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In a semi-arid climate, groundwater-fed endorheic lakes often have shallow depths and large areas of associated wetlands. These lake-wetland-aquifer systems are sensitive to land use and climate changes. Difficulties of modeling such systems include (a) tracking several thousand lakes and wetlands (LWs), and (b) evaluating major drivers of trends and fluctuations in LW areal coverage. Integrated hydrologic modeling of such systems is prohibitively expensive. We combine traditional groundwater flow modeling (MODFLOW), driven by modern and projected groundwater recharge (GR) estimates, with digital elevation model (DEM)-based geographic information system (GIS) terrain analyses. This approach can be a useful diagnostic tool for estimates of decade-averaged LW areas and numbers, and stream baseflow, facilitating conceptualization and design of more detailed modeling studies. We apply the approach to simulate dynamics of thousands of groundwater-fed LWs in the Nebraska Sand Hills, USA, under GCM-projected GR changes. Compared to the baseline period of 2000–2009, the model indicates an increase by 35% (from 2083 to 2804 km2) of decade-averaged LW area by the end of the 21st century under the Median GR projection, with GR averaging 55.3 mm/yr from 2010 through 2099 (5% increase). Under the same GR projection, the overall numbers of LW is simulated to increase by 18% (from 7331 to 8666). The uncertainty range in the hydrologic response of the system caused by differences among GR projections (Wet and Dry) are also evaluated.

Groundwater recharge (GR) controls vegetation, geomorphology, groundwater, wetlands and surface flow, and ultimately, the ecology and economics of semi-arid regions. Therefore, it is critical to assess hydroclimate model scenarios and the uncertainty in future GR to force regional groundwater models. We use basic statistics of downscaled Global Circulation Model (GCM)-projected cumulative potential GR (GRp) for selecting representative projections. Cumulative GRp is the net recharge (difference between precipitation (P) and evapotranspiration (ET) rates) over the projection period. The approach is illustrated with an example in the Nebraska Sand Hills (NSH), the largest dune region in the Western Hemisphere, where sandy soils are not conducive to overland flow. Changes in decadal-average GRp at 1/8° (∼12-km) scale were estimated from spatially downscaled, bias-corrected temperature and P output from 16 commonly used GCMs for years 2010 to 2099. These changes accounted for three greenhouse gas emissions scenarios, and projections were subsequently used as input to the Variable Infiltration Capacity (VIC) land surface hydrology model. For each of the 48 GCM/VIC hydroclimate projections, cumulative GRp was calculated and averaged over the study area. Three projections (those with cumulative average GRp nearest the median and ±1 standard deviation) were selected as representing Median, Wet and Dry conditions. These projections allow for rapid screening of the sensitivity of regional groundwater models, using readily available downscaled GCM-projected climate changes, thereby optimizing modeling efforts. Future GRp was calculated for the NSH using the selected GR projections by adjusting the 2000–2009 baseline GRp estimates at 1-km scale. The latter was inferred from a previously calibrated groundwater model, with ET based on remote sensing (MODIS) temperature data, and matching regional baseflows. In the NSH by 2099, the Median projection indicates an increase in GRp of 3 mm/yr (+5%) relative to the 2000–2009 baseline of 52.6 mm/yr. The Wet projection has an average increase of 22 mm/yr (+42%), and the Dry projection shows an average decrease of 15 mm/yr (−29%), relative to the baseline. Effects of projection period duration and time-step averaging on selection of GR projections with this approach are discussed. The new detailed GRp projections clarify varying trends of past large-scale analyses of the Northern High Plains region and indicate the possibility for substantial future changes in the NSH hydrologic system. This approach can be extended to other arid-to-humid regions with available GCM hydroclimate projections.

