ALBERT

Notes: Ground water quality networks for monitoring phreatic drinking water wellfields are generally established for two main purposes: (1) the short-term safeguarding of public water supply and (2) signaling and predicting future quality changes in the extracted ground water. Six monitoring configurations with different well locations and different screen depths and lengths were evaluated using a numerical model of the 3D ground water flow toward a partially penetrating pumping well in a phreatic aquifer. Travel times and breakthrough curves for observation and pumping wells were used to judge the effectiveness of different design configurations for three monitoring objectives: (1) early warning; (2) prediction of future quality changes; and (3) evaluation of protection measures inside a protection zone. Effectiveness was tested for scenarios with advective transport, first-order degradation, and linear sorption. It is shown that the location and especially the depth of the observation wells should be carefully chosen, taking into account the residence time from the surface to the observation well, the residual transit times to the extraction well, and the transformation and retardation rates. Shallow monitoring was most functional for a variety of objectives and conditions. The larger the degradation rates or retardation, the shallower should the monitoring be for effective early warning and prediction of future ground water quality. The general approach followed in the current study is applicable for many geohydrological situations, tuning specific monitoring objectives with residence times and residual transit times obtained from a site-specific ground water flow model.

Notes: Heat carried by ground water serves as a tracer to identify surface water infiltration, flow through fractures, and flow patterns in ground water basins. Temperature measurements can be analyzed for recharge and discharge rates, the effects of surface warming, interchange with surface water, hydraulic conductivity of streambed sediments, and basin-scale permeability. Temperature data are also used in formal solutions of the inverse problem to estimate ground water flow and hydraulic conductivity. The fundamentals of using heat as a ground water tracer were published in the 1960s, but recent work has significantly expanded the application to a variety of hydrogeological settings. In recent work, temperature is used to delineate flows in the hyporheic zone, estimate submarine ground water discharge and depth to the salt-water interface, and in parameter estimation with coupled ground water and heat-flow models. While short reviews of selected work on heat as a ground water tracer can be found in a number of research papers, there is no critical synthesis of the larger body of work found in the hydrogeological literature. The purpose of this review paper is to fill that void and to show that ground water temperature data and associated analytical tools are currently underused and have not yet realized their full potential.

Notes: In the upper Midwest, USA, elevated arsenic concentrations in public drinking water systems are associated with the lateral extent of northwest provenance late Wisconsin-aged drift. Twelve percent of public water systems located within the footprint of this drift (212 of 1764) exceed 10 μg/L arsenic, which is the U.S. EPA's drinking water standard. Outside of the footprint, only 2.4% of public water systems (52 of 2182) exceed 10 μg/L arsenic. Both glacial drift aquifers and shallow bedrock aquifers overlain by northwest provenance late Wisconsin-aged sediment are affected by arsenic contamination. Evidence suggests that the distinct physical characteristics of northwest provenance late Wisconsin-aged drift—its fine-grained matrix and entrained organic carbon that fosters biological activity—cause the geochemical conditions necessary to mobilize arsenic via reductive mechanisms such as reductive desorption and reductive dissolution of metal oxides. This study highlights an important and often unrecognized phenomenon: high-arsenic sediment is not necessary to cause arsenic-impacted ground water—when “impacted” is now defined as 〉10 μg/L. This analysis also demonstrates the scientific and economic value of using existing large but imperfect statewide data sets to observe and characterize regional-scale environmental problems.

Notes: Ground water in deep confined aquifers is one of the major water resources for agricultural, industrial, and domestic uses in the North China Plain. Detailed information on ground water age and recharge is vital for the proper management of these water resources, and to this end, we used carbon 14 of dissolved inorganic carbon and tritium in water to measure the age and determine the recharge areas of ground water in the North China Plain. These isotopic data suggest that most ground water in the piedmont part of the North China Plain is 〈40 years old and is recharged locally. In contrast, ground water in the central and littoral portions of the North China Plain is 10,000 to 25,000 years old. The δ18O (δD) values of this ground water are 1.7‰ (11‰) less than that in the piedmont plain ground water and possibly reflect water recharged during a cooler climate during the last glaciation. The temperature of this recharge, based on δ18O values, ranges from 3.7°C to 8.4°C, compared to 12°C to 13°C of modern recharge water. The isotopic data set combined indicates that ground water in the central and littoral part of the North China Plain is being mined under non–steady state conditions.

Notes: This paper describes reactive transport simulations conducted to assess the impact of mineral fouling on the hydraulic behavior of continuous-wall permeable reactive barriers (PRBs) employing granular zero-valent iron (ZVI) in carbonate-rich alluvial aquifers. The reactive transport model included a geochemical algorithm for simulating corrosion and mineral precipitation reactions that have been observed in ZVI PRBs. Results of simulations show that porosity and hydraulic conductivity of the ZVI decrease over time and that flows are redistributed throughout the PRB in response to fouling of the pore space. Under typical conditions, only subtle changes occur within the first 10 years (i.e., duration of the current field experience record with PRBs), and the most significant changes do not occur until the PRB has operated for at least 30 years. However, changes can occur sooner (or later) if the rate at which mineral-forming ions are delivered to the PRB is higher (or lower) than that expected under typical conditions (i.e., due to higher/lower flow rate or inflowing ground water that has higher/lower ionic strength). When the PRB is more permeable than the aquifer, the median Darcy flux in the PRB does not change appreciably over time because the aquifer controls the rate of flow through the PRB. However, seepage velocities in the PRB increase, and residence times decrease, due to porosity reductions caused by accumulation of minerals in the pore space. When fouling becomes extensive, bypassing and reductions in flow rate in the PRB occur.

Notes: A total of 176 wells in sand-and-gravel glacial aquifers in central Illinois were sampled for arsenic (As) and other chemical parameters. The results were combined with archived and published data from several hundred well samples to determine potential sources of As and the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the As concentrations. High concentrations were confined to areas smaller than 1 km in diameter. Arsenic and well depth were uncorrelated. Arsenic solubility appeared to be controlled by oxidation-reduction (redox) conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing water does As accumulate in solution. In wells in which total organic carbon (TOC) was below 2 mg/L and sulfate (SO42−) was present, As concentrations were low or below the detection limit (0.5 μg/L). Arsenic concentrations 〉10 μg/L were almost always found in wells where TOC was 〉2 mg/L and SO42− was absent or at low concentrations, indicating post–SO42−reducing conditions. Iron (Fe) is common in the aquifer sediments, and Fe oxide reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as Fe oxide is reduced.

Notes: Historically, specific capacity information has been used to calculate aquifer transmissivity when pumping test data are unavailable. This paper presents a simple computer program written in the MATLAB programming language that estimates transmissivity from specific capacity data while correcting for aquifer partial penetration and well efficiency. The program graphically plots transmissivity as a function of these factors so that the user can visually estimate their relative importance in a particular application. The program is compatible with any computer operating system running MATLAB, including Windows, Macintosh OS, Linux, and Unix. Two simple examples illustrate program usage.

Notes: Hydrogeology is a field closely related to the needs of society. Many problems of current national and local interest require predictions of hydrogeological system behavior, and in a number of important cases, the period of prediction is tens to hundreds of thousands of years. It is argued that the demand for such long-term hydrogeological predictions casts a new light on the future needs of hydrogeological research. Key scientific issues are no longer concerned only with simple processes or narrowly focused modeling or testing methods but also with assessment of prediction uncertainties and confidence, couplings among multiple physicochemical processes occurring simultaneously at a site, and the interplay between site characterization and predictive modeling. These considerations also have significant implications for hydrogeological education. With this view, it is asserted that hydrogeological directions and education need to be reexamined and possibly refocused to address specific needs for long-term predictions.

Notes: Alternative fractional models of contaminant transport lead to a new travel time formula for arbitrary concentration levels. For an evolving contaminant plume in a highly heterogeneous aquifer, the new formula predicts much earlier arrival at low concentrations. Travel times of contaminant fronts and plumes are often obtained from Darcy's law calculations using estimates of average pore velocities. These estimates only provide information about the travel time of the average concentration (or peak, for contaminant pulses). Recently, it has been shown that finding the travel times of arbitrary concentration levels is a straightforward process, and equations were developed for other portions of the breakthrough curve for a nonreactive contaminant. In this paper, we generalize those equations to include alternative fractional models of contaminant transport.

Notes: A methodology using ordinal logistic regression is proposed to predict the probability of occurrence of heavy metals in ground water. The predicted probabilities are defined with reference to the background concentration and the maximum contaminant level. The model is able to predict the occurrence due to different influencing variables such as the land use, soil hydrologic group (SHG), and surface elevation. The methodology was applied to the Sumas-Blaine Aquifer located in Washington State to predict the occurrence of five heavy metals. The influencing variables considered were (1) SHG; (2) land use; (3) elevation; (4) clay content; (5) hydraulic conductivity; and (6) well depth. The predicted probabilities were in agreement with the observed probabilities under existing conditions. The results showed that aquifer vulnerability to each heavy metal was related to different sets of influencing variables. However, all heavy metals had a strong influence from land use and SHG. The model results also provided good insight into the influence of various hydrogeochemical factors and land uses on the presence of each heavy metal. A simple economic analysis was proposed and demonstrated to evaluate the cost effects of changing the land use on heavy metal occurrence.

Notes: Artificial neural networks (ANNs) were developed for accurately predicting potentiometric surface elevations (monitoring well water level elevations) in a semiconfined glacial sand and gravel aquifer under variable state, pumping extraction, and climate conditions. ANNs “learn” the system behavior of interest by processing representative data patterns through a mathematical structure analogous to the human brain. In this study, the ANNs used the initial water level measurements, production well extractions, and climate conditions to predict the final water level elevations 30 d into the future at two monitoring wells. A sensitivity analysis was conducted with the ANNs that quantified the importance of the various input predictor variables on final water level elevations. Unlike traditional physical-based models, ANNs do not require explicit characterization of the physical system and related physical data. Accordingly, ANN predictions were made on the basis of more easily quantifiable, measured variables, rather than physical model input parameters and conditions. This study demonstrates that ANNs can provide both excellent prediction capability and valuable sensitivity analyses, which can result in more appropriate ground water management strategies.

