bluuebunny/crossref_metadata_2025 huggingface dataset:
From Models to Performance Assessment: The Conceptualization Problem
Authors: Bredehoeft John D.
Published: December 2005

Today, models are ubiquitous tools for ground water analyses. The intent of this paper is to explore philosophically the role of the conceptual model in analysis. Selection of the appropriate conceptual model is an a priori decision by the analyst. Calibration is an integral part of the modeling process. Unfortunately a wrong or incomplete conceptual model can often be adequately calibrated; good calibration of a model does not ensure a correct conceptual model. Petroleum engineers have another term for calibration; they refer to it as history matching. A caveat to the idea of history matching is that we can make a prediction with some confidence equal to the period of the history match. In other words, if we have matched a 10‐year history, we can predict for 10 years with reasonable confidence; beyond 10 years the confidence in the prediction diminishes rapidly. The same rule of thumb applies to ground water model analyses. Nuclear waste disposal poses a difficult problem because the time horizon, 1000 years or longer, is well beyond the possibility of the history match (or period of calibration) in the traditional analysis. Nonetheless, numerical models appear to be the tool of choice for analyzing the safety of waste facilities. Models have a well‐recognized inherent uncertainty. Performance assessment, the technique for assessing the safety of nuclear waste facilities, involves an ensemble of cascading models. Performance assessment with its ensemble of models multiplies the inherent uncertainty of the single model. The closer we can approach the idea of a long history with which to match the models, even models of nuclear waste facilities, the more confidence we will have in the analysis (and the models, including performance assessment). This thesis argues for prolonged periods of observation (perhaps as long as 300 to 1000 years) before a nuclear waste facility is finally closed.
Ground Water/Surface Water Interaction in a Fractured Rock Aquifer
Authors: Oxtobee Jaime P.A. , Novakowski Kent S.
Published: December 2005

In a recent field study of ground water/surface water interaction between a bedrock stream and an underlying fractured rock aquifer, it was determined that the majority of ground water discharge occurred through sparsely located vertical fractures. In this paper, the dominant mechanisms governing ground water/surface water exchange in such an environment are investigated using a numerical model. The study was conducted using several conceptual models based on the field study results. Although the field results provided the motivation for the modeling study, it was not intended to match modeling and field results directly. In addition, the extent of capture zones for discharging or recharging fractures was explored. The results of this study are intended to provide a better understanding of contaminant migration in the vicinity of bedrock streams. Based on the numerical results, the rate of ground water discharge (or recharge) was found to depend on the aperture size of the discharging feature, and on the distribution of hydraulic head with depth within the fracture network. It was determined that the extent of both the capture zone and reverse capture zone for an individual fracture can be extremely large, and will be determined by the height of the stream stage, the fracture apertures of the network, and the hydraulic‐head distribution within the network. Because both the stream stage and the hydraulic‐head distribution are transient, the size of the capture zone and/or the reverse capture zone for an individual fracture may change significantly over time. As a result, the migration path for contaminants within the fracture network and between the surface and subsurface will also vary significantly with time.
Monitoring Well Interception with Fractures in Clayey Till
Authors: Jørgensen Peter R. , Klint Knud Erik S. , Kistrup Jens P.
Published: December 2005

When using monitoring wells for investigation of contaminant sources in clayey till, there is a high risk that fractures may cause mobile contaminants to bypass the monitoring wells. This paper indicates that the probability of interception between monitoring wells and hydraulic conductive fractures is often significantly less than 50%. Based on a field experiment and application of a calibrated discrete fracture matrix diffusion numerical model (FRAC3Dvs), the paper also evaluates pesticide‐monitoring results for different positions of monitoring well screen relative to fractures. For well screens situated 0.25 and 2 m from a conductive fracture, the first concentrations of the pesticide metabolite (2,6 dichlorobenzamide, “BAM”)would be measured two years and 18 years, respectively, after the contaminant had been transported into an underlying aquifer. In this way, underlying aquifers may be subjected to contamination by downward moving contamination without being observed in monitoring wells in the till.
Long‐Term Disturbance of Ground Water Chemistry Following Well Installation
Authors: Kim Kangjoo
Published: December 2005

Ground water samples collected from a multilevel sampler shortly after its construction showed significantly higher alkalinity and concentrations of calcium and magnesium than those from nearby wells installed 10 years earlier. The sampler was drilled using a conventional hollow‐stem power auger in a sandy, silicate aquifer lying beneath an isthmus between two lakes in northern Wisconsin. Ground water in the study area is of low ionic strength and its chemistry is dominated by silicate mineral weathering. Periodic sampling over two years following installation of the sampler showed that the higher solute concentrations had subsequently decreased. Oxygen isotope signature and other solute species, such as sulfate and chloride, were comparable to those of older wells and did not show any notable trends over time. Independent variation of other chemical species that cannot be derived from aquifer minerals, and the similarly high concentrations in older wells shortly after their installation, suggest that rapid dissolution of fresh mineral surfaces and hyperfine particles generated during drilling has induced the enhanced concentrations. This observation is consistent with the field equivalent of laboratory mineral dissolution experiments that show initially increased dissolution rates that decay over time. Well installations for geochemical sampling in dominantly silicate material may require longer times to reach an equilibrium state than has been previously thought.
Steady Flow to a Well Near a Stream with a Leaky Bed
Authors: Bakker Mark , Anderson Erik I.
Published: December 2005