Despite potential evaporation rates in excess of the local precipitation, dry climates often support saline lakes through groundwater inputs of water and associated solutes. These groundwater-fed lakes are important indicators of environmental change, in part because their shallow water levels and salinity are very sensitive to weather and climatic variability. Some of this sensitivity arises from high rates of open-water evaporation, which is a dominant but poorly quantified process for saline lakes. This study used the Bowen ratio energy budget method to calculate open-water evaporation rates for Alkali Lake, a saline lake in the Nebraska Sandhills region (central United States), where numerous groundwater-fed lakes occupy the landscape. Evaporation rates were measured during the warm season (May – October) over three consecutive years (2007–2009) to gain insights into the climatic and limnological factors driving evaporation, as well as the partitioning of energy balance components at seasonal and interannual time scales. Results show a seasonal peak in evaporation rate in late June of 7.0 mm day −1 (on average), with a maximum daily rate of 10.5 mm day −1 and a 3-year mean July-September (JAS) rate of 5.1 mm day −1 , which greatly exceeds the long-term JAS precipitation rate of 1.3 mm day −1 . Seasonal variability in lake evaporation closely follows that of net radiation and lake surface temperature, with sensible heat flux and heat storage variations being relatively small, except in response to short-term, synoptic events. Interannual changes in the surface energy balance were weak, by comparison, although a 6-fold increase in mean lake level over the three years (0.05–0.30 m) led to greater heat storage within the lake, an enhanced JAS lake-air temperature gradient, and greater sensible heat loss. These large variations in water level were also associated with large changes in absolute salinity (from 28 to 118 g kg −1 ), with periods of high salinity characterized by reductions in mass transfer estimates of evaporation rate by up to 20%, depending on atmospheric conditions and absolute salinity. Energy balance estimates of evaporation, on the other hand, were found to be less sensitive to variations in salinity. These results provide regional insights for lakes in the Nebraska Sandhills region and implications for estimation of the energy and water balance of saline lakes in similar arid and semi-arid landscapes.

Summary Availability and uncertainty in input data are the primary constraints of groundwater modeling. Analytical models assimilate the key and important data, but capture the major traits of the watershed. We study a baseflow-dominated stream, Frenchman Creek in southwestern Nebraska, USA, which has experienced large streamflow reductions since the 1960s and is a subject of various actions on water rights appropriation. The new element of the model is simultaneous analytical consideration of groundwater pumping and land use change effects. Analytical stream depletion rate calculations by various methods show that pumping from the 462 irrigation wells in the basin consumed a large amount of baseflow. The simulated streamflow at the outlet of Frenchman Creek with minimal calibration compares favorably with observed streamflow and indicates the viability of an analytical approach to watersheds with limited hydrogeologic data.

Summary Despite the importance of groundwater recharge (GR), its accurate estimation still remains one of the most challenging tasks in the field of hydrology. In this study, with the help of inverse modeling, long-term (6 years) soil moisture data at 34 sites from the Automated Weather Data Network (AWDN) were used to estimate the spatial distribution of GR across Nebraska, USA, where significant spatial variability exists in soil properties and precipitation (P). To ensure the generality of this study and its potential broad applications, data from public domains and literature were used to parameterize the standard Hydrus-1D model. Although observed soil moisture differed significantly across the AWDN sites mainly due to the variations in P and soil properties, the simulations were able to capture the dynamics of observed soil moisture under different climatic and soil conditions. The inferred mean annual GR from the calibrated models varied over three orders of magnitude across the study area. To assess the uncertainties of the approach, estimates of GR and actual evapotranspiration (ETa) from the calibrated models were compared to the GR and ETa obtained from other techniques in the study area (e.g., remote sensing, tracers, and regional water balance). Comparison clearly demonstrated the feasibility of inverse modeling and large-scale (>104 km2) soil moisture monitoring networks for estimating GR. In addition, the model results were used to further examine the impacts of climate and soil on GR. The data showed that both P and soil properties had significant impacts on GR in the study area with coarser soils generating higher GR; however, different relationships between GR and P emerged at the AWDN sites, defined by local climatic and soil conditions. In general, positive correlations existed between annual GR and P for the sites with coarser-textured soils or under wetter climatic conditions. With the rapidly expanding soil moisture monitoring networks around the globe, this study may have important applications in aiding water resources management in different regions.

