bluuebunny/crossref_metadata_2025 huggingface dataset:
ON ANALYSIS OF STEP‐DRAWDOWN DATA
Authors: Gupta A. Das
Published: August 2005

Abstract. A procedure is described for the analysis of step‐ drawdown pumping test data. The method uses the general form of the drawdown equation during a step with the superposition of the effects of incremental pumping between steps. An interactive computer code based on this procedure is developed to estimate aquifer characteristics and well‐loss factor from the observed time‐drawdown data at different discharge rates. Analysis of specific step‐drawdown data indicates the adaptability of the code for practical application.
Horizontal Anisotropy of the Principal Ground‐Water Flow Zone in the Salinas Alluvial Fan, Puerto Rico
Authors: Quiñones‐Aponte Vicente
Published: August 2005

Well drawdown data from an anisotropic aquifer in the Salinas alluvial fan were collected and analyzed with a computer program called TENSOR2D. The program uses ordinary and weighted least‐squares optimization procedures to solve the system of simultaneous equations needed to define the theoretical transmissivity ellipse. Results from the alluvial fan aquifer at Salinas, Puerto Rico indicated that when applying the ordinary least‐squares method:

(1) the major and minor transmissivity tensors are 52, 339 and 17,800 feet squared per day (ft2/d) respectively;

(2) the square root of the major and minor diffusivity tensors are 12,066 and 7,036 ft2/d, (3) the ratio of horizontal anisotropy is 2·94 to 1·00, and (4) the direction of major anisotropy tensor is along a line 31 degrees west from the true north. From the application of the weighted least‐squares method: (1) the major and minor transmissivity tensors are 38,840 and 23,987 ft2/d, (2) the square root of the major and minor diffusivity tensors are 11,282 and 8,866 ft2/d, (3) the ratio of horizontal anisotropy is 1·62 to 1·00, and (4) the direction of major anisotropy tensor is along a line 28 degrees west of the true north. Prediction of drawdown data was made by coupling the anisotropy ellipse with the Hantush modified leaky‐confined or Theis model. Drawdown data predicted by using the theoretical directional diffusivity obtained with the weighted least‐squared fit gave a more accurate representation of the actual drawdown data than when using the test‐data directional diffusivity.
Source of Ground Water in the Deserts of Northern Chile: Evidence of Deep Circulation of Ground Water from the Andes
Authors: Magaritz M. , Aravena R. , Peña H. , Suzuki O. , Grilli A.
Published: August 2005

Ground water in the Pampa del Tamarugal basin part of the desert of Northern Chile is the main water resource for cities and agricultural activities in the region. As the area received virtually no precipitation, the source of the ground water is rains in the high Andes. Aquifer recharge has been linked to a drainage system associated with this part of the Andes. Ground‐water flow is from northeast to southwest, and a flow component from east to west is also observed in some parts of the basin. In general, the water becomes very saline towards the western part of the basin, giving rise to salt lakes (salares) in ground‐water discharge areas. However, isotopic, chemical, and geological evidence plus ground‐water temperatures distribution, suggest the existence of a different recharge mechanism linked to a regional ground‐water flow system. It is suggested that low‐salinity water emerges from ground water recharged in the high Andes through a basement fault system underneath the basin. This model has implications for the future exploration for new fresh‐water resources.
Particle–Tracking Analysis of Flow Paths and Traveltimes from Hypothetical Spill Sites Within the Capture Area of a Wellfield
Authors: Bair E. Scott , Sheets Rodney A. , Eberts Sandra M.
Published: August 2005

Velocity fields computed from simulated flows from a steady–state, three–dimensional numerical model of a glacialdrift/carbonate–bedrock aquifer system were used in conjunction with a particle–tracking program to delineate traveltimerelated capture areas of a municipal wellfield and to compute flow paths and advective traveltimes from hypothetical spill sites located along interstate highways and heavily traveled State routes. Results indicate the municipal wells operated by the City of Columbus, Ohio, capture flow from certain highway segments but other discharge points, including quarrydewatering systems, the Scioto River, and Big Walnut Creek, capture flow from other highway segments. Advective traveltimes from three hypothetical spill sites to the municipal wells range from 900 to 5,100 days under normal pumping rates and from 450 to 3,100 days under estimated maximum pumping rates. The 2,000–day capture area of the wellfield is approximately 30 percent larger (16 square miles) and contains approximately 4.5 more miles of highway under estimated maximum pumping rates compared to normal pumping rates.
Numerical Generation of Flow Nets — The FLOWNS Model
Authors: Bramlett Wayne , Borden Robert C.
Published: August 2005

Abstract. A simple‐to‐use computer program (FLOWNS) has been developed for generating flow nets for any saturated rectangular domain with any combination of constant head or constant flux (including zero) boundary conditions. The program approximates with discrete values the continuous distributions of potential and stream function using finite‐difference approximations of Laplace's equation. The hydraulic conductivity distribution may be anisotropic and/or heterogeneous. A contouring program is required to generate the final stream and equipotential lines.
A Device for In Situ Determination of Geochemical Transport Parameters 1. Retardation
Authors: Gillham R. W. , Robin M.J.L. , Ptacek C. J.
Published: August 2005

The retardation factor for dissolved chemicals in ground water is an important parameter in predicting the rate of migration of contaminants and in the design of remediation schemes for contaminated aquifers. Conventional methods for evaluating retardation factors include laboratory batch and column tests, and in the case of nonpolar organic contaminants, correlations with the organic carbon content of the aquifer material. In these procedures it is difficult to insure the geochemical integrity of the sediment material and the solutions used in the tests, leading to uncertainties concerning the applicability of the results. Alternatively, large‐scale tracer tests can be conducted; however, because of the time and cost requirements, these procedures have generally been limited to research applications.

