bluuebunny/crossref_metadata_2025 huggingface dataset:
Definition of a Critical Confining Zone Using Surface Geophysical Methods
Authors: Eddy‐Dilek Carol A. , Looney Brian B. , Hoekstra Pieter , Harthill Norman , Blohm Mark , Phillips David R.
Published: August 2005

Definition of the hydrologic framework in layered sediments of fluvial and deltaic origin is a difficult challenge for environmental characterization and remediation programs due to the lithologic and stratigraphic heterogeneities inherent in these settings. We set out to use complementary geophysical surveys to determine the nature and extent of a deep confining unit at the Savannah River Site, South Carolina. Time Domain Electromagnetic (TDEM) soundings were used to define the electrical conductance of the clayey confining unit (aquitard), and shear‐wave reflection seismic was used to define the stratigraphic framework. Based on correlations with borehole geophysical logs and sieve data, the shear‐wave seismic proved capable of defining relatively fine layering in the coastal plain sediments, the upper and lower surfaces of a critical confining unit, and erosional features on the surface of the confining unit. The TDEM surveys defined the presence or absence of the clay facies of the confining unit. Moreover, by constraining the interpretation of the TDEM data with the thickness of the confining unit derived from the seismic data, we mapped the extent of the unit, showing where the clay is thicker, where it probably was never deposited, and where it was eroded by downcutting channels. These results have significant implications on the design and optimization of remedial systems.
A Stochastic‐Analytical Framework for Safety Assessment of Waste Repositories: 1. Theory
Authors: Selroos Jan‐Olof
Published: August 2005

A model framework for safety assessment of waste repositories is derived on the basis of stochastic‐analytical solution techniques. The approach is computationally simple but maintains a relatively detailed description of processes and uncertainty. The framework consists of separate subunits for the engineered system, the geo sphere, and impact evaluation in the biosphere. The impact in the biosphere is probabilistically assessed using the derived first two statistical moments of the mass flux or cumulative mass flux out from the geo sphere. Various regulatory criteria related to the mass fluxes or contaminant travel times may be used for the impact assessment. The subunits of the framework are presented in the context of a repository for high‐level nuclear waste.
The Use of Upward Hydraulic Gradients to Arrest Downward DNAPL Migration in Rock Fractures
Authors: Chown Jennifer C. , Kueper Bernard H. , McWhorter David B.
Published: August 2005

Upward water flow can arrest the downward migration of dense, non aqueous phase liquids (DNAPLS) through rough‐walled fractures provided that a sufficient hydraulic gradient exists. An exact analytical solution to predict the arresting gradient demonstrates that there is little difference between the gradient required to arrest DNAPL migration near the top of the fracture, and the gradient required to arrest migration once DNAPL has extended to the bottom of the fracture. Laboratory experiments involving the migration of tetrachloroethylene (PCE) through two samples of fractured limestone demonstrate the ability of upward water flow to arrest downward DNAPL migration under both wetting and drainage conditions. It is suggested that upward gradients can be generated beneath contaminated regions of the subsurface to provide a “hydraulic bottom.” A hydraulic bottom at a site would protect against potential downward mobilization of DNAPL in response to the application of aggressive remediation technologies such as surfactant flooding, alcohol flooding, and steam flooding. Upward gradients applied during drilling may also protect against downward mobilization of DNAPL in the formation while drilling through source zones.
Effects of Unsaturated Zone on Aquifer Test Analysis in a Shallow‐Aquifer System
Authors: Halford Keith J.
Published: August 2005

A comparison between two hypothetical flow models of an unconfined aquifer, one saturated and the other variably saturated, indicates that the variably saturated model which explicitly models drainage from the unsaturated zone provides a better conceptual framework for analyzing unconfined aquifer test data and better estimates of the lateral and vertical hydraulic conductivity in fine‐grained sands. Explicitly accounting for multiple aquifers, well‐bore storage, and the effects of delayed drainage from the unsaturated zone increases confidence in aquifer property estimates by removing some assumptions and allowing for the inclusion of early time data and water‐table observations in an aquifer test analysis. The inclusion of the unsaturated zone expands the number of parameters to be estimated, but reasonable estimates of lateral and vertical hydraulic conductivity and specific storage of the unconfined aquifer can be obtained. For the cases examined, only the van Genuchten parameter a needed to be determined by the test, because the parameters n and 9r had a minimal effect on the estimates of hydraulic conductivities, and literature values could be used for these parameters. Estimates of lateral and vertical hydraulic conductivity using MODFLOW were not as good as the VS2DT based estimates and differed from the known values by as much as 30 percent.

