ALBERT

Description: Over the last century, afforestation in Ireland has increased from 1% of the land area to 10%, with most plantations on upland drained blanket peatlands. This land use change is considered to have altered the hydrological response and water balance of upland catchments with implications for water resources. Due to the difficulty of observing these long-term changes in the field, the aim of this study was to: utilise a hydrological model to simulate the rainfall-runoff processes of an existing pristine blanket peatland; and then to simulate the hydrology of the peatland if it were drained and afforested. The hydrological rainfall-runoff model (GEOtop) was calibrated and validated for an existing small (76 ha) pristine blanket peatland in the southwest of Ireland for the two year period 2007–2008. The current hydrological response of the pristine blanket peatland catchment with regard to streamflow and water table (WT) levels was captured well in the simulations. Two land-use change scenarios of afforestation were also examined; (A) a young 10-year-old and (B) a semi mature 15-year-old Sitka Spruce forest. Scenario A produced similar streamflow dynamics to the pristine peatland whereas total annual streamflow from Scenario B was 20% lower. For Scenarios A and B, on an annual average basis, the WT was drawn down by 16 and 20 cm below that observed in the pristine peatland respectively. The maximum WT draw down in Scenario B was 61 cm and occurred in the summer months, resulting in a significant decrease in summer streamflow. Occasionally in the winter (following rainfall) the WT for Scenario B was just 2 cm lower than the pristine peatland, which when coupled with the drainage networks associated with afforestation led to higher peak streamflows. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Three deepwater hydrocarbon seep sites in the northern Gulf of Mexico that feature near-seafloor gas hydrates, MC118 (depth = 900 m), GC600 (depth = 1250 m) and GC185 (depth = 550 m), were investigated during the Remote Sensing and Sea-Truth Measurements of Methane Flux to the Atmosphere (HYFLUX) study in July 2009. Continuous measurements of air and sea surface concentrations of methane were made to obtain high spatial and temporal resolution of the diffusive net sea-to-air fluxes. The atmospheric methane fluctuated between 1.70 and 2.40 parts per million (ppm) during the entire cruise except for high concentrations (up to 4.01 ppm) sampled during the end of the occupation of GC600 and the transit between GC600 and GC185. In conjunction with air-mass back trajectory analysis, these high concentrations are likely from a localized methane source to the atmosphere. Methane concentrations in surface seawater and methane net sea-to-air fluxes show high temporal and spatial variability within and between sites. The presence of ethane and propane in the surface seawater indicates a thermogenic source in the plume areas, suggesting the surface methane could be at least partly attributable to transport from the deepwater hydrocarbon seeps. Results from interpolations within the survey areas show the daily methane fluxes to the atmosphere at the three sites range from 0.744 to 300 mol d−1. Extrapolating the highest daily sea-to-air flux of methane to other deepwater seeps in the northern Gulf of Mexico suggests that the net diffusive sea-to-air flux from deepwater hydrocarbon seeps in this region is an insignificant source to the atmospheric methane.

Description: Models of dual-domain mass transfer (DDMT) are used to explain anomalous aquifer transport behavior such as the slow release of contamination and solute tracer tailing. Traditional tracer experiments to characterize DDMT are performed at the flowpath scale (meters), which inherently incorporates heterogeneous exchange processes; hence, estimated “effective” parameters are sensitive to experimental design (i.e., duration and injection velocity). Recently, electrical geophysical methods have been used to aid in the inference of DDMT parameters because, unlike traditional fluid sampling, electrical methods can directly sense less-mobile solute dynamics and can target specific points along subsurface flowpaths. Here, we propose an analytical framework for graphical parameter inference based on a simple petrophysical model explaining the hysteretic relation between measurements of bulk and fluid conductivity arising in the presence of DDMT at the local scale. Analysis is graphical and involves visual inspection of hysteresis patterns to (1) determine the size of paired mobile and less-mobile porosities, and (2) identify the exchange rate coefficient through simple curve fitting. We demonstrate the approach using laboratory column experimental data, synthetic streambed experimental data, and field tracer-test data. Results from the analytical approach compare favorably with results from calibration of numerical models and also independent measurements of mobile and less-mobile porosity. We show that localized electrical hysteresis patterns resulting from diffusive exchange are independent of injection velocity, indicating that repeatable parameters can be extracted under varied experimental designs, and these parameters represent the true intrinsic properties of specific volumes of porous media of aquifers and hyporheic zones.

