bluuebunny/crossref_metadata_2025 huggingface dataset:
Selecting MODFLOW Cell Sizes for Accurate Flow Fields
Authors: Haitjema Henk , Kelson Vic , de Lange Wim
Published: December 2005

Contaminant transport models often use a velocity field derived from a MODFLOW flow field. Consequently, the accuracy of MODFLOW in representing a ground water flow field determines in part the accuracy of the transport predictions, particularly when advective transport is dominant. We compared MODFLOW ground water flow rates and MODPATH particle traces (advective transport) for a variety of conceptual models and different grid spacings to exact or approximate analytic solutions. All of our numerical experiments concerned flow in a single confined or semiconfined aquifer. While MODFLOW appeared robust in terms of both local and global water balance, we found that ground water flow rates, particle traces, and associated ground water travel times are accurate only when sufficiently small cells are used. For instance, a minimum of four or five cells are required to accurately model total ground water inflow in tributaries or other narrow surface water bodies that end inside the model domain. Also, about 50 cells are needed to represent zones of differing transmissivities or an incorrect flow field and (locally) inaccurate ground water travel times may result. Finally, to adequately represent leakage through aquitards or through the bottom of surface water bodies it was found that the maximum allowable cell dimensions should not exceed a characteristic leakage length λ, which is defined as the square root of the aquifer transmissivity times the resistance of the aquitard or stream bottom. In some cases a cell size of one‐tenth of λis necessary to obtain accurate results.
Numerical Simulation of a Natural Attenuation Experiment with a Petroleum Hydrocarbon NAPL Source
Authors: Brauner J. Steven , Widdowson Mark A.
Published: December 2005

A three‐dimensional solute transport model with biological reactions is presented for simulating the natural attenuation study (NATS) at the Columbus Air Force Base in eastern Mississippi. NATS consisted of the release of a petroleum‐based nonaqueous phase liquid (NAPL) and subsequent monitoring of BTEX (benzene, toluene, ethylbenzene, p‐xylene), naphthalene, decane, and bromide in a shallow, unconfined aquifer. Conceptual and mathematical models were developed for NAPL source release, sequential aerobic/anaerobic biodegradation, and sorption during NATS. A multiple species, solute transport code (SEAM3D) was used to simulate fully three‐dimensional transport and aerobic, nitrate‐reducing, ferrogenic, and methanogenic hydrocarbon biodegradation. Simulation results matched individual BTEX concentration distributions collected five‐ and nine‐months following NAPL release. SEAM3D mass‐balance calculations at t = nine months indicated that 49% of the hydrocarbon mass that dissolved into the aqueous phase was consumed by biodegradation, 13% of this mass was sorbed, and the remaining 38% was present in the aqueous phase. Mass calculations at t = nine months further indicated that aerobic biodegradation accounted for the majority of hydrocarbon biodegradation (46% of the biodegraded mass), followed by ferrogenesis (28%), nitrate‐reduction (21%), and methanogenesis (5%). Model results were particularly sensitive to the NAPL release rate, the initial ferric iron (Fe[III]) concentration, hydrocarbon utilization rates, initial condition for the anaerobic microbial populations, and dispersivity.
Analytical Solution for Subsurface Gas Flow to a Well Induced by Surface Pressure Fluctuations
Authors: Rossabi Joseph , Falta Ronald W.
Published: December 2005

A simple analytical model is presented for predicting subsurface gas flow to a vadose‐zone well in response to atmospheric pressure fluctuations (barometric pumping). The effective radial permeability (kr) in the vicinity of the well is determined during model calibration using less than two weeks worth of data. By combining the flow solution with a solution for the vertical gas pressure, only atmospheric pressure data are required to predict the induced flow through a well. The ability to quantitatively predict naturally induced flow in vadose‐zone wells by simple and inexpensive measurements is invaluable for systems using barometric pumping for remediation.
Back Diffusion of Chlorinated Solvent Contaminants from a Natural Aquitard to a Remediated Aquifer Under Well‐Controlled Field Conditions: Predictions and Measurements
Authors: Liu Chongxuan , Ball William P.
Published: December 2005