Notes: A field-scale demonstration project was conducted to evaluate the capability of eastern cottonwood trees (Populus deltoides) to attenuate trichloroethene (TCE) contamination of ground water. By the middle of the sixth growing season, trees planted where depth to water was 〈3 m delivered enough dissolved organic carbon to the underlying aquifer to lower dissolved oxygen concentrations, to create iron-reducing conditions along the plume centerline and sulfate-reducing or methanogenic conditions in localized areas, and to initiate in situ reductive dechlorination of TCE. Apparent biodegradation rate constants for TCE along the centerline of the plume beneath the phytoremediation system increased from 0.0002/d to 0.02/d during the first six growing seasons. The corresponding increase in natural attenuation capacity of the aquifer along the plume centerline, from 0.0004/m to 0.024/m, is associated with a potential decrease in plume-stabilization distance from 9680 to 160 m. Demonstration results provide insight into the amount of vegetation and time that may be needed to achieve cleanup objectives at the field scale.

Notes: Israel and the Palestinian Authority share the southern Mediterranean coastal aquifer. Long-term overexploitation in the Gaza Strip has resulted in a decreasing water table, accompanied by the degradation of its water quality. Due to high levels of salinity and nitrate and boron pollution, most of the ground water is inadequate for both domestic and agricultural consumption. The rapid rate of population growth in the Gaza Strip and dependence upon ground water as a single water source present a serious challenge for future political stability and economic development. Here, we integrate the results of geochemical studies and numerical modeling to postulate different management scenarios for joint management between Israel and the Palestinian Authority. The chemical and isotopic data show that most of the salinity phenomena in the Gaza Strip are derived from the natural flow of saline ground water from Israel toward the Gaza Strip. As a result, the southern coastal aquifer does not resemble a classic “upstream-downstream” dispute because Israel's pumping of the saline ground water reduces the salinization rates of ground water in the Gaza Strip. Simulation of different pumping scenarios using a monolayer, hydrodynamic, two-dimensional model (MARTHE) confirms the hypothesis that increasing pumping along the Gaza Strip border combined with a moderate reduction of pumping within the Gaza Strip would improve ground water quality within the Gaza Strip. We find that pumping the saline ground water for a source of reverse-osmosis desalination and then supplying the desalinated water to the Gaza Strip should be an essential component of a future joint management strategy between Israel and the Palestinian Authority.

Notes: The goal for any property rights system is to achieve equity, efficiency, and certainty. Trying to achieve these goals for ground water is difficult because a ground water right is not exclusive. To make matters more complicated, ground water is often under the jurisdiction of more than one political unit. The result is transboundary conflicts. Two critical elements must be included in any system of ground water rights. The system must define how the ground water can be used and define the relationships that each user and each use has with the other users and uses in the system. Unfortunately, these relationships are seldom completely defined and are made more complex by the transboundary scales at which they operate. As ground water moves horizontally across boundaries, different users or different jurisdictions have sequential control, creating conflicts between the first users and subsequent ones. Other problems occur because of vertical relationships, with more than one person or entity having control over ground water at the same time. This simultaneous exercise of authority can create conflicts between an individual who possesses a right to use ground water and a state or federal agency that regulates the same water. Transboundary conflicts occur at different scales and include conflicts between neighboring property owners as well as conflicts between countries. Scale, the property rights structure, and the nature of the relationship between users influence the way transboundary ground water conflicts are resolved.

Notes: Recent legislation required regional grassroots water resources planning across the entire state of Texas. The Texas Water Development Board (TWDB), the state's primary water resource planning agency, divided the state into 16 planning regions. Each planning group developed plans to manage both ground water and surface water sources and to meet future demands of various combinations of domestic, agricultural, municipal, and industrial water consumers. This presentation describes the challenges in developing a ground water model for the Llano Estacado Regional Water Planning Group (LERWPG), whose region includes 21 counties in the Southern High Plains of Texas. While surface water is supplied to several cities in this region, the vast majority of the regional water use comes from the High Plains aquifer system, often locally referred to as the Ogallala Aquifer. Over 95% of the ground water demand is for irrigated agriculture. The LERWPG had to predict the impact of future TWDB-projected water demands, as provided by the TWDB, on the aquifer for the period 2000 to 2050. If detrimental impacts were noted, alternative management strategies must be proposed. While much effort was spent on evaluating the current status of the ground water reserves, an appropriate numerical model of the aquifer system was necessary to demonstrate future impacts of the predicted withdrawals as well as the effects of the alternative strategies. The modeling effort was completed in the summer of 2000. This presentation concentrates on the political, scientific, and nontechnical issues in this planning process that complicated the modeling effort. Uncertainties in data, most significantly in distribution and intensity of recharge and withdrawals, significantly impacted the calibration and predictive modeling efforts. Four predictive scenarios, including baseline projections, recurrence of the drought of record, precipitation enhancement, and reduced irrigation demand, were simulated to identify counties at risk of low final ground water storage volume or low levels of satisfied demand by 2050.

Notes: In Europe, a long history of cooperation over transboundary rivers—most notably the Rhine and Danube rivers—exists. To help foster cooperation and communication vis-à-vis transboundary ground water, the United Nations Economic Commission for Europe (UNECE), as part of its ground water program, conducted a survey on transboundary aquifers in Europe. The survey produced 25 responses from 37 countries and identified 89 transboundary aquifers. Respondents reported on the degree of ground water use within their own boundaries, transboundary aspects (agreements, joint commissions, etc.) of ground water, and transboundary aquifers themselves. The inventory proved useful, but a number of problems were identified: different map scales and symbols, difficulty in identifying transboundary aquifers, inconsistent labeling of aquifers, and data discrepancies. The UNECE ground water program also drafted guidelines for monitoring and assessment of transboundary ground water. These guidelines are not legally binding but have been adopted by 25 countries, deal mainly with monitoring and assessment, and are being implemented through a number of pilot projects. Other organizations—the United Nations Scientific, Educational and Cultural Organization, the Food and Agriculture Organization, the International Association of Hydrogeologists, and the European Union—are all supporting the investigation of transboundary aquifers in an effort to facilitate data sharing and coordinated management of these valuable resources.

Notes: Ground water can facilitate earthquake development and respond physically and chemically to tectonism. Thus, an understanding of ground water circulation in seismically active regions is important for earthquake prediction. To investigate the roles of ground water in the development and prediction of earthquakes, geological and hydrogeological monitoring was conducted in a seismogenic area in the Yanhuai Basin, China. This study used isotopic and hydrogeochemical methods to characterize ground water samples from six hot springs and two cold springs. The hydrochemical data and associated geological and geophysical data were used to identify possible relations between ground water circulation and seismically active structural features. The data for δ18O, δD, tritium, and 14C indicate ground water from hot springs is of meteoric origin with subsurface residence times of 50 to 30,320 years. The reservoir temperature and circulation depths of the hot ground water are 57°C to 160°C and 1600 to 5000 m, respectively, as estimated by quartz and chalcedony geothermometers and the geothermal gradient. Various possible origins of noble gases dissolved in the ground water also were evaluated, indicating mantle and deep crust sources consistent with tectonically active segments. A hard intercalated stratum, where small to moderate earthquakes frequently originate, is present between a deep (10 to 20 km), high–electrical conductivity layer and the zone of active ground water circulation. The ground water anomalies are closely related to the structural peculiarity of each monitoring point. These results could have implications for ground water and seismic studies in other seismogenic areas.

Notes: An exact, closed-form analytical solution is developed for calculating ground water transit times within Dupuit-type flow systems. The solution applies to steady-state, saturated flow through an unconfined, horizontal aquifer recharged by surface infiltration and discharging to a downgradient fixed-head boundary. The upgradient boundary can represent, using the same equation, a no-flow boundary or a fixed head. The approach is unique for calculating travel times because it makes no a priori assumptions regarding the limit of the water table rise with respect to the minimum saturated aquifer thickness. The computed travel times are verified against a numerical model, and examples are provided, which show that the predicted travel times can be on the order of nine times longer relative to existing analytical solutions.

Notes: A nitrate-reactive porous media layer comprising wood particles with very high hydraulic conductivity (K∼ 1 cm/s) was used to successfully treat nitrate in a shallow sand-and-gravel aquifer in southern Ontario. Nitrate concentrations of 1.3 to 14 mg/L as N in the aquifer were attenuated to 〈0.5 mg/L as N in the reactive layer. Borehole dilution testing indicated that ground water velocities in the reactive layer, although variable, averaged five times higher than in the surrounding aquifer, suggesting that the layer was capturing ground water flow from deeper in the aquifer. The use of high-K reactive media opens up the possibility of installing permeable reactive barriers as horizontal layers in the shallow water table zone that do not necessarily have to penetrate the full depth of a contaminant plume to be effective. Model simulations show that the depth of capture of a high-K layer increases as the layer width in the direction of flow increases. Shallower emplacement could decrease barrier costs at some sites.

Notes: Longitudinal dispersivity (α) data were compiled from 109 different authors for different types of geological media. The data were subdivided into different subsets. Dispersivity values for consolidated media were subdivided as basalts, granites, sandstones, and carbonate rocks, while unconsolidated sediments were subdivided into three reliability classes. The data sets provided here may provide ground water practitioners a preliminary guide to estimate dispersivity values at various scales and to guide and verify theories on scaling behavior. Based on the data set presented here, the relationship that empirically best described the dispersivity data in regard to scale of measurement was in the form of a power law. The scaling exponent for consolidated and unconsolidated geological media varied between 0.40 and 0.92, and 0.44 and 0.94, respectively. Higher reliability subsets of data for the unconsolidated sediments and more frequently tested rock formations indicate that the scaling exponent is at the lower end of the observed range, close to 0.5. No significant difference in scaling exponent was found among different media, and no clear evidence exists for the presence of an upper bound or asymptotic behavior on the relationship for any of the analyzed media.

