bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling Solute Transport in Ground Water At or Near Freezing
Authors: Zukowski Matthew D. , Tumeo Mark A.
Published: August 2005

Under freezing or near‐freezing conditions, temperature‐induced viscosity changes, solute immobilization, and solute exclusion can affect field‐scale solute transport in ground water. This can lead to concentration profiles significantly different from those predicted by models which do not account for these processes. A model (GWFREEZE) is presented which incorporates these processes into a two‐dimensional transport equation for saturated porous media subject to a hydraulic head field that does not vary with time.

Ground‐water temperature can be expected to vary widely in some natural and artificial freezing and near‐freezing situations. Spatially varying temperature fields cause spatial variations in the fluid viscosity field, especially near 0° C, where viscosity varies faster with temperature than at higher temperatures. GWFREEZE models these phenomena with a viscosity‐dependent hydraulic conductivity that is a function of temperature. Solute exclusion is modeled by a variable exclusion coefficient (Ke) which represents the percentage of solute excluded at the freezing front. The freezing front is modeled as a planar no‐flow boundary behind which solutes are immobilized and at which solutes are excluded.
Assessment of Porous Media Heterogeneity by Inverse Plume Analysis
Authors: Poeter Eileen P. , Belcher Wayne R.
Published: August 2005

Results from analysis of models of heterogeneous porous media indicate that inverse plume analysis has potential for use in delineating zones of differing characteristics within porous media even when the zones are not penetrated by a borehole. By analyzing the concentrations at pairs of points in a contaminant plume, the spatial distributions of apparent transverse dispersivity and source size are determined. Since dispersivity is an intrinsic property of a porous medium, spatial distributions of the apparent dispersivities obtained from inverse analysis of plumes are an indication of transitions in the character of the porous medium.

Dispersivity values obtained by inverse plume analysis in heterogeneous porous media are “apparent” because the equation on which the analysis is based was developed under the assumption that the porous media is isotropic and homogeneous. In the case of heterogeneous porous media, the dispersivity value obtained at a given location by inverse plume analysis is affected by the spreading of the plume through different materials. Therefore, variations in dispersivity should be interpreted qualitatively for delineation of heterogeneity and not quantitatively as for input to numerical transport models.
A Method for Estimating Effective Porosity and Ground‐Water Velocity
Authors: Hall Stephen H. , Luttrell Stuart P. , Cronin William E.
Published: August 2005

Estimates of ground‐water velocity, based either on Darcy's law or on the single‐well drift and pumpback tracer method, require prior knowledge of effective porosity. That is, after field data have been collected, the equation for ground‐water velocity, using either method, still contains the two unknowns, velocity and porosity. If the local hydraulic gradient is known and if a drift and pumpback tracer test is conducted at a well whose hydraulic conductivity has been determined, two independent functional relationships between velocity and porosity are established. By treating these functions as nonlinear simultaneous equations, a unique solution for the local velocity and porosity can be obtained.
Hydrogeology of Effluent Disposal Zones, Floridan Aquifer, South Florida
Authors: Haberfeld Joseph L.
Published: August 2005

The increasing population of south Florida is creating a need for greater waste‐water disposal capacity. The people of Florida are dependent on ground water for drinking, irrigation, and other uses. The emphasis on preservation of surface‐water quality means that many surface waters are not allowed to receive effluent from municipal or industrial sources. Since the 1940s, the highly transmissive zones of the nonpotable portion of the Floridan aquifer have served as disposal zones for oil field brine and later for industrial and municipal effluent.

The Floridan aquifer of south Florida is a carbonate Cenozoic age aquifer. Transmissivities between 0.5 and 21 million gpd/ft exist in cavernous and fractured sequences in the aquifer. Confining zones that overlie the disposal intervals have transmissivities and hydraulic conductivities one to three orders of magnitude less than the injection intervals. These confining zones prevent low density municipal effluent from rising upward through the overlying saline waters.

This paper provides an integration of previously published work with recent site‐specific data collected from deep injection wells drilled in the last two decades.
Multilayer Sampling in the Water‐Table Region of a Sandy Aquifer
Authors: Kaplan Edward , Banerjee Sujit , Ronen Daniel , Magaritz Mordeckai , Machlin Albert , Sosnow Michael , Koglin Eric
Published: August 2005

Depth profiles of organic pollutants, metals, and anions were obtained using a ground‐water multilayer sampler (MLS) in a sandy unconfined aquifer contaminated by liquid fuels. The sampler incorporates dialysis membranes and is capable of obtaining vertical profiles with a resolution on the order of few centimeters. Of the eight solutes examined (metals, organics, and anions), all except sulfate exhibited microscale heterogeneities in their concentration over a vertical interval of 200 cm. Chloride, manganese, and iron showed a long linear concentration gradient versus depth which is probably the result of vertical mixing by rainfall input. Maximum concentrations of toluene, xylene, and xylidine at the water table were 17.1, 8.3, and 17.1 mg/l, respectively, decreasing with depth. For toluene and xylene the maximum concentrations found at the water table with the MLS are 40 and 4 times higher, respectively, than those detected at depths greater than 200 cm in water samples obtained with a pump.
Literature Review and Model (COMET) for Colloid/Metals Transport in Porous Media
Authors: Mills William B. , Liu Sally , Fong Fred K.
Published: August 2005