We present an explicit analytic solution for steady, two‐dimensional ground water flow to a well near a leaky streambed that penetrates the aquifer partially. Leakage from the stream is approximated as occurring along the centerline of the stream. The problem domain is infinite and pumping on one side of the stream induces flow on the other side. The solution includes the effects of uniform flow in the far field and a sloping hydraulic head in the stream. We use the solution to investigate the interaction between ground water and surface water in the stream, the effects of pumping on the opposite side of the stream, and the effects of the leaky streambed on the capture zone envelope of the well. We develop a relationship between parameters such that the pumping well will not capture water from the stream, or from the opposite side of the stream. When the discharge of the well is large enough to capture water from the stream, the shape of the capture zone envelope depends on flow conditions on the side of the stream opposite the well.
A Simple Constant‐Head Injection Test for Streambed Hydraulic Conductivity Estimation
Authors: Cardenas M. Bayani , Zlotnik Vitaly A.
Published: December 2005

A fast, efficient constant‐head injection test (CHIT) for in situ estimation of hydraulic conductivity (K) of sandy streambeds is presented. This test uses constant‐head hydraulic injection through a manually driven piezometer. Results from CHIT compare favorably to estimates from slug testing and grain‐size analysis. The CHTT combines simplicity of field performance, data interpretation, and accuracy of K estimation in flowing streams.
A Comparison of Two Stochastic Inverse Methods in a Field‐Scale Application
Authors: Larocque Marie , Delay Fred , Banton Olivier
Published: December 2005

Inverse modeling is a useful tool in ground water flow modeling studies. The most frequent difficulties encountered when using this technique are the lack of conditioning information (e.g., heads and transmissivities), the uncertainty in available data, and the nonuniqueness of the solution. These problems can be addressed and quantified through a stochastic Monte Carlo approach. The aim of this work was to compare the applicability of two stochastic inverse modeling approaches in a field‐scale application. The multi‐scaling (MS) approach uses a downscaling parameterization procedure that is not based on geostatistics. The pilot point (PP) approach uses geostatistical random fields as initial transmissivity values and an experimental variogram to condition the calibration. The studied area (375 km2) is part of a regional aquifer, northwest of Montréal in the St. Lawrence lowlands (southern Québec). It is located in limestone, dolomite, and sandstone formations, and is mostly a fractured porous medium. The MS approach generated small errors on heads, but the calibrated transmissivity fields did not reproduce the variogram of observed transmissivities. The PP approach generated larger errors on heads but better reproduced the spatial structure of observed transmissivities. The PP approach was also less sensitive to uncertainty in head measurements. If reliable heads are available but no transmissivities are measured, the MS approach provides useful results. If reliable transmissivities with a well inferred spatial structure are available, then the PP approach is a better alternative. This approach however must be used with caution if measured transmissivities are not reliable.
Enhanced Attenuation of Septic System Phosphate in Noncalcareous Sediments
Authors: Robertson W.D.
Published: December 2005

Review of phosphate behavior in four mature septic system plumes on similar textured sand has revealed a strong correlation between carbonate mineral content and phosphate concentrations. A plume on calcareous sand (Cambridge site, 27 wt % CaCO3 equiv.) has proximal zone PO4 concentrations (4.8 mg/L P average) that are about 75% of the septic tank effluent value, whereas three plumes on noncalcareous sand (Muskoka, L. Joseph, and Nobel sites, <1 wt % CaCO3 equiv.) have proximal zone phosphate concentrations (<0.1 mg/L P) that are consistently less than 2% of the effluent values. Phosphate attenuation at the noncalcareous sites appears to be an indirect result of the development of acidic conditions (site average pH 3.5 to 5.9) and elevated Al concentrations (up to 24 mg/L), which subsequently causes the precipitation of Al‐P minerals such as variscite (AlPO4. 2H2O). This is supported by scanning electron microscope analyses, which show the widespread occurrence of (Al+P)—rich secondary mineral coatings on sand grains below the infiltration beds. All of these septic systems are more than 10 years old, indicating that these attenuation reactions have substantial longevity.