Notes: Fracture trends (defined as kilometer-scale linear features interpolated between field observations of fractures along their strikes) often have a dominant orientation. Finding a correlation between this orientation and hydraulic data could shed light on their hydraulic influence. A significant correlation between head residuals from first-order regional drift and the orientation of 2- to 4-km-long fracture trends was found in a study site in the Negev, Israel, using the semivariogram cloud analysis. Correlation of head residuals rather than the head itself implies that the orientation of the fracture trends controls the anisotropy and heterogeneity at this scale, mainly because the fracture trends define the orientation of blocks, which differ in their hydraulic properties. Preferential transmissive pathways are probably shorter than the full extent of the fracture trends, causing a relatively high head difference along the trends on the 2- to 4-km scale. Fracture trend density and additional data from short-range hydraulic tests helped characterize two blocks separated by a fault zone. The identification of hydraulic features on a kilometer scale is necessary for better modeling of regional ground water flow and transport. Hydraulic tests at this scale are not feasible, thereby rendering combined analyses of head and structural data, such as the one presented here, essential.

Notes: For years, researchers have sought index and other methods to predict aquifer sensitivity and vulnerability to nonpoint pesticide contamination. In 1995, an index method and map were developed to define aquifer sensitivity to pesticide leaching based on a combination of soil and hydrogeologic factors. The soil factor incorporated three soil properties: hydraulic conductivity, amount of organic matter within individual soil layers, and drainage class. These properties were obtained from a digital soil association map. The hydrogeologic factor was depth to uppermost aquifer material. To test this index method, a shallow ground water monitoring well network was designed, installed, and sampled in Illinois. The monitoring wells had a median depth of 7.6 m and were located adjacent to corn and soybean fields where the only known sources of pesticides were those used in normal agricultural production. From September 1998 through February 2001, 159 monitoring wells were sampled for 14 pesticides but no pesticide metabolites. Samples were collected and analyzed to assess the distribution of pesticide occurrence across three units of aquifer sensitivity. Pesticides were detected in 18% of all samples and nearly uniformly from samples from the three units of aquifer sensitivity. The new index method did not predict pesticide occurrence because occurrence was not dependent on the combined soil and hydrogeologic factors. However, pesticide occurrence was dependent on the tested hydrogeologic factor and was three times higher in areas where the depth to the uppermost aquifer was 〈6 m than in areas where the depth to the uppermost aquifer was 6 to 〈15 m.

Notes: In Nebraska, a large number (〉200) of shallow sand-point and cased wells completed in coarse alluvial sediments along rivers and lakes still are used to obtain drinking water for human consumption, even though construction of sand-point wells for consumptive uses has been banned since 1987. The quality of water from shallow domestic wells potentially vulnerable to seepage from septic systems was evaluated by analyzing for the presence of tracers and multiple isotopes. Samples were collected from 26 sand-point and perforated, cased domestic wells and were analyzed for bacteria, coliphages, nitrogen species, nitrogen and boron isotopes, dissolved organic carbon (DOC), prescription and nonprescription drugs, or organic waste water contaminants. At least 13 of the 26 domestic well samples showed some evidence of septic system effects based on the results of several tracers including DOC, coliphages, NH4+, NO3−, N2, δ15N[NO3−] and boron isotopes, and antibiotics and other drugs. Sand-point wells within 30 m of a septic system and 〈14 m deep in a shallow, thin aquifer had the most tracers detected and the highest values, indicating the greatest vulnerability to contamination from septic waste.

Notes: Large stainless steel chambers were designed, constructed, and installed to make in situ mesocosms (ISMs) of aquifer sediments. The advantages of the ISMs over in situ microcosms, their smaller counterparts, are that ISMs may be sampled for longer times with larger sampling volumes. The former advantage makes ISMs a good tool to study the natural attenuation of contaminants with slow degradation rates. The latter advantage allows samples to be large enough that they may be analyzed for contaminants and reaction products, plus major ions normally associated with “complete” water analyses. Having such a large suite of analytes provides insights into associated mineral saturation conditions and possible reaction pathways that may not be readily apparent. As an example of their utility, a tracer test was done in a pair of ISMs to test our hypothesis that S(−I) in pyrite was a major electron donor for denitrification in the Elk Valley aquifer in eastern North Dakota. During the 9-month experiment, sediment data, ground water saturation indices, and trends in the geochemical evolution of the bromide tracer, calcium, inorganic carbon, magnesium, nitrate, and sulfate concentrations indicate that 58% of the denitrification was caused by S(−I) as the electron donor. These data also suggest that ferrous iron and organic carbon may have served as electron donors for the denitrification. The apparent zero-order denitrification rate was 16 mM/d (r2= 0.93) with a δ15N isotopic enrichment factor of −20.4‰ (r2= 0.998).

Notes: Temperature is one of the most frequently measured physical phenomena in environmental research. Many laboratory and field investigations of temperature use commercially available temperature loggers that come from the manufacturer precalibrated. After-market calibration of temperature loggers provides more confidence in measured data. This paper proposes a general method for calibrating temperature loggers in order to obtain consistent, high-quality data. A procedure and derivation of equations for performing logger calibration is presented. Spreadsheets are described for calculating correction coefficients and adjusting proxy data to a reference temperature. A temperature logger calibration example demonstrates the importance of instrument calibration and the improvement in accuracy. The procedure and spreadsheets presented here can be used for calibration and correction of data from any type of thermistor.

Notes: A process-based preferential flow transport model was implemented in a geographic information system to locate areas in the landscape with high risk of contamination by agrochemicals, especially pesticides. Protecting ground water resources necessitates a reliable ground water quality monitoring strategy. It is valuable to be able to focus monitoring on areas with the highest risk of contamination because monitoring ground water is an expensive activity, especially at the landscape scale. The objective of this project was to develop a tool that quantifiably estimates distributed ground water contamination risk in order to develop reliable, cost-effective ground water observation networks. The tool is based on a mechanistic model of chemical movement via preferential flow and uses land cover data, information about chemical properties, and modeled recharge to estimate the concentration of chemical reaching the ground water at each point in the landscape. The distributed risk assessment tool was tested by comparing the model-predicted risk with observed concentrations from 40 sampling wells in Cortland County, New York, for atrazine (pesticide) and nitrate, the latter assumed to be an indicator of agricultural pollution. The tool predictions agreed well with observed nitrate concentrations and pesticide detections. An Internet-based version of this tool is currently being developed for ready application to New York State.

Notes: This study examines the effects of sequential filtration on the particle abundance and lead concentrations in ground water from four monitoring wells in New Jersey with a history of high turbidity, elevated metal concentrations, or where differences in metal concentrations exist between filtered and unfiltered samples. In these monitoring wells, both transportable suspended particles, such as colloidal particles that are suspended in solution, and nontransportable particles that are disturbed during sample collection but not considered mobile transportable species may be present in solution with potential overlap in particle size distribution. Filtration, particularly the operational pore size (25 to 0.45 μm) of the filter, was evaluated as a method to obtain a representative sample of the transportable metal, as defined by the dissolved phase and particles that are persistently suspended in solution. Two monitoring wells at the Denzer-Schaefer site, a silty/clay aquifer with high particle concentrations (〉8900 mg/L) from samples taken with bailers and a low-flow purge (LFP) pump, showed that a filter of pore size 25 μm could remove 60% to 90% of soil-derived particles, with minimal loss of suspended particles from solution. The two monitoring wells within the highly conductive Picatinny Arsenal sand aquifer provided higher particle abundance with the samples collected with bailers (4300 to 6500 mg/L) than with the LFP pump (4 to 11 mg/L), indicating greater artificial particle disruption with a bailer. At Picatinny Arsenal, the major portion of nontransportable particles in the ground water samples could be removed by filtration through a 25–μm pore size filter, with a minimal loss in suspended particles. Filtration of ground water through a 25–μm pore size filter followed by acidification at the sampling site would provide investigators a tool to examine particle transport in aquifers where there exists the potential for artificial particle disruption during sampling.

Notes: Continuous-flow and batch experiments were conducted using a column reactor system containing Hanford Aquifer material in order to assess the potential of in situ bioremediation of carbon tetrachloride (CT) at the Hanford site in south-central Washington state. Benzoate and acetate were evaluated as primary substrates both in the presence and absence of nitrate as a potential electron acceptor. Each of the four resulting test conditions was first run under continuous-flow mode until a pseudo–steady state was achieved and then switched to batch mode. The longer residence time of the batch portion of the test resulted in more complete transformations and helped elucidate zones of variable activity within the column. Reductions in CT concentration and chloroform (CF) production were observed in all test conditions. Benzoate generally supported faster and more complete CT transformations than acetate, even though substrate use and denitrification was more rapid for acetate. Sulfate was present in all cases; yet, sulfate reduction was never observed, even during extended absences of nitrate and nitrite. CT transformation was most rapid near the point of injection, with rates decreasing toward the effluent end of the column. These results indicate that the microbial population at Hanford is capable of transforming CT in the subsurface. However, methods to control the production of CF may be necessary before this technology can be successfully implemented in the field.

Notes: To manage risk or to implement natural attenuation as a remedy, regulatory agencies must understand the processes that attenuate methyl-tert-butyl ether (MTBE) in ground water. Most case studies and laboratory studies in the literature indicate that natural biodegradation is not important; however, recent reports indicate that natural biodegradation of MTBE plays an important role under certain conditions. In an MTBE plume at a retail gasoline station in Parsippany, New Jersey, the long-term monitoring data indicated that the concentration of MTBE was slowly declining over time in the wells that were within the footprint of the plume. The ratio of tert-butyl alcohol (TBA) to MTBE increased with distance from the source area, and the ratio of TBA to MTBE in individual monitoring wells in the plume increased over time. This anecdotal evidence of natural biodegradation of MTBE to TBA at field scale was confirmed with a microcosm study. Core material from the interior of the plume was used to construct the microcosms. Following an initial lag period of 58 d, the concentration of MTBE decreased from more than 1460 μg/L to less than 10 μg/L within 199 d of incubation. As concentrations of MTBE declined in the microcosms, concentrations of TBA increased. The decrease in concentration of MTBE in the microcosms could be accounted for by an increase in the concentration of TBA.