Vertical profiles of tetrachloroethene (or perchloroethylene, PCE) and trichloroethene (TCE) were used to validate a diffusion process in a natural aquitard at Dover Air Force Base, Delaware. PCE and TCE distributions in the aquitard underlying an unconfined aquifer were sampled from core tubes obtained at four times over the course of a 35–month field investigation within “test cells” that were isolated from the surrounding ground water by means of grout‐sealed steel sheetpile barriers (Mackay et al. 2000). For the final 23 months of this period, boundary conditions at the aquiferJaquitard interface were such that a “back diffusion” of contaminants from the aquitard was induced. Modeling predictions of concentration changes were made on the basis of the earliest coring results and an assumption of sorption‐retarded diffusion and using laboratory information about sorption and diffusion characteristics of the media. The predictive modeling was complicated by the fact that “initial” and “final” PCE and TCE distributions in the aquitard were measured at different (albeit proximate) coring locations, such that results reflect spatial variations in aquitard characteristics. This problem was solved by means of an inverse interpretation that involved spatial “translation” of observed profiles on the basis of the laboratory characterizations and assuming a common aquifer‐side contaminant history. Predictions indicated substantial change in PCE and TCE concentrations within the upper aquitard (near the aquiferJaquitard interface) and the development of a back‐diffusion profile up into the aquifer. Modeling also predicted comparatively minor profile changes in the deeper aquitard, and especially in the deep layer where sorption was strongest. All of these predicted effects were observed in the coring results. Although not exact, the agreement between predictions and observations was sufficiently good to justify the basic tenets of the diffusion model and to support a conclusion that major processes of advection and Jor transformation were unimportant within the 35‐month time scale of this work.
Kriging Water Levels with a Regional‐Linear and Point‐Logarithmic Drift
Authors: Tonkin Matthew J. , Larson Steven P.
Published: December 2005

Ground water levels measured in the vicinity of pumping wells are kriged using a regional‐linear and point‐logarithmic drift, the latter derived from the approximation to the Theis equation for drawdown in response to a pumping well. Kriging is widely used throughout the hydrogeologic discipline, most commonly as the preferred method for constructing gridded hydrogeologic datasets suitable for contouring. Residuals arising from using the most common (linear) drift to krige water levels in the vicinity of extraction wells often indicate large local departures from the linear drift, which correlate with areas of drawdown. The combined regional‐linear and point‐logarithmic drift accounts for these drawdowns using a logarithmic approximation for the curvature of the poten‐tiometric surface. The drift model approximates the principal physical processes that govern ground water flow and ultimately govern the autocorrelation of ground water elevation data. This approach produces maps of contoured water levels that more realistically represent physical conditions and allow for improved interpretation of measured water‐level data by including features and information known to be present. Additional benefits include an improved estimate of the regional (background) hydraulic gradient and generation of an approximately flow‐conserved grid suitable for two‐dimensional particle tracking.
The Water Budget Myth Revisited: Why Hydrogeologists Model
Authors: Bredehoeft John D.
Published: December 2005

Within the ground water community, the idea persists that if one can estimate the recharge to a ground water system, one then can determine the size of a sustainable development. Theis addressed this idea in 1940 and showed it to be wrong‐yet the myth continues. The size of a sustainable ground water development usually depends on how much of the discharge from the system can be “captured” by the development. Capture is independent of the recharge; it depends on the dynamic response of the aquifer system to the development. Ground water models were created to study the response dynamics of ground water systems; it is one of the principal reasons hydrogeologists model.
A Subregional‐Scale Method to Assess Aquifer Vulnerability to Pesticides
Authors: Schlosser Stephanie A. , McCray John E. , Murray Kyle E. , Austin Brad
Published: December 2005

A method to predict aquifer vulnerability to pesticide contamination at the subregional scale was developed. The assessment method was designed to incorporate relevant hydrologic and pesticide‐transport information and to use generally available data. The method assumes steady‐state advection of pesticides in the vadose zone, including sorption and biological decay. The solution is presented as a vulnerability index (VI) that increases as the aquifer vulnerability increases. The hydrologic input data for the VI model are obtained from the soil survey geographic database. Pesticides were grouped into three leachability classes using a leachabiiity ratio (half‐life divided by organic carbon partition coefficient). Pesticide transformation is assumed to occur in the surface layer. The influence of vertical transport in the remainder of the vadose zone has been incorporated by applying a multiplying factor to the VI that varies with depth to ground water. Hydraulic conductivity is used as a surrogate for soil‐water velocity for practical purposes. The performance of the VI model is evaluated using ground water data from Weld County, Colorado. The model is demonstrated to be successful at predicting relative vulnerability, defined as the magnitude of pesticide concentration and percent of wells in a unit that exhibit a pesticide detection. Areas of low, medium, and high vulnerability are assigned. Results indicate that the vulnerability classifications are most dependent on the leachability ratio, hydraulic conductivity, and organic carbon content.
Role of Ground Water in Geomorphology, Geology, and Paleoclimate of the Southern High Plains, USA
Authors: Wood Warren W.
Published: December 2005