A field lysimeter experiment demonstrated that this reaction sequence can be readily incorporated into engineered waste water treatment systems. We feel this important P removal mechanism has not been adequately recognized, particularly for its potential significance in reducing P loading from septic systems in lakeshore environments.
Hydrostratigraphic Modeling of a Complex, Glacial‐Drift Aquifer System for Importation into MODFLOW
Authors: Herzog Beverly L. , Larson David R. , Abert Curtis C. , Wilson Steven D. , Roadcap George S.
Published: December 2005

Deposition from at least three episodes of glaciation left a complex glacial‐drift aquifer system in central Illinois. The deepest and largest of these aquifers, the Sankoty‐Mahomet Aquifer, occupies the lower part of a buried bedrock valley and supplies water to communities throughout central Illinois. Thin, discontinuous aquifers are present within glacial drift overlying the Sankoty‐Mahomet Aquifer. This study was commissioned by local governments to identify possible areas where a regional water supply could be obtained from the aquifer with minimal adverse impacts on existing users.

Geologic information from more than 2200 existing water well logs was supplemented with new data from 28 test borings, water level measurements in 430 wells, and 35 km of surface geophysical profiles. A three‐dimensional (3‐D) hydrostratigraphic model was developed using a contouring software package, a geographic information system (GIS), and the 3‐D geologic modeling package, Earth Vision®. The hydrostratigraphy of the glacial‐drift sequence was depicted as seven uneven and discontinuous layers, which could be viewed from an infinite number of horizontal and vertical slices and as solid models of any layer. Several iterations were required before the 3‐D model presented a reasonable depiction of the aquifer system. Layers from the resultant hydrostratigraphic model were imported into MODFLOW, where they were modified into continuous layers. This approach of developing a 3‐D hydrostratigraphic model can be applied to other areas where complex aquifer systems are to be modeled and is also useful in helping lay audiences visualize aquifer systems.
An Evaluation of Conditioning Data for Solute Transport Prediction
Authors: Scheibe Timothy D. , Chien Yi‐Ju
Published: December 2005

The large and diverse body of subsurface characterization data generated at a field research site near Oyster, Virginia, provides a unique opportunity to test the impact of conditioning data of various types on predictions of flow and transport. Bromide breakthrough curves (BTCs) were measured during a forced‐gradient local‐scale injection experiment conducted in 1999. Observed BTCs are available at 140 sampling points in a three‐dimensional array within the transport domain. A detailed three‐dimensional numerical model is used to simulate breakthrough curves at the same locations as the observed BTCs under varying assumptions regarding the character of hydraulic conductivity spatial distributions, and variable amounts and types of conditioning data. We present comparative results of six cases ranging from simple (deterministic homogeneous models) to complex (stochastic indicator simulation conditioned to cross‐borehole geophysical observations). Quantitative measures of model goodness‐of‐fit are presented. The results show that conditioning to a large number of small‐scale measurements does not significantly improve model predictions, and may lead to biased or overly confident predictions. However, conditioning to geophysical interpretations with larger spatial support significantly improves the accuracy and precision of model predictions. In all cases, the effects of model error appear to be significant in relation to parameter uncertainty.
Including Geophysical Data in Ground Water Model Inverse Calibration
Authors: Dam Dorte , Christensen Steen
Published: December 2005

A nonlinear regression method is developed that can be used to estimate parameters of a ground water flow model from a combination of observations of hydrological variables and observations of geophysical properties that are functionally related with the hydraulic conductivity. The procedure estimates: parameters characterizing the hydraulic conductivity field (e.g., zonal or pilot point values); geophysical properties that have been observed and that are functionally related with the hydraulic conductivity parameters; and a few parameters of the function that relates the hydraulic conductivity parameters with the geophysical properties (the type of function is assumed known). A fidelity factor, σr2, of a term of the minimized objective function reflects the faith one has in the validity of this functional relationship. The estimation methodology has been tested by means of synthetic models. The experimental results demonstrate that the number of estimated hydraulic conductivity parameters can be increased by adding geophysical observations to the set of hydrological observations that are traditionally used for model calibration. The improvement of the estimated hydraulic conductivity field and the simulated hydraulic head field can be significant but is dependent on the number, the locations, and the uncertainty of geophysical observations. The sensitivity of the estimation results to the value of σr is small for the studied problems except when the uncertainty of geophysical observations is high. In the latter case, a large σr value was found to be optimal to avoid that hydraulic conductivity estimates are closely tied to corresponding but highly uncertain geophysical observations.
Stepwise Use of GFLOW and MODFLOW to Determine Relative Importance of Shallow and Deep Receptors
Authors: Feinstein Daniel , Dunning Charles , Hunt Randall J. , Krohelski Jim
Published: December 2005