Notes: Steam injected below the water table tends to move upward because of buoyancy. This limits the horizontal steam zone development, which determines the optimal spacing between injection wells. In this study, observations indicating steam override at a full-scale remediation of an unconfined aquifer are analyzed by numerical modeling using the code T2VOC. A simplified three-dimensional (3D) numerical model is set up, which qualitatively shows the same mechanisms as observed at the site. By means of the model, it is found that it will be possible to achieve a larger horizontal extent of the steam zone in a layered geology compared to the homogeneous case. In the homogeneous case, the steam injection rate increases dramatically when the injection pressure is increased, which is necessary to achieve a larger horizontal development. The development of the steam zone under unconfined conditions is found to be a complex function of the geologic layering, the water table at steady-state extraction, and the injection/extraction system. Because of this complexity, it will be difficult to predict steam behavior without site-specific 3D numerical modeling.

Notes: Trichlorofluoroethene (TCFE) was used as a reactive tracer to determine the in situ rate of reductive dechlorination in treatment zones impacted by three large-diameter permeable columns (LDPCs) that were installed at a trichloroethene (TCE)–contaminated site. The LDPCs were part of a pilot study to evaluate the effectiveness of hydrogen, lactate, and zero-valent iron for remediating TCE-contaminated ground water. The rate of TCFE reductive dechlorination was determined for each LDPC by means of push-pull tests conducted in each treatment layer. In addition, the distribution of TCFE's lesser chlorinated transformation products was determined. The rates of TCFE reductive dechlorination ranged from 0.05/d to 0.20/d and corresponded to half-lives ranging from 3.5 to 13.9 d. cis-Dichlorofluoroethene was the dominant transformation product detected in all the tests, which is consistent with the findings from pilot tests conducted in the LDPCs prior to the TCFE push-pull tests. cis-Chlorofluoroethene (CFE) and 1,1-CFE also were detected and indicate the potential for vinyl chloride to form under all treatment regimes. Significant production of fluoroethene (FE), the analog of ethene, was observed for only one of the hydrogen treatments. Unambiguous and sensitive detection of the lesser chlorinated products, such as CFE and FE, is possible because TCFE and its transformation products are not found in the background ground water at contaminated sites. Good agreement between the rates and transformation product profiles for TCFE and TCE in both field and laboratory experiments indicates the suitability of TCFE as a surrogate for predicting the rates of TCE reductive dechlorination.

Notes: Monitoring data collected over a 6-year period show that a plume of chlorinated ethene–contaminated ground water has contracted significantly following treatment of the contaminant source area using in situ oxidation. Prior to treatment (1998), concentrations of perchloroethene (PCE) exceeded 4500 μg/L in a contaminant source area associated with a municipal landfill in Kings Bay, Georgia. The plume emanating from this source area was characterized by vinyl chloride (VC) concentrations exceeding 800 μg/L. In situ oxidation using Fenton's reagent lowered PCE concentrations in the source area below 100 μg/L, and PCE concentrations have not rebounded above this level since treatment. In the 6 years following treatment, VC concentrations in the plume have decreased significantly. These concentration declines can be attributed to the movement of Fenton's reagent–treated water downgradient through the system, the cessation of a previously installed pump-and-treat system, and the significant natural attenuation capacity of this anoxic aquifer. While in situ oxidation briefly decreased the abundance and activity of microorganisms in the source area, this activity rebounded in 〈6 months. Nevertheless, the shift from sulfate-reducing to Fe(III)-reducing conditions induced by Fenton's treatment may have decreased the efficiency of reductive dechlorination in the injection zone. The results of this study indicate that source-area removal actions, particularly when applied to ground water systems that have significant natural attenuation capacity, can be effective in decreasing the areal extent and contaminant concentrations of chlorinated ethene plumes.

Notes: Bulk age determinations, based upon chlorofluorocarbons and sulphur hexafluoride measurements of samples from twenty-one chalk groundwater supplies in southern England, Indicate that waters of relatively recent age predominate In both unconfined and partially confined situations. Water from pumping stations located on chalk below Palaeogene cover can be distinguished hydrochemically, and a likely interpretation is that these supplies are receiving a small proportion of recharge via induced via induced leakage. Whilst water which is abstracted from the chalk always involves mixing processes, for a sub-set of confined supplies, ‘piston’ flow could be inferred as a dominant mechanism - resulting in bulk groundwater ages of a few decades. Other supplies are the product of complex mixing. Although low-level chlorofluorocarbon enrichment was encountered for half the catchments sampled, they and sulphur hexafluoride appear to provide independent corroboration to microbiological indicators of the presence of rapid recharge.

Notes: Groundwater residence-time survey results on 21 public water supplies in the chalk aquifer in southern England are compared with a previous Cryptosporidium risk assessment which was carried out on the same supplies for regulatory-compliance purposes in 1999. The results indicate that residence-time indicators could provide useful corroborative evidence for rapid recharge hazard - not only in those settings already identified by microbiological surveillance, but also in the more difficult-to-identify situation where potential rapid pathways have been identified but the bacteriological indicators are negative or ambiguous. However, groundwater-mixing processes under pumping conditions are complex, especially in the chalk, and will always require interpretation informed by an understanding of the local hydrogeological and operational setting.

Notes: Effective implementation is crucial to the success of public policy. This paper focuses on the implementation of the EU Drinking Water Directive (80/778/EEC) in England and Wales and the Republic of Ireland. It demonstrates that the consumer can both positively and negatively affect implementation. It is concluded that, if water providers and regulators wish to improve their ability to shape and effectively implement water policy, they must engage with the consumer in a more informative and educational manner.

Notes: A computer model was developed to evaluate the impact of various technologies for water conservation in domestic households, in terms of the Impact on the operation of downstream infrastructure. These technologies, which include (a) low-flush toilets, (b) greywater re-use, and (c) re-use of rainwater from roof runoff for toilet flushing, were compared using indicators of sustainability to measure water consumption, sewerage-system operational performance and process treatment efficiency. The results demonstrated that rainwater re-use is potentially the most sustainable strategy in terms of the benefits associated with water conservation and reduction in sewage discharges from combined-sewer overflows (CSOs). The benefits were observed without the problems associated with increased sedimentation in sewers during dry weather, associated with other water-conservation strategies such as reduced-flush toilets, greywater re-use and the resultant increase in pollutants from CSOs during wet weather.

Notes: System dynamics is a computer-aided approach to evaluating the interrelationships of different components and activities within complex systems. Recently, system dynamics models have been developed in areas such as policy design, biological and medical modeling, energy and the environmental analysis, and in various other areas in the natural and social sciences. The Idaho National Engineering and Environmental Laboratory, a multipurpose national laboratory managed by the Department of Energy, has developed a system dynamics model in order to evaluate its utility for modeling large complex hydrological systems. We modeled the Bear River basin, a transboundary basin that includes portions of Idaho, Utah, and Wyoming. We found that system dynamics modeling is very useful for integrating surface water and ground water data and for simulating the interactions between these sources within a given basin. In addition, we also found that system dynamics modeling is useful for integrating complex hydrologic data with other information (e.g., policy, regulatory, and management criteria) to produce a decision support system. Such decision support systems can allow managers and stakeholders to better visualize the key hydrologic elements and management constraints in the basin, which enables them to better understand the system via the simulation of multiple “what-if” scenarios. Although system dynamics models can be developed to conduct traditional hydraulic/hydrologic surface water or ground water modeling, we believe that their strength lies in their ability to quickly evaluate trends and cause-effect relationships in large-scale hydrological systems, for integrating disparate data, for incorporating output from traditional hydraulic/hydrologic models, and for integration of interdisciplinary data, information, and criteria to support better management decisions.

Notes: Transboundary aquifers are as important a component of global water resource systems as are transboundary rivers; yet, their recognition in international water policy and legislation is very limited. Existing international conventions and agreements barely address aquifers and their resources. To rectify this deficiency, the International Association of Hydrogeologists and UNESCO's International Hydrological Programme have established the Internationally Shared (transboundary) Aquifer Resource Management (ISARM) Programme. This multiagency cooperative program has launched a number of global and regional initiatives. These are designed to delineate and analyze transboundary aquifer systems and to encourage riparian states to work cooperatively toward mutually beneficial and sustainable aquifer development. The agencies participating in ISARM include international and regional organizations (e.g., Organization of American States, United Nations Environment Programme, United Nations Economic Commission for Europe, Food and Agriculture Organization, and South African Development Community). Using outputs of case studies, the ISARM Programme is building scientific, legal, environmental, socioeconomic, and institutional guidelines and recommendations to aid sharing nations in the management of their transboundary aquifers. Since its start in 2000, the program has completed inventories of transboundary aquifers in the Americas and Africa, and several ISARM case studies have commenced.

Notes: Submarine ground water discharge (SGD) rates were measured continuously by automated seepage meters to evaluate the process of ground water discharge to the ocean in the coastal zone of Suruga Bay, Japan. The ratio of terrestrial fresh SGD to total SGD was estimated to be at most 9% by continuous measurements of electrical conductivity of SGD. Semidiurnal changes of SGD due to tidal effects and an inverse relation between SGD and barometric pressure were observed. Power spectrum density analyses of SGD, sea level, and ground water level show that SGD near shore correlated to ground water level changes and SGD offshore correlated to sea level changes. SGD rates near the mouth of the Abe River are smaller than those elsewhere, possibly showing the effect of the river on SGD. The ratio of terrestrial ground water discharge to the total discharge to the ocean was estimated to be 14.7% using a water balance method.

Notes: Ground water nitrate concentrations on Nantucket Island, Massachusetts, were analyzed to assess the effects of land use on ground water quality. Exploratory data analysis was applied to historic ground water nitrate concentrations to determine spatial and temporal trends. Maximum likelihood Tobit and logistic regression analyses of explanatory variables that characterize land use within a 1000-foot radius of each well were used to develop predictive equations for nitrate concentration at 69 wells. The results demonstrate that historic nitrate concentrations downgradient from agricultural land are significantly higher than nitrate concentrations elsewhere. Tobit regression results demonstrate that the number of septic tanks and the percentages of forest, undeveloped, and high-density residential land within a 1000-foot radius of a well are reliable predictors of nitrate concentration in ground water. Similarly, logistic regression revealed that the percentages of forest, undeveloped, and low-density residential land are good indicators of ground water nitrate concentration 〉2 mg/L. The methodology and results outlined here provide a useful tool for land managers in communities with shallow water tables overlain with highly permeable materials to evaluate potential effects of development on ground water quality.