This paper describes an in situ column procedure for measuring retardation factors. The column is installed in advance of the cutting head of hollow‐stem augers, minimizing geochemical disturbance of the sediment, and the sediment within the column is exposed only to natural ground water from the immediate vicinity of the test device. By maintaining geochemical integrity, the method has considerable advantages over laboratory procedures, while being substantially less costly than conventional field tracer tests. Two tests are described in which retardation factors for strontium and for five halogenated organic compounds, obtained using the in situ device, were found to be in good agreement with results obtained by conventional methods.
Expert Testimony for the Plaintiffs in the Case that Brought Ohio Ground‐Water Law into the 20th Century
Authors: Bair E. Scott , Norris Stanley E.
Published: August 2005

Abstract. The 1984 Ohio Supreme Court ruling on Cline v. American Aggregates changed Ohio's ground‐water law from an 1861 ruling based on the English Rule of absolute ownership to a doctrine of reasonable use which recognizes that landowners have the privilege to use the water beneath their land but can be held liable to others if their use of ground water causes others unreasonable harm. The Ohio Supreme Court did not define unreasonable harm in the Cline case. As a result, unreasonable harm is being established through case law, the first such case being the relitigation of Cline v. American Aggregates.

Depositions and expert testimony presented during the Cline cases, which represent a group of 56 landowners in a single lawsuit, showed the diverse ways in which the quantity and quality of ground water used for domestic‐water supplies were affected by operation of a dewatering system at an aggregate mine. The testimony highlighted the complex interaction between regional hydrodynamics, variations in local hydrogeologic settings, well‐construction methods, spatial variations in infiltration and recharge, and vertical leakage with the effects of the regional decline in water levels produced by the dewatering system.
On the Relation Between Saturated Conductivity and Capillary Retention Characteristics
Authors: Mishra S. , Parker J. C.
Published: August 2005

Abstract. A simple closed‐form expression relating saturated hydraulic conductivity to the van Genuchten capillary retention model parameters is derived. Application of this equation to an experimental data set shows reasonable agreement between measured and predicted saturated conductivity values. The proposed equation provides a consistent theoretical basis for estimating both saturated and unsaturated hydraulic conductivity from statistical pore structure models.
“Where's the Benzene?”— Examining California Ground‐Water Quality Surveys
Authors: Hadley Paul W. , Armstrong Richard
Published: August 2005

Thousands of leaking underground gasoline tanks have been found throughout California from which a tremendous amount of gasoline has leaked into ground water over the last half century. The most water‐soluble constituent of gasoline is benzene, and it typically contaminates ground water below leaking underground tanks. In a state‐mandated program 7,167 wells serving water‐supply systems throughout California were tested for a broad panel of organic contaminants. Of the wells tested, 812 (11.3%) had detectable concentrations of at least one of the contaminants tested for. Detectable concentrations of benzene were reported for only 10 wells. Similar findings were reported in surveys focused on San Gabriel and San Fernando Basins. While many processes influence the environmental fates of organic contaminants in ground water, the most likely explanation for the nonoccurrence of benzene is that it is destroyed near its source by biodegradation.
Ground‐Water Discharge to Lakes: Focusing in Embayments
Authors: Cherkauer Douglas S. , McKereghan Peter F.
Published: August 2005

Seepage of ground water to Green Bay and Lake Michigan from a fractured dolomite aquifer beneath the Door Peninsula of Wisconsin is examined to determine the influences of embayments on the relative amounts of ground‐water discharge. A digital simulation has been used to calculate the quantity and distribution of that discharge. The simulated results are based on the net recharge which occurs on the land portion of the study area. They compare favorably with discharges calculated by flow net using observed hydraulic gradients and conductivities, and are in relative agreement with direct measurements made by seepage meters. The simulation has not been verified (due to lack of independent observations), but the similarity of the discharges from the three methods is argued to indicate the model's validity.

In the study area, 66,000 m3/day of ground water is discharged directly to Green Bay (26,000 m3/day) and Lake Michigan (40,000 m3/day). Almost 60% of this total occurs in 20 embayments along the shore, even though they occupy only about 33% of the shoreline length as measured parallel to the long axis of the peninsula.