A stepwise modeling approach is implemented in which a regional one‐layer analytic element model is used to simulate the flow system and to furnish boundary conditions for an extracted local three‐dimensional model. In this case study the stepwise approach is used to evaluate the fate of recharge in the Menomonee Valley adjacent to Lake Michigan. Two major receptors exist for recharge that flows through contaminated valley fill: the surface water estuary and a tunnel system constructed ∼75 to 110 m below land surface to store storm runoff. The primary objective of the modeling is to delineate the contributing areas of recharge to each receptor. Of interest is the ability of the one‐layer regional model to furnish flux boundary conditions to the local three‐dimensional model despite the presence of vertical flow conditions at the boundaries of the local model. Sensitivity analysis suggests that the local model was insensitive to the vertical distribution of the flux. Each step of the modeling approach demonstrates that both receptors play an important role in capturing valley recharge. The pattern of capture of the one‐layer model differed in shape from that delineated by the multi‐layer local model in the presence of a flow system with pronounced vertical anisotropy and with sinks drawing water from different elevations.
Coupling Basin‐ and Site‐Scale Inverse Models of the Espanola Aquifer
Authors: Keating Elizabeth H. , Vesselinov Velimir V. , Kwicklis Edward , Lu Zhiming
Published: December 2005

Large‐scale models are frequently used to estimate fluxes to small‐scale models. The uncertainty associated with these flux estimates, however, is rarely addressed. We present a case study from the Española Basin, northern New Mexico, where we use a basin‐scale model coupled with a high‐resolution, nested site‐scale model. Both models are three‐dimensional and are analyzed by codes FEHM and PEST. Using constrained nonlinear optimization, we examine the effect of parameter uncertainty in the basin‐scale model on the nonlinear confidence limits of predicted fluxes to the site‐scale model. We find that some of the fluxes are very well constrained, while for others there is fairly large uncertainty. Site‐scale transport simulation results, however, are relatively insensitive to the estimated uncertainty in the fluxes. We also compare parameter estimates obtained by the basin‐ and site‐scale inverse models. Differences in the model grid resolution (scale of parameter estimation) result in differing delineation of hydrostratigraphic units, so the two models produce different estimates for some units. The effect is similar to the observed scale effect in medium properties owing to differences in tested volume. More important, estimation uncertainty of model parameters is quite different at the two scales. Overall, the basin inverse model resulted in significantly lower estimates of uncertainty, because of the larger calibration dataset available. This suggests that the basin‐scale model contributes not only important boundary condition information but also improved parameter identification for some units. Our results demonstrate that caution is warranted when applying parameter estimates inferred from a large‐scale model to small‐scale simulations, and vice versa.
Ground Water Flow in a Desert Basin: Challenges of Simulating Transport of Dissolved Chromium
Authors: Andrews Charles B. , Neville Christopher J.
Published: December 2005

A large chromium plume that evolved from chromium releases in a valley near the Mojave River was studied to understand the processes controlling fate and migration of chromium in ground water and used as a tracer to study the dynamics of a basin and range ground water system. The valley that was studied is naturally arid with high evapotranspiration such that essentially no precipitation infiltrates to the water table. The dominant natural hydrogeologic processes are recharge to the ground water system from the Mojave River during the infrequent episodes when there is flow in the river, and ground water flow toward a playa lake where the ground water evaporates. Agricultural pumping in the valley from the mid‐1930s to the 1970s significantly altered ground water flow conditions by decreasing water levels in the valley by more than 20 m. This pumping declined significantly as a result of dewatering of the aquifer, and water levels have since recovered modestly.

The ground water system was modeled using MODFLOW, and chromium transport was simulated using MT3D. Several innovative modifications were made to these modeling programs to simulate important processes in this ground water system. Modifications to MODFLOW include developing a new well package that estimates pumping rates from irrigation wells at each time step based on available drawdown. MT3D was modified to account for mass trapped above the water table when the water table declines beneath nonirrigated areas and to redistribute mass to the system when water levels rise.
Simulating Ground Water‐Lake Interactions: Approaches and Insights
Authors: Hunt Randall J. , Haitjema Henk M. , Krohelski James T. , Feinstein Daniel T.
Published: December 2005

Approaches for modeling lake‐ground water interactions have evolved significantly from early simulations that used fixed lake stages specified as constant head to sophisticated LAK packages for MODFLOW. Although model input can be complex, the LAK package capabilities and output are superior to methods that rely on a fixed lake stage and compare well to other simple methods where lake stage can be calculated. Regardless of the approach, guidelines presented here for model grid size, location of three‐dimensional flow, and extent of vertical capture can facilitate the construction of appropriately detailed models that simulate important lake‐ground water interactions without adding unnecessary complexity. In addition to MODFLOW approaches, lake simulation has been formulated in terms of analytic elements. The analytic element lake package had acceptable agreement with a published LAK1 problem, even though there were differences in the total lake conductance and number of layers used in the two models. The grid size used in the original LAK1 problem, however, violated a grid size guideline presented in this paper. Grid sensitivity analyses demonstrated that an appreciable discrepancy in the distribution of stream and lake flux was related to the large grid size used in the original LAK1 problem. This artifact is expected regardless of MODFLOW LAK package used. When the grid size was reduced, a finite‐difference formulation approached the analytic element results. These insights and guidelines can help ensure that the proper lake simulation tool is being selected and applied.
A Finite‐Volume ELLAM for Three‐Dimensional Solute‐Transport Modeling
Authors: Russell Thomas F. , Heberton Caroline I. , Konikow Leonard F. , Hornberger George Z.
Published: December 2005