Summary Due to its complex interactions with various processes and factors, soil moisture exhibits significant spatial variability across different spatial scales. In this study, a modeling approach and field observations were used to examine the soil control on the relationship between mean ( θ ¯ ) and standard deviation (σθ) of soil moisture content. For the numerical experiments, a 1-D vadose zone model along with van Genuchten parameters generated by pedotransfer functions was used for simulating soil moisture dynamics under different climate and surface conditions. To force the model, hydrometeorological and physiological data that spanned over three years from five research sites within the continental US were used. The modeling results showed that under bare surface conditions, different forms of the θ ¯ – σ θ relationship as observed in experimental studies were produced. For finer soils, a positive θ ¯ – σ θ relationship gradually changed to an upward convex and a negative one from arid to humid conditions; whereas, a positive relationship existed for coarser soils, regardless of climatic conditions. The maximum σθ for finer soils was larger under semiarid conditions than under arid and humid conditions, while the maximum σθ for coarser soils increased with increasing precipitation. Moreover, vegetation tended to reduce θ ¯ and σθ, and thus affected the θ ¯ – σ θ relationship. A sensitivity analysis was also conducted to examine the controls of different van Genuchten parameters on the θ ¯ – σ θ relationship under bare surface conditions. It was found that the residual soil moisture content mainly affected σθ under dry conditions, while the saturated soil moisture content and the saturated hydraulic conductivity largely controlled σθ under wet conditions. Importantly, the upward convex θ ¯ – σ θ relationship was mostly caused by the shape factor n that accounts for pore size distribution. Finally, measured soil moisture data from a semiarid region were retrieved from the Automated Weather Data Network. The observed moisture data showed that based on soil texture, a positive θ ¯ – σ θ relationship existed for sandy soils, while an upward convex one was observed for silty soils. The difference in the observed θ ¯ – sigma θ relationship can be attributed to the differences in water holding capacities between sand and silt, which is consistent with the modeling results. The field data also revealed that increasing spatial variability in soil texture led to increased variability in soil moisture (e.g., the maximum σθ). Therefore, the effect of soil texture for verifying remotely sensed soil moisture products should be considered.

Summary To meet the challenge of estimating spatially varying groundwater recharge (GR), increasing attention has been given to the use of vadose zone models (VZMs). However, the application of this approach is usually constrained by the lack of field soil hydraulic characteristics (SHCs) required by VZMs. To tackle this issue, SHCs based on the van Genuchten or Brooks–Corey model are generally estimated by pedotransfer functions or taken from texture based class averages. With the increasing use of this method, it is important to elucidate the controls of SHCs on computing GR mostly due to the high nonlinearity of the models. In this study, it is hypothesized that the nonlinear controls of SHCs on computing GR would vary with climatic conditions. To test this hypothesis, a widely used VZM along with two SHCs datasets for sand and loamy sand is used to compute GR at four sites in the continental Unites States with a significant gradient of precipitation (P). The simulation results show that the distribution patterns of mean annual GR ratios ( GR ‾ / P ‾ , where GR ‾ and P ‾ are mean annual GR and P, respectively) vary considerably across the sites, largely depending on soil texture and climatic conditions at each site. It is found that GR ‾ / P ‾ is mainly controlled by the shape factor n in the van Genuchten model and the nonlinear effect of n on GR ‾ / P ‾ varies with climatic conditions. Specifically, for both soil textures, the variability in GR ‾ / P ‾ is smallest at the Andrews Forest with the highest P ‾ (191.3 cm/year) and GR ‾ / P ‾ is least sensitive to n; whereas, the variability in GR ‾ / P ‾ at the Konza Prairie ( P ‾ = 84.2 cm/year) is the largest and GR ‾ / P ‾ is most sensitive to n. With further decreasing P ‾ , the nonlinear effect of n weakens at the Barta Brothers ( P ‾ = 57.3 cm/year) and Sevilleta ( P ‾ = 20.3 cm/year), leading to smaller GR ‾ / P ‾ variability at those two sites than at the Konza Prairie. The results also reveal that GR ‾ / P ‾ in finer soils with smaller n values decreases more rapidly with decreasing P ‾ .

