bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Artificial Ground‐Water Recharge on Aquifer Parameters Using Cavity Well
Authors: Taneja D. S. , Khepar S. D.
Published: August 2005

Cavity wells are shallow wells without strainers and are installed in confined aquifers. These wells are commonly installed in alluvial tracts in the Indian subcontinent, and can be used as discharge as well as recharge wells. However, during recharge, the value of aquifer parameters may change. This study was undertaken to address these changes. A cavity well was developed in a simulated circular sand tank model of a confined aquifer. The cavity thus developed was observed to be a part of a hemisphere. An analytical solution of the hydraulics of this cavity well was developed. The well was initially discharged and then recharged at various rates, and experimental data on drawdown and rise of head were collected. Aquifer parameters were calculated by substituting these data in the analytical solution. It was observed that during recharge through the well, the hydraulic conductivity and specific storage coefficient of the aquifer decreased by 15% and 13%, respectively in relation to those of discharge. Also, the aquifer parameters were found to be least affected with the change in rate of recharge or discharge.
A Test of the In Situ Permeable Flow Sensor at Savannah River, SC
Authors: Ballard Sanford , Barker Glenn T. , Nichols Ralph L.
Published: August 2005

A test of the In Situ Permeable Flow Sensor was conducted in which ground‐water flow velocity measurements made by the flow sensors were directly compared to velocity estimates obtained using standard hydrologic techniques. Two flow sensors were deployed in a confined aquifer in close proximity to a well which was screened over the entire vertical extent of the aquifer. When the well was pumped at four different pumping rates, the horizontal component of the flow velocity measured by the flow sensors was directed toward the pumping well, within the uncertainty in the measurements, and the magnitude of the horizontal component of the velocity increased linearly with pumping rate, as predicted by theoretical considerations. The measured magnitudes differed from predicted values, calculated with the assumption that the hydraulic properties of the aquifer were homogeneous and isotropic, by less than a factor of two. Vertical components of ground‐water flow observed with the flow sensors are qualitatively consistent with the vertical distribution of hydraulic conductivity estimated from grain‐size analysis but are significantly larger in magnitude than predicted. This is likely due to the creation of a vertical conduit of increased hydraulic conductivity during emplacement of the probes. The results suggest that while the flow sensors measured the local ground‐water flow velocity vector during the test quite accurately, care must be exercised to disturb the formation as little as possible during emplacement. The technology has many potential uses, particularly in the area of environmental site characterization and remediation process monitoring.
Optimal Ground‐Water Remediation Design Using Multiple Control Technologies
Authors: Marryott Robert A.
Published: August 2005

Combined optimization‐simulation models for designing ground‐water remediation systems generally focus on design strategies that use only one remedial technology, typically pumping. In practice, many field remediation programs use pumping along with one or more additional ground‐water control technologies, such as drains, trenches, slurry walls, and low permeability caps. Simulated annealing provides a flexible optimization framework that can incorporate a number of different remedial technologies into the design process. Using simulated annealing, additional remedial technologies can be added to or subtracted from the optimization at any time. This flexibility can provide for remedial design evaluations that are more consistent with current field implementations. The use of simulated annealing is demonstrated by a series of hypothetical remediation design problems that incorporate four different remedial technologies. The computed optimal design solution depends on the relative costs of the remedial tools as specified by the cost function and on the effects that each tool has on the system behavior. A sensitivity analysis can reveal which costs are critical to the outcome of the optimization.
Analytical Modeling of Effects of Application Frequency on Pesticide Concentrations in Wells
Authors: Beltman W.H.J. , Boesten J.J.T.I. , van der Zee S.E.A.T.M. , Quist J. J.
Published: August 2005

Pesticide transport from the soil surface to the ground‐water table was modeled by means of an analytical solution of the convection‐dispersion equation, assuming steady water flow, a linear sorption isotherm and first‐order transformation kinetics. Pesticide transport in the saturated zone to a well was described by means of an analytical solution of the conservation equation for a cylindrical flow system, assuming steady flow, no dispersion, linear sorption, and first‐order transformation. We derived an analytical solution for this coupled unsaturated‐saturated system. We limited our analysis to systems that are difficult to protect with a protection zone. The strongest fluctuations in the pesticide concentration in the well were calculated for the lowest frequency of application, shortest characteristic travel time in the saturated zone and shortest half‐life in the saturated zone. A simplified model, ignoring the variation of travel times in the unsaturated zone, resulted in concentration fluctuations in the well. The peak concentrations differed less than two times from those calculated with the analytical coupled unsaturated‐saturated model, except for cases with the combination of a short characteristic travel time and a short half‐life in the saturated zone.
Improving the Quality of Parameter Estimates Obtained from Slug Tests
Authors: Butler James J. , McElwee Carl D. , Liu Wenzhi
Published: August 2005