Discharge is focused into embayments by distortion of equipotential lines caused by a bay's penetration into the ground‐water system. In the study area, it is shown that flow amplifications of up to 500% vary directly with two measures of the shoreline shape at bays, horizontal penetration (defined as the ratio of a bay's axial length to the distance to the ground‐water divide), and vertical penetration (the ratio of the maximum bay depth to aquifer thickness). Both factors have equal influence on the amplification of discharge. A graphical means of predicting the effect of coastal configuration on other bays is presented, but its applicability outside the study area is unknown.
Impact of the Proposed Qattara Reservoir on the Moghra Aquifer of Northwestern Egypt
Authors: Rizk Zein S. , Davis Arden D.
Published: August 2005

This paper examines hydrogeologie effects of proposed diversion of water from the Mediterranean Sea to the Qattara Depression for generation of electricity in northwestern Egypt. The Lower Miocene Moghra aquifer directly underlies the Qattara Depression, extending to the east and dipping beneath younger formations to the north. The Qattara Depression is a sink for ground‐water flow from the Nile Delta aquifer in the east, the Mediterranean Sea in the north, and the Nubian artesian aquifer in the south. In this study, a two‐dimensional finite‐difference model was used in conjunction with available meteorologie, geologic, and hydrologie data to characterize the present ground‐water conditions in this aquifer and to make predictions about the water‐table rise which could occur as a result of the proposed Qattara Reservoir. Model predictions indicate that creation of the reservoir could cause a 30‐meter rise of the Moghra aquifer's water table in the central part of the Qattara Depression. This water‐table rise decreases to the north and east. Transient simulations indicate that steady‐state conditions in the modeled area of the Moghra aquifer are not likely to be reached during the expected 100‐year lifetime of the reservoir.
Model for Predicting Virus Movement Through Soils
Authors: Tim Udoyara S. , Mostaghimi Saied
Published: August 2005

A numerical model, VIROTRANS, is developed for simulating the vertical movement of water and virus through soils treated with waste‐water effluents and sewage sludges. The expression describing transient flow of water is coupled with the convective‐dispersive equation for subsurface solute transport. The resulting methodology is a coupled set of partial differential equations that describe the transient flow of water and suspended virus particle movement through variably saturated media. Solutions to the partial differential equations are accomplished by a Galerkin finite element method. Several example problems are used to provide a quantitative verification and validation of the model. The model simulations are compared to an analytical solution and to experimental measurements of soil moisture content and poliovirus 1 transport. The comparisons show reasonable agreement between model simulations and measured data. Sensitivity of the model's prediction to variations in pertinent input parameters are also analyzed.
A Douglas‐Jones Predictor‐Corrector Program for Simulating One‐Dimensional Unsaturated Flow in Soil
Authors: Babajimopoulos C.
Published: August 2005

Abstract. A fully documented program to represent one‐dimensional unsaturated flow in soil is described. The program is based on a Douglas‐Jones finite‐difference implicit method to solve the Richards equation. An implicit linearization scheme is used to estimate the hydraulic conductivity and specific moisture capacity functions. Predicted moisture content profiles compared with two Galerkin finite‐element solutions and field observations on a Panoche clay loam soil show very good agreement.
Comparison Between Model Simulations and Field Results for In‐Situ Biorestoration of Chlorinated Aliphatics: Part 1. Biostimulation of Methanotrophic Bacteria
Authors: Semprini Lewis , McCarty Perry L.
Published: August 2005

A nonsteady‐state model is presented for the in‐situ biostimulation of a microbial population in saturated porous media. The model includes basic processes of microbial growth, utilization of electron donor and acceptor, advective transport, dispersion, and sorption in porous media. Model simulations are compared with results from a series of controlled field experiments at the Moffett Naval Air Station, where the growth of an indigenous population of methane‐utilizing bacteria (methanotrophs) was stimulated by the controlled addition of dissolved methane and oxygen (DO) into a semiconfined aquifer. Simulations provide good matches to the observed transient uptake of methane and DO, demonstrating that the observed response resulted from the growth of methanotrophs in the test zone. Simulations duplicate results from alternate pulsed addition of methane (electron donor) and oxygen (electron acceptor), used as a means for distributing microbial growth throughout the test zone. The model permits estimation of changes in microbial population distribution at various stages of the two‐year experiment. Temporal changes in model‐fitted kinetic parameters indicate that the microbial population evolved to one that more effectively utilized the methane at lower concentrations. These analyses demonstrate that a relatively simple model, which includes basic microbial and transport processes, can be of use in the design and evaluation of in‐situ biotreatment processes. The model user, however, must provide judgment in the selection of appropriate input parameters, as well as being aware of model limitations.
Delineation of a Discontinuous Aquitard with Vertical Electrical Soundings, San Bernardino Valley, Southern California
Authors: Owen William P. , Park Stephen K. , Lee Tien‐Chang
Published: August 2005