The hydraulic properties of a surficial aquifer system were estimated through a series of aquifer tests conducted at Cecil Field Naval Air Station in Jacksonville, Florida. Aquifer test results were analyzed by calibrating a variably saturated, radial flow model to the measured drawdowns. Parameter estimation was performed by minimizing the difference between simulated and measured drawdowns with an optimization routine coupled to VS2DT and was constrained by assuming that the hydraulic properties of each aquifer or confining unit were homogeneous. Given the hydrogeologic conditions at the field site, estimating the hydraulic properties of the aquifers and confining units with analytically derived type curves would have been inappropriate. Estimates of the lateral hydraulic conductivity from the VS2DT solution were more consistent with the observed geology than estimates from Theis analyses, which ranged from 20 to 80 percent more than the final estimates. The unsaturated zone affected an aquifer test conducted in a leaky aquifer about 100 feet below land surface more than the other two aquifer tests because about half of the pumped water came from the overlying, unconfined aquifer.
The Role of Sorption in the Transport of Klebsiella oxytoca Through Saturated Silica Sand
Authors: Hendry M. J. , Lawrence J. R. , Maloszewski P.
Published: August 2005

Experiments were conducted to investigate the role of sorption during the transport of the bacterium Klebsiella oxytoca through saturated silica sand. The sorption process was visualized at the pore scale in a minicell (3 mm deep × 6.0 mm wide × 7.0 cm long) using scanning confocal laser microscopy. The sorption process was also studied by conducting column experiments at three scales (3.8, 10, and 40 cm long). Results of image analyses of the sorbed and unattached cells in pore throats and the bacterial breakthrough data from the column experiments exhibited similar trends. Breakthrough peaks were attenuated with respect to the input concentrations and well‐defined tailing was observed. Visualization suggested that the sorption process was dominated by reversible and irreversible sorption (kirr). In the case of reversible sorption, the rate of forward sorption (kf) was different from the rate for reversible sorption (kr). Visualization also showed that the bacterial coverage on the sand grains, although extensive, covered < 0.5% of the available surface area. A 1D solution for advective‐dispersive transport was used to estimate kirr, kf, and kr with appropriate values for the coefficient of hydrodynamic dispersion and average linear pore‐water velocity (determined from CI data). Simulated best fits to the bacterial peaks were good for the 3.8 cm columns but underestimated peak heights in the 10 and 40 cm columns by one order of magnitude. Best‐fit kirr values decreased with increasing scale (0.6,0.13, and 0.062 hr‐1 for the 3.8,10, and 40 cm columns, respectively) and showed that a kirr value determined at one scale cannot be used to determine concentrations of K. oxytoca with time at another scale. These results suggested that kirr was a function of to (length of column over velocity). The equivalent irreversible sorption parameter (A, where A = to· kirr) was a constant (mean value of 3.36) for the three scales investigated. This observation suggested that the use of the value A, determined at one scale of investigation, may prove effective in approximating the value of kirr predicting bacterial transport at other scales. Best‐fit determinations yielded the same kf and kr values at all three scales (0.1 and 0.02 hr‐1). This suggested that reversible sorption may be independent of column length. This study emphasized the need for more comprehensive investigations of the role of sorption in the transport of microorganisms in the subsurface.
Alternative Spatial Estimators for Ground‐Water and Soil Measurements
Authors: Walker Douglas D. , Loftis Jim C.
Published: August 2005