Summary Deep drainage of water below plant root zones (potential groundwater recharge) will become groundwater recharge (GR) after a delay (or lag time) in which soil moisture traverses the vadose zone before reaching the water table. Depending on the thickness of the vadose zone, the magnitude of deep drainage, and soil hydraulic properties, lag times will vary broadly, exceeding decades to centuries in semi-arid and arid environments. Yet, studies of future climate change impacts to GR have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone. We investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of GR throughout the semi-arid Nebraska Sand Hills (NSH) region (∼50,000 km2). We propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate. Understanding of such effects is needed for groundwater modeling, analysis of climate change impacts on groundwater, and the effective management and sustainability of future water resources. Lag time estimates are made using the pressure-based vertical velocity of soil moisture changes, equivalent to a kinematic wave approximation of Richards’ equation. The underlying assumptions (unit hydraulic gradient and relatively slow changes in climate) are supported by observations in the High Plains aquifer region, encompassing the NSH. The analysis relies on four sources of input data, including: Spatially distributed high resolution (1.1-km) GR rates (as the difference in 2000–2009 mean precipitation and evapotranspiration—estimated using a novel MODIS-based approach); thickness of the vadose zone—based on 30-m digital elevation models of the land surface and water table elevations; statistically downscaled estimates of future (2010–2099) potential GR rates from two hydroclimate model projections (WRCP CMIP3) with opposing GR trends; and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van Genuchten–Mualem equation. We found that vadose zone lag times (with a mean GR rate of 78 mm/yr for 2000–2009) have an estimated spatial mean of 5.28 yrs (vertical velocity of 5.68 m/yr). End-of-century lag times (with mean potential GR rates of 128 and 61 mm/yr for 2090–2099) have estimated spatial means of 3.78 and 6.55 yrs (vertical velocities of 8.13 and 4.69 m/yr) for wet and dry scenarios, respectively. Findings suggest that the highly conductive materials (primarily sand deposits) and high potential GR rates in the NSH lead to relatively quick drainage of vadose zone water on the order of several years (less than a decade) for the entire 21st century. However, considerable spatial variations of lag times exist due to gradients in potential GR rates and the highly variable vadose zone thickness in the NSH.

Summary A complementary relationship (CR) between potential ( ET p ) and actual ( ET a ) evapotranspiration is obtained using two independent approaches, and the role of soils in formulating the CR is also investigated. The first approach employs a process-based vadose zone model that links ET p and ET a by explicitly considering the role of soils. To drive the model, hydrometeorological, soil, and remotely sensed physiological data are utilized. The second approach utilizes the products of the MODIS_ ET (MOD16) data set that is derived from remotely sensed data by mainly considering above-ground land surface processes. Both methods are applied for 42 sites across Nebraska, USA. The results from both approaches demonstrate the CR; however, according to previously reported ET a and ET p values, MODIS_ ET tends to underestimate ET a in wet years and systematically overestimate ET p across Nebraska, which leads the wet surface evapotranspiration ( ET w ) to decrease with increasing annual humidity index Φ (i.e., the ratio of annual precipitation over annual ET p ). In comparison, ET w estimated from the vadose zone model is similar to ET w calculated from the Priestley–Taylor equation, and remains relatively constant over a wide range of annual Φ with a mean of 105.1 cm/year. Both approaches show that ET w is not sensitive to soil properties, which provides a rationale for combining historical data from different locations to study the impact of climate variability on evapotranspiration processes at annual time scales.