The slug test is one of the most commonly used field methods for obtaining in situ estimates of hydraulic conductivity. Despite its prevalence, this method has received criticism from many quarters in the ground‐water community. This criticism emphasizes the poor quality of the estimated parameters, a condition that is primarily a product of the somewhat casual approach that is often employed in slug tests. Recently, the Kansas Geological Survey (KGS) has pursued research directed at improving methods for the performance and analysis of slug tests. Based on extensive theoretical and field research, a series of guidelines have been proposed that should enable the quality of parameter estimates to be improved. The most significant of these guidelines are: (1) three or more slug tests should be performed at each well during a given test period; (2) two or more different initial displacements (H0) should be used at each well during a test period; (3) the method used to initiate a test should enable the slug to be introduced in a near‐instantaneous manner and should allow a good estimate of Ho to be obtained; (4) data‐acquisition equipment that enables a large quantity of high quality data to be collected should be employed; (5) if an estimate of the storage parameter is needed, an observation well other than the test well should be employed; (6) the method chosen for analysis of the slug‐test data should be appropriate for site conditions; (7) use of pre‐and post‐analysis plots should be an integral component of the analysis procedure, and (8) appropriate well construction parameters should be employed. Data from slug tests performed at a number of KGS field sites demonstrate the importance of these guidelines.
In Situ Hydraulic Conductivity of Clay and Silty‐Clay Fluvial‐Deltaic Sediments, Texas Gulf Coast
Authors: Capuano R. M. , Jan R. Z.
Published: August 2005

Two aquifer pumping tests (13 and 24 hours) and a tracer injection test were performed on shallow (7.5 m) clay and silty‐clay sediments of the fluvial‐deltaic Beaumont Formation from the U.S. Gulf Coast. The in situ horizontal hydraulic conductivity determined from these tests is found to be approximately 10−3 cm/sec. This value is one to three orders of magnitude larger than that of typical silts and clays, and two to four orders of magnitude larger than the vertical hydraulic conductivity measured on core in laboratory permeameter experiments. A uniformity of the drawdown curves from different observation wells strongly suggests that hydraulic conductivity is relatively homogeneous and isotropic in the horizontal plane at the scale of the test. This relatively large in situ horizontal hydraulic conductivity for these clay‐rich sediments is attributed to macroporosity rather than matrix porosity.
Flow to a Well in a Water‐Table Aquifer: An Improved Laplace Transform Solution
Authors: Moench Allen F.
Published: August 2005

An alternative Laplace transform solution for the problem, originally solved by Neuman, of constant discharge from a partially penetrating well in a water‐table aquifer was obtained. The solution differs from existing solutions in that it is simpler in form and can be numerically inverted without the need for time‐consuming numerical integration. The derivation involves the use of the Laplace transform and a finite Fourier cosine series and avoids the Hankel transform used in prior derivations. The solution allows for water in the overlying unsaturated zone to be released either instantaneously in response to a declining water table as assumed by Neuman, or gradually as approximated by Boulton's convolution integral. Numerical evaluation yields results identical with results obtained by previously published methods with the advantage, under most well‐aquifer configurations, of much reduced computation time.
Site Characterization by Neuronets: An Application to the Landfill Siting Problem
Authors: Basheer Imad A. , Reddi Lakshmi N. , Najjar Yacoub M.
Published: August 2005

For a large number of geotechical/ground‐water applications, determining the spatial distribution of the subsoil properties constitutes a major challenge in both design and construction phases. Models that are frequently used for characterization of site properties emphasize the use of probabilistic models that incorporate a number of model parameters to be evaluated. In this paper, neural networks (or neuronets) are used to map the variation of permeability for purposes of identifying boundaries of landfill to be constructed on a real site. The neural network, as a simple technique, was found to be able to logically predict the variation. The sensitivity of the produced permeability maps to both the quality and number of observations was also studied to investigate the accuracy of the proposed mapping methodology. The use of neuronets as a mapping tool can help identify the regions within a site where additional subsoil exploration is warranted.
Extension of the “Pore Tree” Model to Describe Transport in Soil
Authors: Simons Girard A.
Published: August 2005

The “pore tree” model of pore structure was originally developed for catalyst and sorbent grains to allow coupled reactions and diffusion into and out of nonpermeable porous media in the absence of convection through the media. The pore tree model is extended herein to describe the permeable pore structure which characterizes the subsurface transport of gas and water in soil, the dispersion of contaminants, and in situ remediation. The interconnectivity of the pore structure is obtained via a statistical determination of the “branches” that are common to several trees to allow convection and diffusion through the large scale (permeable) structure in addition to diffusion and reactions in the smaller scale (nonpermeable) structure. The extended pore tree model has successfully explained measurement errors in the permeability of soil due to the measurement scale size and has successfully predicted the bulk gaseous diffusivity in partially saturated soil as a function of a saturation scale size. The extended pore tree model provides an analytic description of the pore structure of soil upon which bulk transport, small scale diffusion, and coupled chemical reactions may be added to accurately describe contaminant transport and in situ remediation.
A Two‐Dimensional MODFLOW Numerical Approximation of Mise‐A‐La‐Masse Electrical Flow Through Porous Media
Authors: Osiensky James L. , Williams Roy E.
Published: August 2005