Detail in mapping studies of spatial data results from spatial interpolation of field or laboratory measurements. The interpolation method used can markedly affect the interpolated estimates and thus the appearance of the resulting map. A popular method, oridinary kriging, is based on several assumptions, including normally distributed observations. For data containing outliers or “hotspots,” which are common in ground‐water monitoring, kriging may not be the most appropriate interpolation method. A robust, nonparametric alternative was developed, tested, and demonstrated using case‐study data from the San Luis Valley in Colorado. The method uses a robust measure of spatial correlation, called the medogram, as an inverse‐distance weight for estimates based on LAD (least‐absolute‐deviation) regression. For both synthetic and real case‐study data containing a few high values (outliers or “hotspots”), this LAD method showed improved performance over ordinary kriging. While performance of the LAD method is slightly better than kriging in terms of the mean squared interpolation error for the “contaminated” data sets studied, the significant advantage of the LAD approach is that it does not produce concentric contours around observations and thus provides more realistic maps.
Single‐Well, “Push‐Pull” Test for In Situ Determination of Microbial Activities
Authors: Istok J. D. , Humphrey M. D. , Schroth M. H. , Hyman M. R. , O'Reilly K. T.
Published: August 2005

A single‐well, “push‐pull” test method is proposed for the in situ determination of microbial metabolic activities in ground‐water aquifers. The method consists of the pulse‐type injection (“push”) of a test solution into the saturated zone of an aquifer through the screen of an existing monitoring well followed by the extraction (“pull”) of the test solution/ground‐water mixture from the same well. The test solution contains a tracer and one or more reactive solutes selected to investigate specific microbial activities. During the injection phase, the test solution flows radially away from the monitoring well into the aquifer. Within the aquifer, biologically reactive components of the test solution are converted to various products by the indigenous microbial community. During the extraction phase, flow is reversed and solute concentrations are measured to obtain breakthrough curves, which are used to compute the quantities of reactant(s) consumed and/or product(s) formed during the test and reaction rates. Tests were performed to determine rates of aerobic respiration, denitrification, sulfate reduction, and methanogenesis in a petroleum contaminated aquifer in western Oregon. High rates of oxygen, nitrate, nitrite, and hydrogen utilization and nitrite, and carbon dioxide production support the hypothesis that petroleum contamination has resulted in an increase in microbial activity in the anaerobic portion of the site. The results suggest that the push‐pull test method should be useful for obtaining quantitative information on a wide range of in situ microbial processes.
Detailed Pumping Test to Characterize a Fractured Bedrock Aquifer
Authors: Gernand Jeffrey D. , Heidtman Jeffrey P.
Published: August 2005

Bedrock fractures and results of a 24‐well, 21‐day pumping test were analyzed to characterize a low‐yielding fractured gneiss aquifer. Several analytical techniques for evaluating aquifer tests in fractured bedrock are reviewed and applied. The time‐drawdown curve for the pumping well and the early time‐drawdown responses at several nearby observation wells indicate linear flow through a single fracture. In contrast, analysis by radial flow techniques of the late time‐drawdown responses at all observation wells suggests that flow occurs through interconnected fractures. Taken together, these time‐drawdown responses are consistent with a model incorporating linear (single fracture) flow at the pumping well and radial (interconnected fracture) flow in an outer region.

The elliptical cone of depression reveals a direction of enhanced drawdown which closely parallels one of five observed fracture sets. However, this fracture set comprises fewer than 15 percent of all fractures present. The drawdown response should not be extrapolated to characterize the entire aquifer, because it does not result from an overriding structural feature which would influence the aquifer‐wide hydraulic response.

The results indicate the advisability of performing structural analysis along with aquifer analysis to characterize a bedrock aquifer. Further, the results indicate that in an aquifer where the hydraulic response at a pumping well suggests linear single fracture flow, if more observation wells or later time data are available it may be found that the aquifer responds with radial flow. The results also suggest that in certain bedrock aquifers, the larger the area of investigation, the more likely the aquifer will be shown to behave consistently with radial flow analytical techniques. In order to obtain the appropriate data, the goals of a bedrock pumping test and the structural data should be carefully considered before the test duration and observation well network are planned.
On the Performance of Computational Methods for the Assessment of Risk from Ground‐Water Contamination
Authors: Hamed Maged M. , Bedient Philip B.
Published: August 2005