Vertical electrical soundings were used to map the presence or absence of an aquitard separating a shallow, contaminated aquifer from a deeper, uncontaminated one in San Bernardino Valley, California. Correlation of vertical electrical soundings with lithologie logs from adjacent wells allowed us to also map local variations in the elevation of the water table Comparison of known waste sites, the distribution of the aquitard, and elevation of the water table yielded probable directions of contaminant transport. We conclude that there is a significant hazard to municipal water wells due to possible transport of contaminants between the shallow and deeper aquifers.
Short‐Term Plume Containment: Multiobjective Comparison
Authors: Peralta Richard C. , Ward Robert L.
Published: August 2005

Multiobjective strategies and models for optimizing the short‐term containment of a 2‐D ground‐water contaminant plume are compared. These are designed for cases in which an appropriate means of treating the contaminated water is unavailable or unauthorized, and the management goal is to prevent plume movement, without pumping contaminated water and without importing or exporting water. Because optimal unsteady strategies are usually more efficient than optimal steady strategies at achieving goals at a prespecified time, the models optimize the needed time‐varying rates of extracting and injecting water. The objective function can use coefficients to consider, one at a time, the objectives of minimizing final hydraulic gradients between observation wells and the plume source, minimizing the cost of pumping, or minimizing the volume of pumping. Alternatively, the model can simultaneously consider the gradient objective and one of the other two objectives in developing a compromise strategy. If the plume must be contained only for the period of optimal unsteady pumping, either the minimizing cost or the minimizing pumping volume objective is most appropriate. For short periods, a minimum pumping strategy, which is the easiest to use, is almost the same as a least‐cost strategy. If the plume must be contained for a longer period, the hydraulic objective becomes increasingly important because it causes a more horizontal potentiometric surface, and less additional pumping is needed to maintain that surface after the end of unsteady pumping.
Effect of Nitrate Addition on Biorestoration of Fuel‐Contaminated Aquifer: Field Demonstration
Authors: Hutchins S. R. , Downs W. C. , Wilson J. T. , Smith G. B. , Kovacs D. A. , Fine D. D. , Douglass R. H. , Hendrix D. J.
Published: August 2005

A spill of JP‐4 jet fuel at the U.S. Coast Guard Air Station in Traverse City, Michigan, contaminated a water‐table aquifer. An infiltration gallery (30 ft × 30 ft) was installed above a section of the aquifer containing 700 gal JP‐4. Purge wells recirculated three million gallons of ground water per week through the infiltration gallery at a rate designed to raise the water table above the contaminated interval. Ground water containing ambient concentrations of oxygen and nitrate was first recirculated for 40 days. Concentrations of benzene in monitoring wells beneath the infiltration gallery were reduced from 760 to <1μ/1. Concentrations of toluene, ethylbenzene,m,p‐xylene, ando‐xylene were reduced from 4500 to 17, 840 to 44, 2600 to 490, and 1400 to 260 μ/1, respectively. Average core concentrations of benzene, toluene, ethylbenzene,m,p‐xylene, ando‐xylene were reduced from 0.84 to 0.032, 33 to 0.13, 18 to 0.36, 58 to 7.4, and 26 to 3.2 mg/kg, respectively. Ground water amended with nitrate (10 mg/1 nitrate‐nitrogen) and nutrients was then recirculated for 76 days. Final core concentrations of benzene, toluene, ethylbenzene,m,p‐xylene, ando‐xylene were 0.017,0.036,0.019,0.059, and 0.27 mg/kg, respectively. Final aqueous concentrations were <1 μ/1 for benzene and toluene, 6 μ/1 for ethylbenzene, and 20 to 40 μ/1 for the xylene isomers, in good agreement with predicted values based on residual fuel content and partitioning theory. Although alkylbenzene concentrations have been substantially reduced, the test plot is still contaminated with the weatheredfuel. Based on stoichiometry, approximately 10 times more nitrate was consumed than could be accounted for by BTX degradation alone, indicating that other compounds were also degraded under denitrifying conditions.
Theoretical Evaluation of Modified Wenner Array for Shallow Resistivity Exploration
Authors: Goyal Vikas C. , Niwas Sri , Gupta Pravin K.
Published: August 2005

A mathematical study has been carried out for critical evaluation of the earlier reported Modified Wenner method for shallow resistivity exploration. It provides a theoretical basis for quantitative interpretation of the measured resistivity data and quantification of the depth of investigation at various potential electrode separations. Synthetic resistivity data for a few three‐layer models have been generated and compared with the Wenner, Schlumberger, and two‐electrode curves. It is established that the MWA and TEA lead to identical theoretical response.
Embedded Knowledge in Software: 1. Description of a System for Contaminant Transport Modeling
Authors: McClymont G. L. , Schwartz F. W.
Published: August 2005

Computer software with embedded knowledge has the potential to improve the utility and usability of computer models. We explore these opportunities with the software package Expert ROKEY that includes EXPAR, a knowledge‐based system to assist in the preparation of input data for a simple mass transport model. EXPAR is partitioned into two levels. At the top level is a set of computer forms that contain groups of related parameters and corresponding entry fields. Users can either volunteer parameter values or request help from an assistance program. Elaboration programs provide supporting information for each parameter and brief tutorials for the mass transport processes included in the model. The sets of assistance and elaboration programs represent the bottom level of EXPAR. This model preprocessor contains useful features like tutorial and data‐base information, systematic procedures for deriving parameter values, capabilities for checking the completeness and consistency of data, and a virtually transparent user interface with the computer, which are applicable to codes of all types.
Modeling the Density‐Driven Movement of Liquid Wastes in Deep Sloping Aquifers
Authors: Dorgarten Hans‐Wilhelm , Tsang Chin‐Fu
Published: August 2005

