sorption induced by the lysimeter material itself, by standard quartz sand added to the BLs, and by two types of glass beads placed around the porous cup of the TLs. Generally, BLs were more suitable for interstitial water sampling than TLs. Among the TLs, the one made of nylon was more reliable than the two others. Box lysimeters are a good choice to obtain accurate and comparable results for Cu, Pb, Sb, and Zn analysis. However, they have to be backfilled with natural soil found at the exact location of the lysimeter and not with standard quartz sand. Nylon TLs can also be used for water sampling for Sb, Pb, and Zn analysis but not for Cu. Glass beads may interact with dissolved metals and their use around the suction cup in TLs can be avoided by using the fine fraction of natural sediments (〈125 μm) found at the location of the porous cup. Water samples from TLs do not need to be filtered for dissolved metal analysis because they are already filtered by the porous cup.

Description: A natural gradient SF 6 tracer experiment provided an unprecedented evaluation of long distance gas transport in the deep unsaturated zone (UZ) under controlled (known) conditions. The field-scale gas tracer test in the 110-m-thick UZ was conducted at the U.S. Geological Survey’s Amargosa Desert Research Site (ADRS) in southwestern Nevada. A history of anomalous (theoretically unexpected) contaminant gas transport observed at the ADRS, next to the first commercial low-level radioactive waste disposal facility in the United States, provided motivation for the SF 6 tracer study. Tracer was injected into a deep UZ borehole at depths of 15 and 48 m, and plume migration was observed in a monitoring borehole 9 m away at various depths (0.5–109 m) over the course of 1 yr. Tracer results yielded useful information about gas transport as applicable to the spatial scales of interest for off-site contaminant transport in arid unsaturated zones. Modeling gas diffusion with standard empirical expressions reasonably explained SF 6 plume migration, but tended to underpredict peak concentrations for the field-scale experiment given previously determined porosity information. Despite some discrepancies between observations and model results, rapid SF 6 gas transport commensurate with previous contaminant migration was not observed. The results provide ancillary support for the concept that apparent anomalies in historic transport behavior at the ADRS are the result of factors other than nonreactive gas transport properties or processes currently in effect in the undisturbed UZ.

Description: The saturated hydraulic conductivity, K s , is a fundamental characteristic of subsurface flow and the hydrologic cycle. However, direct measurement of K s is time consuming. Recently, air permeability measurements at 50 and 100 cm of H 2 O tension, a relatively dry condition in coarse and medium-textured soils, have been used to estimate K s . In this short communication, we present a theoretical framework for relating K s to k a (), the air permeability as a function of porosity, , under completely dry conditions. We used power-law scaling from continuum percolation theory to develop a theoretical relationship between K s and k a (). Our result, similar in form to a published logarithmic equation with empirical coefficients, gives a physical interpretation to these empirical coefficients.

Description: Shallow saturated subsurface flow, frequently observed on hillslopes of headwater catchments in humid temperate climates, often dominates hydrologic responses of the catchments to major rainfall events. Typically, these responses are significantly affected by the presence of preferential flow. Reliable prediction of runoff from hillslope soils under such conditions remains a challenge. In this study, two approaches to modeling hillslope responses to rainstorms, which differ in dimensionality and thus also in the complexity of geometric, material, and boundary conditions, were tested and used on the hillslope discharge data observed in an experimental trench. In the one-dimensional (1D) approach, 1D variably saturated vertical soil water flow is combined with 1D lateral saturated flow above the soil–bedrock interface. In this approach, vertical flow is modeled by means of a dual-continuum concept involving two coupled Richards’ equations (representing flow in the soil matrix and in the preferential pathways), while lateral flow is described by the diffusion wave equation. In the two-dimensional (2D) approach, the movement of water in a variably saturated hillslope segment is modeled as vertical planar flow (i.e., the vertical and lateral flow components are fully integrated into one flow system). Similar to the 1D approach, the preferential flow effects are implemented in the 2D model by means of the dual-continuum concept. The two model approaches (1D and 2D) resulted in similar hillslope discharge hydrographs, characterized by short-term runoff peaks followed by zero-discharge periods, but the 2D model showed closer agreement between observed and simulated soil water pressure heads near the trench. The sensitivity analysis of soil and bedrock properties confirmed a significant influence of the bedrock saturated hydraulic conductivity on simulated hillslope discharge. The simpler 1D approach, based on the combination of 1D vertical flow and 1D lateral flow, was found to provide a useful approximation of the more complex and flexible 2D system and to be far more efficient in terms of computing time.

Description: Anisotropy in unsaturated hydraulic conductivity is saturation-dependent. Accurate characterization of soil anisotropy is very important in simulating flow and contaminant transport (e.g., radioactive nuclides at the U.S. Department of Energy’s Hanford Site). A tensorial connectivity–tortuosity (TCT) concept describes the hydraulic conductivity tensor of the unsaturated anisotropic soils as the product of a scalar variable, the symmetric connectivity tortuosity tensor, and the saturated hydraulic conductivity tensor. In this study, a model based on the TCT concept is used to quantify soil anisotropy in unsaturated hydraulic conductivity. The TCT model can describe different types of soil anisotropy; for example, the anisotropy coefficient can monotonically increase or decrease with saturation and can vary from greater than unity to less than unity and vice versa. Soil anisotropy is independent of soil water retention properties and can be characterized by the ratio of the saturated hydraulic conductivities and the difference of the tortuosity–connectivity coefficients in two directions. The anisotropy coefficient is log-linearly proportional to the effective saturation. This log-linear relationship allows the saturation-dependent anisotropy to be determined using regression with the measurements of the directional hydraulic conductivities at a minimum of two water content levels, one of which may be at full saturation. The model was tested using measured or simulated directional hydraulic conductivities.