Study of ground water in the Southern High Plains is central to an understanding of the geomorphology, deposition of economic minerals, and climate change record in the area. Ground water has controlled the course of the Canadian and Pecos rivers that isolated the Southern High Plains from the Great Plains and has contributed significantly to the continuing retreat of the westward escarpment. Evaporative and dissolution processes are responsible for current plateau topography and the development of the signature 20,000 small playa basins and 40 to 50 large saline lake basins in the area. In conjunction with eolian processes, ground water transport controls the mineralogy of commercially valuable mineral deposits and sets up the distribution of fine efflorescent salts that adversely affect water quality. As the water table rises and retreats, lunette and tufa formation provides valuable paleoclimate data for the Southern High Plains. In all these cases, an understanding of ground water processes contributes valuable information to a broad range of geological topics, well beyond traditional interest in water supply and environmental issues.
Is the Forward Problem of Ground Water Hydrology Always Well Posed?
Authors: Valota Giorgio , Giudici Mauro , Parravicini Guido , Ponzini Giansilvio , Romano Emanuele
Published: December 2005

Complex aquifer systems are often modeled with quasi‐three‐dimensional models, which consider two‐dimensional horizontal flow in the aquifers and one‐dimensional vertical flow through aquitards. When the aquifer system consists of a phreatic aquifer and one or more semiconfined aquifers connected by aquitards, the discrete model consists of a nonlinear system of algebraic equations, because the transmissivity of the phreatic aquifer depends on the phreatic head. If the water extraction is very high, the phreatic aquifer can be depleted and the equations of the model must be modified accordingly. There are not simple and general criteria to state if the phreatic aquifer is depleted before solving the system of equations. Therefore, the iterative procedures (e.g., relaxation methods), used to find the solution to the forward problem, must handle these particular conditions and can suffer several problems of convergence. These problems can be caused by the choice of the initial head values or of the relaxation coefficient of the iterative algorithms; however, they can also be caused by the nonexistence or nonuniqueness of the solution to the system of nonlinear equations. The study of existence and uniqueness of the general problem is very difficult and, therefore, we consider a simplified problem, for which the discrete model can be handled analytically. The results of the numerical experiments show that the solution to the forward problem can be nonunique. Only for some cases it is possible to invoke physical arguments to eliminate tentative solutions.
Spatial and Temporal Variability of Ground Water Recharge in Central Australia: A Tracer Approach
Authors: Harrington Glenn A. , Cook Peter G. , Herczeg Andrew L
Published: December 2005

Two environmental tracer methods are applied to the Ti‐Tree Basin in central Australia to shed light on the importance of recharge from floodouts of ephemeral rivers in this arid environment. Ground water carbon‐14 concentrations from boreholes are used to estimate the average recharge rate over the interval between where the ground water sample first entered the saturated zone and the bore. Environmental chloride concentrations in ground water samples provide estimates of the recharge rate at the exact point in the landscape where the sample entered the saturated zone. The results of the two tracer approaches indicate that recharge rates around one of the rivers and an extensive flood‐plain are generally higher than rates of diffuse recharge that occurs in areas of lower topographic relief. Ground water 2H/1H and 18O/16O compositions are all depleted in the heavier isotopes (δ2H = ‐67%0 to ‐50%0; 518O = ‐9.2%0 to ‐5.7%0) compared with the long‐term, amount‐weighted mean isotopic composition of rainfall in the area (δ2H = ‐33.8%0; δ18O = ‐6.3%0). This indicates that recharge throughout the basin occurs only after intense rainfall events of at least 150 to 200 mm/month. Finally, a recharge map is developed to highlight the spatial extent of the two recharge mechanisms. Floodout recharge to the freshest ground water (TDS <1000 mg/L) is ∼1.9 mm/year compared with a mean recharge rate of ∼0.2 mm/year to the remainder of the basin. These findings have important implications for management of the ground water resource.
A Global Experimental System Approach of Karst Springs1 Hydrographs and Chemographs
Authors: Grasso D. Alessandro , Jeannin Pierre‐Yves
Published: December 2005

Investigation techniques for karst flow systems are based mainly on the study of different signals leaving the system caused by natural or induced external influences. Each signal represents one of the systems outputs (e.g., hydraulic, chemical, physical, or isotopic responses) that reflect the characteristics of the entire system.

In this paper, we present a method to infer information about the structure of karst systems. It is based on a simultaneous analysis of chemical and hydraulic responses. Beside the classical piston flow at the beginning of a flood pulse, we define a chemically based recession flow phase. During this phase, field data show that the concentration of total dissolved solids can be considered as an exponential function of the logarithm of flow. This relationship allows two parameters to be defined, one of which is dependent on the structure and degree of development of the karst conduit network, the other is dependent mainly on bioclimatic factors. Data collected from seven karst springs are used to support ideas introduced in the paper.
Errors with Small Volume Elastic Seepage Meter Bags
Authors: Schincariol Robert A. , McNeil John D.
Published: December 2005