One method for the disposal of hazardous liquid wastes is by injection into deep aquifers. Although these aquifers may be separated from underground sources of drinking water by thick formations of low permeability, their mobility due to different migration mechanisms has to be studied carefully, since the injected wastes remain toxic over periods of thousands of years. One possible mechanism for waste movement is density‐driven flow and transport, due to density differences between the waste and the surrounding water in the injection zone. In the present paper the importance of this phenomenon is studied mathematically by means of analytical and numerical calculations for typical deep injection conditions. The analytical estimates reveal that density‐driven movement of liquid wastes in sloping aquifers can be much stronger than plume migration due to natural hydraulic gradients. This finding is emphasized by the results of a two‐dimensional vertical finite element model, which is applied for detailed numerical simulations. Results show that during the initial stage, waste can be expected to spread into all directions due to density‐induced stratification effects. Later on, it mainly moves laterally along the slope of either aquifer top or aquifer bottom, depending on the waste density. If regional ground‐water flow is directed the same way, transport is accelerated. If regional ground‐water flow is in the opposite direction, on the other hand, transport to both sides must be expected to occur. Thus, the aquifer slope and regional hydraulic gradient may be equally significant factors in estimating potential migration of disposed liquid wastes.
Subsurface Partitioning of Volatile Organic Compounds: Effects of Temperature and Pore‐Water Content
Authors: Kerfoot H. B.
Published: August 2005

Equations describing the three‐phase equilibrium of contaminants between the pore‐gas, pore‐water, and solid‐sorbed phases in the unsaturated zone are presented. These relationships are then used to derive general expressions for the temperature‐dependence of the equilibrium in a three‐phase system, and those relationships are applied to derive expressions for the temperature‐dependence of the concentrations in the three phases when the total contaminant mass present and the gas‐filled and water‐filled porosities are constant. The temperature‐dependence of the three‐phase equilibrium is dictated by the enthalpies of phase transition and the appropriate equilibrium constants. As the temperature increases, contaminants move from the more‐dense to the less‐dense phases, so that the pore‐gas‐phase concentration increases to an upper limit; the pore‐water‐phase concentration increases, reaches a maximum, and then decreases; and the solid‐sorbed concentration decreases. Equations describing the effects of changes in the water‐filled porosity, such as those that would be caused by infiltration of either clean or contaminated pore water, on the gas‐phase and pore‐water‐phase concentrations of contaminants are presented. For compounds with a Henry's Law constant (H, dimensionless) greater than one, an increase in water‐filled porosity results in an increase in the pore‐gas concentration, while for compounds with H less than one, such a change results in a decrease in the pore‐gas concentration. The applicability of the thermodynamic relationships developed is limited by the assumption of ideal‐gas behavior, isothermal phase transition, and isobaric conditions. The stoichiometric relationships developed are limited by the assumption of complete mixing. They are used to develop equations that allow calculation of the equilibrium fraction of the total contaminant mass present in each phase on the basis of pore‐gas monitoring data, site properties, and equilibrium constants. Based on observed natural seasonal changes in subsurface temperatures and moisture contents, the effects predicted in the derived equations could result in significant changes in gas‐phase concentrations without changes in the total contaminant mass present.
The Behavior of Dense, Nonaqueous Phase Liquids in Fractured Clay and Rock
Authors: Kueper Bernard H. , McWhorter David B.
Published: August 2005

This paper examines the behavior of dense, nonaqueous phase liquids (DNAPLs) in fractured clay and rock. The conditions under which a DNAPL will enter an initially water‐saturated, rough‐walled fracture are outlined and expressed in a number of ways, including the height to which a DNAPL pool can accumulate above a fracture prior to initial entry. To study the behavior of DNAPL in a rough‐walled fracture following initial entry, numerical simulations are carried out both in the plane of a fracture using a discrete representation of fracture roughness, and at a larger scale of averaging using an equivalent homogeneous porous media approach. The simulations illustrate that DNAPL will migrate through the larger aperture regions of a fracture plane, and that the DNAPL has the potential to enter progressively smaller aperture fractures with depth as it migrates. Additional numerical simulations indicate that the time taken for a nonaqueous phase liquid to traverse a fractured aquitard is inversely proportional to the fracture aperture, the fracture dip from the horizontal, and the height of the pool collected above the aquitard. It is also demonstrated that upward hydraulic gradients across a fractured aquitard can significantly slow the downward rate of DNAPL migration while downward water gradients enhance the rate of DNAPL migration across the aquitard.
Assessment of Uncertainty in Time‐Related Capture Zones Using Conditional Simulation of Hydraulic Conductivity
Authors: Varljen M. D. , Shafer J. M.
Published: August 2005