Christensen et al. (2009) studied the optimal design of pumping tests whose goal is to estimate hydrogeologic parameters affecting stream depletion caused by pumping in a leaky aquifer. Their analysis relies on the analytical solutions of Zlotnik and Tartakovsky (2008), which are based on the assumptions of negligible drawdown in the source bed of a leaky aquifer and of horizontal flow in an aquifer of infinite extent. We conduct a series of two- and three-dimensional simulations to identify the validity and range of applicability of these assumptions, and to quantify their impact on estimation of aquifer drawdown, stream depletion, their sensitivities to the hydrogeologic parameters, and on selection of optimal locations to observe drawdown during a pumping test. Quantitative criteria for the use of these analytical solutions are formulated in terms of the source-bed transmissivity and the distance from the well to prescribed-head boundaries other than the stream.

Summary We investigate the conditions and criteria for identifying flow regimes in shallow groundwater-dominated lakes in arid environments. Relying on regional and local water-table configurations, we develop a steady-state model of lake-aquifer systems. Using dimensional analysis and numerical simulations, we investigate relationships between groundwater inflow and outflow. A new framework uses the concepts of gradient ratio G , defined as the ratio of regional-to-local head gradients, and the topohydrologic offset (THO), defined as the difference between actual lake level and regional groundwater level. It is shown that a critical value of gradient ratio, G c , exists for each aquifer thickness and anisotropy in hydraulic conductivity. In cases where the regional gradient is larger than the local gradient and G > G c , the lake is “flushed” in a flow-through regime. When the local gradient is larger than the regional head gradient and G G c , the lake gains and large evaporative losses occur. G c becomes progressively larger with increasing aquifer thickness and anisotropy in hydraulic conductivity. Variation in the ratio of groundwater inflow-to-outflow as G approaches G c is consistent with observed variability in lake salinity. Transition from flow-through conditions to other regimes in groundwater-dominated lakes in arid environments may occur with subtle changes in the gradient ratio, regional water-table slope, lake size, and THO. Finally, we discuss the feasibility of using a geographic information system to delineate the potential for lake to retain solutes in areas with numerous lakes, similar to Nebraska, USA.

Summary We derive a closed form theory of water level fluctuations caused by slug tests in a well with an annulus that extends to the piezometric surface. This hydraulics can be created by small diameter slug test equipment in a large diameter well, with an annulus established by borehole casing and the riser pipe of the equipment. The conservation of mass equation couples aquifer hydraulics, core hydraulics inside the riser pipe (which include inertia), and annular hydraulics outside the riser pipe (which include inertia and linear friction). The linear friction is needed to attenuate oscillations between the core and annulus, which would otherwise persist indefinitely after damping of aquifer discharge. A characteristic velocity is adopted for the annular friction, so that aquifer permeability is the sole calibration parameter. We calibrate the theory with overdamped and underdamped experiments run in a limestone aquifer at the Hydrogeological Experimental Site of Poitiers in France by Audouin, O., Bodin, J. 2007. Analysis of slug tests with high frequency oscillations. Journal of Hydrology 334, 282–289. The data and analytical theory yield plausible, accurate permeability estimates of 4.1 × 10 −12 and 1.7 × 10 −11 m 2 . Portable, small diameter slug testing devices may accordingly be used in permanent, large diameter cased boreholes to estimate aquifer permeability, although the annular effects can obscure aquifer hydraulics. The annular effects may also occur in slug tests on wells with skins, bypasses, and packers, although different boundary and matching conditions would be needed to model this hydraulics.