This paper summarizes topics related to colloid transport in subsurface media. The ultimate objective of the paper is to present a model that can be used to evaluate the significance of colloid facilitated transport on the mobility of metals. Field and laboratory studies are first reviewed to evaluate evidence that colloids are transported in subsurface media. Second, researchers active in the field are contacted to identify areas of ongoing research, and to solicit opinions concerning the level of understanding of mechanisms that control colloid migration. Third, the literature on colloid transport mechanisms is reviewed, with particular emphasis on colloid (and particle) filtration and on colloid stability as influenced by electrical repulsion and Van der Waals attraction. Fourth, a conceptual colloids‐metal transport model (COMET) is developed and incorporated into EPA's CML model, a model that simulates solute migration from a landfill in the unsaturated zone to a receptor (i.e., drinking‐water well) in the saturated zone. Among the major features of the COMET model are the capability to simulate multiple metal species either dissolved or adsorbed to mobile colloids (in conjunction with results from a geochemical equilibrium model), the capability to simulate the influence of multiple colloid types, and to adjust source concentration and duration in the presence of colloids that migrate from a source. These capabilities are embodied in equations (2) and (3) in the main text of this paper.

As expected, results from the COMET simulations indicate that mobile phase metal concentrations (dissolved concentration plus concentration adsorbed to mobile colloids) increase as colloid concentrations increase, and arrival times of soluble metal species (solutes) to stationary receptors decrease. When the partition coefficients for solute‐colloid adsorption and solute‐soil matrix adsorption are the same, neither the increase in mobile phase concentration nor decrease in travel time is always significant. However, when partition coefficients for solute‐colloid adsorption are greater than partition coefficients for solute‐soil matrix adsorption, travel times to stationary receptors can dramatically decrease and total mobile phase concentrations dramatically increase.
Persistence of Selected Pesticides in Ground‐Water Samples
Authors: Cavalier T. C. , Lavy T. L. , Mattice J. D.
Published: August 2005

This study was conducted to determine the stability of five commonly used herbicides fortified into freshly collected ground water. No aquifer materials were present. Ground‐water samples obtained from three sites in eastern Arkansas were fortified with an aqueous mixture containing either the chloroacetamide herbicides, alachlor, metolachlor, and propanil, or the phenoxy herbicides, 2,4‐D and dichlorprop. Experiments were conducted at tempersitures comparable to Arkansas' extreme ground‐water temperatures, 15 and 22° C, and at initial concentrations of 1 and 5 μg/1. Degradation occurring for each herbicide was determined at two‐month intervals, for a period of 18 months.

Results indicated that degradation did not begin immediately, but generally required a lag period of several months. Of the five herbicides studied, propanil degraded the fastest and to the greatest extent. After eight months, 44% to 69% of propanil at the lower fortification level and 85% to 99% at the higher level had degraded, The remaining four herbicides exhibited little or no degradation at the lower concentration during the first 12 months. By 18 months, however, significant degradation of each herbicide had occurred. Degradation generally occurred earlier for each herbicide at the higher fortification level. With the exception of 2,4‐D, a higher percentage of herbicide was broken down at the greater fortification level. Degradation was more extensive at the elevated storage temperature. In general, the degradation rate was faster in ground water taken from shallower depths. Zero‐order rate constants and half‐lives are presented. Half‐lives, including lag periods, ranged from 196 to 1907 days.
Laboratory Column Measurement of VOC Retardation Factors and Comparison with Field Values
Authors: Priddle M. W. , Jackson R. E.
Published: August 2005

A laboratory column technique was developed to measure retardation factors (Rf) of volatile organic compounds (VOC) using geological material obtained from an outwash aquifer. The site is a former solvent disposal pit near Ottawa, Canada. The experimental column was designed as a closed system from introduction of the sample to the collection and analysis of effluent samples to prevent loss through volatilization of the VOCs. The retardation of the volatile compounds 1,4‐dioxane, tetrahydrofuran, diethyl ether, 1,2‐dichloroethane, chloroform, 1,1‐dichloroethene, and benzene was measured. The centers of mass of both the contaminant and conservative tracer breakthrough curves were compared to determine Rf. The measured Rf values ranged from 1.1 (1,4‐dioxane) to 14.3 (benzene) compared with values of 1.6 and 8.8 derived from contaminant plume lengths. Considering the uncertainty of the disposal history at the site, the values are of reasonable agreement in comparison.
Geochemical Evolution in the Cambrian‐Ordovician Sandstone Aquifer, Eastern Wisconsin: 1. Major Ion and Radionuclide Distribution
Authors: Weaver T. R. , Bahr J.
Published: August 2005