A stochastic approach to steady‐state time‐related capture zone determination that is based on the conditional simulation of hydraulic conductivity is presented. Conditional simulation of hydraulic conductivity preserves the spatial correlation structure of the real hydraulic conductivity field as well as the measured, or known, values of hydraulic conductivity at sampling locations, thereby optimizing the use of available data and providing the most representative results possible. The stochastic analysis methodology utilizing conditional simulation is applied to a hypothetical ground‐water system with a water supply well to estimate uncertainty in the 1‐year and 10‐year capture zones of the well. The results demonstrate the influence of known hydraulic conductivity values on the estimates of uncertainty, as well as the behavior of the “zone of uncertainty” of the capture zone as a function of travel time and orientation of regional flow. The results have significant implications regarding strategies for sampling aquifer properties (e.g., hydraulic conductivity) for capture zone delineation and for interpretation of the effects of hydraulic conductivity uncertainty on capture zones.
Analysis of Ground‐Water Remedial Alternatives at a Superfund Site
Authors: Zheng C. , Bennett G. D. , Andrews C. B.
Published: August 2005

This paper presents a simple yet effective approach and methodology for the problem of evaluating ground‐water remedial alternatives at a waste disposal site, and discusses the application of this approach in a case study. The attainment areas, which represent the areas (outside the contaminant source itself) in which remediation is required, are first identified. Simulation of ground‐water flow in three dimensions, augmented by fluid particle tracking, is utilized to evaluate the travel times of ground water through the attainment areas. The mixed linear reservoir or “batch flush” model is then used to estimate the number of pore volumes which must be flushed through each attainment area in order to achieve remediation. The travel times are used in conjunction with this pore volume figure to estimate the time required for cleanup under each alternative. By comparing the cleanup time, the costs, and other features of each alternative, a cost‐effective remedy for the study site can be determined. While the development of remedial alternatives and the choice of the most cost‐effective remedy are highly site‐specific, the approach and methodology outlined in this paper have general applicability. The results presented herein also provide insight into the difficulties and special considerations associated with modeling and analyzing remedial alternatives.
A Monte Carlo‐Based Approach for Determining Traveltime‐Related Capture Zones of Wells Using Convex Hulls as Confidence Regions
Authors: Bair E. Scott , Safreed Connie M. , Stasny Elizabeth A.
Published: August 2005

Determination of traveltime‐related capture zones of wells in instances where there is a lack of adequate or reliable site‐specific values of hydraulic parameters and/or the heterogeneous character of the geologic materials may lead to designation and management of wellhead‐protection areas of dubious function. In these instances, Monte Carlo simulation can be used to determine traveltime‐related capture zones that account for the uncertainty in values of hydraulic and geologic parameters that cannot be accounted for by deterministically based flow models.

Monte Carlo simulations using 100 randomly generated values of hydraulic conductivity and effective porosity were used to determine one‐year capture zones based on determination of percentile confidence regions from reverse‐tracked flowpaths emanating from a well completed in a leaky‐confined aquifer. The mean of the lognormal distribution used to generate the hydraulic‐conductivity values was taken to be 3.89 ft/d, the log of the average value from an aquifer test, and a standard deviation of 1.0 ft/d was used. The normal distribution used to generate the effective porosity values had a mean of 25 percent and a standard deviation of 3.5 percent. The values of hydraulic conductivity and effective porosity were ordered and then paired to induce correlation. Other relevant hydraulic and geologic parameters were fixed at average or observed values from the available data.

Simulations were made using an analytical flow model in conjunction with a particle‐tracking program to obtain 100 sets of endpoint coordinates for 36 reverse‐tracked, one‐year flowpaths emanating from the well. Wellhead‐protection areas based on the 90th‐ and 75th‐percentile confidence regions of the distribution of 3,600 endpoints were determined by deleting the 10 and 25 most‐extreme (distant) endpoints, respectively, from the 100 sets of flowpath endpoints of the 36 flowpaths. Delineation of the wellhead‐protection areas was based on determination of the convex hull of the remaining endpoints. The distribution of remaining endpoints around the well also was used to determine optimum locations for construction of sentinel wells to detect contaminants moving toward the well along the most probable flowpaths.
Multilevel Slug Tests with Comparisons to Tracer Data
Authors: Melville Joel G. , Molz Fred J. , Güven Oktay , Widdowson Mark A.
Published: August 2005

Multilevel slug tests were conducted in a confined, granular aquifer near Mobile, Alabama. Tracer injection and travel time experiments, more direct measurements of transport in the aquifer, were also conducted at the same location. The objective was to compare results of the two field investigations to ascertain the accuracy and utility of slug testing to measure transport properties of the aquifer. Nondimensional hydraulic conductivity profiles inferred from the slug tests and the tracer experiments were similar. The similarity of the profiles supported the utility of multilevel slug testing to characterize the hydraulic conductivity profile in the aquifer.