In river basins with several tributaries, wells in alluvial valleys are often located near the wedge-shaped confluence of two tributaries. Therefore, stream depletion characteristics, namely stream depletion rate (SDR) and stream depletion volume (SDV), induced by the pumping wells are distributed between two tributaries upstream from the confluence. Existing methods for the evaluation of stream depletion characteristics are applicable only for a wedge-shaped aquifer with special or uncommon wedge angles or for steady-state conditions. In water resources management, temporal stream depletion characteristics are important for the adjudication of water rights. A new and practically important method for analytical evaluation of transient SDR and SDV is developed for a wedge-shaped aquifer with arbitrary wedge angles. Results are obtained for each of two tributaries or the tributary segment lengths. A simple expression for the SDR fraction that originates from each tributary is presented for long period of pumping after which 100% of the pumping rate is supplied by stream depletion. The sensitivity analyses for different parameters are performed to examine their affects on the influence period of the total SDR and the results indicated that the SDR is sensitive to the angles of wedge-shaped aquifer and the pumping well. The results and sensitivity analysis presented in this study may provide useful information in the assessment of water balance or exploitation of river basins.

Summary In the Nebraska Sand Hills (NSH), the largest vegetated dune field in the Western Hemisphere, dunes have relatively high hydraulic conductivities, and surficial aquifers can partially retain a significant fraction of precipitation as groundwater recharge and return it for preservation of plants. This important mechanism for maintaining the stability of the ecosystem in the NSH requires knowledge of saturated hydraulic conductivity ( K fs ) in the vadose zone. A unique 3D dataset for the distribution of K fs was collected within the framework of the Grassland Destabilization Experiment (GDEX) at the Barta Brothers Ranch site, Nebraska, using direct in situ measurements. The results show that K fs increases with depth towards the lower boundary of the root zone (up to 2 m). For the topographic effects, highlands generally present higher K fs values than lowlands, and there exists a significant correlation between K fs and absolute elevation. It is found that the effect of short-term vegetation disturbance on K fs was minimal that may not hold in the long run.

Summary The dipole flow test (DFT) was applied to investigate the effects of entrapped air on quasi-saturated hydraulic conductivity ( K ) in a cylindrical aquifer model (sand tank). The sand tank was equipped with 58 spatially distributed piezometers and screened well to accommodate the dipole probe (DP). The experiments included tests with and without pre-saturation of the sand medium with CO 2 , a technique intended to minimize the impacts of entrapped air on K . A mathematical model was developed and applied to interpret the piezometer drawdown data. Estimation of K from DP chamber drawdowns produced good results when the medium was pre-saturated with CO 2 . However, without CO 2 pre-saturation, the effect of entrapped air was apparent from the nonlinear drawdown responses to the pumping rate. In general, the quasi-saturated K was approximately 50% of the fully saturated K .

Analytical solutions of drawdown in unconfined aquifers are widely applied for determining the specific yield, S y , and the horizontal and the vertical hydraulic conductivity K r and K z , respectively. In many previous studies, estimates of S y and K z were observed to be highly variable and physically unrealistic. This has been attributed to the conceptualization of flow above the declining water table and aquifer heterogeneity in the applied models. We present the analysis of time-drawdown data from a pumping test instrumented with depth-differentiated observation piezometers arranged in clusters. Applying homogeneous anisotropic aquifer models in combination with nonlinear least squares parameter identification techniques, the data were analyzed in different groups: analysis of data from individual piezometer clusters and simultaneous analysis of the entire data set from all piezometer clusters (global analysis). From the cluster analyses, estimates of S y and K z exhibit large variances and depart from a priori estimates inferred from the hydrostratigraphy. Parameter estimates from the global analysis do not fall within the parameter bounds (minimum and maximum values) defined by the cluster analyses. While heterogeneity appears to be the important reason for large parameter variances, we discuss the influence of rarely considered aquifer return flow on drawdown and the inconsistent results from the cluster and global analyses. We corroborate our findings with data on hydraulic gradients, slug test data, and results from the application of a more realistic numerical flow model.