Notes: While the discussion of model uncertainty has centered on spatial heterogeneity, it is possible that ground water models have not enjoyed much success as predictive tools often because the sources that were eventually imposed in the field differed from those represented in the simulations. This is because deterministic prediction of future conditions is often inaccurate due to the random nature of contaminant sources, in terms of their timing, location, and magnitude. This paper presents a stochastic framework for accommodating random contaminant sources in conventional, deterministic advection-dispersion transport models. The contaminant sources are first classified into two types: those occurring continuously with a deterministic component and random variations and those occurring randomly at instantaneous discrete-time intervals. For the first type, the governing partial differential equation (PDE) is replaced by a stochastic PDE. The random variations are modeled by Gaussian noise or Brownian motion, and the solution is obtained by using Ito's integration technique. For the second type, Markovian analysis is used for discrete-time contamination events. Both approaches use a deterministic transport model to generate response functions at any observation location and time. The response functions are then integrated to yield probabilistic description of contaminant transport, from which key statistical properties such as mean, standard deviation, and confidence interval can be drawn.

Notes: Calibration of ground water transport models is often performed using results of field tracer experiments. However, little attention is usually paid to the influence, on resulting breakthrough curves, of injection conditions and well-aquifer interactions, more particularly of the influence of the possible trapping of the tracer in the injection wellbore. Recently, a new mathematical and numerical approach has been developed to model injection conditions and well-aquifer interactions in a very accurate way. Using an analytical solution derived from this model, a detailed analysis is made of the evolution of the tracer input function in the aquifer. By varying injection conditions from one simulation to another, synthetic breakthrough curves are generated with the SUFT3D ground water flow and transport finite-element simulator. These tests show clearly that the shape of the breakthrough curves can be dramatically affected by injection conditions. Using generated breakthrough curves as “actual” field results, a calibration of hydrodispersive parameters is performed, neglecting the influence of injection conditions. This shows that neglecting the influence of actual injection conditions can lead to (1) errors on fitted parameters and (2) misleading identification of the active transport processes. Conclusions and guidelines are drawn in terms of proposed methodologies for better controlling the tracer injection in the field, in order to minimize risk of misinterpretation of results.

Notes: This paper describes a technique for applying the transition probability geostatistics method for stochastic simulation to a MODFLOW model. Transition probability geostatistics has some advantages over traditional indicator kriging methods including a simpler and more intuitive framework for interpreting geologic relationships and the ability to simulate juxtapositional tendencies such as fining upward sequences. The indicator arrays generated by the transition probability simulation are converted to layer elevation and thickness arrays for use with the new Hydrogeologic Unit Flow package in MODFLOW 2000. This makes it possible to preserve complex heterogeneity while using reasonably sized grids and/or grids with nonuniform cell thicknesses.

Notes: When fugitive methane migrates upward along boreholes of oil and gas wells, it may migrate into shallow ground water or pass through overlying soil to the atmosphere. Prior to this study, there was little information on the fate of fugitive methane that migrates into ground water. In a field study near Lloydminster, Alberta, Canada, we found hydrogeochemical evidence that fugitive methane from an oil well migrated into a shallow aquifer but has been attenuated by dissimilatory bacterial sulfate reduction at low temperature (∼5°C) under anaerobic conditions. Evidence includes spatial and temporal trends in concentrations of methane and sulfate in ground water and associated trends in concentrations of bicarbonate and sulfide. Within 10 m of the oil well, sulfate concentrations were low, and sulfate was enriched in both 34S and 18O. Sulfate concentrations had a strong positive correlation with δ13C values of bicarbonate, and sulfide was depleted in 34S compared to sulfate. These data indicate that bacterial sulfate reduction occurred near the production well. Near the oil well, elevated concentrations of bicarbonate were observed, and the bicarbonate was depleted in 13C. Modeling indicates that the main source of this excess 13C-depleted bicarbonate is oxidized methane. In concert with the sulfate concentration and isotope data, these results support an interpretation that in situ bacterial oxidation of methane has occurred, linked to bacterial sulfate reduction. Bacterial sulfate reduction may play a major role in bioattenuation of fugitive natural gas in ground water in western Canada.

Notes: Matrix diffusion can attenuate the rate of plume migration in fractured bedrock relative to the rate of ground water flow for both conservative and nonconservative solutes of interest. In a system of parallel, equally spaced constant aperture fractures subject to steady-state ground water flow and an infinite source width, the degree of plume attenuation increases with time and travel distance, eventually reaching an asymptotic level. The asymptotic degree of plume attenuation in the absence of degradation can be predicted by a plume attenuation factor, β, which is readily estimated as R′ (φm/φf), where R′ is the retardation factor in the matrix, φm is the matrix porosity, and φf, is the fracture porosity. This dual-porosity relationship can also be thought of as the ratio of primary to secondary porosity. β represents the rate of ground water flow in fractures relative to the rate of plume advance. For the conditions examined in this study, β increases with greater matrix porosity, greater matrix fraction organic carbon, larger fracture spacing, and smaller fracture aperture. These concepts are illustrated using a case study where dense nonaqueous phase liquid in fractured sandstone produced a dissolved-phase trichloroethylene (TCE) plume ∼300 m in length. Transport parameters such as matrix porosity, fracture porosity, hydraulic gradient, and the matrix retardation factor were characterized at the site through field investigations. In the fractured sandstone bedrock examined in this study, the asymptotic plume attenuation factors (β values) for conservative and nonconservative solutes (i.e., chloride and TCE) were predicted to be ∼800 and 12,210, respectively. Quantitative analyses demonstrate that a porous media (single-porosity) solute transport model is not appropriate for simulating contaminant transport in fractured sandstone where matrix diffusion occurs. Rather, simulations need to be conducted with either a discrete fracture model that explicitly incorporates matrix diffusion, or a dual-continuum model that accounts for mass transfer between mobile and immobile zones. Simulations also demonstrate that back diffusion from the matrix to fractures will likely be the time-limiting factor in reaching ground water cleanup goals in some fractured bedrock environments.

Notes: According to common understanding, the advective velocity of a conservative solute equals the average linear pore-water velocity. Yet direct monitoring indicates that the two velocities may be different in heterogeneous media. For example, at the Camp Dodge, Iowa, site the advective velocity of discrete Cl− plumes was less than one tenth of the average pore-water velocity calculated from Darcy's law using the measured hydraulic gradient, effective porosity, and hydraulic conductivity (K) from large-scale three-dimensional (3D) techniques, e.g., pumping tests. Possibly, this difference reflects the influence of different pore systems, if the K relevant to transient solute flux is influenced more by lower-K heterogeneity than a steady or quasi-steady water flux.To test this idea, tracer tests were conducted under controlled laboratory conditions. Under one-dimensional flow conditions, the advective velocity of discrete conservative solutes equaled the average pore-water velocity determined from volumetric flow rates and Darcy's law. In a larger 3D flow system, however, the same solutes migrated at ∼65% of the average pore-water velocity. These results, coupled with direct observation of dye tracers and their velocities as they migrated through both homogeneous and heterogeneous sections of the same model, demonstrate that heterogeneity can slow the advective velocity of discrete solute plumes relative to the average pore-water velocity within heterogeneous 3D flow sytems.

Notes: Naturally occurring long-term mean annual recharge to ground water in Nebraska was estimated by a novel water-balance approach. This approach uses geographic information systems (GIS) layers of land cover, elevation of land and ground water surfaces, base recharge, and the recharge potential in combination with monthly climatic data. Long-term mean recharge 〉 140 mm per year was estimated in eastern Nebraska, having the highest annual precipitation rates within the state, along the Elkhorn, Platte, Missouri, and Big Nemaha River valleys where ground water is very close to the surface. Similarly high recharge values were obtained for the Sand Hills sections of the North and Middle Loup, as well as Cedar River and Beaver Creek valleys due to high infiltration rates of the sandy soil in the area. The westernmost and southwesternmost parts of the state were estimated to typically receive 〈 30 mm of recharge a year.

Notes: Proper management of ground water resources requires knowledge of the rates and spatial distribution of recharge to aquifers. This information is needed at scales ranging from that of individual communities to regional. This paper presents a methodology to calculate recharge from readily available ground surface information without long-term monitoring. The method is viewed as providing a reasonable, but conservative, first approximation of recharge, which can then be fine-tuned with other methods as time permits.Stream baseflow was measured as a surrogate for recharge in small watersheds in southeastern Wisconsin. It is equated to recharge (R) and then normalized to observed annual precipitation (P). Regression analysis was constrained by requiring that the independent and dependent variables be dimensionally consistent. It shows that R/P is controlled by three dimensionless ratios: (1) infiltrating to overland water flux, (2) vertical to lateral distance water must travel, and (3) percentage of land cover in the natural state. The individual watershed properties that comprise these ratios are now commonly available in GIS data bases.The empirical relationship for predicting R/P developed for the study watersheds is shown to be statistically viable and is then tested outside the study area and against other methods of calculating recharge. The method produces values that agree with baseflow separation from streamflow hydrographs (to within 15% to 20%), ground water budget analysis (4%), well hydrograph analysis (12%), and a distributed-parameter watershed model calibrated to total streamflow (18%). It has also reproduced the temporal variation over 5 yr observed at a well site with an average error 〈 12%.

Notes: Three-dimensional grids representing a heterogeneous, ground water system are generated at 10 different resolutions in support of a site-scale flow and transport modeling effort. These grids represent hydrostratigraphy near Yucca Mountain, Nevada, consisting of 18 stratigraphic units with contrasting fluid flow and transport properties. The grid generation method allows the stratigraphy to be modeled by numerical grids of different resolution so that comparison studies can be performed to test for grid quality and determine the resolution required to resolve geologic structure and physical processes such as fluid flow and solute transport. The process of generating numerical grids with appropriate property distributions from geologic conceptual models is automated, thus making the entire process easy to implement with fewer user-induced errors. The series of grids of various resolutions are used to assess the level at which increasing resolution no longer influences the flow and solute transport results. Grid resolution is found to be a critical issue for ground water flow and solute transport. The resolution required in a particular instance is a function of the feature size of the model, the intrinsic properties of materials, the specific physics of the problem, and boundary conditions. The asymptotic nature of results related to flow and transport indicate that for a hydrologic model of the heterogeneous hydrostratigraphy under Yucca Mountain, a horizontal grid spacing of 600 m and vertical grid spacing of 40 m resolve the hydrostratigraphic model with sufficient precision to accurately model the hypothetical flow and solute transport to within 5% of the value that would be obtained with much higher resolution.