Two‐dimensional, finite‐difference, saturated, ground‐water flow models such as MODFLOW can be useful in simulating “mise‐a‐la‐masse” electrical flow through conductive porous media. A new technique for using an existing numerical code to develop an approximate solution for point source electrical flow is presented. Incorporating a change in variables from ground‐water flow to electrical flow within the MODFLOW input file allows the ground‐water flow code to model the effects of resistivity contrasts directly. By adjusting the input data as presented herein, spherical electrical flow through contaminated porous media can be approximated in two dimensions.
A New Method to Determine Storage Coefficient from Pumping Test Recovery Data
Authors: Banton Olivier , Bangoy Lumony Mulongo
Published: August 2005

A new method is presented to determine the transmissivity and the storage coefficient from recovery data measured after a pumping period of an aquifer test. Previously available methods can determine the storage coefficient during the pumping period only or under restrictive conditions. This method requires observations from a minimum of two points (e.g., the pumping well and an observation well). For each time during recovery, the drawdowns observed in the two wells are first plotted with regard to the distance from the pumping well centrum, giving an intercept A and a slope B for each pair of data. The plot of A values with regard to time allows to calculate the transmissivity value, while the plot of B values versus time gives the storage coefficient value. The method is applied to two sets of field data, and the results obtained are close to those obtained using the classical methods (Theis or Jacob) applied to the drawdown data obtained during the pumping period. This method well determines transmissivity and storativity from recovery data obtained at short times after the completion of pumping, i.e., for times shorter than those which can be used by the Cooper‐Jacob approximation.
Subsurface Stormflow Modeling with Sensitivity Analysis Using a Latin‐Hypercube Sampling Technique
Authors: Gwo J. P. , Toran L. E. , Morris M. D. , Wilson G. V.
Published: August 2005

During storm events, the shallow disposal facilities at Oak Ridge National Laboratory become inundated, and subsurface stormflow may intermittently extract radionuclides from the surrounding soils and the wastes themselves. It is estimated that 90% of the rain water infiltrating the soil horizons becomes subsurface stormflow, yet stormflow is a poorly understood process. The objectives of this research are to model stormflow: (1) to identify important parameters for waste site monitoring and data collections; (2) to evaluate remediation designs; and (3) to investigate the effect of local heterogeneities on stormflow paths. Numerical models of a proposed waste disposal site were developed, and a Latin‐hypercube simulation technique was used to study the uncertainty of model parameters. Sensitivity analyses of model parameters suggested that hydraulic conductivity was the most influential parameter. However, local heterogeneities may alter flow patterns and result in complex recharge and discharge processes. Hydraulic conductivity, therefore, may not be used as the only reference for subsurface flow monitoring and engineering designs. Neither of two engineering designs, capping and French drains, was found to be effective in hydrologically isolating downslope waste trenches. However, combinations of both designs may prove more effective than either one alone.
Seepage Relationships Between Lake Chad and the Chad Aquifers
Authors: Isiorho S. A. , Matisoff G. , Wehn K. S.
Published: August 2005

Lake Chad, a closed basin lake, is the only large surface‐water body in the African Sahel. Future water resource development in the Chad Basin requires an understanding of the hydrogeology of the basin. In this paper we examine the hydraulic relationship between the lake and the phreatic aquifer of the Chad Formation. Electrical resistivity data from 30 surveys were combined with over 60 open well measurements to construct a water‐table map of the southwestern quadrant of the lake. The map indicates that the Lake Chad water level is at a higher hydraulic level than the phreatic aquifer and that the ground‐water flow is southwest, away from the lake. The average measured seepage rate into the phreatic aquifer in the southwestern and southern parts of the lake was 7.1 × 10−3 m/d (median = 1.3 × 10−3 m/d) which is about 21% of the annual water input to the lake. This measured seepage flux (9.96 × 109 m3/yr) can account for about 107% of the annual solute input to the lake. Solute transport model simulations indicate about 32% (15.3 × 109 m3/yr) of the input water and 152% (2.87 × 1012 g/yr) of the total solute input can be removed by ground‐water recharge (seepage) from the lake. These results provide an explanation to account for the freshness of Lake Chad's water despite its surficial closed basin geologic setting: solutes delivered to the lake via surface‐water inflow leave the lake via ground‐water recharge. These results also indicate recharge from the lake (∼ 1010 m3/yr) represents an enormous amount of water available as a ground‐water resource in the African Sahel.
Variations in Capture‐Zone Geometry of a Partially Penetrating Pumping Well in an Unconfined Aquifer
Authors: Bair E. Scott , Lahm Terry D.
Published: August 2005

Design of effective and efficient pump‐and‐treat systems requires capture zones of recovery wells to closely circumscribe the contaminant plume. Overestimation of capture zones incurs the undesired expense of treating clean water. Underestimation of capture zones enables contaminants to escape downgradient. Recovery wells in unconfined aquifers commonly penetrate only part of the aquifer either because of its large saturated thickness or the shallow vertical extent of contamination. The capture‐zone geometry of a partially penetrating pumping well can differ greatly from that of a fully penetrating one because of vertical flow components near the well. The differences in geometry are far greater if the medium is anisotropic (Kh≠ Kv).