The effect of parameter uncertainty and overly conservative measures on risk assessment has been addressed in numerous researches. Most of the work conducted to date is based on the use of the classic Monte Carlo simulation method (MCS) as a probabilistic modeling tool. Although the MCS is robust and asymptotically convergent, it lacks computational efficiency when the simulated probability is small. Furthermore, the sensitivity information can only be obtained with additional computational effort. First‐ and second‐order reliability methods (FORM and SORM) have been developed in the structural analysis field and have been recently applied to ground‐water contaminant transport and remediation problems. In this work, we extend the application of the reliability methods to the probabilistic assessment of cancer risk due to ground‐water contamination. Results of the reliability methods compared well with a published case study of PCE contamination of a ground‐water supply in California. The target risk level is extended over a larger range, and the sensitivity of the probability of failure to the relevant random variables is analyzed. The application of the methods to another case study, cancer risk due to the ingestion of benzene contaminated water, further illustrates a systematic way of directly accounting for the intrinsic uncertainty of the transport and fate model parameters involved in the risk assessment procedure. The probability of exceeding the target risk level in this case was found to be most sensitive to the uncertainty in the parameters describing the ground‐water transport process.
Water‐Level Fluctuations Due to Barometric Pressure Changes in an Isolated Portion of an Unconfined Aquifer
Authors: Hare Paul Wm. , Morse Ralph E.
Published: August 2005

The potential effect of barometric pressure changes on the water‐level elevations in wells tapping unconfined aquifers is often ignored or considered to be negligibly small. However, under certain circumstances, wells in unconfined aquifers can have barometric efficiencies greater than those typically observed for wells in confined aquifers. This is illustrated by results of a study performed at a federal Superfund site where a contaminated portion of the unconfined aquifer is isolated by a soil‐bentonite cutoff wall and clay cap. A well located within the containment system displays water‐level fluctuations which mirror barometric pressure changes. During the study, water‐level elevation ranged 36.6 centimeters (cm) [1.20 feet (ft)] in response to barometric pressure changes of 2.87 centimeters mercury (cm‐Hg) [1.13 inches mercury (in‐Hg)]–a barometric efficiency of about 94 percent (%). A well screened similarly outside the containment system did not display any observable barometric pressure effects. Vertical transmission of barometric pressure changes to the water table inside the containment system is retarded by the clay cap, and the soil‐bentonite cutoff wall retards the lateral transmission of barometric pressure changes from the unsaturated zone outside the containment system. Water‐level fluctuations of the magnitude observed during this study may need to be considered when developing monitoring programs to assess the performance of remedial systems.
Occurrence of Selected Herbicides and Herbicide Degradation Products in Iowa's Ground Water, 1995
Authors: Kolpin Dana W. , Kalkhoff Stephen J. , Goolsby Donald A. , Sneck‐Fahrer Debra A. , Thurman E. Michael
Published: August 2005

Herbicide compounds were prevalent in ground water across Iowa, being detected in 70% of the 106 municipal wells sampled during the summer of 1995. Herbicide degradation products were three of the four most frequently detected compounds for this study. The degradation product alachlor ethanesulfonic acid was the most frequently detected compound (65.1%), followed by atrazine (40.6%), and the degradation products deethylatrazine (34.9%), and cyanazine amide (19.8%). The corn herbicide acetochlor, first registered for widespread use in the United States in March 1994, was detected in a single water sample. No reported herbicide compound concentrations for this study exceeded currem U.S. Environmental Protection Agency's maximum contaminant levels or health advisory levels for drinking water, although the herbicide degradation products examined have yet to have such levels established.

The occurrence of herbicide compounds had a significant, inverse relation to well depth and a significant, positive relation to dissolved‐oxygen concentration. It is felt that both well depth and dissolved oxygen are acting as rough surrogates to ground‐water age, with younger ground water being more likely to contain herbicide compounds. The occurrence of herbicide compounds was substantially different among the major aquifer types across Iowa, being detected in 82.5% of the alluvial, 81.8% of the bedrock/ karst region, 40.0% of the glacial‐drift, and 25.0% of the bedrock/nonkarst region aquifers. The observed distribution was partially attributed to variations in general ground‐water age among these aquifer types. A significant, inverse relation was determined between total herbicide compound concentrations in ground water and the average soil slope within a 2‐km radius of sampled wells. Steeper soil slopes may increase the likelihood of surface runoff occurring rather than ground‐water infiltration–decreasing the transport of herbicide compounds to ground water. As expected, a significant positive relation was determined between intensity of herbicide use and herbicide concentrations in ground water.
Two‐Well Test Method for Determining Hydraulic Properties of Aquifers
Authors: Clement T. P. , Truex M. J. , Hooker B. S.
Published: August 2005