Notes: This paper presents the design of the passive-discrete water sampler (PDWS) which has been developed to facilitate investigations of flow partitioning in fractured rocks. The PDWS continuously isolates seeping water into discrete samples while monitoring the seepage rate. The PDWS was used in a flow and transport experiment that investigated fracture-matrix interactions. During the experiment, a mix of conservative tracers with significantly different diffusion coefficients (lithium bromide [LiBr] and pentafluorobenzoic acid [PFBA]) was introduced along a fault located in fractured tuffs, and water seeping through the lower end of the fault was collected by the PDWS and analyzed for tracer concentrations. Preliminary results from this investigation show that samples of effluent captured by the PDWS effectively retained temporal changes in the chemical signature, while providing seepage rates.

Notes: The combination of flowmeter and depth-dependent water-quality data was used to evaluate the quantity and source of high-chloride water yielded from different depths to eight production wells in the Pleasant Valley area of southern California. The wells were screened from 117 to 437 m below land surface, and in most cases, flow from the aquifer into the wells was not uniformly distributed throughout the well screen. Wells having as little as 6 m of screen in the overlying upper aquifer system yielded as much as 50% of their water from the upper system during drought periods, while the deeper parts of the well screens yielded 15% or less of the total yield of the wells. Mixing of water within wells during pumping degraded higher-quality water with poorer-quality water from deeper depths, and in some cases with poorer-quality water from the overlying upper aquifer system. Changes in the mixture of water within a well, resulting from changes in the distribution of flow into the well, changed the quality of water from the surface discharge of wells over time. The combination of flowmeter and depth-dependent water quality data yielded information about sources of high-chloride water to wells that was not available on the basis of samples collected from nearby observation wells. Changing well design to eliminate small quantities of poor-quality water from deeper parts of the well may improve the quality of water from some wells without greatly reducing well yield.

Notes: The occurrence of methyl tert-butyl ether (MTBE) and gasoline hydrocarbons was examined in three types of studies of ground water conducted by the U.S. Geological Survey: major aquifer surveys, urban land-use studies, and agricultural land-use studies. The detection frequency of MTBE was dependent on the study type, with the highest detection frequency in urban land-use studies. Only 13 ground water samples from all study types, or 0.3%, had concentrations of MTBE that exceeded the lower limit of the U.S. EPA's Drinking-Water Advisory. The detection frequency of MTBE was highest in monitoring wells located in urban areas and in public supply wells. The detection frequency of any gasoline hydrocarbon also was dependent on study type and generally was less than the detection frequency of MTBE. The probability of detecting MTBE in ground water was strongly associated with population density, use of MTBE in gasoline, and recharge. Ground water in areas with high population density, in areas where MTBE is used as a gasoline oxygenate, and in areas with high recharge rates had a greater probability of MTBE occurrence. Also, ground water from public supply wells and shallow ground water underlying urban land-use areas had a greater probability of MTBE occurrence compared to ground water from domestic wells and ground water underlying rural land-use areas. The probability of detecting MTBE in ground water was weakly associated with the density of leaking underground storage tanks, soil permeability, and aquifer consolidation, and only concentrations of MTBE 〉0.5 μg/L were associated with dissolved oxygen.

Notes: An approximate analytical solution to the advection-dispersion equation was derived to describe solute transport during spherical-flow conditions in single-well push-pull tests. The spherical-flow case may be applicable to aquifer tests conducted in packed intervals or partially penetrating wells. Using results of two-dimensional numerical simulations, we briefly illustrate the applicability of the derived spherical-flow solution and provide a comparison with its cylindrical-flow counterpart. Good agreement between simulated extraction-phase breakthrough curves and the spherical-flow solution was found when the length of the injection/extraction region was small compared to both aquifer thickness and maximum solute frontal position at the end of the injection phase. On the other hand, discrepancies between simulated breakthrough curves and the spherical-flow solution increased with increasing anisotropy in hydraulic conductivities. Several inherent limitations embedded in its derivation such as assumptions of isotropy and homogeneity warrant the cautious use of the spherical-flow solution.

Notes: A common assumption of ground water models formulated using a block-centered finite-difference method is that a well is located at the center of a cell regardless of its actual location. Due to this assumption, errors are introduced in the spatial distribution of simulated heads. This paper presents an alternative approach for assigning the pumping rates of wells that are located off cell centers. This approach consists of assigning the pumping rate not only to the cell in which the well is located but also to adjacent cells, taking into account the length of the well screen, the hydraulic conductivity, and the distance from the well to the center of its cell. The advantage of this alternative approach over the conventional one is illustrated with a test problem of a synthetic aquifer. Statistical measures of error indicate a much better model fit when pumping rates of wells are distributed over several cells.

Notes: Discrete-fracture and dual-porosity models are infrequently used to simulate solute transport through fractured unconsolidated deposits, despite their more common application in fractured rock where distinct flow regimes are hypothesized. In this study, we apply four fracture transport models—the mobile-immobile model (MIM), parallel-plate discrete-fracture model (PDFM), and stochastic and deterministic discrete-fracture models (DFMs)—to demonstrate their utility for simulating solute transport through fractured till. Model results were compared to breakthrough curves (BTCs) for the conservative tracers potassium bromide (KBr), pentafluorobenzoic acid (PFBA), and 1,4-piperazinediethanesulfonic acid (PIPES) in a large-diameter column of fractured till. Input parameters were determined from independent field and laboratory methods. Predictions of Br BTCs were not significantly different among models; however, the stochastic and deterministic DFMs were more accurate than the MIM or PDFM when predicting PFBA and PIPES BTCs. DFMs may be more applicable than the MIM for tracers with small effective diffusion coefficients (De) or for short timescales due to differences in how these models simulate diffusion or incorporate heterogeneities by their fracture networks. At large scales of investigation, the more computationally efficient MIM and PDFM may be more practical to implement than the three-dimensional DFMs, or a combination of model approaches could be employed. Regardless of the modeling approach used, fractures should be incorporated routinely into solute transport models in glaciated terrain.

Notes: This paper describes a methodology for resolving transboundary water disputes that arise when people/states/nations sharing a resource that crosses legal/political jurisdictions disagree about the use of the resource. Laws and treaties written in an attempt to settle disputes are frequently neither enforced nor effective, and disagreements continue. Crises, arising through resource overuse or shortages, worsen the conflict and typically result in further discord, lawsuits, depletion of the resource, and even open-armed hostility. Many water management experts call for either private/market-based or state/command-and-control resource management systems, but these eventually break down during crisis. The crises therefore necessitate the adoption of a more effective institutional arrangement to address and resolve present and future problems. A better alternative to management by private or state entities and the resolution of conflicts by the mere application of law is a cooperative approach. The Rowland-Ostrom Framework, introduced in this paper, incorporates Ostrom's eight design principles for sustainable common pool resource management within the context of crisis that involves an urgent threat to the quantity or quality of a resource such as water, as described by the author. This paper demonstrates that although established 15 years ago, Ostrom's design principles remain applicable today for effective, sustainable transboundary water management, and the Rowland-Ostrom Framework is a model for the equitable use of shared water resources throughout the world.

Notes: A substantial body of research has been conducted on transboundary water, transboundary water law, and the mitigation of transboundary water conflict. However, most of this work has focused primarily on surface water supplies. While it is well understood that aquifers cross international boundaries and that the base flow of international river systems is often derived in part from ground water, transboundary ground water and surface water systems are usually managed under different regimes, resulting in what has been described as “hydroschizophrenia.” Adding to the problem, the hydrologic relationships between surface and ground water supplies are only known at a reconnaissance level in even the most studied international basins, and thus even basic questions regarding the territorial sovereignty of ground water resources often remain unaddressed or even unasked. Despite the tensions inherent in the international setting, riparian nations have shown tremendous creativity in approaching regional development, often through preventive diplomacy, and the creation of “baskets of benefits,” which allow for positive-sum, integrative allocations of joint gains. In contrast to the notion of imminent water wars, the history of hydropolitical relations worldwide has been overwhelmingly cooperative. Limited ground water management in the international arena, coupled with the fact that few states or countries regulate the use of ground water, begs the question: will international borders serve as boundaries for increased “flows” of hydrologic information and communication to maintain strategic aquifers, or will increased competition for shared ground water resources lead to the potential loss of strategic aquifers and “no flows” for both ground water users?

Notes: Intermontane basins in the Trans-Pecos region of westernmost Texas and northern Chihuahua, Mexico, are target areas for disposal of interstate municipal sludge and have been identified as possible disposal sites for low-level radioactive waste. Understanding ground water movement within and between these basins is needed to assess potential contaminant fate and movement. Four associated basin aquifers are evaluated and classified; the Red Light Draw Aquifer, the Northwest Eagle Flat Aquifer, the Southeast Eagle Flat Aquifer, and the El Cuervo Aquifer. Encompassed on all but one side by mountains and local divides, the Red Light Draw Aquifer has the Rio Grande as an outlet for both surface drainage and ground water discharge. The river juxtaposed against its southern edge, the basin is classified as a topographically open, through-flowing basin. The Northwest Eagle Flat Aquifer is classified as a topographically closed and drained basin because surface drainage is to the interior of the basin and ground water discharge occurs by interbasin ground water flow. Mountains and ground water divides encompass this basin aquifer on all sides; yet, depth to ground water in the interior of the basin is commonly 〉500 feet. Negligible ground water discharge within the basin indicates that ground water discharges from the basin by vertical flow and underflow to a surrounding basin or basins. The most likely mode of discharge is by vertical, cross-formational flow to underlying Permian rocks that are more porous and permeable and subsequent flow along regional flowpaths beneath local ground water divides. The Southeast Eagle Flat Aquifer is classified as a topographically open and drained basin because surface drainage and ground water discharge are to the adjacent Wildhorse Flat area. Opposite the Eagle Flat and Red Light Draw aquifers is the El Cuervo Aquifer of northern Chihuahua, Mexico. The El Cuervo Aquifer has interior drainage to Laguna El Cuervo, which is a phreatic playa that also serves as a focal point of ground water discharge. Our evidence suggests that El Cuervo Aquifer may lose a smaller portion of its discharge by interbasin ground water flow to Indian Hot Springs, near the Rio Grande. Thus, El Cuervo Aquifer is a topographically closed basin that is either partially drained if a component of its ground water discharge reaches Indian Hot Springs or undrained if all its natural ground water discharge is to Laguna El Cuervo.