To estimate the capture‐zone geometry of a partially penetrating pumping well, a steady‐state, finite‐difference model was constructed to simulate flow to a well in a regional flow system. The model was used to simulate differences in the velocity field created by changes in (1) the depth of well penetration, (2) the magnitude of the regional hydraulic gradient, and (3) the degree of anisotropy. Following each simulation particle tracking was performed to determine the maximum width, depth, and distance to the stagnation point of the capture zone. Graphs were developed between capture‐zone width, relative capture‐zone depth, distance to the stagnation point, versus the ratio of Q/q (pumping rate/specific discharge). The graphs enable estimates to be made of these geometric parameters for a variety of pumping rates, regional hydraulic gradients, hydraulic conductivities, anisotropy ratios, and degrees of partial penetration. The results show that for isotropic conditions, particularly for small ratios of Q/q and wells that penetrate less than 40 percent of the aquifer, the shape of a capture zone can deviate significantly from that of a fully penetrating well. For anisotropic conditions, these differences are more pronounced and apply to a wider range of Q/q ratios and well penetration depths.

In sequences of sediment, anisotropy is produced by textural, architectural, and stratigraphic elements that occur at different scales. Capture zones should be estimated using Kh:Kv ratios determined from pumping tests. This assures that the measured degree of anisotropy is commensurate with the scale of elements encountered by the stress created by a pumping well. Tabulated Kh:Kv ratios from analysis of pumping tests in unconfined aquifers suggest there are few isotropic sand and gravel aquifers. Recognition of this characteristic and consideration of the effects of partial penetration and regional hydraulic gradient on the geometry of capture zones may lead to the design of more efficient and effective pump‐and‐treat systems.
A Controlled Field Test of Surfactant‐Enhanced Aquifer Remediation
Authors: Fountain John C. , Starr Robert C. , Middleton Thomas , Beikirch Michael , Taylor Craig , Hodge Dennis
Published: August 2005

The presence of dense nonaqueous phase liquids is one of the principal problems associated with current ground‐water remediation efforts. Standard pump‐and‐treat methods are ineffective largely because of the low aqueous solubilities of DNAPL components. Surfactants can increase DNAPL solubility and hence have the potential for increasing the rate of DNAPL dissolution in pump‐and‐treat systems. To test the effectiveness of surfactants under field conditions, a controlled field test at Canadian Forces Base Borden was undertaken. Results indicate surfactant‐enhanced aquifer remediation can rapidly remove the majority of DNAPL using simple modifications of a pump‐and‐treat system. As in all pump‐and‐treat systems, the efficiency is a function of the hydraulic conductivity. The persistence of high DNAPL concentrations at specific elevations within the aquifer throughout the test indicates that little vertical movement of DNAPL occurred as a result of the introduction of the surfactant. Since the test was stopped when small amounts of DNAPL still remained, the limit of removal was not investigated.
Aquifer Response to Record Low Barometric Pressures in the Southeastern United States
Authors: Landmeyer James E.
Published: August 2005

A late‐winter cyclone classified as one of the most intense of the 20th century moved across the Southeastern states of Georgia and South Carolina and onto the Northeast during March 12–14, 1993. Record low barometric pressures were recorded in Augusta, Georgia (28.93 inches of mercury) and Columbia, South Carolina (28.63 inches of mercury) on March 13, 1993, and pressures returned to normal values (near 30 inches of mercury) within one day following these record lows. This relatively unusual event provided an opportunity to examine the attendant water‐level response in continuously monitored ground‐water wells in regional Atlantic Coastal Plain, Piedmont, and Blue Ridge aquifers in the Southeast. Water levels in all wells examined responded inversely to the short duration, extreme drop in barometric pressure. Barometric efficiencies (Δground‐water level/Δbarometric‐pressure level) calculated were dependent on depth to screened‐ or open‐interval midpoint (highest correlation coefficient, r2= 0.89) and, to a lesser extent, total thickness of confining material above the aquifer tapped (highest r2= 0.65). Wells in crystalline‐rock aquifers had a correlation with depth to open‐interval midpoint (r2= 0.89) similar to the sedimentary aquifers examined. The magnitude of barometric efficiency was also strongly related to a well's increased distance from aquifer outcrop areas in the Cretaceous aquifers in South Carolina (r2= 0.95) and the upper Brunswick aquifer in Georgia (r2= 0.90), because these aquifers are more deeply buried toward the coast. This relation between barometric efficiency, well depth, and extent of confinement suggests that barometric efficiency determinations can provide useful information to hydrologists concerned with examining an aquifer's degree of confinement and corresponding isolation from land surface, particularly when the aquifer is used as a source for public supply.
Numerical Simulation of Deep‐Well Injection in Geismar, Louisiana
Authors: Jin Peikang , Barber Michael E. , Flowers George C.
Published: August 2005