Ground water from wells in the Cambrian‐Ordovician sandstone aquifer of eastern Wisconsin often contains elevated concentrations of dissolved solids, chloride, and sulfate and locally approaches or exceeds the current drinking‐water standard for combined radium activity. Samples were collected from wells located along an approximate flow line in order to gain an improved understanding of the chemical evolution of ground water in this system. The concentrations of chloride, sulfate, and sodium increase where the aquifer is confined by the Maquoketa Shale, and the highest combined radium activity is also found in this area. Charge balance calculations and geochemical modeling indicate that dissolution of trace evaporite minerals and cation exchange are important mechanisms controlling major ion distribution. Isotopic equilibrium calculations and analysis of well cuttings from one well with alpha‐sensitive film indicate that the dissolved radionuclide activities are consistent with a uranium source occurring in shaly intervals of the sandstone aquifer at maximum local concentrations of 5 ppm.
Delineation of Traveltime‐Related Capture Areas of Wells Using Analytical Flow Models and Particle‐Tracking Analysis
Authors: Bair E. Scott , Springer Abraham E. , Roadcap George S.
Published: August 2005

Traveltime‐related capture areas of wells can be estimated in Hydrogeologie settings with predominantly two‐dimensional flow regimes using image‐well theory, the principle of superposition, an appropriate well‐hydraulics; equation, and particle‐tracking analysis. These analytical methods can be used to simulate the response of an aquifer to pumping stress by incorporating site‐specific values of hydraulic properties, the effects of geologic and hydrologie boundaries, nonuniform regional hydraulic gradients, and well interferences. This is done by developing a conceptual model of the flow system, incorporating an image‐well solution (if needed), computing drawdowns at the intersections of a rectangular grid superposed over the area of interest, and subtracting computed drawdowns from regional prepumping head values assigned to the grid intersections. The resulting flow model can be calibrated by comparing predicted heads with measured heads. The calibrated head distribution then is used in a particle‐tracking analysis to determine flowpath trajectories and traveltime‐related capture areas of wells.

Analytical flow models based on the Theis equation and the Hantush‐Jacob equation were constructed from conceptual flow models based on geologic, hydrologie, and pumpage data from wellfields completed in a confined/unconfined stratified‐drift aquifer and in a leaky‐confined fractured‐carbonate aquifer. Predicted head distributions from the calibrated flow models were used in a particle‐tracking analysis to compute flowpath trajectories and traveltime‐related capture areas. The results compare favorably with known flowpaths from sources of contamination proximal to each wellfield.
Acid Neutralization by Gulf Coast Sediments
Authors: Fink Linn E. , Aubert Winton G. , Bernard William J.
Published: August 2005

The capacity of sandstone to neutralize injected acid was determined for material from a waste disposal well in Jefferson Parish, Louisiana. Injection zone sandstone was found to neutralize approximately 1.51×10−3 grams H+ ions per gram of sand. Kinetics parameters were determined for reactions of acid with confining layer shale material. Reaction rate coefficients and activation energies of acid‐shale systems agree with values previously determined for reactions of acids with relatively pure clay minerals.
Estimation of Storativity from Recovery Data
Authors: Ballukraya P. N. , Sharma K. K.
Published: August 2005

In estimating aquifer parameters from pumping test data, it is often found advantageous to use recovery, rather than drawdown, measurements. An equation, derived from Cooper‐Jacob's (1946), is suggested for estimating storativity using residual drawdown measurements from an observation well.
Geochemical Evolution in the Cambrian‐Ordovician Sandstone Aquifer, Eastern Wisconsin: 2. Correlation Between Flow Paths and Ground‐Water Chemistry
Authors: Weaver T. R. , Bahr J. M.
Published: August 2005

A finite‐difference ground‐water flow model linked to a particle‐tracking routine was used to determine ground‐water flow paths and residence times in the Cambrian‐Ordovician sandstone aquifer of eastern Wisconsin. The modeled region was a cross section along an approximate flow line that included the boundary between unconfmed and confined conditions in the aquifer. Modeling results indicate that lower conductivity units within the sandstone aquifer produce vertically stratified flow in the confined region. These simulation results help explain chemical signatures of ground water in different parts of the aquifer. Three distinct regions of the flow system are identified: the unconfmed zone where vertical mixing across the aquifer yields a homogeneous chemistry, the shallow part of the confined zone where sodium and sulfate charged water from the Maquoketa Shale mixes with water that has migrated from the unconfmed area, and the deeper region of the confined aquifer, containing older water that entered the system in the unconfmed area. Radium activities in ground water increase with distance along flow paths predicted by the simulation, consistent with a low concentration source of solid phase uranium and release of decay products to ground water by desorption and dissolution throughout the sandstone aquifer.
Determining the Mean Hydraulic Gradient of Ground Water Affected by Tidal Fluctuations
Authors: Series Michael E.
Published: August 2005