Description: The magnetic resonance sounding (MRS) method is usually applied for delineation and characterization of aquifer system stratification. Its unique property, distinct from other hydrogeophysical methods, is the direct sensitivity to water content in the subsurface. The inversion of MRS data yields the subsurface water content distribution without need of a petrophysical model. Recent developments in instrumentation, i.e., decreased instrumental dead times and advanced noise cancellation strategies, enable the use of this method for investigating the vadose zone. A possible way to interpret MRS measurements with focus on water retention (WR) parameters is an inversion approach that directly provides WR parameters by modeling the capillary fringe (CF inversion). We have developed this kind of inversion further to account for different WR models and present a sensitivity study based on both synthetic and real field data. To assess the general applicability of the CF inversion, we analyzed the resolution properties for different measurement layouts and the parameter uncertainties for different realistic scenarios. Under moderate noise conditions and if the water table position is known, all WR parameters except the residual water content can be reliably estimated. The relative accuracy of the estimated pore distribution index estimation is better for larger CF. Small measurement loops of 5-m diameter achieve the best resolution for shallow investigation depths of 〈10 m.

Description: One strategy for maintaining irrigated agricultural productivity in the face of diminishing resource availability is to make greater use of marginal quality waters and lands. A key to sustaining systems using degraded irrigation waters is salinity management. Advanced simulation models and decision support tools can aid in the design and management of water reuse systems, but at present model predictions and related management recommendations contain significant uncertainty. Sensitivity analyses can help characterize and reduce uncertainties by revealing which parameter variations or uncertainties have the greatest impact on model outputs. In this work, the elementary effects method was used to obtain global sensitivity analyses of UNSATCHEM seasonal simulations of forage corn ( Zea mays L.) production with differing irrigation rates and water compositions. Sensitivities were determined with respect to four model outcomes: crop yield, average root zone salinity, water leaching fraction, and salt leaching fraction. For a multiple-season, quasi-steady scenario, the sensitivity analysis found that overall the most important model parameters were the plant salt tolerance parameters, followed by the solute dispersivity. For a single-season scenario with irrigation scheduling based on soil water deficit, soil hydraulic parameters were the most important; the computed salt leaching fraction was also strongly affected by the initial ionic composition of the exchange phase because of its impact on mineral precipitation. In general, parameter sensitivities depend of the specifics of a given modeling scenario, and procedures for routine use of models for site-specific degraded irrigation water management should include site-specific uncertainty and sensitivity analyses. The elementary effects method used in this work is a useful approach for obtaining parameter sensitivity information at relatively low computational cost.

Description: We have developed a screening method for simplifying groundwater models by delineating areas within the domain that can be represented using steady-state groundwater recharge. The screening method is based on an analytical solution for the damping of sinusoidal infiltration variations in homogeneous soils in the vadose zone. The damping depth is defined as the depth at which the flux variation damps to 5% of the variation at the land surface. Groundwater recharge may be considered steady where the damping depth is above the depth of the water table. The analytical solution approximates the vadose zone diffusivity as constant, and we evaluated when this approximation is reasonable. We evaluated the analytical solution through comparison of the damping depth computed by the analytic solution with the damping depth simulated by a numerical model that allows variable diffusivity. This comparison showed that the screening method conservatively identifies areas of steady recharge and is more accurate when water content and diffusivity are nearly constant. Nomograms of the damping factor (the ratio of the flux amplitude at any depth to the amplitude at the land surface) and the damping depth were constructed for clay and sand for periodic variations between 1 and 365 d and flux means and amplitudes from nearly 0 to 1 x 10 –3 m d –1 . We applied the screening tool to Central Valley, California, to identify areas of steady recharge. A MATLAB script was developed to compute the damping factor for any soil and any sinusoidal flux variation.

Description: Mercury intrusion porosimetry (MIP) is commonly used to determine soil pore size distributions (PSDs) in a wide, but incomplete, range of equivalent pore diameters. Mono- and multifractal analyses of soil PSDs are useful for analyzing the soil porous system. The main objective of this work was to characterize PSDs from tropical soils using the multifractal approach. Duplicate PSDs were measured by MIP for 54 horizons collected from 19 soil profiles in Minas Gerais, Brazil. Although 10 of the studied soils were Ferralsols (Oxisols or Latosols), other soil groups such as Nitisols, Acrisols, Alisols, Luvisols, Planosols, Cambisols, Andisols, and Leptosols also were sampled. Different patterns of PSDs were observed, depending on soil type, texture, and weathering intensity. Textural porosity (0.005–0.2 μm) was positively and significantly correlated to clay and Al 2 O 3 contents and soil specific surface area. Comparison of multifractal analysis from Hg intrusion curves and N 2 adsorption isotherms (NAIs) showed entropy dimension values and width of the singularity spectra smaller for the former than for the latter. Multifractal analysis of MIPs and NAIs allowed a thorough characterization of the complexity and heterogeneity of the soil pore space at different scales and was useful to provide new insights for portraying the soil pore system.

Description: The soil water retention curve (SWRC) describes a soil’s constitutive relation between matric suction and volumetric water content. The suction stress characteristic curve (SSCC) describes a soil’s constitutive relation between suction stress and volumetric water content. Both the SWRC and SSCC are often established assuming a soil is nondeformable. Under a shallow field environment, however, the total stress can change several hundreds of kilopascals due to self-weight variation with depth, surface structural loading, or soil excavation, leading to changes in volumetric strain and consequently the SWRC and SSCC. This work experimentally examined the effect of a confining stress on the SWRC and SSCC under both drying and wetting conditions through a silty sand. The SWRCs were measured under different confining stresses up to 200 kPa, allowing assessment of the uniqueness of the SWRC or SSCC. We found that under wetting conditions, even though the SWRCs and SSCCs were different under different confining stresses, a unique SWRC or SSCC could be defined if the effective degree of saturation was used. Under drying conditions, the uniqueness of the SWRC or SSCC held approximately; the differences mainly reflecting in the air-entry pressure. Furthermore, the failure envelope by the effective stress representation based on the SWRC was unique for either wetting or drying and was well represented by the saturated failure envelope. The SSCC inferred from the shear strength tests was similar to that from the SWRC measurement under the respective wetting or drying states. Therefore, the SWRC and SSCC are practically independent of the confining stress up to 200 kPa for a silty sand.