The slug test data collected in three wells were interpreted using a radial flow model (Cooper et al., 1967) and a finite element model which considered both radial and vertical flow (Widdowson et al., 1989, 1990). Hydraulic conductivity values inferred from the measured data considering both radial and vertical flow were approximately two‐thirds of the values inferred assuming radial flow.
A Comparison of Sorption Coefficients Determined by Batch, Column, and Box Methods on a Low Organic Carbon Aquifer Material
Authors: Maclntyre William G. , Stauffer Thomas B. , Antworth Christopher P.
Published: August 2005

Results of three independent experimental methods for determination of sorption coefficients were compared on sieved aquifer material samples from Columbus AFB, Columbus, Mississippi. These aquifer materials contained <0.1% organiccarbon, and sorption coefficients for naphthalene ranged from 0.06 to 0.20 I/kg. There was low correlation between organic carbon contents and sorption coefficients. Good agreement was found among sorption coefficients measured by batch, dynamic column, and dynamic box methods. Similarity of the coefficients was attributed to measured fast sorption kinetics and to linearity of isotherms observed for naphthalene sorption on Columbus aquifer material. Results indicate that channeling, wall and end effects, and particle loss were insignificant in the dynamic systems. Since laboratory sorption coefficient measurements were found impractical on whole sand‐gravel aquifer samples, an equation is proposed to convert laboratory measured sorption coefficients on the separated sand (<2 mm) fraction to sorption coefficients on wholesand‐gravel aquifer materials.
Comparative Hydrogeology of Fresh‐Water Lenses of Bermuda and Great Exuma Island, Bahamas
Authors: Vacher H. L. , Wallis T. N.
Published: August 2005

The islands of Bermuda and Great Exuma are similar in that they are both composed of Quaternary marginal‐marine, calcarenitic limestones. These limestones are predominantly eolianite. Topographic depressions between eolian ridges are locally lower than present sea level; consequently, the ground water is coupled to inland marshes and ponds. The islands differ climatically. Rainfall in Bermuda exceeds potential evapotranspiration by about 0.1 m/yr, while in Great Exuma, potential evapotranspiration exceeds rainfall by some 0.5 m/yr. As a result, the inland ponds of Great Exuma are brackish to hypersaline, and fresh‐water lenses are localized beneath the topographic highs between the ponds and the shoreline. In contrast, the marshes of Bermuda are not strong discharge sites, and the surrounding fresh‐water lenses are geologically rather than topographically controlled. Dupuit‐Ghyben‐Herzberg modeling indicates that if the climates of Bermuda and Great Exuma were reversed, the characteristics of their fresh‐water lenses would be reversed as well. This comparison indicates that climate, as reflected in the water budget, is one of the important controlling variables in the occurrence and geometry of fresh‐water lenses in low‐lying subtropical islands.
Hydraulic Barriers in Pleistocene Buried‐Valley Aquifers
Authors: Shaver Robert B. , Pusc Steve W.
Published: August 2005

Aquifer‐test analysis, evaluation of potentiometric data, and numerical simulation of ground‐water flow are useful techniques for detecting hydraulic barriers in Pleistocene buried‐valley aquifers. Using these techniques, hydraulic barriers were identified in both the Spiritwood and Sundre‐New Rockford buried‐valley aquifers in North Dakota. The existence of hydraulic barriers in these two aquifers was confirmed by geologic data obtained from test drilling.

Two types of hydraulic barriers have been distinguished based on orientation with respect to the principal axes of buried valleys. Longitudinal hydraulic barriers are relatively narrow, linearly discontinuous ridges comprised of shale bedrock or till that are aligned roughly parallel to the long axes of buried valleys. Transverse hydraulic barriers are narrow, continuous barriers comprised of silt and clay aligned roughly perpendicular to the long axes of buried valleys.

The principal directions of ground‐water flow in buried‐valley aquifers commonly are parallel to longitudinal hydraulic barriers and perpendicular to transverse hydraulic barriers. As a result, under natural conditions, transverse hydraulic barriers generally are manifested by larger potentiometric surface discontinuities.

Pleistocene drainageways explain the occurrence and formation of hydraulic barriers in buried‐valley aquifers. Longitudinal hydraulic barriers are erosional features that appear to be formed by stream‐valley bifurcation in melt‐water channels or glacial‐lake spillways. Transverse hydraulic barriers appear to be formed by catastrophic flooding along spillways from the rapid drainage of glacial lakes and subsequent deposition of silt and clay deposits by sluggish, underfit streams.
Estimating Aquifer Properties by Nonlinear Least‐Squares Analysis of Pump Test Response
Authors: Johns R. A. , Semprini L. , Roberts P. V.
Published: August 2005