Notes: Estimation of available ground water is a basic aspect of ground water management. Mathematical modeling is one of the methods that can be effectively used to obtain such estimates. A numerical model was used to calculate available ground water in the Zohor depression—an aquifer transcending national boundaries between the Slovak Republic and Austria. The aquifer, formed by Quaternary sediments overlying Neogene sequences, is composed of various clays interbedded with layers of sand, gravel, sandstones, and conglomerates. The AQUA computer model package was used to simulate flow in the aquifer. For model compilation, the following data were used: (1) effective precipitation; (2) surface water levels in surface water gauging profiles; and (3) withdrawal amounts. Hydraulic parameters of the aquifer were estimated based on information from 86 wells located in the area. The model was verified using data on ground water levels from a monitoring network. The simulation of the aquifer system permitted the estimation of the available ground water in the study area, showing that an additional 587 L/s can be abstracted. Ground water inflows to the Morava River, which flows through the region, range from 745 to 3100 L/s.

Notes: Variations in the concentrations and isotopic compositions (δ13CDIC) of dissolved inorganic carbon (DIC) reflect contamination and biogeochemical cycling of the carbon in ground water. In order to understand contamination and biogeochemical cycling of DIC, we carried out research on the geochemistry of ground water of Guiyang, the capital city of Guizhou Province, China. Results show that ground water is mainly characterized by SO4·HCO3-Ca·Mg and HCO3-Ca·Mg chemical compositions. The hydrochemical characteristics of these types of water are mainly controlled by lithology of the aquifers. HCO3− is the dominant species of DIC in ground water and has lower concentrations and more negative values of δ13CDIC in the high-flow (summer monsoon) season, as compared to the low-flow season. This indicates that DIC is relatively enriched in carbon of biological origin in the high-flow season as compared to the low-flow season and that biological activities are the predominant control on shifts of stable carbon isotope values. The evidence that the δ13CDIC values of ground water decrease with increasing concentrations of anthropogenic species shows that the carbon isotopic composition of DIC can be a useful tracer of contamination, in addition to biogeochemical cycling of inorganic carbon in ground water. Results from this study show that ground water is impacted by significant levels of contamination from human activities, especially in the urban areas, as well as the northeast and west suburbs, in Guiyang city, southwest China.

Notes: Recharge events that deliver electron acceptors such as O2, NO3, SO4, and Fe3+ to anaerobic, contaminated aquifers are likely important for natural attenuation processes. However, the specific influence of recharge on (bio)geochemical processes in ground water systems is not well understood. The impact of a moderate-sized recharge event on ground water chemistry was evaluated at a shallow, sandy aquifer contaminated with waste fuels and chlorinated solvents. Multivariate statistical analyses coupled with three-dimensional visualization were used to analyze ground water chemistry data (including redox indicators, major ions, and physical parameters) to reveal associations between chemical parameters and to infer processes within the ground water plume. Factor analysis indicated that dominant chemical associations and their interpreted processes (anaerobic and aerobic microbial processes, mineral precipitation/dissolution, and temperature effects) did not change significantly after the spring recharge event of 2000. However, the relative importance of each of these processes within the plume changed. After the recharge event, the overall importance of aerobic processes increased from the fourth to the second most important factor, representing the variability within the data set. The anaerobic signatures became more complex, suggesting that zones with multiple terminal electron–accepting processes (TEAPs) likely occur in the same water mass. Three-dimensional visualization of well clusters showed that water samples with similar chemical associations occurred in distinct water masses within the aquifer. Water mass distinctions were not based on dominant TEAPs, suggesting that the recharge effects on TEAPs occurred primarily at the interface between infiltrating recharge water and the aquifer.

Notes: Heat was used as a natural tracer to characterize shallow ground water flow beneath a complex wetland system. Hydrogeologic data were combined with measured vertical temperature profiles to constrain a series of two-dimensional, transient simulations of ground water flow and heat transport using the model code SUTRA (Voss 1990). The measured seasonal temperature signal reached depths of 2.7 m beneath the pond. Hydraulic conductivity was varied in each of the layers in the model in a systematic manual calibration of the two-dimensional model to obtain the best fit to the measured temperature and hydraulic head. Results of a series of representative best-fit simulations represent a range in hydraulic conductivity values that had the best agreement between simulated and observed temperatures and that resulted in simulated pond seepage values within 1 order of magnitude of pond seepage estimated from the water budget. Resulting estimates of ground water discharge to an adjacent agricultural drainage ditch were used to estimate potential dissolved organic carbon (DOC) loads resulting from the restored wetland. Estimated DOC loads ranged from 45 to 1340 g C/(m2 year), which is higher than estimated DOC loads from surface water. In spite of the complexity in characterizing ground water flow in peat soils, using heat as a tracer provided a constrained estimate of subsurface flow from the pond to the agricultural drainage ditch.

Notes: Uncertainty of hydrogeologic conditions makes it important to consider alternative plausible models in an effort to evaluate the character of a ground water system, maintain parsimony, and make predictions with reasonable definition of their uncertainty. When multiple models are considered, data collection and analysis focus on evaluation of which model(s) is(are) most supported by the data. Generally, more than one model provides a similar acceptable fit to the observations; thus, inference should be made from multiple models. Kullback-Leibler (K-L) information provides a rigorous foundation for model inference that is simple to compute, is easy to interpret, selects parsimonious models, and provides a more realistic measure of precision than evaluation of any one model or evaluation based on other commonly referenced model selection criteria. These alternative criteria strive to identify the true (or quasi-true) model, assume it is represented by one of the models in the set, and given their preference for parsimony regardless of the available number of observations the selected model may be underfit. This is in sharp contrast to the K-L information approach, where models are considered to be approximations to reality, and it is expected that more details of the system will be revealed when more data are available. We provide a simple, computer-generated example to illustrate the procedure for multimodel inference based on K-L information and present arguments, based on statistical underpinnings that have been overlooked with time, that its theoretical basis renders it preferable to other approaches.

Notes: Ground water nitrate-nitrogen (NO3−-N) concentrations in the area of Helena, Montana, have increased in both magnitude and extent over the past three decades. It is hypothesized that increases are due to land-use changes associated with urbanization. Population in the Helena area increased by 28.7% between 1990 and 2000, with a commensurate increase in subsurface waste water disposal. Aquifer NO3−-N trends were examined using standard statistical methods applied to identical data sets compiled from 10 publicly funded investigations carried out between 1971 and 2003. Although these analyses indicated an overall increase in NO3−-N over time, conventional statistical techniques applied to flawed data sets are not appropriate for analysis, nor do they illustrate combined temporal and spatial trends of anthropogenic aquifer impacts. In order to use publicly available data collected over decades, geographic information system spatial analysis using inverse distance–weighted interpolation was employed. Isopleth maps graphically depicting temporal changes in NO3−-N concentrations and distribution throughout the study area (35,340 ha) were created. Analysis of these maps revealed increases in NO3−-N concentration and extent over each separate decade. NO3−-N increases were most evident in areas overlying bedrock aquifers and locales with high density and unpermitted septic systems. NO3−-N concentrations did not appear to be increasing extensively in areas overlying the shallow alluvial aquifer or along major stream corridors. Urban development and concurrent loss of native and agricultural properties were the main changes in land use in the Helena, Montana, area over the 32-year study period.

Notes: Two small-diameter (≤0.5-inch/13-mm O.D.) bladder pumps were evaluated under the U.S. EPA Environmental Technology Verification Program to assess their ability to provide representative water quality samples for several volatile organic compounds (VOCs) and major element cations. Both a pneumatic bladder pump (PnBP) and a newly designed mechanical bladder pump (MBP) were evaluated. Since the MBP does not require compressed air to operate, the field logistics are simplified and cost of conducting low-flow sampling can be reduced. Replicate samples were collected from an above-ground standpipe spiked with selected VOCs and cations. Low-flow sampling protocol was followed during collection of replicate samples from six direct push (DP)–installed monitoring wells at Tyndall Air Force Base, Florida. Simple statistical methods are used to evaluate the sample quality provided by the pumps as compared to reference samples. F-tests are used to evaluate pump sample precision, and t-tests are applied to evaluate comparability of the pump sample results to the colocated reference or port sample results. Statistical analysis reveals that the samples collected with the pumps provide precision comparable to that observed in the reference samples. Significantly greater variability was observed between the pump and reference samples collected from the DP wells than was observed during the standpipe trials. The results and statistics from the trials at the standpipe indicate that both the PnBP and MBP provide water quality samples that are comparable to the reference samples, for the analytes investigated.

Notes: A simple conceptual model is presented that leads to a quantitative description of the behavior of light non–aqueous phase liquid (LNAPL) in fine-grained soil (FGS). The occurrence of large (15 feet) (4.6 m) LNAPL accumulations in observation wells in FGS and of LNAPL located below the water table is explained by macropore theory and capillarity of the FGS. Using soil capillary data, fluid property data, and a simple spreadsheet model, the LNAPL saturation in a soil profile and LNAPL recovery were predicted for a field study site. The predicted LNAPL distribution, saturation, and recovery matched the field observations and actual LNAPL recovery. Measured LNAPL saturations were 〈2%, while model-predicted values were 〈3%. The model predicted recovery of ∼530 gallons (2009 L). After 1.5 years of continuous operation, a three-phase, high-vacuum extraction system recovered 150 gallons (568 L) of LNAPL. Application of a model that assumes homogeneity of the soil that is heterogeneous at a small scale may seem to be a misapplication; however, conceptualizing the model domain at a sufficiently large scale (3 to 6 feet; 0.9 to 1.8 m) allows for the FGS to be viewed as a homogeneous medium with small effective porosity.