This uniquely designed study investigates a fundamental issue—the feasibility of predicting stream depletion rates using linear uniform two-dimensional models. Required input for these models includes the hydraulic parameter estimates of the aquifer and the stream–aquifer interface, which may be obtainable through pumping test data analysis. This study utilizes pumping test data collected near the naturally meandering Prairie Creek, Platte River watershed, Nebraska, USA. Drawdown data were obtained in eight piezometer clusters, located on both sides of the stream, each containing three piezometers screened at different aquifer depths. Parameter estimates and, thus, stream depletion predictions varied over a wide range. Large parameter variance and a low degree of goodness of fit between the calculated and measured data encountered during the analysis suggest deficiencies of the uniform aquifer models in describing significant physical processes. This was also shown by additional field experiments that indicate lateral and vertical aquifer heterogeneity. Hydrogeological and sedimentological considerations of the meandering stream architecture (point bar versus cut bank) and the application of a linear piecewise-homogeneous model yielded a higher degree of goodness of fit and higher confidence in stream depletion predictions. Aquifer heterogeneity appears to be the major reason for uncertainty in stream depletion predictions, though other possible sources of uncertainty should be considered. These include the model linearity, the Dupuit assumption, the simplified representation of the stream–aquifer interface, the approximation of the stream as a straight line or a strip, and the impact of regional groundwater flow.

Recent numerical studies have demonstrated that the conventional interpretation of the borehole flowmeter test (BFT) may lead to considerable errors in estimates of the horizontal hydraulic conductivity ( K r ) due to neglect of head loss across the electromagnetic borehole flowmeter (EBF). Even in uniform aquifers, the conventional interpretation underestimates K r at the base and overestimates K r at the top of the aquifer. In this paper, we derive exact analytical solutions for hydraulic head and streamlines induced by the BFT in a confined homogeneous aquifer. The solutions explicitly consider head loss across the EBF. The derived analytical solutions for head distribution in the vicinity of the pumping well and for volumetric flux to the well sections above and below the EBF can be used to interpret field BFT data. In uniform aquifers, this approach can be applied to obtain estimates of K r from the conventional interpretation. Applications of this approach to the BFT field data set from a highly heterogeneous aquifer indicate that the constraint of aquifer homogeneity limits the applicability of this approach, but it can provide useful insights into the mechanism of flux redistribution near the borehole during the BFT.

Flow structure around a recirculation well in a uniform anisotropic aquifer is investigated using potential theory and Stokes’ stream function techniques. A vertical recirculation well consists of two screened sections (chambers) separated by an impermeable casing: one section extracts water from the aquifer and the other section injects the water back into the aquifer. Analytical formulae are derived for the drawdown and the stream function for a model in which the extraction and injection chambers of the well are modeled as uniformly distributed cylindrical sinks and sources, respectively. Travel times are analyzed for water particles traveling from the injection chamber to the extraction chamber along streamlines containing various percentages of the flow. We consider a disturbed zone (skin) having properties which are different from those in the uniform anisotropic aquifer owing to disturbance of the area near the well during installation. The effects of this skin on the streamlines and travel times are analyzed for various lengths separating the chambers and for various skin conductivities. The method completely eliminates the use of numerical finite-difference or finite-element methods and can be used for optimization of technological parameters in a remediation system.

A multi-level slug test model and a method for the evaluation of vertical profiles of hydraulic conductivity in highly permeable formations are developed. A double-packer system is employed to estimate local hydraulic conductivity. Depending on the formation and double-packer system parameters, the water level recovery in the tested well can exhibit a monotonic or oscillatory response. To discern information on aquifer properties from artifacts introduced by the measurement system, the theory of flow in an aquifer and a double-packer system is developed. The mathematical model incorporates features of the Springer and Gelhar (1991) model and reduces to the Bouwer and Rice (1976) model in a special case. The model involves equations of momentum and mass conservation for the double-packer system with quasi-steady well-aquifer interaction equations. The method is uniformly applicable for both monotonic and oscillatory well responses and can produce profiles of hydraulic conductivity for the tested well. The criterion is given to determine the type of well response for given slug test conditions.