In this study, nonlinear least‐squares analysis of pump test data is used as a method for estimating aquifer parameters and choosing a conceptual model of the aquifer. The goal of this study is to investigate the potential advantages and disadvantages of using statistical algorithms for pump test analyses. Four different aquifer models were fitted to an extensive set of pump test data to characterize the aquifer at a single aquifer test site. These models were the Theis model, the equipotential boundary model, the confined leaky aquifer model, and the water‐table aquitard model. For several of the data sets, more than one aquifer model was found to match the pump test response with the same residual least‐squares error, and a review of the local hydrogeology was required to choose the leaky aquifer model as the most appropriate aquifer model. A few data sets, however, did not agree well with the leaky aquifer model. Using the fitting routine, the water‐table aquitard model was found to significantly improve the fit to these data sets. Further analysis of hydrogeologic features at the site indicated that the duration of the pump tests which matched the leaky aquifer model was probably insufficient to exhibit deviation from the leaky aquifer model. The water‐table aquitard model is, therefore, chosen as the most suitable model for this site, and parameters for the aquifer are estimated using the results from the fitting procedure. In comparison, an earlier graphical interpretation of these pump tests did not distinguish the improved fit which could be obtained with the water‐table aquitard model. However, the graphical analyses were valuable as they did provide preliminary estimates of the aquifer transmissivity, aiding in accepting or rejecting the results of the fitting analysis. Thus, we found that nonlinear least‐squares analysis can complement, but does not replace, graphical interpretation methods. The nonlinear least‐squares analysis can provide a means for analyzing pump tests which may not have been easily interpreted using traditional techniques, as the nonlinear least‐squares analysis attempts to match all the test data at both large and small values of time. However, this analysis also illustrates that fitting methods cannot be totally automated, but rather must be interpreted in light of other hydrogeologic data in order to arrive at a reasonable model for the aquifer.
The Topographic Approach to Locating High‐Yield Wells in Crystalline Rocks: Does It Work?
Authors: Yin Zhi‐Yong , Brook George A.
Published: August 2005

Despite convincing evidence that fracture trace mapping can be extremely useful in locating sites for high‐yield wells, well drillers persist in using the “lay of the land” to select well sites. This paper examines: (1) the value of topographic variables in predicting the productivity of wells in crystalline rocks; and (2) whether topographic data for well sites can increase the accuracy of well yield predictions based on fracture trace data alone. Topographic and fracture trace data for 29 wells in the Georgia Piedmont and Blue Ridge area were studied. Regression analysis revealed that the LeGrand (1967) point value (TOPO) explained only 0.3% of the variation in the productivity of the 29 wells. More explanation was afforded by the less subjective topographic variables, distance from the well to the nearest valley or draw axis (DISTV), and the difference in elevation between the nearest valley or draw axis and the well site (ELEV). Together these variables still explained only 15.3% of the variation in well productivity.

Brook (1988) showed that a single fracture trace variable, distance from the well to the nearest fracture trace intersection (DISTIN), explained 59% of the variation in the productivity of the 29 wells and that the two variables well depth and DISTIN explained 81% of the variation in well productivity. Addition of the topographic variables TOPO, DISTV, and ELEV to the models of well productivity developed by Brook (1988) did not greatly improve the explanation. None of the added variables were significant at the 0.05 confidence level. Results, therefore, indicate that well productivity is only poorly related to topographic characteristics, perhaps explaining why so many of the wells drilled in crystalline rocks using the “lay of the land” have a low yield or are dry regardless of their topographic locations.
Predicting Ground‐Water Nitrate‐Nitrogen Impacts
Authors: Hantzsche Norman N. , Finnemore E. John
Published: August 2005

The buildup of nitrates in upper ground‐water zones is a potential cumulative effect of on‐site sewage disposal practices which is not addressed by standard siting and design criteria. Literature concerning the contribution and fate of nitrogen beneath septic tank disposal fields is reviewed. From these findings, convenient, simplified methods are developed for estimating long‐term ground‐water nitrate increases on an area‐wide basis. The methods are presented in a manner useful to engineers, planners, and regulatory agencies for routine evaluation of existing and proposed land developments and for design of large, common disposal systems. Typical solutions are shown graphically to illustrate the relative importance of various factors, including development density, rainfall recharge, and soil denitrification. Predicted values are compared with actual monitoring data for three California communities to verify the reasonableness of the suggested methods. Several possible regulatory applications are suggested.
Parameter Estimation of the Coefficients in Soil Hydraulic Equations from Steady‐State Unsaturated Flow
Authors: Richie E. B. , Schnabel R. R.
Published: August 2005

A parameter estimation procedure which determines the coefficients of soil hydraulic equations (soil‐water characteristic and hydraulic conductivity curves) is developed. The parameters are determined by nonlinear least‐squares regression of steady‐state water potential measured at several points along an unsaturated horizontal soil column to an equation, herein derived, which describes water potential continuously along the column. To estimate parameter values, total mass of water in the soil column at steady‐state, saturated water content, and steady‐state flow rate must also be measured.

The parameter estimation procedure is verified using synthetic data and is applied in the laboratory. The coefficients appearing in the van Genuchten and Campbell soil hydraulic equations are estimated by the procedure. Alternative methodologies are proposed to augment this procedure and to provide user flexibility. The coefficients in the difficult‐to‐ measure wet range can be determined from measurements taken at steady‐state without independently measuring water retention.