Tidal fluctuations in surface‐water bodies produce progressive pressure waves in adjacent aquifers. As these pressure waves propagate inland, ground‐water levels and hydraulic gradients continuously fluctuate, creating a situation where a single set of water‐level measurements cannot be used to accurately characterize ground‐water flow. For example, a time series of water levels measured in a confined aquifer in Atlantic City, New Jersey, showed that the hydraulic gradient ranged from. 01 to. 001 with a 22‐degree change in direction during a tidal day of approximately 25 hours. At any point where ground water tidally fluctuates, the magnitude and direction of the hydraulic gradient fluctuates about the mean or regional hydraulic gradient. The net effect of these fluctuations on ground‐water flow can be determined using the mean hydraulic gradient, which can be calculated by comparing mean ground‐ and surface‐water elevations. Filtering methods traditionally used to determine daily mean sea level can be similarly applied to ground water to determine mean levels. Method (1) uses 71 consecutive hourly water‐level observations to accurately determine the mean level. Method (2) approximates the mean level using only 25 consecutive hourly observations; however, there is a small error associated with this method. The exact magnitude of this error is usually unknown, and therefore the accuracy of the mean level is also unknown. Method (1) should be used if a higher degree of accuracy is required.
The Effect of Tidal Fluctuation on a Coastal Aquifer in the UK
Authors: Erskine A. D.
Published: August 2005

Tidal fluctuation is shown to affect the piezometer readings taken in a nearshore aquifer around a working nuclear power station. Continuous monitoring of a network of 40 piezometers provided relations between the tidal cycle and the piezometer readings. These relations can be expressed in terms of a time lag and amplitude scaling factor. Using these parameters, tidal interference was filtered from the day‐to‐day piezometer readings. The early detection of ground‐water drawdown below the working station caused by construction of a new nuclear power station on an adjacent site could thus be achieved.

In parallel with the field work, the theoretical relationship between the tide and piezometric response was shown to be of the same form as that observed in the field. Values of the aquifer characteristics could then be deduced and compared with those measured in conventional pumping tests. The transmission of the pressure oscillation is shown to be faster in areas of higher permeability and in the deeper parts of the aquifer. This second effect is thought to be caused by the presence of the phreatic surface.
Capture Zones for Simple Aquifers
Authors: McElwee Carl D.
Published: August 2005

Abstract. The protection and cleanup of aquifers is a matter of high priority for all states and the federal government. One concept that is receiving increased attention is that of wellhead protection. Capture zones showing the area influenced by a well within a certain time are useful for both aquifer protection and cleanup. If hydrodynamic dispersion is neglected, a deterministic curve defines the capture zone. Analytical expressions for the capture zones can be derived for simple aquifers. However, the capture zone equations are transcendental and cannot be explicitly solved for the coordinates of the capture zone boundary. Fortunately, an iterative scheme allows the solution to proceed quickly and efficiently even on a modest personal computer. Three forms of the analytical solution must be used in an iterative scheme to cover the entire region of interest, after the extreme values of the x coordinate are determined by an iterative solution. The resulting solution is a discrete one, and usually 100‐1000 intervals along the x‐axis are necessary for a smooth definition of the capture zone. The presented program is written in FORTRAN and has been used in a variety of computing environments. No graphics capability is included with the program; it is assumed the user has access to a commercial package. The superposition of capture zones for multiple wells is expected to be satisfactory if the spacing is not too close. Because this program deals with simple aquifers, the results rarely will be the final word in a real application. However, the program is useful as a first phase in developing wellhead protection or aquifer cleanup schemes.
Semianalytical Solutions for Radial Dispersion in a Three‐Layer Leaky Aquifer System
Authors: Chen Chia‐Shyun
Published: August 2005

Waste water injected into aquifers bounded by aquitards can potentially leak into the aquitards and eventually reach other layers. Based on the mathematical framework of an earlier model, the current study deals with steady‐state and transient solutions for solute transport from an injection well into a three‐layer leaky aquifer system consisting of a pumped and an unpumped aquifer separated by an aquitard. Solutes are assumed to be transported by horizontal advection within the pumped aquifer, by advection and dispersion in a vertical flow field in the aquitard, and by molecular diffusion under hydrostatic conditions in the unpumped aquifers. Proper leaky aquifer well hydraulics are used to describe the ground‐water flow fields in the model, and radioactive (biological) decay and linear isotherm adsorption are considered. This model study shows that adsorption in the pumped aquifer has an overall retardation effect on solute movement in the whole system. Adsorption in the aquitard does not significantly affect solute movement in the pumped aquifer. Type curves are prepared to indicate whether the unpumped aquifer would be contaminated by the injection into the pumped aquifer. Pumping rate is an important hydraulic factor determining the leakage conditions of an aquifer (in addition to some hydrogeological conditions of the systems). Analytical solutions are obtained for steady‐state conditions. For transient conditions, however, only semianalytical solutions can be determined by numerically inverting the solutions of the model in the Laplace transform domain.
Some Additional Nonparametric Prediction Limits for Ground‐Water Detection Monitoring at Waste Disposal Facilities
Authors: Gibbons Robert D.
Published: August 2005