Industrial waste water has been injected into a permeable part of the Upper Miocene injection reservoirs near Geismar, Louisiana since 1971. The injection zone is a series of alternating extremely permeable sands and impermeable clays, hydraulically isolated from the surficial aquifer system by a thick layer of clay, which acts as a confining layer. The study area encompasses four industrial facilities and 12 active injection wells. Numerical simulations were performed to evaluate injection‐well design and potential for movement of injected waste water within the hydrogeologic system. Numerical modeling was conducted in both cylindrical coordinates and Cartesian coordinates. A convection cell developed around a single prototype injection well in cylindrical‐coordinate system with a buoyant injection rising to form a lens within the injection zone. Around an ideal, fully penetrating well cased into the injection sand and open from a depth of 5,056 feet to 5,456 feet, simulations show the injectant moves upward to a depth of 4,866 feet, forms a lens about 400 feet thick, and spreads radially outward to a distance of 1,670 feet after 10 years. Comparison simulations of injection through wells having open depth intervals of 5,056 to 5,256 feet and 5,256 to 5,456 feet demonstrate that such changes in well design have little effect on the areal spread of the injectant lens or the rate of upward movement. Multiple injection well simulations in Cartesian coordinates indicate that about 5,10, and 80 percent of the study area would be underlain by injected waste water after 10,20, and 100 years of injection, respectively. Outward movement of waste water proceeds even after injection stops. The rate of outward movement under the post‐operational period (after injection ceases) is much smaller than under the operational period (injection period).
Two‐Dimensional Advective Transport in Ground‐Water Flow Parameter Estimation
Authors: Anderman Evan R. , Hill Mary C. , Poeter Eileen P.
Published: August 2005

Nonlinear regression is useful in ground‐water flow parameter estimation, but problems of parameter insensitivity and correlation often exist given commonly available hydraulic‐head and head‐dependent flow (for example, stream and lake gain or loss) observations. To address this problem, advective‐transport observations are added to the ground‐water flow, parameter‐estimation model MODFLOWP using particle‐tracking methods. The resulting model is used to investigate the importance of advective‐transport observations relative to head‐dependent flow observations when either or both are used in conjunction with hydraulic‐head observations in a simulation of the sewage‐discharge plume at Otis Air Force Base, Cape Cod, Massachusetts, USA. The analysis procedure for evaluating the probable effect of new observations on the regression results consists of two steps: (1) parameter sensitivities and correlations calculated at initial parameter values are used to assess the model parameterization and expected relative contributions of different types of observations to the regression; and (2) optimal parameter values are estimated by nonlinear regression and evaluated. In the Cape Cod parameter‐estimation model, advective‐transport observations did not significantly increase the overall parameter sensitivity; however: (1) inclusion of advective‐transport observations decreased parameter correlation enough for more unique parameter values to be estimated by the regression; (2) realistic uncertainties in advective‐transport observations had a small effect on parameter estimates relative to the precision with which the parameters were estimated; and (3) the regression results and sensitivity analysis provided insight into the dynamics of the ground‐water flow system, especially the importance of accurate boundary conditions. In this work, advective‐transport observations improved the calibration of the model and the estimation of ground‐water flow parameters, and use of regression and related techniques produced significant insight into the physical system.
Advection‐Dispersion Analysis of Solute Transport in Undisturbed Soil Monoliths
Authors: Jensen Karsten Høgh , Destouni Georgia , Sassner Mona
Published: August 2005

The results of an extended analysis of the chloride breakthrough curves (BTCs) obtained from transport experiments on undisturbed soil monoliths previously reported by Sassner et al. (1994) are discussed. Parameter values for different forms of the advection‐dispersion equation (ADE) were obtained by curve‐fitting using the nonlinear least‐squares optimization code CXTFIT developed by Parker and van Genuchten (1984). Good fits to the experimental BTCs from the individual soil monoliths were obtained for three different forms of the ADE, allowing for anion exclusion, immobile water, and both of these processes, respectively. Although the more complex forms of the ADE possess more flexibility to provide a slightly more refined fit to the individual breakthrough curves, larger uncertainty is associated with the fitted parameter values. Due to the limited amount of information manifested in the BTCs it is not possible on the basis of this information to distinguish which of the models is physically more meaningful. Furthermore, when more parameters are included in the optimization procedure, the resulting values are more uncertain. The same ADE models were also fitted with equally good results to a hypothetical large‐scale BTC derived by flux‐averaging the responses from the individual monoliths. However, regardless of the form of the ADE used, the resulting parameter values for the large‐scale transport (i.e., not only the dispersivity value that is expected to increase) were inconsistent with the corresponding parameter values obtained for the individual BTCs. This supports previous indications that ADE models may not be accurate for predicting large‐scale transport in heterogeneous soil systems corresponding to e.g. the scale of an agricultural field or a grid element in a numerical catchment model because model parameters are not determinable from independent measurements on a scale corresponding to a practical manageable core size or to standard intrumentation.
Determination of Nonvolatile Organic Carbon in Aquifer Solids After Carbonate Removal by Sulfurous Acid
Authors: Heron G. , Barcelona M. J. , Andersen M. L. , Christensen T. H.
Published: August 2005