A three‐dimensional finite‐volume ELLAM method has been developed, tested, and successfully implemented as part of the U.S. Geological Survey (USGS) MODFLOW‐2000 ground water modeling package. It is included as a solver option for the Ground Water Transport process. The FVELLAM uses space‐time finite volumes oriented along the streamlines of the flow field to solve an integral form of the solute‐transport equation, thus combining local and global mass conservation with the advantages of Eulerian‐Lagrangian characteristic methods. The USGS FVELLAM code simulates solute transport in flowing ground water for a single dissolved solute constituent and represents the processes of advective transport, hydrodynamic dispersion, mixing from fluid sources, retardation, and decay. Implicit time discretization of the dispersive and source/sink terms is combined with a Lagrangian treatment of advection, in which forward tracking moves mass to the new time level, distributing mass among destination cells using approximate indicator functions. This allows the use of large transport time increments (large Courant numbers) with accurate results, even for advection‐dominated systems (large Peclet numbers). Four test cases, including comparisons with analytical solutions and benchmarking against other numerical codes, are presented that indicate that the FVELLAM can usually yield excellent results, even if relatively few transport time steps are used, although the quality of the results is problem‐dependent.
MAROS: A Decision Support System for Optimizing Monitoring Plans
Authors: Aziz Julia J. , Ling Meng , Rifai Hanadi S. , Newell Charles J. , Gonzales James R.
Published: December 2005

The Monitoring and Remediation Optimization System (MAROS), a decision‐support software, was developed to assist in formulating cost‐effective ground water long‐term monitoring plans. MAROS optimizes an existing ground water monitoring program using both temporal and spatial data analyses to determine the general monitoring system category and the locations and frequency of sampling for future compliance monitoring at the site. The objective of the MAROS optimization is to minimize monitoring locations in the sampling network and reduce sampling frequency without significant loss of information, ensuring adequate future characterization of the contaminant plume. The interpretive trend analysis approach recommends the general monitoring system category for a site based on plume stability and site‐specific hydrogeologic information. Plume stability is characterized using primary lines of evidence (i.e., Mann‐Kendall analysis and linear regression analysis) based on concentration trends, and secondary lines of evidence based on modeling results and empirical data. The sampling optimization approach, consisting of a two‐dimensional spatial sampling reduction method (Delaunay method) and a temporal sampling analysis method (Modified CES method), provides detailed sampling location and frequency results. The Delaunay method is designed to identify and eliminate redundant sampling locations without causing significant information loss in characterizing the plume. The Modified CES method determines the optimal sampling frequency for a sampling location based on the direction, magnitude, and uncertainty in its concentration trend. MAROS addresses a variety of ground water contaminants (fuels, solvents, and metals), allows import of various data formats, and is designed for continual modification of long‐term monitoring plans as the plume or site conditions change over time.
Modeling the Buildup of Iron Oxide Encrustations in Wells
Authors: Houben Georg J.
Published: December 2005

Ferric iron encrustations are a common problem that seriously affects the performance of wells and drains. Their formation is induced by the mixing of reduced ground water containing ferrous iron with oxic shallow ground water and exposure to air. The process of ferrous iron oxidation is a kinetically controlled reaction. The reaction rate has a quadratic dependency on pH. The precipitating oxides have an autocatalytic effect that further enhances reaction progress. This paper describes the application of kinetic models to the problem of encrustation formation. Influences of pH, residence time, and autocatalysis were modeled. The overall influence of the autocatalytic reaction path is particularly strong when initial amounts of iron oxides are present. Autocatalysis provides a good explanation on the development of well yield commonly measured in the field. Encrustation precipitation is slow at first, but speeds up after a sufficient amount of iron oxide has built up. An incomplete removal of iron oxide encrustations during rehabilitation leads to a renewed increase in catalytic efficiency and encrustation precipitation.
Use of Temperature Profiles and Stable Isotopes to Trace Flow Lines: Nagaoka Area, Japan
Authors: Salem Zenhom El‐said , Taniguchi Makoto , Sakura Yasua
Published: December 2005

