bluuebunny/crossref_metadata_2025_split huggingface dataset:
Implementing Spatially Variable Anisotropy in MODFLOW
Authors: Kladias Michael P. , Ruskauff Gregory J. 
Published: August 2005

AbstractThe U.S. Geological Survey modular ground‐water flow model (MODFLOW) allows column to row anisotropy to vary only in different layers. Modifications to the code were made to allow each grid block in the model to have its own anisotropy, and still retain backward compatibility with the original formulation. Memory usage increases because of the need to allocate storage for every grid block in the model versus a single entry for each layer. The example shown requires about 30 percent more memory for the spatially variable versus original anisotropy formulations. Storing anisotropy for each grid block is the same as specifying both column and row components of hydraulic conductivity or transmissivity.
Comparison of Neutron Moisture Gauges and a Neutron Tool for Use in Monitoring Wells
Authors: Bishop C. W. , Porro I. 
Published: August 2005

AbstractResearch at the Idaho National Engineering Laboratory investigated the movement of water through a thick fractured basalt vadose zone. Measurements taken in 20‐cm diameter boreholes completed with 10.2‐cm diameter PVC casing and in specially constructed test containers filled with basalt showed that 50‐m Ci neutron moisture gauges could distinguish differences in moisture levels in the basalt formation beyond the well's annular space. Measurements made at depths up to 55 m in similarly constructed monitoring wells in the field showed that moisture gauge readings lead to the same interpretation of moisture movement as readings made with a 3‐Ci neutron tool.
Statistical Evaluation of the Vulnerability of Ground‐Water Wells, A Case Study of Data from the Strasbourg Polygone Pumping Field
Authors: Jameta Ph. , Sampson P. D. , Vincent F. 
Published: August 2005

AbstractThe case study presented in this paper illustrates how statistical methods can help to understand the relationships between ground‐water wells and pollutant sources in alluvial systems. The observation of a time series of chloride concentrations in a specific well in Strasbourg, France, influenced by the infiltration of chlorinated water from the Rhine river, made it possible to derive a two‐predictor model of the water quality in the well. This model combines a linear dependence upon a time‐lagged chloride concentration in the river, reflecting the steady‐state water balance in the well, and a nonlinear dependence upon average lagged water output pumped from the well. This simple model explains 86% of the variance in well chloride concentration measured weekly over a two year period.
Electrical Imaging of Fractures Using Ground‐Water Salinity Change
Authors: Slater Lee D. , Binley Andrew , Brown David 
Published: August 2005

AbstractCross‐borehole Eletrical Resistivity Tomography (ERT) has been applied to map the spatial distribution of an electrically conductive zone within a hydrologically complex limestone environment. Advantage has been taken of a “natural” conductive tracer provided by salt‐water migration resulting from long‐term pumping at a “well” within the fieldsite. The resistivity distribution of 13 between‐borehole planes has been reconstructed from borehole resistance measurements, and comparisons between resistivity images obtained prior to, and during, saline migration have been made. The distribution of resistivity obtained from the a priori measurements revealed the location of electrically conductive pathways between boreholes which correlate with cavity features obtained from Borehole Television (BHTV) logging. The continuity of these pathways between boreholes suggests the presence of a laterally extensive electrically conductive zone. The reconstruction of differences in resistivity caused by the salt‐water migration confirmed the presence of this conductive zone and has shown that the saline water passed primarily through this zone. This preferential flow illustrates the hydrological importance of this zone and suggests that it is not only an electrically conductive, but also a highly hydraulically conductive feature.
Model Simulations in Support of Field Scale Design and Operation of Bioremediation Based on Cometabolic Degradation
Authors: Lang Margaret M. , Roberts Paul V. , Semprini Lewis 
Published: August 2005