A steady‐state test to determine the hydraulic properties of an aquifer is presented. The test uses an injection‐extraction well‐pair to establish a two‐dimensional, steady‐state flow field in the tested region. At least one observation well is needed to measure the hydraulic head within the flow field. Depending on the number of observation wells, a single‐observation‐well method or a distance‐head method is used to analyze the head data. Two field examples demonstrate the potential application of the test. The field experiments were done at the U.S. Department of Energy Hanford site near Richland, Washington. The two‐well test procedure described in this work provides a useful alternative for characterizing aquifers while deploying in situ ground‐water remediation systems. One of the most attractive features of this test, especially at contaminated aquifers, is that there is no water to dispose. The test is simple and economical, and it also avoids several other limitations associated with typical pumping or slug tests.
An Assessment of Conventional In Situ Air Sparging Pilot Tests
Authors: Johnson Paul C. , Johnson Richard L. , Neaville Christopher , Hansen Erik E. , Stearns Steve M. , Dortch Ira J.
Published: August 2005

Short‐term (< 24 h) pilot tests play a key role in the selection and design of in situ air sparging systems. During air injection, changes in dissolved oxygen in ground water, water levels in wells, soil gas pressures, and soil gas contaminant concentrations are measured. These parameters are assumed to be indicators of air sparging feasibility and performance. In this work we assess the validity of this critical assumption. Data are presented from a study site where a typical short‐term test was conducted for three days, followed by continued operation of a full‐scale system for 110 days. Conventional sampling practices were augmented with more discrete and detailed assessment methods. In addition, a tracer gas was used to better investigate air distributions, vapor flow paths, and vapor recovery efficiency. The data illustrate that conclusions regarding the performance and applicability of air sparging can vary dramatically depending on the monitoring approach used. Samples collected for chemical analyses from conventional monitoring wells were clearly affected by the air sparging during, and for some time after, air injection. Results from this site suggest that typical pilot test measurements might be useful for assessing infeasibility, but do not yield sufficient information to extrapolate to long‐term performance.
Simulation of Regional Flow and Salinity Intrusion in an Integrated Stream‐Aquifer System in Coastal Region: Southwest Region of Bangladesh
Authors: Nobi N. , Gupta A. Das
Published: August 2005

A numerical model for simulation of the regional flow and salt‐water intrustion in an integrated stream‐aquifer system in coastal regions is developed, considering the dynamic interaction between the streams and the aquifer. The stream‐aquifer model comprised of a two‐dimensional depth‐average finite‐element model of the aquifer system and a quasi‐steady node and reach model of the river network. The applicability of the model was demonstrated, through simulation of the spatial and temporal distributions of flow and salinity in the estuaries and in the underlying aquifer of the Southwest Region of Bangladesh. The important management aspects of water transfer and additional pumping and their effects on the system were evaluated. The interactions between the streams and the aquifer significantly influenced the flow and salt‐water intrusion in the aquifer and the river network. An increased abstraction of ground water in the area caused a significant increase in the estuarine salinity. The salinity intrusion in the estuaries of the area, except in the southwest corner, could be reduced significantly by diverting the available water from the Ganges through the boundary river Gorai.
Delineation of Ground‐Water Flow Systems in the Southern Great Basin Using Aqueous Rare Earth Element Distributions
Authors: Johannesson Kevin H. , Stetzenbach Klaus J. , Hodge Vernon F. , Kreamer David K. , Zhou Xiaoping
Published: August 2005

The rare earth element (REE) signature of ground waters from both felsic volcanic rocks on the Nevada Test Site and from the regional Paleozoic carbonate aquifer of southern Nevada resemble the REE signature of the rocks through which they flow. Moreover, the REE signatures of Ash Meadows ground waters are similar to those of springs in the Furnace Creek region of Death Valley but different from shallow ground waters from predominantly tuffaceous alluvial deposits in the Amargosa Desert, perched ground waters from felsic volcanic rocks, and ground waters that have only flowed through the regional Paleozoic carbonate aquifer. The similar REE patterns of Ash Meadows and Furnace Creek ground waters support previous investigations that suggested ground waters discharging from the Furnace Creek springs are similar to the ground waters emerging from the Ash Meadows springs. The REE patterns indicate that the contribution of ground water from the Amargosa Desert to the Furnace Creek springs is of minor importance. Our REE analyses along with previous stable isotope, ground‐water potentiometric surface relationships, and geologic structure analyses support ground‐water flow from east to west in the fractured and faulted carbonate rocks beneath Ash Meadows, the Amargosa Desert, and the southern end of the Funeral Mountains. Our observations are contrary to some previous investigations that identified shallow ground waters from the central and northwestern Amargosa Desert as a substantial component of the ground water that discharges from the Furnace Creek springs.
Simulating Reservoir Leakage in Ground‐Water Models
Authors: Fenske Jon P. , Leake S. A. , Prudic David E.
Published: August 2005