In this study, we use borehole temperature data and stable isotopes to delineate the flow system and estimate the effect of urbanization in the Nagaoka area of Japan. Temperature profiles were measured four times in observation wells during the period 2000–2001 and compared with those measured in the same wells during the period 1977–1983 (Taniguchi 1986). Water was sampled in both observation and pumping wells during the same period. The temporal and spatial variability in temperature indicate clearly the effect of urban warming and heavy pumping on the ground water system. Urban warming caused higher temperatures recently as compared to the older values, and pumping caused induced recharge from the river to the ground water. The stable isotope data show the ground water flow system is divided into shallow, intermediate, and deep systems, and that land use and infiltration rate are affecting the shallow flow system.
Small‐Scale Electrical Resistivity Tomography of Wet Fractured Rocks
Authors: LaBrecque Douglas J. , Sharpe Roger , Wood Thomas , Heath Gail
Published: December 2005

This paper describes a series of experiments that tested the ability of the electrical resistivity tomography (ERT) method to locate correctly wet and dry fractures in a meso‐scale model. The goal was to develop a method of monitoring the flow of water through a fractured rock matrix.

The model was a four by six array of limestone blocks equipped with 28 stainless steel electrodes. Dry fractures were created by placing pieces of vinyl between one or more blocks. Wet fractures were created by injecting tap water into a joint between blocks. In electrical terms, the dry fractures are resistive and the wet fractures are conductive. The quantities measured by the ERT system are current and voltage around the outside edge of the model. The raw ERT data were translated to resistivity values inside the model using a three‐dimensional Occam's inversion routine. This routine was one of the key components of ERT being tested.

The model presented several challenges. First, the resistivity of both the blocks and the joints was highly variable. Second, the resistive targets introduced extreme changes the software could not precisely quantify. Third, the abrupt changes inherent in a fracture system were contrary to the smoothly varying changes expected by the Occam's inversion routine. Fourth, the response of the conductive fractures was small compared to the background variability.

In general, ERT was able to locate correctly resistive fractures. Problems occurred, however, when the resistive fracture was near the edges of the model or when multiple fractures were close together. In particular, ERT tended to position the fracture closer to the model center than its true location. Conductive fractures yielded much smaller responses than the resistive case. A difference‐inversion method was able to correctly locate these targets.
Determining the Permeability of Fractured Rocks Based on Joint Mapping
Authors: Zhang Faming , Zhou Zhifang , Huang Yongqing , Chen Zuyu
Published: December 2005

A typical fractured rock mass is intersected by several sets of discontinuities, which provide the main flowpath for ground water. Due to the limitations of data obtained by conventional field measurements, it is often difficult to estimate the anisotropic permeability tensor associated with the joints existing in the rock mass. For that reason, determining permeability tensors for fractured rocks is an important topic in rock mass hydraulics. Based on field surveys, joint parameters can be analyzed by using probabilistic and statistical tools, and three‐dimensional mapping of the jointed rock mass. Through analysis of a single joint's hydraulic characteristics, the principal value of the permeability tensor for the jointed rock mass can be determined by using Monte Carlo methods and the searching percolation trace method, which is developed in this paper. The study reports on practical examples demonstrating that results from the methods discussed in this paper are in agreement with those from field hydrogeological surveys and measurements.
Colonization by Aerobic Bacteria in Karst: Laboratory and In Situ Experiments
Authors: Personné J.‐C. , Poty F. , Mahler B.J. , Drogue C.
Published: December 2005

Experiments were carried out to investigate the potential for bacterial colonization of different substrates in karst aquifers and the nature of the colonizing bacteria. Laboratory batch experiments were performed using limestone and PVC as substrates, a natural bacterial isolate and a known laboratory strain (Escherichia coli [E. coli]) as inocula, and karst ground water and a synthetic formula as growth media. In parallel, fragments of limestone and granite were submerged in boreholes penetrating two karst aquifers for more than one year; the boreholes are periodically contaminated by enteric bacteria from waste water. Once a month, rock samples were removed and the colonizing bacteria quantified and identified. The batch experiments demonstrated that the natural isolate and E. coli both readily colonized limestone surfaces using karst ground water as the growth medium. In contrast, bacterial colonization of both the limestone and granite substrates, when submerged in the karst, was less intense. More than 300 bacterial strains were isolated over the period sampled, but no temporal pattern in colonization was seen as far as strain, and colonization by E. coli was notably absent, although strains of Salmonella and Citrobacter were each observed once. Samples suspended in boreholes penetrating highly fractured zones were less densely colonized than those in the borehole penetrating a less fractured zone. The results suggest that contamination of karst aquifers by enteric bacteria is unlikely to be persistent. We hypothesize that this may be a result of the high flow velocities found in karst conduits, and of predation of colonizing bacteria by autochthonous zooplankton.
Radon (222Rn) in Ground Water of Fractured Rocks: A Diffusion/Ion Exchange Model
Authors: Wood Warren W. , Kraemer Thomas F. , Shapiro Allen
Published: December 2005