Description: Microtensiometer probes with a tip radius 〈1 mm were used for direct measurement of the change in pore water pressure caused by tensile loading at different strain rates in soils. These probes responded rapidly to changes in pore water pressure during testing and demonstrated that the applied tensile stress was transmitted almost entirely through the pore water, as would be expected. Above a strain rate of 1% min –1 , viscous effects became significant, leading to a significant increase in the fracture stress. The results are described using an extended version of the Kelvin–Voigt model of rheological behavior. At low strain rates, capillary forces dominate the fracture stress. Above the critical strain rate, the viscosity of the soil also contributes to the fracture stress.

Description: The thermodynamical analysis presented here follows from the work of Coussy et al., who proposed a thermodynamically consistent model for unsaturated soils that is based on a Bishop-like effective stress to describe the stress–strain relationship while the water saturation (or the capillary pressure) is involved in a saturation-induced hardening in addition to the mechanical hardening. We extended this model to include the effect of interfaces in the mechanical behavior and we showed that the Bishop-like stresses involved in the elastic and plastic responses respectively can take different expressions. The Modified Cam–Clay model used for saturated soils is extended to unsaturated soils through the use of these Bishop-like stresses. This model is compared to some experimental results reported from the literature.

Description: A three-scale model is proposed to describe electro-chemo-mechanical couplings in unsaturated swelling clays characterized by two porosity levels and three separate length scales. The nanoscale portrait consists of charged clay particles separated by a nanoporous network saturated by a binary monovalent aqueous electrolyte solution. Local ion distribution and electric potential are governed by the Poisson–Boltzmann problem. At the microscale, the system is represented by swollen clay clusters separated from each other by a network of micropores filled with a mixture of bulk water and air. Under mechanical equilibrium characterized by the competition between disjoining forces of electrochemical nature and capillary attraction effects, we derived a novel form of the effective stress principle in the asymptotic limit of scale separation. Such form includes a new macroscopic equivalent pore pressure weighted by a two-scale effective Bishop coefficient that incorporates the effects of the adsorbed water at the secondary nanopore level in addition to the contributions of the water–air interfaces at the micropore level. Within the thermodynamic context for constructing macroscopic constitutive laws based on stress–strain variables, the three-scale model leads to a set of three work-conjugated state variables. In addition to the contact stress between particles and the new effective Bishop-type component, the novel form includes salinity as an additional stress-conjugated variable and a three-scale version of the electrochemical swelling stress. The potential of the multiscale approach in capturing the complex features of unsaturated expansive clays is illustrated by numerical reconstruction of the effective coefficients in a simplified isotropic microstructure.

Description: We designed and developed an experimental setup in which acoustic to electromagnetic (EM) wave conversions at interfaces can be measured. Theoretical results are obtained with an electrokinetic full-waveform theoretical model, where use was made of the Sommerfeld approach. Using bimodal samples, different fluid–solid interface effects and saturating fluids were investigated. The contrast between water and water-saturated porous glass samples is larger than the contrast between water and oil-saturated porous glass samples. The contrast between water and water-saturated Fontainebleau sandstone is larger than the contrast between oil and water-saturated Fontainebleau sandstone.

Description: The effective stress in clays possessing a significant proportion of one of the clay minerals kaolinite, illite, or montmorillonite was determined based on the suction stress characteristic curves (SSCCs) of the clays. The SSCCs were determined based on the drying soil-water characteristic curves of the clays for a suction range of 0.03 to about 219.0 MPa. One-dimensional compressibility behavior of initially saturated clays was also studied by loading clay specimens up to a maximum vertical stress of 21.0 MPa. The effective stress–void ratio responses of the clays during the drying process were compared with their saturated counterparts. The shapes of SSCCs and the magnitudes of minimum and maximum suction stress were strongly dependent on the mineralogy and the properties of the clays. For the clays with kaolinite and illite as the dominant clay minerals, the suction stress decreased, remained nearly constant, and then increased with an increase in the applied suction, whereas it decreased monotonically with increasing suction for the montmorillonite clay. A decrease in the suction stress caused an increase in the effective stress, which in turn reduced the volume of the clays. For applied suctions smaller than the air-entry value of any clay, equal magnitudes of suction stress and effective stress produced a similar volume change of the clay. The study clearly showed that suction changes beyond the air-entry value are less effective in producing volume changes in unsaturated soils, primarily because of a decrease in the effective stress due to an increase in the suction stress.

Description: A pore-scale model for water retention and the specific air–water interfacial area is developed to quantitatively assess the contribution of interfacial forces to effective stress in coarse-grained unsaturated soils. The simulated air–water interfacial area for several sand-sized materials shows non-monotonic behavior as a function of saturation, with the peak interface area between 10 and 25% saturation. Comparisons are made between suction stress characteristic curves calculated with and without accounting for interfacial forces calculated using surface tension and the simulated interfacial area. The contribution of interfacial forces to effective stress is most significant at low saturation ( S 〈 ~0.2) and for wetting processes because the air–water interface area is largest. The percent difference between suction stress for drying processes calculated with and without accounting for interfacial forces is zero at 100% saturation, negligible for saturation greater than ~70%, increases to ~10% at 50% saturation, and reaches a maximum of ~50% at 10% saturation. During wetting, the difference increases to as much as 140% at 10% saturation. While these differences can be appreciable for some geotechnical problems, the practical implications of neglecting interfacial forces in effective stress formulations for the mechanical behavior of unsaturated soil depend on the range of saturation, suction stress, and normal stress encountered in typical field environments.