In a previous paper Gibbons (1990) derived nonparametric prediction limits that are applicable to ground‐water monitoring applications. The nonparametric prediction limit was defined as the maximum of n historical measurements (e.g., taken from upgradient wells), denoted X(max, n), and was designed to include one of m future measurements in each of k monitoring wells. In this way, an owner /operator who failed an initial sample but passed any one of m −1 resamples would be considered in compliance. In this paper, these limits are modified so that m −1 out of m future measurements in each of k monitoring wells are included. In this way, the owner/operator is required to pass allm −1 resamples in order to demonstrate compliance in the event of an initial failure. The paper provides a detailed tabulation of confidence levels corresponding to selections of n (4‐100), k (1‐100), and m (2‐4). Power characteristics of the nonparametric approach relative to the parametric alternative are presented for cases in which the assumed distribution is correct and incorrect.
Revisiting the Membrane Analog—A Conceptual and Communication Tool
Authors: Arnold Forest
Published: August 2005

Membrane analogs were used before the advent of computers as physical models of stresses on the water‐table surface. Measurement of the membrane deflection can be related to equations representing pumping stresses. Although numerical models using computer techniques have replaced membrane analogs due to their greater accuracy and versatility, the mem brane analog remains an inexpensive way to conceptualize the effect of different boundary conditions, nearby surface‐water bodies and pumping stresses, and is useful in preparing input data sets for numerical modeling. The membrane analog is also a visual way to communicate modeling results and modeling scenarios to nonmodelers. A small office‐sized membrane analog can be constructed for less than $25.00 and can be stored on a shelf or under a desk, for ready access.
Assessing Transmissivity from Specific Capacity in a Large and Heterogeneous Alluvial Aquifer
Authors: Razack M. , Huntley David
Published: August 2005

Transmissivity is often estimated from specific capacity data because of the expense of conducting standard aquifer tests to obtain transmissivity and the relative availability of specific capacity data. Most often, analytic expressions relating specific capacity to transmissivity derived by Thomasson and others (1960), Theis (1963), or Brown (1963) are used in this analysis. This paper focuses on a test of these relations using a large (215 pairs) data set from a heterogeneous aquifer.

The analytic solutions predicting transmissivity from specific capacity do not agree well with the measured transmissivities, apparently due to turbulent well loss within the production wells, which is not taken into account by any of the analytic solutions. Empirical relations are better than the theoretical relations. Log‐log functions have greater correlation coefficients than linear functions. The best relation found for the data set chosen for this study has a correlation coefficient of 0.63, but the prediction interval was about 1.2 log cycles, indicating that the range of probable transmissivities corresponding to a single specific capacity was more than one order of magnitude. Tests with smaller subsets of data suggest that correlations based on data sets of 10 points or less are of limited value.
Coincidence of Well Fields and Tectonic Basins in the Precambrian Shield Area of Southeast Botswana
Authors: Dietvorst E.J.L. , De Vries J. J. , Gieske A.
Published: August 2005

A compilation of ground‐water exploration data in southeastern Botswana reveals that the occurrence of ground water is not primarily controlled by large‐scale fracture systems and lithologies, but by folds in the Precambrian basement. Two sets of superimposed synforms and antiforms generate a “chess‐board” pattern of dome and basin structures. Successful boreholes and well fields appear to be preferentially situated in the areas where synforms intersect, suggesting that tectonic basins coincide with ground‐water basins. Small‐scale fractures and favorable lithologies doubtless improve the permeability within a ground‐water basin, but appear to be subordinate in determining the distribution of large‐scale aquifer characteristics.
Thermodynamic Framework for Evaluating PAH Degradation in the Subsurface
Authors: McFarland Michael J. , Sims Ronald C.
Published: August 2005

A method is presented to evaluate the influence of redox conditions and pH on mineralization of polycyclic aromatic hydrocarbons (PAH) in subsurface environments. Microbial yields based on the free energy liberated from heterotrophic PAH metabolism are estimated under various environmental conditions using a simple bioenergetic growth model. The types and chemical forms of electron acceptors addressed in this paper include oxygen, nitrate, sulfate, carbon dioxide, iron (Fe+3and FeOOH), and manganese (Mn+4 and MnO2). PAHs addressed include naphthalene (2‐fused aromatic rings), anthracene (3‐fused aromatic rings), phenanthrene (3‐fused aromatic rings), and pyrene (4‐fused aromatic rings).

Calculated free energy changes demonstrated that sequential utilization of electron acceptors will follow the order Mn+4, O2, NO3−1, Fe+3, MnO2, FeOOH, SO4−2, and CO2. The behavior in microbial growth yield predictions were found to mimic the change in free energy liberated with the use of different electron acceptors. Despite release of free energy under all conditions evaluated, the small energy liberated during PAH mineralization under sulfate‐reducing and methanogenic conditions suggests that heterotrophic degradation of PAH compounds under these conditions is unlikely to occur.