The use of small volume elastic collection bags (condoms) has become popular in seepage meter studies in recent years, despite minimal field or laboratory validation of their use and, specifically, the impact of their elasticity on seepage measurements. A laboratory study was initiated after field results using small elastic collection bags produced seepage data that did not correlate with hydrometric data. The laboratory data demonstrate that condoms undergo significant mechanical relaxation during seepage measurement times typically observed in field settings. Unlike conventional nonelastic collection bags, which mechanically relax over several minutes, the condoms suffered from a slow mechanical relaxation or equilibration. Over nine hours, condoms gained 43 mL of water, ‐50% of maximum workable volume (between mechanical relaxation effect and elastic limit), under stagnant flow conditions. This long‐term equilibration invalidates simple subtraction of equilibration volumes from collection volumes as a correction technique. Previously published studies using flexible small‐volume elastic measurement bags (condoms) have not reported a mechanical relaxation effect. Overall, because the condom's small workable volume and inherent variability, we would not recommend any small‐volume elastic measurement bags for quantitative seepage measurements.
Influence of Fracture Anisotropy on Ground Water Ages and Chemistry, Valley and Ridge Province, Pennsylvania
Authors: Burton William C. , Plummer L. Niel , Busenberg Eurybiades , Lindsey Bruce D. , Gburek William J.
Published: December 2005

Model ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium‐3 (3H/3He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding‐plane parting and east striking, steeply dipping axial‐plane spaced cleavage. In the area of the piezometer arrays, which trend north‐south on the north limb of the fold, north dipping bedding‐plane parting is a more dominant fracture set than is steeply south dipping axial‐plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding‐plane parting has younger 3H/3He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.
A Field Demonstration of the Simulation Optimization Approach for Remediation System Design
Authors: Zheng Chunmiao , Wang P. Patrick
Published: December 2005

While significant progress has been made in the theoretical development of the simulation/optimization (S/O) approach for ground water remediation design, its application to large, field‐scale problems has remained limited. To demonstrate the applicability and usefulness of the S/O approach under real field conditions, an optimization demonstration project was conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts, involving the design of a pump‐and‐treat system for the containment and cleanup of a large trichloroethylene (TCE) plume. The optimization techniques used in this study are based on evolutionary algorithms coupled with a response function approach for greater computational efficiency. The S/O analysis was performed parallel to a conventional trial‐and‐error analysis based on simulation alone. The results of this study demonstrate that not only would it be possible to remove more TCE mass under the same amount of pumping assumed in the trial‐and‐error design, but also substantial cost savings could be achieved by reducing the number of wells needed and adapting dynamic pumping. In spite of the large model size of more than 500,000 nodes and a long planning horizon of 30 years, the optimization modeling was carried out successfully on desktop PCs. This field demonstration project clearly illustrates the potential benefits of applying optimization techniques in remediation system design.
A Simple Correction for Slug Tests in Small‐Diameter Wells
Authors: Butler James J.
Published: December 2005

A simple procedure is presented for correcting hydraulic conductivity (K) estimates obtained from slug tests performed in small‐diameter installations screened in highly permeable aquifers. Previously reported discrepancies between results from slug tests in small‐diameter installations and those from tests in nearby larger‐diameter wells are primarily a product of frictional losses within the small‐diameter pipe. These frictional losses are readily incorporated into existing models for slug tests in high‐K aquifers, which then serve as the basis of a straightforward procedure for correcting previously obtained K estimates. A demonstration of the proposed procedure using data from a series of slug tests performed in a controlled field setting confirms the validity of the approach. The results of this demonstration also reveal the detailed view of spatial variations in K that can be obtained using slug tests in small‐diameter installations.
Measurement System for Systematic Hydrological Characterization of Unsaturated Fractured Welded Tuff in a Mined Underground Tunnel
Authors: Cook P.J. , Salve R. , Freifeld B.M. , Tsang Y.T.
Published: December 2005

A field investigation of unsaturated flow through a lithophysal unit of fractured welded tuff containing litho‐physal cavities has been initiated. To characterize flow in this spatially heterogeneous medium, a systematic approach has been developed to perform tests in boreholes drilled at regular intervals in an underground tunnel (drift). The purpose of the testing is to quantify the amounts of water seeping into the drift versus the amount of water moving around the drift when released into boreholes at many equidistant locations along the drift. In this paper, we describe the test equipment system that has been built for this purpose. Because the field‐scale measurements—of liquid flow in the unsaturated, fractured rocks—require continuous testing for periods of days to weeks, the control of test equipment has been fully automated, allowing operation with no human presence at the field site. Preliminary results from the first set of tests indicate that, while the effects of evaporation on characterization of hydrological properties of the rock can be significant, these effects can be controlled and quantified. These tests give insight into the role of the cavities as potential storage during the initial transient flow prior to the breakthrough of water at the drift crown, as well as the role of connected fractures that provide the subsequent quasi‐steady flow. In addition to the stated purpose of realizing the flow partitioning, the results yield values for the effective porosity in the pathways for liquid flow in the regions tested thus far.
Geochemical Effects on Metals Following Permanganate Oxidation of DNAPLs
Authors: Crimi Michelle L , Siegrist Robert L.
Published: December 2005