Description: The theory of poroelastic behavior in a deformable porous medium containing two immiscible, viscous, compressible fluids was applied to the three-dimensional consolidation of unsaturated soils. Three coupled partial differential equations were developed that feature the displacement vector of the solid phase and the excess pore water and air pressures as dependent variables. These equations generalize the classic Biot consolidation model, which applies to saturated soils, with effective stress emerging naturally from a pure compliance formulation of the relation between stress and strain. Under uniaxial strain and constant total compaction stress, the equations simplify to two coupled diffusion equations for the excess pore water and air pressures. Analytical solutions describing the response to instantaneous compression under both permeable and semipermeable boundary drainage conditions were obtained using the Laplace transform. Numerical calculations of pore water pressure, effective stress, and total settlement were made for a soil with clay texture as a representative example. The results show that excess pore water pressure dissipates faster at higher initial water content, leading to higher effective stress. The loading efficiency also was found to be highly sensitive to initial water saturation.

Description: To predict the shear strength of unsaturated soils, several equations have been proposed in the literature. The majority of these equations have the air entry value as the controlling parameter. In this study, an experimental program is planned to investigate the effect of confining stress on the air entry and air expulsion values, in the drying and wetting process, respectively. To evaluate the influence of stress state on these parameters, the soil water retention curve (SWRC) of the soil is determined under two different applied net stresses. For this purpose, a novel miniature pressure plate apparatus was designed to determine the SWRCs under different net stresses. The soil specimens are sheared in a suction-controlled triaxial apparatus, and the shear stress of the soil is measured. The measurements are performed under net confining stresses identical to the net stresses for which the SWRCs are obtained. Furthermore, the shear strength behavior of unsaturated soil samples are investigated in drying and wetting branches, and the values of effective internal friction angle and cohesion are determined for each path. Comparison of the obtained values to those of saturated samples revealed a rather significant difference. Finally, the shear strength values obtained from these measurements are compared with the predicted shear strength from some of the existing equations. In addition, variations of the parameter during drying and wetting processes and its dependence on the air entry/expulsion value were investigated and compared with reported equations. Moreover, a relationship is proposed for the effective stress parameter that can better capture the effect of net stress and, consequently, results in better predictions for the considered soil type.

Description: We conducted conventional triaxial and uniaxial tensile tests using partially saturated samples of an artificial compacted mixture of kaolin and sand. Test results were analyzed by applying the effective stress principle in partially saturated soils. The results showed that suction stresses derived by triaxial and tensile tests were consistent at saturations 〉0.1, while the prediction of suction stress from the drying and wetting soil water characteristic curves of the mixture failed. Experimental results indicated that soil structure effects result in differences in upscaling the microscopic interaction effects to macroscopic strength at different suction levels. These soil structure effects seem to differ for shear strength and tensile strength.

Description: For the saturated case with only one pore fluid, either water or air, the roles of both the intergranular stress tensor and the pore fluid stress can be distinguished easily. In the unsaturated case, the capillary water is recognized to induce capillary suction in the pores and capillary-suction-induced interparticle forces. At the macroscale, volume averaging of these forces would lead to the capillary-suction-induced intergranular stress tensor. In its approximate formulation, the concept of the fabric stress tensor is applied, enabling the effect of the spatial distribution of the intergranular fabric on the capillary water bridges as occurring in the drier pendular saturation phase to be accounted for. Subsequently, the combined intergranular stress tensor and the combined pore fluid stress tensor can be derived directly. The constitutive relation of a granular skeleton, composed of elastic particles with mainly frictional interaction, like quartz sands and silts, is considered to remain independent of the degree of saturation. Under such restrictive conditions, only the additional physical parameters of the capillary-suction-induced intergranular stress tensor need to be determined, which can be achieved by means of inverse modeling, taking advantage of all macroscale experimental data and physical modeling for the whole unsaturated range. For clays and peats, with potential physicochemical and biochemical actions and double porosity and/or fibrous microstructures, the constitutive models can be expected to be physically more complicated, thus involving more physically relevant parameters. Hence, clays and peats must be considered to fall outside the scope of the proposed model framework.

Description: This study investigated the mechanical behavior of a wet granular soil at low moisture content in which isolated pendular water bridges (menisci) at the interface of the solid particles give rise to capillary forces in addition to existing interparticle contact forces. We derived a single effective stress tensor encapsulating evolving liquid bridges, packing, interfaces, and water saturation. Apart from the fact that the stress due to contact forces is dependent on the fabric, we found that the so-called suction (capillary) stress arising from liquid bridges is inevitably direction dependent, i.e., anisotropic. The latter is at odds with the common belief that capillary stress is isotropic. We demonstrate that capillary stress is a function of the distribution of liquid bridges, degree of saturation, as well as particle packing, and thus provide an adequate effective stress definition to describe both the constitutive behavior and strength of unsaturated media. We conducted discrete element modeling simulations of triaxial compression tests of pendular-state granular samples at different matric suctions to verify the anisotropic nature of the capillary stress and resulting strength contributions to validate the proposed effective stress equation.

Description: Here we theoretically derive the Terzaghi stress principle for saturated and partially saturated isotropic, porous media with compressible phases. We use previously derived thermodynamic definitions of the drained and unjacketed compressibilities and total differentials to theoretically determine how the total pressure relates to isotropic strain and changes in fluid pressures. We show that under simplifying assumptions we recover the varying forms of the Biot coefficient for saturated porous media and the Bishop parameter for partially saturated porous media. We compare this approach with four modern constructions: the mixture theoretic approaches of Coussy and Borja, Wang’s differential approach, and the thermodynamically constrained averaging theory approach of Gray and Schrefler. In doing so we theoretically clarify the often confused definitions of the solid compressibility coefficient, the unjacketed and drained compressibilities, and the generalized Terzaghi stress principle in differential form.