The precise determination of total organic carbon (TOC) in aquifer materials is important in the estimation of retardation coefficients for organic compound transport. Commonly used methods for TOC determination after carbonate removal by acidification include: weight loss on ignition of solids at 550° C, acid‐dichromate oxidation, and combustion at 800–950° C. Since carbonate carbon frequently is more abundant than organic carbon all TOC methods depend on efficient and complete carbonate removal prior to the TOC quantification step. A method for the determination of TOC in aquifer solids involving carbonate removal by 0.73 M sulfurous acid and subsequent combustion at 800° C was tested on solids from three aquifers and on mineral standards. The carbon quantification by combustion and infrared detection of CO2 was accurate at solid organic carbon contents between 30 and 10,000 μgC/g. The acid treatment added small amounts of carbon to the sample, and siderite removal was incomplete. For samples without siderite or dolomite, the accuracy of TOC determinations was enhanced by grinding the solids. Measurement of very low TOC contents (50–100 /μgC/g) requires supplementary testing of grinding and acid treatment effects.
PMFCT‐2D: A Transport Simulator for Various Grid Peclet Numbers
Authors: Aimo N. J. , Oostrom M.
Published: August 2005

Solute transport simulation using numerical models is an important and widespread tool for evaluation of clean‐up strategies as well as for prediction of future transport. Classical simulation algorithms for advective‐dispersive transport usually introduce large numerical errors where concentrations are lowest. In general, numerical errors tend to spread (disperse) the solute more than physical processes alone. For simulations where the Peclet number (Pe) is greater than about 2, numerical dispersion can be very significant and could lead to erroneous conclusions. Recent numerical techniques for simulating advective transport minimize numerical errors and provide much better solutions. One such technique, Flux‐Corrected Transport (FCT), can preserve sharp concentration fronts by virtually eliminating numerical dispersion. In general, it has been observed that as more detailed knowledge of subsurface flow fields is obtained, smaller dispersivity values are needed to match observed and simulated data. However, for many numerical codes the use of small dispersivities is not practical, because it requires fine grids to keep the grid Peclet number limited. A general purpose transport code, PMFCT‐2D, has been developed, including a fast and efficient FCT algorithm, to simulate advective‐dispersive transport in variably saturated, heterogeneous porous media, with nonuniform aquifer thickness in the third dimension. PMFCT‐2D can be used to accurately simulate high Peclet number transport, including purely advective transport (Pe =∞), resulting from transient or steady‐state flow conditions. The code is easily coupled to any flow simulator via generated velocity, saturation, and cell thickness fields.
Ambiguity in Measuring Matrix Diffusion with Single‐Well Injection/Recovery Tracer Tests
Authors: Lessoff Steven C. , Konikow Leonard F.
Published: August 2005

Single‐well injection/recovery tracer tests are considered for use in characterizing and quantifying matrix diffusion in dual‐porosity aquifers. Numerical modeling indicates that neither regional drift in homogeneous aquifers, nor heterogeneity in aquifers having no regional drift, nor hydrodynamic dispersion significantly affects these tests. However, when drift is coupled simultaneously with heterogeneity, they can have significant confounding effects on tracer return. This synergistic effect of drift and heterogeneity may help explain irreversible flow and inconsistent results sometimes encountered in previous single‐well injection/recovery tracer tests. Numerical results indicate that in a hypothetical single‐well injection/recovery tracer test designed to demonstrate and measure dual‐porosity characteristics in a fractured dolomite, the simultaneous effects of drift and heterogeneity sometimes yields responses similar to those anticipated in a homogeneous dual‐porosity formation. In these cases, tracer recovery could provide a false indication of the occurrence of matrix diffusion. Shortening the shut‐in period between injection and recovery periods may make the test less sensitive to drift. Using multiple tracers having different diffusion characteristics, multiple tests having different pumping schedules, and testing the formation at more than one location would decrease the ambiguity in the interpretation of test data.
Inverse Models: A Necessary Next Step in Ground‐Water Modeling
Authors: Poeter Eileen P. , Hill Mary C.
Published: August 2005