The large microbial growth yields associated with reduction of free metal species (Mn+4, Fe+3) during PAH oxidation suggest a selective advantage for microbes that can tolerate acid conditions and/or participate in metal chelation processes.
Slug Tests with Storage Due to Entrapped Air
Authors: Keller C. Kent , van der Kamp G.
Published: August 2005

Any air entrapped in the completion region of a piezometer endows that piezometer with storage due to the compressibility of the air. Such storage retards the recovery of water levels in the piezometer and leads to underestimation of formation hydraulic conductivity in the interpretation of slug tests. These effects appear to be greatest for piezometers with small‐diameter casings installed in low‐permeability formations where filter pack materials are placed dry around the piezometer intake. This paper presents (a) a method which accounts for storage due to entrapped air in slug test analysis; (b) examples illustrating the method; and (c) some recommendations for piezometer construction and slug test practice.
Difluorobenzoates as Nonreactive Tracers in Soil and Ground Water
Authors: Bowman Robert S. , Gibbens Joseph F.
Published: August 2005

Over the past 10 years, several fluorinated derivatives of benzoic acid have proven their usefulness as nonreactive water tracers. In this study, the suitability of four previously untested isomers of difluorobenzoate (DFBA) as saturated‐ and unsaturated‐zone tracers was examined in a series of laboratory and field experiments. Experimental conditions included batch shaking tests with two different soils, laboratory soil column tests under saturated and unsaturated conditions, a three‐month aquifer tracer test, and a month‐long tracer test in a field soil. Bromide, which is considered nonreactive in most hydrological environments, was included as an index tracer in each experiment. Under every condition, each DFBA isomer snowed the same concentration distribution and mass recovery as Br. These and other previously tested fluorobenzoates were ranked according to their suitability as nonreactive soil and ground‐water tracers.
Competitive Exclusion of Sulfate Reduction by Fe(lll)‐Reducing Bacteria: A Mechanism for Producing Discrete Zones of High‐Iron Ground Water
Authors: Chapelle Francis H. , Lovley Derek R.
Published: August 2005

The Middendorf aquifer of South Carolina exhibits a 40‐kilometer‐wide zone where dissolved ferrous iron concentrations commonly exceed 1 mg/I. Downgradient of this zone, dissolved iron concentrations decrease to less than 0.05 mg/1. Geochemical and microbiologie evidence indicates that this zonation reflects the competitive exclusion of sulfate‐reducing activity by Fe(IH)‐reducing bacteria in the high‐iron zone and the emergence of sulfate reduction as the predominant process in the low‐iron zone. Viable Fe(III)‐ and sulfate‐reducing bacteria coexist throughout the aquifer. However, the observed linear relationship between dissolved iron and dissolved inorganic carbon as well as the lack of sulfate consumption indicates that sulfate‐reducing bacteria are much less active than Fe(III)‐reducing bacteria in the high‐iron zone. Fe(III)‐reducing bacteria appear to exclude sulfate‐reducing activity by maintaining dissolved hydrogen (˜1.0 nM), formate (˜2.0 μM), and acetate (˜1.0 μM) concentrations at levels lower than thresholds required by sulfate‐reducing bacteria. Downgradient of the high‐iron zone, Fe(III)‐reducing activity becomes limited by a lack of Fe(III) oxyhydroxides as Middendorf sediments become progressively more marine in origin. Hydrogen, formate, and acetate concentrations then increase to levels (˜3.0 nM, ˜10.9, and 2.5 μM, respectively) that allow sulfate‐reducing bacteria to become active. Increased sulfide production strips ferrous iron from solution by precipitating ferrous sulfides, and dissolved iron concentrations decrease. The observed high‐iron zonation is thus one manifestation of microbial competition for scarce substrates. The wide occurrence of similar water‐chemistry patterns implies that microbial competition mechanisms are important to the ground‐water geochemistry of many hydrologie systems.
Comparison Between Model Simulations and Field Results for In‐Situ Biorestoration of Chlorinated Aliphatics: Part 2. Cometabolic Transformations
Authors: Semprini Lewis , McCarty Perry L.
Published: August 2005

A nonsteady‐state model is presented and used in simulating the results of field experiments where chlorinated ethenes were cometabolically degraded by methanotrophic bacteria. In cometabolism, contaminants are degraded fortuitously by microbes growing on a primary substrate, which in this case is methane. The model includes microbial processes of microbial growth, utilization of the electron donor (methane) and the electron acceptor (oxygen), and the cometabolic transformation of the chlorinated ethenes coupled with the transport processes of advection, dispersion, and sorption onto the aquifer solids.

Model simulations of the chlorinated ethenes biotransformation resulting from the biostimulation of methanotrophic bacteria agreed well with the field observations. A kinetic model for the cometabolic transformation that included the competitive inhibition by methane was required to match the field observations. The simulations duplicated the cyclic oscillations in concentration of the chlorinated ethenes that resulted from the pulse feeding of methane. Rate‐limited sorption and desorption were also required in order to match the extended tailing in the concentration response that was observed along with the pulsed oscillations in the concentration due to competitive inhibition.