AbstractThis paper addresses questions fundamental to the design and operation of aquifer bioremediation based on cometabolic degradation. A model of a full‐scale, in situ system for bioremediation of chlorinated ethenes relying on cometabolic degradation was developed and applied to a hypothetical aquifer being considered for a large‐scale field demonstration of in situ bioremediation with recirculation. The model was used to identify feasible substrate (electron donor and electron acceptor) delivery schedules. Trichloroethylene (TCE) was the target contaminant. Methane and phenol were considered as electron donors. The delivery of the electron donors and the electron acceptor, oxygen, was varied to evaluate the rate and extent of bioremediation under different substrate delivery schedules. Maximum removal of TCE was predicted when substrates are delivered at ratios near the stoichiometric requirement of electron donor and acceptor for net microbial growth.Additionally, the decrease in TCE removal that results from using substrate delivery schedules other than those achieving the maximum removal of TCE was quantified. This decrease was greater for the methane‐oxygen system because the two gaseous substrates compete for transfer into the recirculated ground water. If one substrate is introduced in excess of the amount required for net microbial growth, it accumulates, thus limiting the ability to introduce the second substrate. This imbalance both limits the introduction of the second substrate and accelerates the accumulation of the substrate added in excess. The phenol‐oxygen system is less sensitive to deviation away from the best observed substrate delivery schedule because phenol is a relatively soluble liquid and its introduction does not compete with the mass transfer of oxygen.
Electromagnetic Investigations of Aquifers in the Grand Brulé Coastal Area of Piton de la Fournaise Volcano, Reunion Island
Authors: Robineau Bernard , Ritz Michel , Courteaud Michel , Descloitres Marc 
Published: August 2005

AbstractA geophysical survey was carried out on the eastern flank of Piton de la Fournaise volcano in the Grand Brulé area using combined audiomagnetotelluric (AMT) and time domain electromagnetic (TDEM) techniques. The aims of the present study were firstly to define the shallow geoelectrical structure, and subsequently to locate large bodies of ground water with anomalous electrical properties. Most of the AMT sites are isotropic across the entire observational bandwidth (7500‐10 Hz), except for a few sites where static‐shift effects are present. Joint one‐dimensional interpretation of the TDEM and AMT data indicates basically three‐layered structures with the following resistivity zones, progressing from the surface downward: (1) a resistive unsaturated surface layer of subaerial basaltic flows with a 2000 to 10,000 ohin‐m resistivity, its base being approximately at mean sea level, (2) an intermediate‐resistivity layer (100‐250 ohm‐m) 50–300 m thick, interpreted as being fresh‐water‐saturated basaltic formations, and (3) a low‐resistivity layer of less than 15 ohm‐m probably due to more saline water at depth and/or higher clay content in crushed materials. In the center of the study area, the results have also shown the presence of a prominent boundary dividing the area into a southern zone where the aquifer sequence is thin (60‐100 m) and a northern zone corresponding to a relatively thick aquifer (>150 m). Geoelectrical models are compared with the available geological information given by two geothermiic exploration drillings. The coastal drill hole gives a crucial information, i.e. the thick intermediate‐resistivity layer in the northern part of Grand Brule can be related to water‐saturated alluvium deposits down to 200 m below sea level. AMT and TDEM soundings have revealed a major paleoalluvial drainage system buried under recent lava flows. A delimitation of sectors with different hydrogeological characteristics can be proposed.
The Problem of Modeling Limestone Springs: The Case of Bagnara (North Apennines, Italy)
Authors: Angelini Paola , Dragoni Walter 
Published: August 2005

AbstractThe Bagnara spring (Central Italy), fed by a fractured, carbonate, and, in some areas, karstic aquifer, was examined. The available information is derived from geological mapping and daily flows over a period of 20 consecutive years. There are no data on the hydrogeological parameters nor on the aquifer hydraulic head, which is known only at the elevation of the spring.The objective of the work was to construct an appropriate mathematical model for the spring despite the scarcity of available information. The MODFLOW code was used to simulate the system following the equivalent porous media approach. The hydraulic conductivity and the specific yield equivalents were estimated by calibrating the model on the master depletion curve and taking into consideration the topographic elevation of the system's surface. The size of the protection area around the spring was investigated on the basis of the isochrons constructed from the results of the model.
Evidence for Ground‐Water Circulation in the Brine‐Filled Aquitard, Oak Ridge, Tennessee
Authors: Nativ Ronit , Halleran Amy , Hunley Arnold 
Published: August 2005