Description: In continuum soil mechanics, the mechanical behavior of an element of soil is related to the effective stress, which is a measure of the average stress transmitted through the solid matrix in the form of contact stresses. In unsaturated soils, the coexistence of water and air within the soil pore space complicates this concept because the microscopic distribution of each fluid phase in soil pores cannot be known. Because it is thus not possible to physically measure effective stress in unsaturated soils, it is often estimated through the experimentally measurable shear strength. A conceptual model based on contact shear forces was built on the contact normal force based models from the literature to link effective stress and shear strength of the continuum to total stress and microscopic pore fluid forces for the simplified case of uniform spherical particles at low water contents. The validity of both models was examined by comparing model predictions of shear strength with the results of drained triaxial compression tests performed on specimens of uniform glass spheres. Although the shear strength calculated by the new model was consistently larger than that calculated via the contact normal force models, values measured in the experiments were even larger. Nevertheless, the test results fit what the new model analytically showed: the friction angle of a soil does not change with desaturation, and introducing moisture in the form of liquid bridges at contacts is fundamentally different from applying an equivalent isotropic stress to the soil externally.

Description: This paper presents an evaluation of the effective stress concept in unsaturated, compacted silt at low degrees of saturation. A set of isothermal, consolidated-drained triaxial tests was performed on silt specimens under different combinations of total suction and net normal stress. The total suction was controlled using an automated humidity system, and variables monitored during equilibration and shearing include anisotropic volume change, axial displacement, temperature, relative humidity at the top and bottom of the specimen, and the gas pressure difference across the specimen. The results from the triaxial tests were analyzed to examine the applicability of predicting the suction stress characteristic curve (SSCC) using parameters from soil water retention curve (SWRC) models fitted to experimental data obtained at low suction magnitudes. The SSCC predicted from the SWRC at low suctions was found to overpredict the suction stress values at high suctions obtained from back-extrapolation of the failure envelope to define the tensile strength. However, small adjustments in the fitting of the SWRC were found to provide a better fit between the SSCC and experimental suction stress data. The suction stress defined using the adjusted SWRC was found to provide a good interpretation of the critical state line in terms of mean effective stress over both high and low suction ranges.

Description: The first law of thermodynamics suggests an energy-conjugate relationship among degree of saturation, suction stress, and density of an unsaturated porous material. Experimental evidence affirms that this constitutive relationship exists and that the water retention curves are dependent on the specific volume or density of the material. This constitutive feature must be incorporated into the mathematical formulation of boundary-value problems involving finite deformation. We present a fully coupled hydromechanical formulation in the finite deformation range that incorporates the variation of degree of saturation with the Kirchhoff suction stress and the Jacobian determinant of the solid-phase motion. A numerical simulation of solid deformation–fluid flow in unsaturated soil with randomly distributed density and degree of saturation demonstrates an intricate but well-established coupling of the hydromechanical processes. As deformation localizes into a persistent shear band, we show that bifurcation of the hydromechanical response manifests itself not only in the form of a softening behavior but also through bifurcation of the state paths on the water-retention surface.

Description: We studied the properties of dissolved organic matter (DOM) in drained boreal peatland in stem-only-harvested (SOH), whole-tree-harvested (WTH), and unharvested sites. The DOM derived from both aerobic and anaerobic peat layers was divided into four different molecular-weight fractions with ultrafiltration, and the fractions were measured for the concentrations of dissolved organic C (DOC), N, and carbohydrates, aromaticity (specific ultraviolet [UV] absorbance at 254 nm), pH, and bioavailability to bacteria. The percentage of DOC in the low-molecular-weight (LMW) fraction was higher in the deeper than in the upper layer. We suggest that easily degradable LMW compounds with low aromatic character and high bioavailability had not degraded in the deeper layer under anaerobic conditions. This had produced a relative enrichment of LMW DOC in the anaerobic layer. A different relation to aerobicity was observed in alternating aerobic conditions in the upper layer alone, where the high-molecular-weight compounds seemed to increase under more anaerobic conditions. Thus the properties and processes of DOM in peatlands seem to be controlled by the aerobicity in a complicated manner. We suggest that operations that affect the water table level in peatland affect also the properties of peat DOM. The properties of DOM that could be connected to clear-cut harvest-induced changes in the recent inputs of C and N were small. Peat DOM was more N rich in harvested than in control sites. Thus it seems possible that N is susceptible to leaching after harvest in naturally relatively N-rich sites and especially in mineral form.

Description: Leaching of dissolved C in arable hummocky ground moraine soil landscapes is characterized by a spatial continuum of more or less erosion-affected Luvisols, Calcaric Regosols at exposed positions, and Colluvic Regosols in depressions. Our objective was to estimate the fluxes of dissolved C in four differently eroded soils as affected by erosion-induced pedological and soil structural alterations. In this model study, we considered landscape position effects by adapting the water table as the bottom boundary condition and erosion effects by using pedon-specific soil hydraulic properties. The one-dimensional vertical water movement was described with the Richards equation using HYDRUS-1D. Solute fluxes were obtained by combining calculated water fluxes with concentrations of dissolved organic and inorganic C (DOC and DIC, respectively) measured from soil solution extracted by suction cups at biweekly intervals. In the 3-yr period (2010–2012), DOC fluxes in the 2-m soil depth were similar at the three non-colluvic locations with –0.8 ± 0.1 g m –2 yr –1 (i.e., outflow) but were 0.4 g m –2 yr –1 (i.e., input) in the depression. The DIC fluxes ranged from –10.2 g m –2 yr –1 for the eroded Luvisol, –9.2 g m –2 yr –1 for the Luvisol, and –6.1 g m –2 yr –1 for the Calcaric Regosol to 3.2 g m –2 yr –1 for the Colluvic Regosol. The temporal variations in DOC and DIC fluxes were controlled by water fluxes. The spatially distributed leaching results corroborate the hypothesis that the effects of soil erosion influence fluxes through modified hydraulic and transport properties and terrain-dependent boundary conditions.