Ground waters from fractured igneous and high‐grade sialic metamorphic rocks frequently have elevated activity of dissolved radon (222Rn). A chemically based model is proposed whereby radium (226Ra) from the decay of uranium (238U) diffuses through the primary porosity of the rock to the water‐transmitting fracture where it is sorbed on weathering products. Sorption of 226Ra on the fracture surface maintains an activity gradient in the rock matrix, ensuring a continuous supply of 226Ra to fracture surfaces. As a result of the relatively long half‐life of 226Ra (1601 years), significant activity can accumulate on fracture surfaces. The proximity of this sorbed 226Ra to the active ground water flow system allows its decay progeny 222Rn to enter directly into the water. Laboratory analyses of primary porosity and diffusion coefficients of the rock matrix, radon emanation, and ion exchange at fracture surfaces are consistent with the requirements of a diffusion/ion‐exchange model. A dipole‐brine injection/withdrawal experiment conducted between bedrock boreholes in the high‐grade metamorphic and granite rocks at the Hubbard Brook Experimental Forest, Grafton County, New Hampshire, United States (42°56′N, 71°43′W) shows a large activity of 226Ra exchanged from fracture surfaces by a magnesium brine. The 226Ra activity removed by the exchange process is 34 times greater than that of 238U activity. These observations are consistent with the diffusion/ion‐exchange model. Elutriate isotopic ratios of 223Ra/226Ra and 238U/226Ra are also consistent with the proposed chemically based diffusion/ion‐exchange model.
Effects of Water Use on Arsenic Release to Well Water in a Confined Aquifer
Authors: Gotkowitz Madeline B. , Schreiber Madeline E. , (Toni) Simo J.A.
Published: December 2005

Field‐based experiments were designed to investigate the release of naturally occurring, low to moderate (< 50 ug/L) arsenic concentrations to well water in a confined sandstone aquifer in northeastern Wisconsin. Geologic, geochemical, and hydrogeologic data collected from a 115 m2 site demonstrate that arsenic concentrations in ground water are heterogeneous at the scale of the field site, and that the distribution of arsenic in ground water correlates to solid‐phase arsenic in aquifer materials. Arsenic concentrations in a test well varied from 1.8 to 22 ug/L during experiments conducted under no, low, and high pumping rates. The quality of ground water consumed from wells under typical domestic water use patterns differs from that of ground water in the aquifer because of reactions that occur within the well. Redox conditions in the well can change rapidly in response to ground water withdrawals. The well borehole is an environment conducive to microbiological growth, and biogeochemical reactions also affect borehole chemistry. While oxidation of sulfide minerals appears to release arsenic to ground water in zones within the aquifer, reduction of arsenic‐bearing iron (hydr)oxides is a likely mechanism of arsenic release to water having a long residence time in the well borehole.
SEEPAGE, a New MODFLOW DRAIN Package
Authors: Batelaan O. , De Smedt F.
Published: December 2005

The prediction of the location of ground water discharge areas is a key aspect for the protection and (re)development of ground water‐dependent wetlands. Ground water discharge areas can be simulated with MODFLOW using the DRAIN package by setting the drain level equal to the topography, while the conductance is mostly set to an arbitrary high value. However, conceptual and practical problems arise in the calculation of the ground water discharge by the DRAIN package as calculated water tables above the land surface, difficult parameterization of the conductance, and large water balance errors. To overcome these problems, a new SEEPAGE package for MODFLOW is proposed. The basic idea of this package is an adaptable constant head cell. It has a variable head, unless the ground water rises above the seepage level, in which case it has a constant head cell. The estimation of the ground water discharge location along a homogeneous, isotropic, linear sloping profile is used to verify the model and to compare it to the DRAIN solution. In an application to three basins in Belgium, it is shown that the SEEPAGE package can be used in combination with the DRAIN package in situations where an upper boundary for a free water table and additional resistance for drainage is required. It is clearly demonstrated that the identification and delineation of regional ground water discharge areas is more accurate using the SEEPAGE package.
Degradates Provide Insight to Spatial and Temporal Trends of Herbicides in Ground Water
Authors: Kolpin Dana W. , Schnoebelen Douglas J. , Thurman E. Michael
Published: December 2005

Since 1995, a network of municipal wells in Iowa, representing all major aquifer types (alluvial, bedrock/karst region, glacial drift, bedrock/nonkarst region), has been repeatedly sampled for a broad suite of herbicide compounds yielding one of the most comprehensive statewide databases of such compounds currently available in the United States. This dataset is ideal for documenting the insight that herbicide degradates provide to the spatial and temporal distribution of herbicides in ground water.