Inverse models using, for example, nonlinear least‐squares regression, provide capabilities that help modelers take full advantage of the insight available from ground‐water models. However, lack of information about the requirements and benefits of inverse models is an obstacle to their widespread use. This paper presents a simple ground‐water flow problem to illustrate the requirements and benefits of the nonlinear least‐squares regression method of inverse modeling and discusses how these attributes apply to field problems. The benefits of inverse modeling include: (1) expedited determination of best fit parameter values; (2) quantification of the (a) quality of calibration, (b) data shortcomings and needs, and (c) confidence limits on parameter estimates and predictions; and (3) identification of issues that are easily overlooked during nonautomated calibration.
The Role of Bedding Fractures in the Hydrogeology of Sedimentary Bedrock—Evidence from the Newark Basin, New Jersey
Authors: Michalski Andrew , Britton Richard
Published: August 2005

Bedding‐plane partings, particularly those enlarged by stress relief, tend to provide principal ground‐water flow pathways which are often overlooked. In order to identify them, the use of a proper conceptual model of the bedrock aquifer system and appropriate methods of hydrogeologic characterization are necessary. Several pervasive bedding partings were identified at a study site located within a dipping sequence of mudstone and shales, typical of the Passaic Formation in the Newark Basin of New Jersey. These bedding fractures constitute the discrete aquifer units of a multiunit, leaky bedrock aquifer system. One such unit of exceptional transmissivity (the “Raritan unit”) was identified and selected for a detailed characterization. Results of three short‐duration pumping tests verified the continuity and relatively uniform transmissivity of the Raritan unit over distances exceeding 1,500 feet. Significant hydrochemical differences between the various aquifer units at this 100‐acre site were found to be consistent with the multiunit structure of the bedrock aquifer system. A similar pattern can be observed in regional hydrochemical data recently published by others. The principal finding, that a few bedding fractures dominate ground‐water flow at many sites in the region, has a major implication on hydrogeologic characterization requirements for the water supply, well‐head protection, and aquifer remediation projects in the Newark Basin and similar areas of sedimentary bedrock.
Fractured‐Aquifer Hydrogeology from Geophysical Logs; The Passaic Formation, New Jersey
Authors: Morin Roger H. , Carleton Glen B. , Poirier Stéphane
Published: August 2005

The Passaic Formation consists of gradational sequences of mudstone, siltstone, and sandstone, and is a principal aquifer in central New Jersey. Ground‐water flow is primarily controlled by fractures interspersed throughout these sedimentary rocks and characterizing these fractures in terms of type, orientation, spatial distribution, frequency, and transmissivity is fundamental towards understanding local fluid‐transport processes. To obtain this information, a comprehensive suite of geophysical logs was collected in 10 wells roughly 46 m in depth and located within a .05 km2 area in Hopewell Township, New Jersey. A seemingly complex, heterogeneous network of fractures identified with an acoustic televiewer was statistically reduced to two principal subsets corresponding to two distinct fracture types: (1) bedding‐plane partings and (2) high‐angle fractures. Bedding‐plane partings are the most numerous and have an average strike of N84°W and dip of 20° N. The high‐angle fractures are oriented subparallel to these features, with an average strike of N79° E and dip of 71° S, making the two fracture types roughly orthogonal. Their intersections form linear features that also retain this approximately east‐west strike. Inspection of fluid temperature and conductance logs in conjunction with flow meter measurements obtained during pumping allows the transmissive fractures to be distinguished from the general fracture population. These results show that, within the resolution capabilities of the logging tools, approximately 51 (or 18 percent) of the 280 total fractures are water producing. The bedding‐plane partings exhibit transmissivities that average roughly 5 m2/day and that generally diminish in magnitude and frequency with depth. The high‐angle fractures have average transmissivities that are about half those of the bedding‐plane partings and show no apparent dependence upon depth. The geophysical logging results allow us to infer a distinct hydrogeologic structure within this aquifer that is defined by fracture type and orientation. Fluid flow near the surface is controlled primarily by the highly transmissive, subhorizontal bedding‐plane partings. As depth increases, the high‐angle fractures apparently become more dominant hydrologically.
Plume Distortion and Apparent Attenuation Due to Concentration Averaging in Monitoring Wells
Authors: Martin‐Hayden James M. , Robbins Gary A.
Published: August 2005