The application of in situ chemical oxidation for dense, nonaqueous phase liquid (DNAPL) remediation requires delivery of substantial levels of oxidant chemicals into the subsurface to degrade target DNAPLs and to satisfy natural oxidant demand. This practice can raise questions regarding changes in subsurface conditions, yet information regarding potential effects, especially at the field scale, has been lacking. This paper describes an evaluation of the effects on metals associated with in situ chemical oxidation using potassium permanganate at Launch Complex 34 (LC34), Cape Canaveral Air Station, Florida. At LC34, high concentrations of permanganate (1 to 2 wt%) were injected into the subsurface as part of a demonstration of DNAPL remediation technologies. In a companion experimental effort at the Colorado School of Mines, field samples were characterized and laboratory batch and mini‐column studies were completed to assess effects of permanganate oxidation on metals in the subsurface one year after completion of the field demonstration. Results indicated there was potential for long‐term immobilization of a portion of introduced manganese and no treatment‐induced loss in subsurface permeability due to deposition of manganese oxides particles, which are a product of the oxidation reactions. Permanganate treatment did cause elevated manganese, chromium, and nickel concentrations in site ground water within the treated region. Some of these metals effects can be attenuated during downgradient flow through uncontaminated and untreated aquifer sediments.
A Critique of the Internal Tracer Method for Estimating Contaminant Degradation Rates
Authors: Martian Peter , Sorenson Kent S. , Peterson Lance N.
Published: December 2005

The internal tracer method for estimating contaminant degradation rates separates the attenuation effects not associated with degradation by using a codisposed recalcitrant internal tracer to normalize the degrading contaminant concentration. The remaining attenuation between the internal tracer and degrading contaminant is attributed to degradation and the degradation rate half‐life is estimated from the first‐order decay equation. An analytical solution of the advection‐ dispersion equation was used to evaluate flow‐and‐transport conditions that could result in incorrect estimates of contaminant degradation rate constants. Flow‐and‐transport characteristics that result in overestimating degradation rates were of particular interest because the internal tracer method often used to demonstrate natural attenuation can achieve remedial objectives. The analytical solution was also used to estimate the magnitude of error associated with using the internal tracer method at an example site and to explain different degradation rates estimated using tracers with different decay rate constants.
Comparison of Heat and Bromide as Ground Water Tracers Near Streams
Authors: Constantz Jim , Cox Marisa H. , Su Grace W.
Published: December 2005

Heat and bromide were compared as tracers for examining stream/ground water exchanges along the middle reaches of the Santa Clara River, California, during a 10‐hour surface water sodium bromide injection test. Three cross sections that comprise six shallow (<1 m) piezometers were installed at the upper, middle, and lower sections of a 17 km long study reach, to monitor temperatures and bromide concentrations in the shallow ground water beneath the stream. A heat and ground water transport simulation model and a closely related solute and ground water transport simulation model were matched up for comparison of simulated and observed temperatures and bromide concentrations in the streambed. Vertical, one‐dimensional simulations of sediment temperature were fitted to observed temperature results, to yield apparent streambed hydraulic conductivities in each cross section. The temperature‐based hydraulic conductivities were assigned to a solute and ground water transport model to predict sediment bromide concentrations, during the sodium bromide injection test. Vertical, one‐dimensional simulations of bromide concentrations in the sediments yielded a good match to the observed bromide concentrations, without adjustment of any model parameters except solute dispersivities. This indicates that, for the spatial and temporal scales examined on the Santa Clara River, the use of heat and bromide as tracers provide comparable information with respect to apparent hydraulic conductivities and fluxes for sediments near streams. In other settings, caution should be used due to differences in the nature of conservative (bromide) versus nonconservative (heat) tracers, particularly when preferential flowpaths are present.
Laboratory and Numerical Investigation of Flow and Transport Near a Seepage‐Face Boundary
Authors: Simpson MJ. , Clement T.P. , Gallop T.A.
Published: December 2005