Notes: In situ biotransformation of BTEX (benzene, toluene, ethylbenzene, o-, m-, and p-xylenes) was investigated for a gasoline spill at Seal Beach Naval Weapons Station (Schroeder 1991) under methanogenic conditions in three controlled-release push-pull experiments. To create methanogenic conditions, anaerobic ground water (710 to 1365 L) was extracted from the anaerobic test zone, treated by deionization to remove nitrate and sulfate, and helium-purged to remove any traces of oxygen. Prior to release through the multiport injection/extraction well, the injection water was amended with BTEX compounds (160 to 367 μg/L) and bromide tracer. Contaminant transformation was observed in three consecutive experiments by withdrawing samples at regular intervals for periods of 73 to 159 d. BTEX removal rates were rapid for toluene and o- and m-xylenes (〈30 d), and slow for benzene, ethylbenzene, and p-xylene degrading (50% removal in 60 to 90 d). Methane was formed in all cases, and the levels of soluble iron, sulfate, and nitrate were too low to account for the levels of BTEX transformation observed. The data confirm that the presence of electron acceptors (oxygen, nitrate, iron, sulfate) is not a precondition for natural attenuation to occur.

Notes: In 2003, the United States Department of Energy completed a full-scale non–aqueous phase liquid (NAPL) remediation of Area A of the Northeast Site at the Young-Rainey STAR Center, Largo, Florida. Area A covered an area of 930 m2 (10,000 square feet) and extended to a depth of 10.7 m (35 feet), representing a total cleanup volume of 9930 m3 (12,960 cubic yards). The site was contaminated with ∼2500 kg (5500 lb) of NAPL constituents such as trichloroethylene, cis-1,2-dichloroethylene, methylene chloride, toluene, and petroleum hydrocarbons. The site consists of a fine-grained sand aquifer underlain by a Hawthorn clay at 9 m (30 feet) depth. The upper 1.5 m (5 feet) of this clay formed part of the remediation volume, as dense non–aqueous phase liquid was present in this layer. The site was remediated using a combination of steam-enhanced extraction and electrical resistance heating. Operations lasted 4.5 months. The site was heated to the target temperatures within 6 weeks, at which time the mass removal rate increased more than 1000-fold. After the target volume had been heated to or near boiling temperatures, pressure cycles were used to increase the mass removal rates, until a final phase of diminishing returns was reached. Postoperational sampling of soil and ground water at randomly selected locations showed the concentrations of all contaminants of concern (COC) to be well below the remedial goals. The majority of the ground water samples were below maximum contaminant level (MCL) for all the COC. The estimate of volatile organic contaminant (VOC) mass removed from the site (1130 kg = 2500 lb) agreed well with the estimate of VOC present before operation (1170 kg = 2580 lb). The postoperational sampling showed that ∼0.5 kg (1 pound) of VOCs remained in the remedial volume, and showed remedial efficiencies of between 99.85% and 99.99% for the four chemicals of concern. Since the postoperational sampling shows all concentrations to be below or close to ground water MCLs, the thermal remedy may be satisfactory for site closure without a polishing phase.

Notes: An analytical solution for steady-state soil vapor extraction (SVE) is derived for both pressure-controlled and flow-controlled well with nonuniform radial flux along the well screen. Since the flow rate and wellhead pressure are not lumped into a single variable, the solutions developed here can predict wellhead pressures required to achieve desired extraction rates. Simulations based on the established P (pressure) or P2 linearization of the governing air flow equation yield practically indistinguishable results for steady SVE. Existing numerical models for ground water flow can therefore be used to represent SVE in the P linearization for more complex vadose zone geometry and heterogeneous soils. The maximum differences between pressure distributions predicted by the nonuniform and uniform flux models occur near the ends of the extraction well screen and diminish with radial distance. Application to field test data indicates that estimates of horizontal and vertical soil-air permeability by the nonuniform flux model become similar to those obtained with the uniform flux model as the extraction well screen is shortened.

Notes: At a number of sites, a plume of methyl tert-butyl ether (MTBE) in ground water has dived below the screen of conventional monitoring wells and escaped detection. Techniques are needed to predict the vertical extent of MTBE in an aquifer. Two techniques that are emerging in the site characterization market are electrical conductivity logging and pneumatic slug testing performed in temporary push wells. These techniques were evaluated at a diving plume of MTBE in the aquifer that supplies water to the village of East Alton, Illinois. The plume stayed near the water table for the first 100 m from the potential sources and then dived below conventional monitoring over the next 100 m. At the location where the MTBE plume dived, the depth to water was 9.1 m below land surface. The first 10 m of material below the water table had an electrical conductivity near 100 mS/m, indicating silts and clays. An electrical conductivity near 25 mS/m, indicating sands or gravels, was encountered at a depth of 10.6 m below the water table, and the sands and gravel extended to a depth of at least 15.2 m below the water table. Pneumatic slug tests measured low hydraulic conductivity in the interval of silt and clay (0.34 and 0.012 m/d) and higher hydraulic conductivity in the interval with sands and gravels (12.5, 11.6, and 11.3 m/d). Ground water with the highest concentration of MTBE was produced just below the contact between the silt and clay and the sands and gravel.

Notes: Methyl tert-butyl ether (MTBE) is one of the most common ground water contaminants in the United States. Ground water contaminated with MTBE is also likely to be contaminated with tert-butyl alcohol (TBA) because TBA is a component of commercial-grade MTBE, can also be used as a fuel oxygenate, and is a biodegradation product of MTBE. In California, MTBE is subject to reporting at concentrations 〉3 μg/L, and TBA has a drinking water action level of 12 μg/L. We describe a simple, automated solid-phase microextraction (SPME) method for the analysis of MTBE and TBA in water. The headspace (HS) of a water sample is extracted with a carboxen/polydimethylsiloxane SPME fiber and the MTBE and TBA are desorbed into a gas chromatograph (GC) and detected using a mass spectrometer (MS). The method is optimized for the routine analysis of MTBE and TBA, with a level of quantitation of 0.3 and 4 μg/L, respectively, in water. The lower level of detection for MTBE is 0.03 μg/L using this method. This HS extraction SPME is applicable to the measurement of both MTBE and TBA at concentrations below regulatory action levels. This method was compared with the certified U.S. EPA Method 5030/8260B (purge and trap/GC/MS) using split samples. Results from the SPME-HS/GC/MS method were directly comparable to those from the U.S. EPA Method 5030/8260B. This method provides a simple, inexpensive, accurate, and sensitive alternative to the U.S. EPA Method 5030/8260B for the analysis of MTBE and TBA in water samples.

Notes: Soil-gas monitoring is a widely used tool to observe the migration of volatile organic compounds (VOCs) at contaminated sites. By combining this technique with natural gradient tracer methods, diffusive contaminant fluxes can be measured in situ, and non–aqueous phase liquid (NAPL) can be detected and roughly quantified. This work describes the new approach and its application at a field site in Denmark with an emplaced NAPL contamination. Soil-gas probes with a low dead volume were installed at 1-m depths in the sandy vadose zone, and a small volume of gas containing conservative and partitioning tracers was injected. Soil-gas samples were withdrawn subsequently during 1 to 4 h and analyzed simultaneously for VOCs and tracers. Tracers detected the NAPL reliably, and the combined data allowed for a close delineation of the source zone. The calculated NAPL saturation deviated by up to a factor of 3 from the analyses of soil cores. Better agreement was found by taking the NAPL composition into consideration, which is, however, generally unknown at the actual field sites. In addition, the tracers were also used to estimate effective diffusion coefficients in situ, which varied by a factor of 2 between various locations. From these data, diffusive contaminant vapor fluxes were quantified without additional laboratory experiments or the use of empirical relationships. The new approach yields a better site investigation with a few additional measurements.

Notes: At sites where soil or ground water contains chemicals of concern, there is the potential for chemical vapors to migrate through the subsurface to nearby basements, buildings, and other enclosed spaces. The 1991 Johnson and Ettinger algorithm and subsequent refinements are often used to assess the significance of this pathway and to establish target cleanup levels. To facilitate its use, the U.S. EPA distributes spreadsheets programmed with the 1991 Johnson and Ettinger algorithm. These user-friendly spreadsheets make the equations more accessible; however, the U.S. EPA spreadsheets require a large number of inputs (〉20), and as a result, relationships between model inputs and outputs are not well understood and users are not able to identify and focus on the critical inputs. The U.S. EPA spreadsheets also allow users to inadvertently enter inconsistent and unreasonable sets of input values, and these often lead to unreasonable outputs. The objective of this work, therefore, is to help users develop a better understanding of the relationships between inputs and outputs so that they can identify critical inputs and also to ensure reasonableness of inputs and outputs. The 1991 Johnson and Ettinger algorithm is introduced, and differences between it and its U.S. EPA spreadsheet implementation are identified. Next, results from a parametric analysis of the algorithm are used to create a flowchart-based approach for identifying the application-specific critical inputs. Use of the flowchart-based approach is then illustrated and validated through comparison with the results of a more traditional sensitivity analysis for four scenarios. Recommendations are also given for the reformulation of inputs to minimize misapplication of the algorithm and the spreadsheets, and reasonable ranges for reformulated input values are discussed.

Notes: Long-term monitoring of sites contaminated with recalcitrant compounds will require the deployment of analytical sensors in an automated system capable of some degree of calibration and quality control. The primary method of providing calibration is to enclose the analytical sensors in a chamber where the environment of the chamber may be controlled. An automated “universal” sampling/calibration/analytical system was developed for mounting analytical sensors in the well or adjacent to the well. The automated universal system allows a sensor to directly analyze the analyte in the water sample or to analyze the analyte in the headspace above the water sample. The ability to analyze the analyte partitioned into the headspace is important for the determination of many volatile species. The analysis of trichloroethene (TCE) is performed by a TCE optrode monitoring the TCE concentrations in the headspace above the water sample. The term optrode refers to a chemically selective fiber-optic sensor. The TCE in the headspace reacts with a colorimetric reagent, producing a colored (red) product. The time history of the development of the colored product is used as the method of determination. The TCE optrode has a limit of detection of 1 part per billion (ppb). There are no interferences from many commonly occurring volatile chlorinated compounds encountered in aquifers such as tetrachloroethene, dichloroethenes, trichloroethane, or dichloroethanes. The system was deployed for monitoring TCE concentrations up to 200 ppb at sites in California and Florida.