Description: Sorptive retention of dissolved organic matter (DOM) at soil particle surfaces controls C flux through the critical zone. Prior studies have shown that pristine Al- and Fe-(oxy)hydroxide surfaces are especially reactive toward DOM sorptive stabilization. However, the impact of progressive and/or preexisting organic surface coatings on further surficial uptake and exchange during repeated DOM infusion episodes remains unclear. In this study, DOM solutions were extracted from organic horizons in grassland (G) and mixed conifer forest (F) vegetation types in the Jemez River Basin Critical Zone Observatory. Extracted DOM solutions were used to sequentially irrigate columns packed with either quartz sand (Qtz), Al-hydroxide-coated quartz sand (Al-Qtz), or Fe-hydroxide-coated quartz sand (Fe-Qtz). Use of distinct DOM sources enabled investigation of how sorption, fractionation, and exchange ensued during reactive transport through mineral media progressively coated with sorbate organic matter (SOM). During initial irrigation of fresh mineral media with G-DOM, the magnitude of DOM sorption (per unit sorbent mass) followed the trend: Al-Qtz ≥ Fe-Qtz 〉 Qtz. Effluent solutions showed diminished molar absorptivity and humification index (HIX) values, indicating preferential uptake of high-molar-mass aromatic constituents. Introduction of F-DOM to G-SOM-coated surfaces revealed competitive desorption of G-SOM from the organo–mineral interface. During F-DOM irrigation, high HIX values were observed in effluent solutions, indicating remobilization of G-SOM by displacement. According to spectroscopic analyses, the displaced G-SOM consisted of aromatic phenolic acids with high excitation–emission "fingerprints" characteristic of fulvic- and humic-acid-like compounds, providing evidence for kinetic DOM exchange reactions.

Description: We investigated the molecular composition of dissolved organic matter (DOM) from various rivers, bogs and soil sites to test for their ecosystems unique molecular patterns. This information is prerequisite for searching of new marker compounds that might help tracing the fate of ecosystem-derived organic matter from terrestrial to marine systems. We used electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) to identify the ecosystem-specific molecular DOM characteristics. We investigated 39 samples and explored their patterns through nonmetric multidimensional scaling (NMDS). Based on the intrasystem similarity, we identified unique molecular formulae for each ecosystem and compared their main characteristics. Our data indicate a pH influence on all samples and possibly a vegetation influence on soil water samples. We interpreted the lack of nitrogen-containing compounds in the surface water and the lower molecular size in the soil water with higher microbial activity and reworking in soils and the lack of aromatic compounds in surface waters as the result of photo degradation. Our results demonstrate that this approach is suitable for resolving ecosystem-specific markers; the tannin molecular formulae seemed to be particularly suited markers for forest systems. However, we emphasize that it is necessary to collect a larger number of samples and reasonable environmental parameters to explain the distinctive molecular features and evaluate new markers. Aside from the ecosystem-specific features, we also found a suite of compounds that were present in all samples, indicating the convergent evolution of terrestrial DOM for a wide gradient of physicochemical and biological features. The molecular formulae of these ubiquitous compounds match those of carboxyl-rich alicyclic molecules (CRAM), but distinction from lignin is only possible by applying methods that elucidated the molecular structure.

Description: Natural fine colloids and nanoparticles have the potential to encapsulate and bind nutrients. Their size and composition is therefore relevant to understand the transport of essential nutrients like phosphorus in an aquatic ecosystem. The aim of this study was to characterize fine colloidal and nanoparticulate bound P of distinct hydromorphological areas in stream water from a forested test site in a small headwater catchment. Asymmetric flow field flow fractionation coupled online to inductively coupled plasma mass spectrometry was applied for size-resolved detection of P, Fe, and Al in the fractions. Online P detection was a challenge due to the low concentrations (in this study down to 0.1 μg/L) in many natural waters. Additionally, the "dissolved" organic matter (DOM) content was derived from the online UV signal. The colloidal P occurred in two size fractions (2–20 and 21–300 nm), which constituted up to 100% of the total river P discharge depending on hydromorphology. For the small size fraction, variations in P concentrations correlated with Al variations; in addition, a high Fe presence in both fractions was accompanied by high P concentrations. Moreover, DOM was detected with P in the presence of Fe and Al, suggesting that Fe and Al are carriers of P and associated with organic matter. The developed methodology enables the inputs and source regions of fine colloidal and nanoparticulate fractions within a small river of a headwater catchment to be traced and conceptually defined for the first time.

Description: Heavy metals (HMs) are toxic to human life and the environment when present in excessive concentrations. Therefore, determining the interactions of HMs with soils and dissolved organic matter (DOM) is essential to predict their fate. To find out the effect of DOM and soil properties (clay minerals, oxides, and bulk organic matter [OM]) on the uptake of Cu, Ni, and Zn, batch adsorption experiments were conducted using five soils sampled from Egypt. The sorption isotherms were well described by the initial mass (IM) isotherm model. The amount and timing of DOM addition was found to play a pivotal role in determining the affinity of the HMs for soil. When DOM and HMs were added simultaneously, the affinity of Cu decreased in Fe-(hydr)oxide-rich soils (by 7%) and increased in soils poor in Fe-(hydr)oxide (by 6–10%). When DOM was added first, followed by HMs, the affinity of Cu strongly increased. In contrast, affinity of both Ni and Zn was enhanced (3–18%) in the presence of DOM, regardless of the timing of DOM addition. The difference is explained by Cu binding to the solid phase and DOM through strong inner-sphere complexes, whereas Ni and Zn adsorbed predominantly through weaker electrostatic interactions. As a result, Cu was able to bind more strongly to previously adsorbed DOM on the solid phase in the case of smectite, while this effect was counteracted by the coating of available specific binding sites on Fe-(hydr)oxides. The study has revealed that Egyptian soils hold great potential to remove HMs from aqueous solutions.