AbstractVarious geologic, hydrologic, and geochemical methods were used to assess active ground‐water circulation in a brine‐filled, deep (> 50 m below land surface) aquitard underlying the Oak Ridge Reservation, Tennessee. In places, the brine which was presumed to be stagnant in the past, contains various contaminants. If ground‐water circulation is viable in the brine‐containing formations, then remediation or containment of the deep‐seated contaminants should be considered a high priority. Data used to determine this included (1) spatial and temporal pressures and hydraulic heads measured in the aquitard, (2) hydraulic parameters of the formations in question, (3) vertical temperature gradients, and (4) spatial and temporal chemical and isotopic composition of the saline ground water. Conclusions suggest that the saline water contained at depth is not isolated (in terms of recharge and discharge) from the overlying active and fresh‐water‐ (< 500 mg/1) bearing units. Consequently, influx of young water (and contamination) from land surface does occur. Potential discharge into the shallow aquifers was assumed where the hydraulic head of the saline water was higher than that in the shallow aquifers, accounting for temperature and salinity anomalies observed close to land surface. The confined water (and dissolved solutes) move along open conduits at relatively high velocity into adjacent, more permeable units.
Modeling Sea‐Water Intrusion with Open Boundary Conditions
Authors: Padilla Francisco , Cruz‐Sanjulián Javier 
Published: August 2005

AbstractThe present study concerns the application of a new numerical approach to describe the fresh‐water /sea‐water relationships in coastal aquifers. Essentially, a solution to the partial differential equation governing the regional motion of a phreatic surface and the resulting interface between fresh water and salt water is analyzed by a Galerkin finite‐element formulation. A single‐phase steady numerical model was applied to approximate, with simple triangular elements, the regional behavior of a coastal aquifer under appropriate sinks, sources, Neumann, outflow face, and open boundary conditions. On the one hand, outflow open boundaries at the coastline were not treated with other classical boundary conditions (Dirichlet or the outflow face approach), but instead with a formal numerical approach for open boundaries inspired in this particular case by the Dupuit approximation of horizontal outflow at the boundary. The solution to this numerical model, together with the Ghyben‐Herzberg principle, allows the correct simulation of fresh‐water heads and the position of the salt‐water interface for a steeply sloping coast. Although the solutions were precise and do not present classical numerical oscillations, this approach requires a previous solution with Dirichlet boundary conditions at the coastline in order to find a good convergence of the solution algorithm. On the other hand, the same precise results were obtained with a more restrictive open boundary condition, similar in a way to the outflow face approach, which required less computer time, did not need a prior numerical solution and could be extended to different coastline conditions. The steady‐state problem was solved for different hypothetical coastal aquifers and fresh‐water usage through three types of numerical tests. Calculated fresh‐water heads, interface positions and discharges show very precise results throughout the domain and especially at the coastline when compared to analytical, experimental, and numerically correct solutions. Therefore, interface positions, fresh‐water heads, and discharges originating from the steady regional behavior of coastal aquifers can be precisely predicted by numerical modeling when open boundaries towards the sea are properly treated for the likely conditions of the coastline.
Determination of Transmissivity from Specific Capacity Tests in a Karst Aquifer
Authors: Mace Robert E. 
Published: August 2005