Laboratory and numerical modeling investigations were completed to study the unconfined ground water flow and transport processes near a seepage‐face boundary. The laboratory observations were made in a radial sand tank and included measurements of the height of the seepage face, flow velocity near the seepage face, travel time distribution of multiple tracer slugs, and streamlines. All the observations were reliably reproduced with a three‐dimensional, axi‐symmetric, variably saturated ground water flow model. Physical data presented in this work demonstrate and quantify the importance of three‐dimensional transport patterns within a seepage‐face zone. The results imply that vertically averaged flow models that employ Dupuit approximations might introduce error in the analysis of localized solute transport near a seepage‐face boundary. The experimental dataset reported in this work will also be of interest for those who are attempting to validate a numerical algorithm for solving ground water and contaminant discharge patterns near a surface‐water boundary.
Analysis of Solute Transport in Flow Fields Influenced by Preferential Flowpaths at the Decimeter Scale
Authors: Zheng Chunmiao , Gorelick Steven M.
Published: December 2005

Several recent studies at the Macrodispersion Experiment (MADE) site in Columbus, Mississippi, have indicated that the relative preferential flowpaths and flow barriers resulting from decimeter‐scale aquifer heterogeneities appear to have a dominant effect on plume‐scale solute transport. Numerical experiments are thus conducted in this study to explore the key characteristics of solute transport in two‐dimensional flow fields influenced by decimeter‐scale preferential flowpaths. A hypothetical but geologically plausible network of 10 cm wide channels of high hydraulic conductivity is used to represent the relative preferential flowpaths embedded in an otherwise homogeneous aquifer. When the hydraulic conductivity in the channels is 100 times greater than that in the remaining portion of the aquifer, the calculated concentration distributions under three source configurations all exhibit highly asymmetrical, non‐Gaussian patterns. These patterns, with peak concentrations close to the source and extensive spreading downgradi‐ent, resemble that observed at the MADE site tracer tests. When the contrast between the channel and nonchannel hydraulic conductivities is reduced to 30:1 from 100:1, the calculated mass distribution curve starts to approach a Gaussian one with the peak concentration near the central portion of the plume. Additional analysis based on a fieldscale model demonstrates that the existence of decimeter‐scale preferential flowpaths can have potentially far‐reaching implications for ground water remediation. Failure to account for them in numerical simulation could lead to over‐estimation of the effectiveness of the remedial measure under consideration.
A Combinatorial Optimization Scheme for Parameter Structure Identification in Ground Water Modeling
Authors: Tsai Frank T‐C. , Sun Ne‐Zheng , Yen William W‐G.
Published: December 2005

This research develops a methodology for parameter structure identification in ground water modeling. For a given set of observations, parameter structure identification seeks to identify the parameter dimension, its corresponding parameter pattern and values. Voronoi tessellation is used to parameterize the unknown distributed parameter into a number of zones. Accordingly, the parameter structure identification problem is equivalent to finding the number and locations as well as the values of the basis points associated with the Voronoi tessellation. A genetic algorithm (GA) is allied with a grid search method and a quasi‐Newton algorithm to solve the inverse problem. GA is first used to search for the near‐optimal parameter pattern and values. Next, a grid search method and a quasi‐Newton algorithm iteratively improve the GA's estimates. Sensitivities of state variables to parameters are calculated by the sensitivity‐equation method. MODFLOW and MT3DMS are employed to solve the coupled flow and transport model as well as the derived sensitivity equations. The optimal parameter dimension is determined using criteria based on parameter uncertainty and parameter structure discrimination. Numerical experiments are conducted to demonstrate the proposed methodology, in which the true transmissivity field is characterized by either a continuous distribution or a distribution that can be characterized by zones. We conclude that the optimized transmissivity zones capture the trend and distribution of the true transmissivity field.
MODFLOW/MT3DMS‐Based Reactive Multicomponent Transport Modeling
Authors: Prommer H. , Barry D.A. , Zheng C.
Published: December 2005

This paper presents a three‐dimensional, MODFLOW/MT3DMS‐based reactive multicomponent transport model for saturated porous media. Based on a split‐operator technique, the model, referred to as PHT3D, couples the transport simulator MT3DMS and the geochemical modeling code PHREEQC‐2. Through the flexible, generic nature of PHREEQC‐2, PHT3D can handle a broad range of equilibrium and kinetically controlled reactive processes, including aqueous complexation, mineral precipitation/dissolution, and ion exchange. The diversity of potential applications is demonstrated through simulation of five existing literature benchmarks and a new three‐dimensional sample problem. The model might be applied to simulate the geochemical evolution of pristine and contaminated aquifers as well as their cleanup. The latter problem class includes the natural and enhanced attenuation/remediation schemes of a wide range of organic and inorganic contaminants. Processes/reactions not included in the standard PHREEQC‐2 database but typical for this type of application (e.g., NAPL dissolution, microbial growth/decay) can be defined and included via the extensible PHREEQC‐2 database file.
Use of an Electromagnetic Seepage Meter to Investigate Temporal Variability in Lake Seepage
Authors: Rosenberry Donald O. , Morin Roger H.
Published: December 2005