Mathematical models that simulate common monitoring well sampling demonstrate the distortions that vertical concentration averaging causes during the mapping and modeling of an idealized, three‐dimensional contaminant plume emanating from a simple source of constant solute concentration. The apparent extent of the plume, mapped using simulations of a regular grid of screened monitoring wells, ranged from a worst case of 0% of the original plume area for long screens (4 m) in a low‐permeability formation to 90% for short screens (1 m) in a high‐permeability formation. When well design and purging procedure were inconsistent among wells, the mapped plume exhibited spurious directional skewing, bifurcation, zones of low concentration, intermittent sources, or multiple sources. Although the study plume was not retarded, calibrating a transport model to the monitoring well data resulted in retardation factors of up to 23. If first‐order decay was assumed, the apparent decay constant was found to be as much as 1.8 × 10‐7 sec‐1 (T1/2= 45 days). Apparent retardation or decay was inconsistent from well to well, depending on the saturated screen length, the degree of screen desaturation during purging, and the distance from the source. The study indicates that the quantitative assessment of contaminant distributions and transport processes requires discrete vertical sampling in the common situation where concentrations vary sharply with depth, even in the most ideal hydrogeologic environment. If screened monitoring wells are used, screen lengths and placements should be appropriate to the contamination situation being assessed and inherent biases must be considered. Even so, vertical concentration averaging biases and the resulting inconsistencies can result in highly misleading evaluations of ground‐water contamination problems.
Quantifying the Effects of Well Development in Unconsolidated Material
Authors: Wendling Gilles , Chapuis Robert P. , Gill Denis E.
Published: August 2005

A fully instrumented physical model was designed and built to reproduce development by surging and monitor its effects during surging and after development. The model simulates a horizontal layer in a confined aquifer with control of vertical overburden pressure. An automatic apparatus produced development by surging in successive phases up to 24 hours. Aquifer tests in steady‐state conditions were performed between successive phases. The paper reports the main results of three experiments performed with Johnson screens 200 mm in diameter; they had slot sizes between the D54 and D70 of the aquifer soil. This soil was placed under controlled conditions, and initial homogeneity was obtained as confirmed by initial control tests. Pore pressures (and thus hydraulic heads) were continuously monitored during development phases and aquifer tests by 22 electronic piezometers at distances between 0 and 1 m from the screen. These piezometers measured water pressures every 0.1 s when required. Solid particles passing through the screen were recovered to study the solid yield and the gradation of particles. Positive and negative values of local gradients reached values up to 400 close to the screen at the beginning of development and decreased with time of development. These high values produced high seepage forces displacing particles in the aquifer. The well yield was increased by a factor of 6 after development. These model test results confirmed empirical criteria on entrance velocity, internal stability criteria, and field values of “sand” production by development. In addition, they enabled a quantification of skin effects to be considered in interpreting an aquifer test.
Delineating the Chemical Composition of the Salinity Source for Saline Ground Waters: An Example from East‐Central Concordia Parish, Louisiana
Authors: Stoessell Ronald K.
Published: August 2005

In the absence of significant water‐rock interaction, the chemical composition of a saline brine polluting a ground‐water aquifer can be delineated from conservative‐mixing relations. An example is the Mississippi River Alluvium Aquifer, within Angelina Plantation in east‐central Concordia Parish, Louisiana, which has high ground‐water salinities, of unknown origin.

Chloride concentrations are linearly correlated with concentrations of major cations and with bromide and iodide. These relationships are consistent with mixing of fresh water with a saline source of either a single brine or multiple brines of similar composition and with a lack of significant water‐rock interactions in the ground water during and after mixing.

The saline source has the following predicted mass ratios for a brine containing 80,000 ppm CI: Na/CI, 0.58; Br/CI, 0.0013; I/CI, 0.00028; Sr/CI, 0.0026; K/CI, 0.0035; CaCI, 0.035; and Mg/CI, 0.0080. The predicted 87Sr/86Sr ratio is 0.70805. The mass ratios fall within the range of Tertiary oil‐field brines in the Louisiana Gulf Coast. The major reservoir for hydrocarbon production in the area is the Wilcox Formation; however, the “predicted” source brine is unlikely to be a “pure” Wilcox brine from Concordia Parish. Ratios of 87Sr/86 Sr are usually lower and bromide, iodide, and calcium concentrations are always lower in the Wilcox brines than predicted in the saline source.
Analyses of Slug Tests and Hydraulic Conductivity Variations in the Near Field of a Two‐Well Tracer Experiment Site
Authors: Mas‐Pla Josep , Jim Yeh T.‐C. , Williams Thomas M. , McCarthy John F.
Published: August 2005

Three‐hundred eight slug tests were conducted in a 5 × 5 m area in a coastal, sandy aquifer at the Georgetown site in South Carolina to characterize three‐dimensional aquifer heterogeneity. Methods developed by Hvorslev, Bouwer and Rice, and Cooper et al. were employed to estimate hydraulic conductivity values from the slug test data. These three methods produced similar spatial distributions of the hydraulic conductivity but quite different values. Overall, the method of Cooper et al. produces higher conductivity values in high permeability zones but lower values in low permeability areas than the Hvorslev method. Variances of the natural log of conductivity values derived from Hvorslev's and Bouwer and Rice's methods agree with those in the other aquifers under similar depositional environments. However, the variance calculated for the data based on the method of Cooper et al. appears unreasonably large. Despite these differences, histograms of the three sets of conductivity values exhibit bimodal distributions, reflecting stratification of the aquifer. Geostatistical analyses show that correlation lengths and statistical anisotropy of the hydraulic conductivity spatial structure varies with depth.