The kinetic parameters for the cometabolic transformation, derived from model fits to the field observations, differed for the different chlorinated ethenes tested. Vinyl chloride (VC) and trans‐dichloroethylene (t‐DCE) had the highest biotransformation rate coefficients (k/Ks), which were in the range of methane itself, the primary substrate for methanotrophic growth. Cis‐dichloroethylene (c‐DCE) and trichloroethylene (TCE) were degraded at rates one and two orders of magnitude lower than methane, respectively. The transformation rate for TCE was consistent with laboratory‐derived rates from microbes enriched and isolated from the test zone and grown under conditions that most represented those in the field. The kinetic model also permitted the activation and the deactivation of microbial mass towards the cometabolic transformation, which was based on cell growth and decay, and first‐order deactivation.
Unsteady Ground‐Water Flow Toward Extended Wells
Authors: Sen Zekâi
Published: August 2005

Ground‐water flow in naturally fractured rocks of low permeability terrain presents special problems which cannot be solved by classical radial flow techniques. Existence of fractures in these rocks increases the conductivity of the media, but the storativity does not increase significantly. Especially, a dominant vertical fracture in hydraulic connection with a well provides an extended surface for ground water to enter the well. Such a well and fracture combination is referred to as an “extended well.” The analytical approach to the ground‐water flow problem of extended wells cannot be achieved through merely radial or linear flow alone if the vertical fracture is of finite length. In fact, in practice, the fracture lengths are almost always of finite length, and the flow pattern is a mixture of radial and linear flows. This paper presents a simplified conceptual model for the ground‐water flow pattern around an extended well leading to an analytical solution with type curves which can be used in determining the aquifer parameters from the field measurements of time‐drawdown data. It is observed that for large times, extended well type curves merge with the Theis curve, and consequently the Jacob straight‐line method becomes applicable. In addition, as the vertical fracture length becomes smaller, the resulting type curve approaches the Theis type curve. The methodology has been applied for ground‐water flow in a folded and fractured limestone formation in the Conasauga Group of East Tennessee.
Simulation of Tidal Effects on Contaminant Transport in Porous Media
Authors: Yim Chan S. , Mohsen M.F.N.
Published: August 2005

A one‐dimensional numerical model is developed with oscillating velocities and dispersions to simulate the migration process of a contaminant plume within tidally influenced aquifers. Model simulations demonstrate that a major effect the tidal fluctuation has on the migration process of a contaminant plume is the exit concentration discharging to the tidal estuary. Tidal fluctuation causes the exit concentration levels to be significantly diluted by the surface‐water body of the estuary. Sensitivity analyses demonstrate that tidal fluctuation hastens the rate of plume migration near the bank of the estuary because of the relatively high advective and dispersive fluxes induced by tides. However, tides affect the migration process only over a short distance from the tidal‐water interface (about 40 ft for the parameters used in this study). If the contaminant plume is located far beyond the interface, tidal fluctuations will not affect the rate of plume migration until an existing regional ground‐water flow velocity brings the plume to the tidally active zone. With or without tides, the rate of contaminant migration increases with higher regional hydraulic gradient. Furthermore, the effects of tidal fluctuations on the transport process become insignificant with higher regional hydraulic gradients.
Influence of Pore Pressure on Apparent Dispersivity of a Fissured Dolomitic Aquifer
Authors: Leap Darrell I. , Belmonte Pamela Mai
Published: August 2005

The U.S. Geological Survey's Amargosa Tracer Calibration Site in southern Nevada has been used for three different recirculating tracer tests using as tracers: (1) tritium, (2) sulfur‐35, and (3) tritium and bromide together. Although the physical setup, well spacings, and thus apparent scale were the same in all tests, the recirculating rates and pore pressures were different. Apparent dispersivities found in the three tests differed considerably, revealing an inverse relationship between computed dispersivity and recirculation rate. These differences are believed to be caused primarily by changes in fissureaperture widths and thus, hydraulic conductivities, with resulting changes in flow rates and directions caused by changes in pore pressure between different tests in the presence of high ambient pore pressure. The results of this study indicate that forced‐gradient tracer tests for determination of dispersivity should not be performed when ambient pore pressure and/or testing pore pressure is a significant percentage of overburden pressure. In addition, apparent dispersivity calculated by forced‐gradient tracer tests can differ considerably from that of unstressed natural situations, and consequent solute‐transport estimates are likely to be in error.
Hydrogeological Decision Analysis: 4. The Concept of Data Worth and Its Use in the Development of Site Investigation Strategies
Authors: Freeze R. Allan , James Bruce , Massmann Joel , Sperling Tony , Smith Leslie
Published: August 2005

In the design of new waste‐management facilities, or in the remedial design for contaminated facilities, there are economic tradeoffs possible between the resources assigned to site investigation, facility design, and site monitoring. Hydrogeological decision analysis provides a framework that allows a sequential, iterative approach to design, in which data acquisition through site investigation alternates with economic analysis of alternative design options. In this framework, the purpose of collecting hydrogeological data is to aid in the making of decisions between the alternative design options. Such decisions are made on the basis of a risk‐cost‐benefit objective function. Additional measurements have worth only if the risk reduction they provide exceeds the cost of obtaining them. The location of measurement points in a site‐investigation network should minimize economic regret with respect to design decisions. This paper provides an introduction to the concepts of data‐worth analysis for two cases: (1) the reduction of uncertainty in aquitard continuity, and (2) the reduction of uncertainty in hydraulic‐conductivity distribution in an aquifer. It is shown how the use of search theory and Bayesian updating can be applied in comparing prior and preposterior analyses to provide a stopping rule for the collection of additional data in site investigation programs at a hypothetical landfill site.
Aquifer Analysis and Modeling in a Fractured, Heterogeneous Medium
Authors: Lee R. R. , Ketelle R. H. , Bownds J. M. , Rizk T. A.
Published: August 2005