A commercially available electromagnetic flowmeter is attached to a seepage cylinder to create an electromagnetic seepage meter (ESM) for automating measurement of fluxes across the sediment/water interface between ground water and surface water. The ESM is evaluated through its application at two lakes in New England, one where water seeps into the lake and one where water seeps out of the lake. The electromagnetic flowmeter replaces the seepage‐meter bag and provides a continuous series of measurements from which temporal seepage processes can be investigated. It provides flow measurements over a range of three orders of magnitude, and contains no protruding components or moving parts. The ESM was used to evaluate duration of seepage disturbance following meter installation and indicated natural seepage rates resumed approximately one hour following meter insertion in a sandy lakebed. Lakebed seepage also varied considerably in response to lakebed disturbances, near‐shore waves, and rainfalls, indicating hydrologic processes are occurring in shallow lakebed settings at time scales that have largely gone unobserved.
Quantifying Ground Water Recharge at Multiple Scales Using PRMS and GIS
Authors: Cherkauer Douglas S.
Published: December 2005

Management of ground water resources requires a method to calculate demonstrably accurate recharge rates at local to regional scales using readily available information bases. Many methods are available to calculate recharge, but most are unable to satisfy all these conditions. A distributed parameter model is shown to meet the stated needs. Such models are input intensive, however, so a procedure to define most inputs from GIS and hydrogeological sources is presented. It simplifies the PRMS calibration observed streamflow hydrographs by reducing degrees of freedom from dozens to four.

For seven watersheds (60 to 500 km2), the GIS‐aided calibrations have average errors of 5% on recharge and 2% on total streamflow, verifying the accuracy of the process. Recharge is also calculated for 63 local‐scale subwater‐sheds (average size 37 km2). For the study area, calculated recharges average 11 cm/yr. Soil and rock conductivity, porosity, and depth to the water table are shown to be the physical properties which dominate the spatial variability of recharge. The model has been extended to uncalibrated watersheds where GIS and climatic information are known. It reproduces total annual discharge and recharge to within 9% and 10%, respectively, indicating the process can also be used to calculate recharge in ungauged watersheds. It has not been tested outside the study area, however.
Surface Flow Boundary Conditions in Modeling Land Subsidence Due to Fluid Withdrawal
Authors: Baú Domenico , Ferronato Massimiliano , Gambolati Giuseppe , Teatini Pietro
Published: December 2005

Land subsidence due to subsurface fluid (water, gas, oil) withdrawal is often predicted by either finite element or finite difference numerical models based on coupled poroelastic theory, where the soil is represented as a semi‐infinite medium bounded by the traction‐free (ground) surface. One of the variables playing a most important role on the final outcome is the flow condition used on the traction‐free boundary, which may be assumed as either permeable or impermeable. Although occasionally justified, the assumption of no‐flow surface seems to be in general rather unrealistic. A permeable boundary where the fluid pressure is fixed to the external atmospheric pressure appears to be more appropriate. This paper addresses the response, in terms of land subsidence, obtained with a coupled poroelastic finite element model that simulates a distributed pumping from a horizontal aquifer confined between two relatively impervious layers, and takes either a permeable boundary surface, i.e., constant hydraulic potential, or an impermeable boundary, i.e., a zero Neumann flow condition. The analysis reveals that land subsidence is rather sensitive to the flow condition implemented on the traction‐free boundary. In general, the no‐flow condition leads to an overestimate of the predicted ground surface settlement, which could even be 1 order of magnitude larger than that obtained with the permeable boundary.
Application of a Bayesian Approach to Stochastic Delineation of Capture Zones
Authors: Feyen L. , Dessalegn A.M. , De Smedt F. , Gebremeskel S. , Batelaan O.
Published: December 2005

This paper presents a Bayesian Monte Carlo method for evaluating the uncertainty in the delineation of well capture zones and its application to a wellfield in a heterogeneous, multiaquifer system. In the method presented, Bayes’ rule is used to update prior distributions for the unknown parameters of the stochastic model for the hydraulic conductivity, and to calculate probability‐based weights for parameter realizations using head residuals. These weights are then assigned to the corresponding capture zones obtained using forward particle tracking. Statistical analysis of the set of weighted protection zones results in a probability distribution for the capture zones. The suitability of the Bayesian stochastic method for a multilayered system is investigated, using the wellfield Het Rot at Nieuwrode, Belgium, located in a three‐layered aquifer system, as an example. The hydraulic conductivity of the production aquifer is modeled as a spatially correlated random function with uncertain parameters. The aquitard and overlying uncon‐fined aquifer are assigned random, homogeneous conductivities. The stochastic results are compared with deterministic capture zones obtained with a calibrated model for the area. The predictions of the stochastic approach are more conservative and indicate that parameter uncertainty should be taken into account in the delineation of well capture zones.
Gas‐Partitioning Tracer Test to Quantify Trapped Gas During Recharge
Authors: Heilweil Victor M. , Kip Solomon D. , Perkins Kim S. , Ellett Kevin M.
Published: December 2005