During 2001, 86 municipal wells in Iowa were sampled and analyzed for 21 herbicide parent compounds and 24 herbicide degradates. The frequency of detection increased from 17% when only herbicide parent compounds were considered to 53% when both herbicide parents and degradates were considered. Thus, the transport of herbicide compounds to ground water is substantially underestimated when herbicide degradates are not considered. A significant difference in the results among the major aquifer types was apparent only when both herbicide parent compounds and their degradates were considered. In addition, including herbicide degradates greatly improved the statistical relation to the age of the water being sampled. When herbicide parent compounds are considered, only 40% of the wells lacking a herbicide detection could be explained by the age of the water predating herbicide use. However, when herbicide degradates were also considered, 80% of the ground water samples lacking a detection could be explained by the age of the water predating herbicide use. Finally, a temporal pattern in alachlor concentrations in ground water could only be identified when alachlor degradates were considered.
A Program for Evaluating Dual‐Equilibrium Desorption Effects on Remediation
Authors: Chen Wei , Lakshmanan Kalyan , Kan Amy T. , Tomson Mason B.
Published: December 2005

Desorption is one of the most critical processes affecting the effectiveness of soil and ground water remediation. None of the currently adopted desorption models can accurately quantify desorption of low‐hydrophobicity organic chemicals, and thus could potentially mislead remediation design and decision‐making. A recently developed dual‐equilibrium desorption (DED) model was found to be much more accurate in quantifying desorption. A screening‐level transport model, DED‐Transport, was developed to simulate the DED effect on behaviors of organic contaminant plumes during remediation. DED‐Transport requires only simple parameters, but is applicable to many remediation scenarios. DED‐Transport can be used as a decision‐support tool in site remediation to more precisely predict the time required for cleanup.
Geochemical Imaging of Flow Near an Artificial Recharge Facility, Orange County, California
Authors: Clark Jordan F. , Hudson G. Bryant , Davisson M. Lee , Woodside Greg , Herndon Roy
Published: December 2005

Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium–3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where ∼2.5 × 108 m3 (200,000 acre‐feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at ∼6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other “time sensitive” contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.
Spatial Interpolation Methods for Nonstationary Plume Data
Authors: Reed Patrick M. , Ellsworth Timothy R. , Minsker Barbara S.
Published: December 2005

Plume interpolation consists of estimating contaminant concentrations at unsampled locations using the available contaminant data surrounding those locations. The goal of ground water plume interpolation is to maximize the accuracy in estimating the spatial distribution of the contaminant plume given the data limitations associated with sparse monitoring networks with irregular geometries. Beyond data limitations, contaminant plume interpolation is a difficult task because contaminant concentration fields are highly heterogeneous, anisotropic, and nonstationary phenomena. This study provides a comprehensive performance analysis of six interpolation methods for scatter‐point concentration data, ranging in complexity from intrinsic kriging based on intrinsic random function theory to a traditional implementation of inverse‐distance weighting. High resolution simulation data of perchloroethylene (PCE) contamination in a highly heterogeneous alluvial aquifer were used to generate three test cases, which vary in the size and complexity of their contaminant plumes as well as the number of data available to support interpolation. Overall, the variability of PCE samples and preferential sampling controlled how well each of the interpolation schemes performed. Quantile kriging was the most robust of the interpolation methods, showing the least bias from both of these factors. This study provides guidance to practitioners balancing opposing theoretical perspectives, ease‐of‐implementation, and effectiveness when choosing a plume interpolation method.
Importance of the Vadose Zone in Analyses of Unconfined Aquifer Tests
Authors: Moench Allen F.
Published: December 2005

Analytical models commonly used to interpret unconfined aquifer tests have been based on upper‐boundary (water table) conditions that do not adequately address effects of time‐varying drainage from the vadose zone. As a result, measured and simulated drawdown data may not agree and hydraulic parameters may be inaccurately estimated. A 72‐hour aquifer test conducted in Cape Cod, Massachusetts, in a slightly heterogeneous, coarse‐grained, glacial outwash deposit was found to be a good candidate for testing models with different upper‐boundary conditions. In general, under the commonly invoked assumption of instantaneous drainage, measured and simulated drawdowns were found to agree with one another only at late time and early time. In the intermediate‐time range, because of delayed drainage, measured drawdowns always exceeded simulated values, most noticeably in piezometers located near the water table. To reduce these discrepancies, an analytical model was developed that can fully account for time‐varying drainage given that the aquifer is not strongly heterogeneous. The approach is flexible as the model, which makes use of empirical relations, does not constrain drainage to follow any particular functional relation. By this approach, measured and simulated drawdowns agree over the complete time range, and the estimated parameters are consistent with prior studies and with what is known about the aquifer geometry, stratigraphy, and composition. By properly accounting for vadose zone drainage, it was found that realistic estimates of all hydraulic parameters, including specific yield, could be obtained with or without the use of late‐time data.