Description: Dissolved organic matter (DOM) affects a wide range of soil processes, but it is generally unknown which factors control its concentration. Here we report a survey of soil solution dissolved organic carbon (DOC) concentrations in agricultural soils with contrasting properties. Eighty-seven agricultural topsoils were sampled throughout Europe. Soil solution was isolated by centrifugation (3500 g) from soils sampled field moist and analyzed directly ( n = 30), from air-dried soils that were rewetted and incubated for 5 wk ( n = 30), or from soils stored moist at 4°C during 1.5 yr ( n = 39). The soil storage and treatment effects on DOC concentrations in membrane filtered soil solution (0.45 μm), tested on identical samples, showed that air-drying followed by wetting increased DOC 2.5- to 7.8-fold compared with freshly sampled field moist samples, whereas cold storage had inconsistent effects. In the entire data set, DOC in the soil solution ranged from 12 to 104 mg C L –1 (10th– 90th percentile), with a median of 33 mg C L –1 . Soil or soil solution properties (including ionic strength) only explained 14% of the variance of log[DOC] in freshly sampled soils, whereas 41% was explained in the entire dataset. The stepwise regression models predict that DOC increases with increasing soil organic matter content and decreasing pH or % base saturation. The specific UV-absorbance of DOM at 254 nm (SUVA) as an indicator of the aromaticity of DOM ranged sevenfold among samples. Soil drying and rewetting decreased the SUVA of DOM, suggesting that DOM released after rewetting originates from decaying biomass rather than from humified organic matter. The DOC concentrations are largely affected by drying–rewetting processes. In freshly sampled soils, DOC concentrations correlate poorly to physicochemical properties of the soil, likely due to biological processes involved.

Description: In this work, the effects of various dissolved organic matter (DOM) sources (piggery effluent [PigE], dairy effluent [DE], sewage effluent [SE], and stormwater [SW]) on the priming effect (PE) of soil C as affected by solid organic amendments (biochar [BC], biosolids [BS], compost, and poultry manure [PM]) and microbial activity were quantified using landfill, arable, and metal-contaminated field and spiked soils. The BC-amended soil caused significantly lower PEs than BS-, compost-, or PM-amended field soils due to its low DOM. A strong positive correlation was observed between the dissolved organic C content and glucose-induced PE of soil C. However, a negative correlation between the PE and dissolved N in different sources of DOM suggested that the PE may also be influenced by the quality of added C sources in the soils. The DE-treated soil with the highest dissolved N resulted in significantly lower PE than PigE-, SE-, and SW-treated soils. Compared with the uncontaminated soils, microbial activity as CO 2 evolution and PE decreased markedly in the metal-contaminated soils, which may be attributed to the heavy metal toxicity. However, the distinct increase in microbial activity in the wastewater-treated contaminated soils suggests the capacity of wastewater to reduce metal toxicity in soils. The findings of this study suggest that although wastewater DOM may reduce the toxic effect to microorganisms, it can have an important effect on the source of CO 2 by stimulating the decomposition of native soil organic matter.

Description: Batch incubation and column percolation experiments with dissolved organic matter solution at different pH values were performed to investigate the effect of organic matter sorption and microbial activity on the wettability of clean quartz sand particles. Sodium azide was used as a biocide to control microbial activity. Changes in wettability were evaluated by contact angle measurements. Wettability of the quartz sand was found to significantly decrease after sorption of organic matter. An adsorbed C content of 〈0.1 g C kg –1 turned out to be effective in increasing the contact angle from initially 30 to 〉70°. Results of the percolation experiments revealed that the change in contact angle was depth dependent, with the greatest increase in the upper part of the columns. The low C/N ratio (〈10) of the organic matter adsorbed in the upper part of the sand columns percolated without biocide addition suggested that the depth dependency was caused not only by sorption of organic matter but also was affected by microbial activity. This finding was supported by an increasing contact angle for sand columns percolated with a solution close to neutral pH (6.7), where more favorable living conditions for bacteria could be assumed. We also observed a trend of a linear inverse relationship between C/N ratio and contact angle, i.e., increasing contact angle with decreasing C/N ratio. Overall, the results indicated that the formation of biogeochemical interfaces can strongly change the original wetting properties of mineral particle surfaces.

Description: Spatio-temporal modifications of the composition of dissolved organic matter (DOM) from a wetland in an agricultural catchment were investigated using thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide coupled to gas chromatography and mass spectrometry (THM-GC–MS). The results were compared with previous data acquired on the same samples using ultraviolet spectroscopy and the stable C isotope ratio. The correlation between molecular markers and bulk-scale descriptors validated the use of THM-GC–MS to investigate the fate of DOM in soils. Molecular proxies, including lignin markers, tannin markers, carbohydrates, and fatty acids, were classified into plant-derived and microbial markers. At the beginning of the hydrologic year, associated with the recharge period, the DOM composition was similar along the depth profile and 〉70% of the analyzed markers were from plant-derived origins. The rise in the water table was associated with a shift from vertical to horizontal water flow due to water saturation and resulted in a clear vertical heterogeneity, i.e., a varying composition throughout the profile. In the surface horizons, the proportion of plant-derived markers remained 〉70%, while in the deep horizon this proportion was 〈30%. Last, the lowering of the water table resulted in the homogenization of the DOM composition along the depth profile.

Description: In temperate spruce forests, dissolved organic matter (DOM) from forest floors is the major source of organic matter entering the mineral soil and thus it determines important soil properties and element cycling through the ecosystem. We examined effects of doubling locally collected throughfall for 6 yr on the concentrations of dissolved organic C (DOC) and properties of DOM (aromaticity, degree of molecule complexity) in the forest floor. Forest floor solutions below the Oi, Oe, and Oa horizons were sampled every 2 to 4 wk using tension lysimeters. For the controls, the average DOC concentrations in 2002 to 2007 were 43.8 ± 2.6 mg L –1 below the Oi, 49.6 ± 2.7 mg L –1 below the Oe, and 61.0 ± 2.0 mg L –1 below the Oa horizon. Doubling throughfall resulted in average DOC concentrations of 37.4 ± 1.8 mg L –1 (Oi), 49.3 ± 1.6 mg L –1 (Oe), and 50.1 ± 8.0 mg L –1 (Oa). The decreases in concentrations due to throughfall addition as well as the effects on DOM properties were, however, not statistically significant. It is commonly assumed that throughfall inputs are linearly related to water fluxes within and from the forest floor. Under that assumption, the results suggest that DOM fluxes are controlled by water fluxes rather than by the quantity of C that can be mobilized from the soil organic matter. Hence, increasing precipitation due to future climate changes presumably will result in enhanced DOM fluxes into the mineral horizons.