Dissolved helium and bromide tracers were used to evaluate trapped gas during an infiltration pond experiment. Dissolved helium preferentially partitioned into trapped gas bubbles, or other pore air, because of its low solubility in water. This produced observed helium retardation factors of as much as 12 relative to bromide. Numerical simulations of helium breakthrough with both equilibrium and kinetically limited advection/dispersion/retardation did not match observed helium concentrations. However, better fits were obtained by including a decay term representing the diffusive loss of helium through interconnected, gas‐filled pores. Calculations indicate that 7% to more than 26% of the porosity beneath the pond was filled with gas. Measurements of laboratory hydraulic properties indicate that a 10% decrease in saturation would reduce the hydraulic conductivity by at least one order of magnitude in the well‐sorted sandstone, but less in the overlying soils. This is consistent with in situ measurements during the experiment, which show steeper hydraulic gradients in sandstone than in soil. Intrinsic permeability of the soil doubled during the first six months of the experiment, likely caused by a combination of dissolution and thermal contraction of trapped gas. Managers of artificial recharge basins may consider minimizing the amount of trapped gas by using wet, rather than dry, tilling to optimize infiltration rates, particularly in well‐sorted porous media in which reintroduced trapped gas may cause substantial reductions in permeability. Trapped gas may also inhibit the amount of focused infiltration that occurs naturally during ephemeral flood events along washes and playas.
Reactive Barriers: Hydraulic Performance and Design Enhancements
Authors: Painter B.D.M.
Published: December 2005

The remediation of contaminated ground water is a multibillion‐dollar global industry. Permeable reactive barriers (PRBs) are one of the leading technologies being developed in the search for alternatives to the pump‐and‐treat method. Improving the hydraulic performance of these PRBs is an important part of maximizing their potential to the industry.

Optimization of the hydraulic performance of a PRB can be defined in terms of finding the balance between capture, residence time, and PRB longevity that produces a minimum‐cost acceptable design. Three‐dimensional particle tracking was used to estimate capture zone and residence time distributions. Volumetric flow analysis was used for estimation of flow distribution across a PRB and in the identification of flow regimes that may affect the permeability or reactivity of portions of the PRB over time. Capture zone measurements extended below the base of partially penetrating PRBs and were measured upgradient from the portion of aquifer influenced by PRB emplacement.

Hydraulic performance analysis of standard PRB designs confirmed previously presented research that identified the potential for significant variation in residence time and capture zone. These variations can result in the need to oversize the PRB to ensure that downgradient contaminant concentrations do not exceed imposed standards. The most useful PRB design enhancements for controlling residence time and capture variation were found to be customized downgradient gate faces, velocity equalization walls, deeper emplacement of the funnel than the gate, and careful manipulation of the hydraulic conductivity ratio between the gate and the aquifer.
Ground Water Whirls
Authors: Hemker Kick , van den Berg Elmer , Bakker Mark
Published: December 2005

Numerical experiments with steady‐state ground water flow models show that spiraling flow lines occur in layered aquifers that have different anisotropic horizontal hydraulic conductivities in adjacent layers. Bundles of such flow lines turning in the same direction can be referred to as ground water whirls. An anisotropic layered block in a field of uniform horizontal flow results in one or more whirls with their axes in the uniform flow direction. The number of whirls depends on the number of interfaces between layers with different anisotropic properties. For flow to a well in an aquifer consisting of two anisotropic layers, with perpendicular major principal directions, whirls are found to occur in quadrants that are bounded by the principal directions of the hydraulic conductivity. The combined effect of flow to a well and a layered anisotropy implies that a single well in a system with a single anisotropic layer within an otherwise isotropic aquifer causes eight whirls. All adjacent whirls rotate in opposite directions.
Estimation of Anaerobic Biodegradation Rate Constants at MGP Sites
Authors: Lewandowski Gordon , Mortimer Georgene
Published: December 2005

Field data at six former manufactured gas plant sites in New Jersey were used to estimate the biodegradation rate constants for the anaerobic processes naturally occurring within the ground water contaminant plumes (primarily iron and sulfate reduction). Those rate constants turned out to be about an order of magnitude smaller than values reported for the same contaminants (primarily benzene and naphthalene) at fuel sites. At four of the sites, there appeared to be sufficient electron acceptor present to eventually degrade the contaminants in the plume. However, the presence of nonaqueous phase liquids tends to offset that capacity by continuing to act as a source of contaminants that can dissolve in the ground water.