A ground‐water flow and contaminant transport modeling study was performed at a proposed low‐level radioactive waste disposal site on the Department of Energy's Oak Ridge Reservation to assess the accuracy of a computer simulation of the flow field in the shallow water‐table aquifer. The migration of a ground‐water tracer, under controlled field conditions, was used as a comparative benchmark of solute transport simulations assuming different aquifer characteristics. A conventional approach to aquifer analysis and modeling, which assumed the domain to be homogeneous with low anisotropy, resulted in an inaccurate simulation of tracer migration. Alternative conceptual models were formulated which considered the effects of geologic heterogeneities on tracer migration which could be verified by additional data acquisition and analysis. Application of the alternative concepts in numerical modeling resulted in more accurate simulations of tracer migration. These results suggested that local geology created discrete flow pathways oriented oblique to the maximum gradient which were unresolved in conventional aquifer analysis. Results of the study showed that standard approaches to aquifer analysis and computer modeling are crudely representative of the flow field and provide nonconservative estimates of contaminant transport at the scale of a waste disposal unit. By adopting an iterative approach to site characterization, conceptual model formulation, and numerical modeling, the level of resolution provided by a ground‐water tracer was accurately simulated.
Tracer Test Evaluation of a Drainage Ditch Capture Zone
Authors: Chambers Lucy W. , Bahr Jean M.
Published: August 2005

Recent analytical and numerical modeling studies (Zheng et al., 1988a, 1988b) suggest that drainage ditches within the central sand plain of Wisconsin may serve to limit the spread of agricultural chemicals. In this study a natural‐gradient tracer test was conducted to determine flow paths around a sand plain drainage ditch. The flow paths define a capture zone for the ditch. These results were used to evaluate the predictive capabilities of an analytical model (Zheng et al., 1988a).

The natural‐gradient tracer test began with injections of iodide at two depths and bromide at an intermediate depth. Movement of these tracers reveals that average ground‐water velocity ranged from 0.4 ft/day at a depth of 20 ft to greater than 1 ft/day near the water table. The delineated capture zone was deeper than 12 ft below the water table. These results are encouraging because they indicate that the capture depth of the ditch should allow for the effective removal of shallow agricultural contaminants. The observed capture depth falls within the range predicted by the analytical model of Zheng et al. (1988a). However, a sensitivity analysis of the analytical method reveals that the predicted range of capture depths is large. This range results from the variability and uncertainty of the model parameters. The analytical solution appears best suited for estimating the minimum capture depth, which is a conservative estimate of contaminant capture.
Applicability of Sharp‐Interface Models for NAPL Transport: 1. Infiltration
Authors: El‐Kadi Aly I.
Published: August 2005

A sharp‐interface, or piston flow model is developed for nonaqueous phase liquids (NAPL) flow through the unsaturated zone. This model, which assumes saturated plug flow downward from the surface, is verified through comparison with an immiscible‐phase model that includes capillarity effects in the formulation. Although significant assumptions are included in the sharp‐interface model, it may be useful for initial screening and in obtaining an order‐of‐magnitude estimate of the time of travel of NAPL in the unsaturated zone. In the absence of data for calibration, guidelines are introduced for estimating the model's parameters.
Unsaturated and Saturated Flow in Response to Pumping of an Unconfined Aquifer: Numerical Investigation of Delayed Drainage
Authors: Akindunni Festus F. , Gillham R. W.
Published: August 2005

In this study, numerical methods are used to examine the capillary fringe hypothesis proposed by Nwankwor et al. (1992) to explain variations in the storage property of an unconfined aquifer during pumping. Based on field observations, the hypothesis suggested that the response of an unconfmed aquifer to the stress imposed by pumping is largely controlled by the magnitude of vertical hydraulic gradients developed above the moving water table. After validation with a 1‐D hysteretic laboratory experiment and a 2‐D axisymmetric variably saturated semianalytical model, the numerical solution was used to simulate the pumping test of Nwankwor et al. (1992). Numerical parameters used for the simulation were chosen from experiments independent of the pumping test. There was good agreement between numerical results and field data at different depths and different radial distances from the pumping well. Numerically obtained variations in the magnitude of vertical gradients above the water table and the resulting variations in values of apparent specific yield also showed similar trends with the field data. The good agreement between the numerical and field results provides convincing evidence that the field observations and the resulting capillary fringe hypothesis concerning time variations of specific yield, are indeed consistent with the physical principles of variably saturated flow. The high storativity values frequently obtained by fitting early‐time data to type curves were shown to correspond to the period of increasing hydraulic gradients, and may not be related to intrinsic properties of the aquifer, such as compressibility. In agreement with the “volume‐balance method” of Nwankwor et al. (1984), it was also shown that the results confirm that the ultimate specific yield of an unconfmed aquifer is more related to the drainable porosity than the low values often obtained from type‐curve analysis.