AbstractSpecific capacity tests are useful for estimating transmissivity in aquifers that have few good‐quality pump tests. In karst aquifers, this has been done by (1) correcting specific capacity for turbulent well loss and using analytical relationships between transmissivity and specific capacity, and (2) correcting specific capacity for well loss and deriving an empirical relationship between transmissivity and specific capacity. This study focuses on the uncertainties of estimating well loss and presents an empirical relationship between transmissivity and uncorrected specific capacity for a karst aquifer.Well loss is difficult to estimate without good‐quality step‐drawdown tests. Pipe‐flow theory tends to underestimate well loss, and an empirical relationship between specific capacity and well‐loss constant has a large prediction interval that leads to well loss exceeding measured drawdown. To overcome uncertainties of estimating well loss, transmissivity and uncorrected specific capacity were related for aquifer tests from the Edwards aquifer in Texas. The resulting best‐fit line is T = 0.76(Sc)1.08 for T and Sc in m2 d–1 with a coefficient of determination, R2, of 0.89 and a 95‐percent prediction interval spanning approximately 1.4 log cycles. Though the prediction interval is large, approximate but useful estimates of transmissivity can be determined because the relationship extends over five orders of magnitude from 1 to 100,000 m2 d‐1. The relationship is applicable in at least one other karst aquifer and therefore may be useful for others.
Utilization of the Hantush Solution for the Simultaneous Determination of Aquifer Parameters
Authors: Chen Xunhong , Ayers Jerry F. 
Published: August 2005

AbstractA method is presented to determine aquifer parameters simultaneously and uniquely based on time‐drawdown data collected from a single observation well in the vicinity of a pumping well partially penetrating a confined leaky aquifer. The Hantush solution is analyzed inversely for the identification of hydraulic properties using the Taylor series and a nonlinear least squares method. Aquifer parameters investigated include vertical and horizontal hydraulic conductivities, storage coefficient, and the leakage factor of the overlying aquitard. The developed method is verified using a hypothetical example and is applied to a field case. Error limits between the determined and true parameter values can be less than 0.1% depending on the quality of the field data.
Using the Deconvolution Approach for Slug Test Analysis: Theory and Application
Authors: Chakrabarty C. , Enachescu C. 
Published: August 2005

AbstractThis paper presents a new analysis methodology for determining hydrogeological parameters from a single well slug test. The procedure involves converting pressure versus time data from a slug test into a form equivalent to that from a constant rate test using an exact relationship. Deconvolution techniques are applied to remove wellbore storage effects. The converted data are then analyzed using existing head and head derivative type curves for constant (downhole) rate conditions. It is demonstrated that the analysis of the converted data yields reliable estimates of aquifer transmissivity from conventional slug tests and even from short duration (i.e., small recovery) tests in low transmissivity rocks. This method allows successful analysis of data from slug tests that would be considered unanalyzable using conventional interpretation techniques. Moreover, the proposed method is able to determine aquifer storativity independent of the influence of wellbore skin factor. This is impossible using existing techniques such as those based on Cooper et al. (1967) or Ramey et al. (1975). The applicability of the proposed methodology is demonstrated using several synthetic and field examples.
Effects of Anisotropy on the Capture Zone of a Partially Penetrating Well
Authors: Zlotnik Vitaly A. 
Published: August 2005

AbstractDimensional analysis is used to show the effects of aquifer anisotropy on capture zone geometry for a partially penetrating well that is screened from the top of a confined aquifer. Dimensionless similarity criteria are identified which allow an extension of results for advective transport modeling from isotropic conditions to anisotropic ones. Using published results of modeling capture zones in isotropic aquifers, it is shown that anisotropy is important in determining the size and shape of the capture zone under two conditions: (1) the pumping rate is small compared to the scaled regional flow velocity, and (2) the well has a low degree of penetration. Examples are used to show that disregard of even modest aquifer anisotropy can significantly affect estimates of capture zone.
Climatic Control of Hydraulic Conductivity of Bahamian Limestones
Authors: Whitaker Fiona F. , Smart Peter L. 
Published: August 2005