Description: Dissolved organic matter (DOM) plays a crucial role in many important processes that take place in terrestrial and aquatic systems. These include carbon and nutrient cycling, pedogenesis, and microbial metabolism. Here we highlight the results of studies that demonstrate the role of DOM in linking terrestrial and aquatic systems. We emphasize three fundamental aspects of the research, which together show the importance of DOM in linking terrestrial and aquatic systems: First, tracing DOM properties during its transport through terrestrial and aquatic systems is a powerful tool for improving our conceptual understanding of the mechanistic drivers of DOM dynamics. Second, linking DOM dynamics to important physical processes such as hydrology provides important insights into the nature of terrestrial–aquatic links. Third, interrelations between DOM dynamics and human impacts on ecosystems highlight how the role of DOM in coupled terrestrial–aquatic systems may change in the future. New measurement and modeling approaches have enabled a more thorough assessment of all three aspects of DOM dynamics. They show that both natural and anthropogenic drivers not only greatly influence DOM dynamics in soils, but owing to the mobility of DOM, also have substantial influence on aquatic systems. This physical connection of soils and surface waters demonstrates the importance of understanding fundamental processes such as nutrient cycling, pedogenesis, and microbial metabolism at a whole-landscape scale.

Description: Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end-member mixing analysis to determine the contributing end-members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end-members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone.

Description: Time domain reflectometry (TDR) is an established method for the determination of apparent dielectric permittivity and water content in soils. Using current waveform interpretation procedures, signal attenuation and variation in dielectric media properties along the transmission line can significantly increase sampling error in estimating the time, t 2 , at which the pulse arrives at the end of the probe. Additionally, manual adjustment of waveform analysis parameters is frequently required in current software to accommodate changes in media properties when processing large time series of TDR measurements. Our objectives were to reevaluate conventional propagation time analysis and difficulties with these methods, introduce the AWIGF (adaptive waveform interpretation with Gaussian filtering) algorithm that circumvents these problems, and compare interpretation methods using waveforms obtained with different TDR instruments and under widely varying media properties. The AWIGF algorithm filters signal noise using Gaussian kernels with an adaptively estimated standard deviation based on the maximum gradient of the reflection at the termination of the probe. Two fitted parameters are required to scale the smoothing level for a given step pulse generator. Additionally, the maximum second derivative is used to evaluate t 2 . The AWIGF-determined t 2 was compared with TACQ, a standard waveform interpretation algorithm. The strategies of AWIGF permitted the determination of t 2 without parameter adjustment when the loss characteristics of the medium changed, such as with an increase in soil water content and bulk electrical conductivity. Using the new method, the sampling error of t 2 was 〈0.06 ns across a wide range of medium properties and less than or equal to that obtained with TACQ. In strongly attenuated waveforms, the water content sampling error determined with AWIGF was 0.005 m 3 m –3 compared with 0.038 m 3 m –3 obtained using TACQ.

Description: Flow and transport parameters of soils in numerical simulations need to be defined at the support scale of computational grid cells. Such support scale can substantially differ from the support scale in laboratory or field measurements of flow and transport parameters. The scale dependence of flow and transport parameters essentially precludes the direct use of measured or pedotransfer-estimated parameter values in numerical simulations. The hypothesis of this work was that a support-based scaling law can be introduced that can convert pedotransfer-estimated saturated hydraulic conductivity values into values to be used over grid cells for finite-element-based simulations of water flow and tracer transport in variably saturated soils. A 4-month-long experiment was conducted at the USDA–ARS experimental site where Cl – as a tracer was applied with a pulse of irrigation water and its transport in groundwater and variably saturated shallow coarse-textured soils was monitored in two rows of wells on a daily basis. The HYDRUS-3D software was used to set and calibrate the Richards model for flow simulations and the convective–dispersive equation for transport simulations. Saturated hydraulic conductivity values were estimated with class pedotransfer functions derived from the USDA database containing results of about 1000 measurements in soils of different textures and bulk densities. A power law scaling for the saturated hydraulic conductivity was suggested based on literature data. When only two parameters of the scaling law rather than nine values of hydraulic conductivity from nine soil materials were calibrated, using the scaled saturated hydraulic conductivity values resulted in an accuracy of simulations that was similar to the accuracy of the calibrated model results. Upscaling of pedotransfer-estimated saturated hydraulic conductivities can provide reasonable estimates for numerical flow and transport modeling in variably saturated soils.

Description: In the vadose zone, preferential flow and transport are much more common than uniform water flow and solute transport. In recent decades, several models have been developed for preferential water flow and physical nonequilibrium solute transport. Among these models, the dual-permeability approach is an interesting tool for the conceptualization and modeling of preferential flow. However, this approach has been mainly studied from a numerical point of view. In this study, we developed a new analytical model for water infiltration into dual-permeability soils. The model is based on the analytical model originally proposed for single-permeability soils. The proposed model relies on the assumption that the water exchange rate at the interface between the matrix and fast-flow regions does not change cumulative infiltration at the soil surface, so that the total cumulative infiltration can be set equal to the sum of independent cumulative infiltrations into each region. This assumption was investigated using numerically generated data. The proposed analytical model was then used to evaluate the effects of fast-flow region hydraulic properties and hydraulic conditions on total cumulative infiltration for the case of single- and multi-tension water infiltration experiments. Finally, both single- and dual-permeability models were evaluated with respect to their ability to fit experimental data and associated problems of non-uniqueness in optimized parameters. The proposed model could serve as a new tool for modeling and characterizing preferential flow in the vadose zone.