Description: This paper presents the results of a field investigation of saturated hydraulic conductivity Ksat and bulk density (ρb) in an Atlantic blanket bog in the southwest of Ireland. Starting at a peatland stream and moving along an uphill transect towards the peatland interior, ρb and Ksat were examined at regular intervals. Saturated horizontal hydraulic conductivity (Khsat) and vertical (Kvsat) was estimated at two depths: 10–20 cm and 30–40 cm below the peat surface while ρb was estimated for the full profile. We consider two separate zones, one a riparian zone extending 10 m from the stream and a second zone in the bog interior. We found that the Ksat was higher (~10-5 m s-1) in the bog interior than the riparian zone (~10-6 m s-1) while the converse applied to bulk density, with lowest density (~0.055 g cm-3) at the interior and highest (~0.11 g cm-3) at the riparian zone . In general, we found Khsat to be approximately twice the Kvsat. These results support the idea that the lower Ksat at the margins control the hydrology of blanket peatlands. It is therefore important that the spatial variation of these two key properties be accommodated in hydrological models if the correct rainfall-runoff characteristics are to correctly modelled. Streamflow analysis over 3 years at the peatland catchment outlet showed that the stream runoff was composed of 8% baseflow and 92% flood flow, suggesting that this blanket peatland is a source rather than a sink for floodwaters. Copyright © 2011 John Wiley & Sons, Ltd.

Description: The distribution of shallow frozen ground is paramount to research in cold regions, and is subject to temporal and spatial changes influenced by climate, landscape disturbance and ecosystem succession. Remote sensing from airborne and satellite platforms is increasing our understanding of landscape-scale permafrost distribution, but typically lacks the resolution to characterise finer-scale processes and phenomena, which are better captured by integrated surface geophysical methods. Here, we demonstrate the use of electrical resistivity imaging (ERI), electromagnetic induction (EMI), ground penetrating radar (GPR) and infrared imaging over multiple summer field seasons around the highly dynamic Twelvemile Lake, Yukon Flats, central Alaska, USA. Twelvemile Lake has generally receded in the past 30 yr, allowing permafrost aggradation in the receded margins, resulting in a mosaic of transient frozen ground adjacent to thick, older permafrost outside the original lakebed. ERI and EMI best evaluated the thickness of shallow, thin permafrost aggradation, which was not clear from frost probing or GPR surveys. GPR most precisely estimated the depth of the active layer, which forward electrical resistivity modelling indicated to be a difficult target for electrical methods, but could be more tractable in time-lapse mode. Infrared imaging of freshly dug soil pit walls captured active-layer thermal gradients at unprecedented resolution, which may be useful in calibrating emerging numerical models. GPR and EMI were able to cover landscape scales (several kilometres) efficiently, and new analysis software showcased here yields calibrated EMI data that reveal the complicated distribution of shallow permafrost in a transitional landscape. Copyright © 2016 John Wiley & Sons, Ltd.

Description: Anomalous solute transport, modeled as rate-limited mass transfer, has an observable geoelectrical signature that can be exploited to infer the controlling parameters. Previous experiments indicate the combination of time-lapse geoelectrical and fluid conductivity measurements collected during ionic tracer experiments provides valuable insight into the exchange of solute between mobile and immobile porosity. Here, we use geoelectrical measurements to monitor tracer experiments at a former uranium mill tailings site in Naturita, Colorado. We use non-linear regression to calibrate dual-domain mass-transfer solute-transport models to field data. This method differs from previous approaches by calibrating the model simultaneously to observed fluid conductivity and geoelectrical tracer signals using two parameter scales: effective parameters for the flowpath upgradient of the monitoring point, and the parameters local to the monitoring point. We use regression statistics to rigorously evaluate the information content and sensitivity of fluid conductivity and geophysical data, demonstrating multiple scales of mass-transfer parameters can simultaneously be estimated. Our results show, for the first time, field-scale spatial variability of mass-transfer parameters along single flowpaths (i.e., exchange-rate coefficient, porosity), and differences between local and upgradient effective parameters; hence our approach provides insight into spatial variability and scaling behavior. Additional synthetic modeling is used to evaluate the scope of applicability of our approach, indicating greater range than earlier work using temporal moments and a Lagrangian-based Damköhler number. The introduced Eulerian-based Damköhler is useful for estimating tracer injection duration needed to evaluate mass-transfer exchange rates that range over several orders of magnitude.

Description: [1]  Widespread lake shrinkage in cold regions has been linked to climate warming and permafrost thaw. Permafrost aggradation, however, has been observed within the margins of recently receded lakes, in seeming contradiction of climate warming. Here, permafrost aggradation dynamics are examined at Twelvemile Lake, a retreating lake in interior Alaska. Observations reveal patches of recently formed permafrost within the dried lake margin, co-located with discrete bands of willow shrub. We test ecological succession, which alters shading, infiltration, and heat transport, as the driver of aggradation using numerical simulation of variably saturated groundwater flow and heat transport with phase change (i.e., freeze-thaw). Simulations support permafrost development under current climatic conditions, but only when net effects of vegetation on soil conditions are incorporated, thus pointing to the role of ecological succession. Furthermore, model results indicate that permafrost aggradation is transitory with further climate warming, as new permafrost thaws within 7 decades.