AbstractData from 244 single‐well pumping tests of the karstified Lucayan Limestone on 14 different islands distributed through the Bahamian Archipelago, demonstrate a strong log‐linear relationship between annual rainfall and the effective mean island effective hydraulic conductivity, which averages over two orders of magnitude higher in the wetter northern Bahamas (mean anual rainfall 1550 mm) than in the more arid southern islands (mean annual rainfall 810 mm). This relationship is independent of the saturated depth of the boreholes tested, although the hydraulic conductivity of the lower part of the formation is significantly greater (about 0.6–0.7 order of magnitude) than the upper unit, reflecting progressive secondary porosity generation with time.The direct effect of increasing ground‐water flux on rates of dissolutional porosity generation in the wetter northern islands is supplemented by the greater geochemical potential for carbonate dissolution resulting from higher primary vegetative production. This both increases soil pCO2 via root respiration (a process augmented by higher soil moisture permitting more microbial activity), and the availability of organic matter for in situ oxidation within the aquifer. Effective hydraulic conductivity may also be directly related to island size. Islands in the north of the Archipelago are larger than those in the south, thus the fresh‐water lens and fresh‐/ salt‐water mixing zone are greater in size and expose more bedrock to dissolution.
Confidence Intervals for Ground‐Water Models Using Linearization, Likelihood, and Bootstrap Methods
Authors: Cooley Richard L. 
Published: August 2005

AbstractFour methods were applied to obtain confidence intervals for selected dependent variables from a nonlinear regression formulation of a ground‐water flow model. The methods examined are (1) the linearization method, (2) the likelihood method, (3) a simple, percentile bootstrap method, and (4) a regression‐based percentile bootstrap method that is a generalization of the first one.The methods were applied to a highly nonlinear regression model of a hypothetical ground‐water system involving an infiltrating stream crossing a heterogeneous aquifer. Confidence intervals were computed for a hydraulic head and a parameter that is proportional to the streamflow infiltration rate. All confidence intervals computed by the likelihood method are virtually exact. Confidence intervals for hydraulic head by the linearization, likelihood, and the second bootstrap methods are similar, but confidence intervals by the first bootstrap method are very different and highly inaccurate. Confidence intervals for the streamflow infiltration parameter by the linearization and first bootstrap methods are identical and inaccurate; confidence intervals by the second bootstrap method lie between those by the likelihood and the other two methods. These characteristics of the confidence intervals result because (1) the regression model is highly nonlinear, and (2) much of the nonlinearity can be removed by some transformation of model parameters.
Simulated Recovery of Light, Nonaqueous Phase Liquid from Unconfined Heterogeneous Aquifers
Authors: Waddill Dan W. , Parker Jack C. 
Published: August 2005

AbstractA multiphase flow model (ARMOS) was used to evaluate the effects of subsurface heterogeneities on the recovery of light, nonaqueous phase liquids (LNAPLs or simply “oil”). Stochastic inputs for the model included the saturated hydraulic conductivity (Ksw), van Genuchten α and n, the water saturation at field capacity (Sm), and the maximum residual oil saturations in the saturated (Sor) and the unsaturated zones (Sog). The turning bands method was used to generate stochastic soil parameters representing three hypothetical sandy loam soils. Oil recovery in the three heterogeneous cases was compared to an “equivalent” homogeneous soil with effective parameters computed as the geometric means of the stochastic parameters. Distributions of the free oil plumes were described over time using statistical and spatial moment analyses. Due to the smoothing effect of the flow process, the predicted well oil thickness (Ho) and free oil specific volume (Vof) were less variable than the input stochastic parameter In(Ksw). However, Ho and Vof became more variable as free oil volume diminished and the oil distribution was controlled more by soil variability than gradients in the fluid levels. At the onset of oil recovery, the free oil area was greater in all three heterogeneous soils than the homogeneous soil. Nevertheless, soil heterogeneities did not greatly affect oil recovery or trapping in the saturated and unsaturated zones. Heterogeneities may have had a greater influence on oil recovery if a smaller spill had been studied or if the vertical dimension had been simulated explicitly. The results suggest that the geometric mean soil properties provide a useful estimate of the potential for oil recovery from oil spills that span a large number of correlation scales.