Leakage to ground water resulting from the expansion and contraction of reservoirs cannot be easily simulated by most ground‐water flow models. An algorithm, entitled the Reservoir Package, was developed for the United States Geological Survey (USGS) three‐dimensional finite‐difference modular ground‐water flow model MODFLOW. The Reservoir Package automates the process of specifying head‐dependent boundary cells, eliminating the need to divide a simulation into many stress periods while improving accuracy in simulating changes in ground‐water levels resulting from transient reservoir stage. Leakage between the reservoir and the underlying aquifer is simulated for each model cell corrresponding to the inundated area by multiplying the head difference between the reservoir and the aquifer with the hydraulic conductance of the reservoir‐bed sediments.
Tracing Recharge from Sinking Streams over Spatial Dimensions of Kilometers in a Karst Aquifer
Authors: Greene Earl A.
Published: August 2005

Stable isotopes of hydrogen and oxygen were used to trace the sources of recharge from sinking streams to wells and springs several kilometers downgradient in the karst Madison aquifer near Rapid City, South Dakota. Temporal sampling of streamflow above the swallets identified a distinct isotopic signature that was used to define the spatial dimensions of recharge to the aquifer. When more than one sinking stream was determined to be recharging a well or spring, the proportions were approximated using a two‐component mixing model. From the isotopic analysis, it is possible to link sinking stream recharge to individual wells or springs in the Rapid City area and illustrate there is significant lateral movement of ground water across surface drainage basins. These results emphasize that well‐head protection strategies developed for carbonate aquifers that provide industrial and municipal water supplies need to consider lateral movement of ground‐water flow from adjacent surface drainage basins.
Tritium/3He Dating of River Infiltration: An Example from the Danube in the Szigetköz Area, Hungary
Authors: Stute M. , Deák J. , Révész K. , Böhlke J. K. , Deseö E. , Weppernig R. , Schlosser P.
Published: August 2005

3H, He, 4He, and Ne data were obtained from a shallow ground‐water system being recharged by bank infiltration from the Danube River in northwestern Hungary. After correting for excess air, 4He and Ne concentrations reflect a recharge temperature of about 9° C., close to the mean annual temperature of the Danube (10.4° C). Values of H plus 3Hetrit (“initial tritium”) as a function of the tritium/He age are consistent with time series measurements of tritium in the Danube. Tritium/ He ages increase linearly as a function of distance from the Danube along ground‐water flow lines. A horizontal flow velocity of about 530 m yr1 was derived from the age gradient. Most of the deviations between measured Danube tritium data and ground‐water tritium/He data can be explained by dispersive mixing.
Sensitivity Models and Design Protocol for Partitioning Tracer Tests in Alluvial Aquifers
Authors: Jin M. , Butler G. W. , Jackson R. E. , Mariner P. E. , Pickens J. F. , Pope G. A. , Brown C. L. , McKinney D. C.
Published: August 2005

Zones of dense, nonaqueous phase liquids (DNAPLs) are difficult to characterize as to their volume, composition, and spatial distribution using conventional ground‐water extraction and soil‐sampling methods. Such incompletely characterized sites have negative consequences for those responsible for their remedial design, e.g., the uncertainties in the optimal placement of ground‐water extraction wells and in the duration of remediation. However, the recent use of the partitioning interwell tracer test (PITT) to characterize DNAPL zones at sites in New Mexico [unsaturated alluvium] and in Ohio, Texas, and Utah [saturated alluvium] demonstrates that the volume and spatial distribution of residual DNAPL can be determined with accuracy. The PITT involves injection of a suite of tracers which reversibly partition to different degrees between the DNAPL and the ground water or soil air resulting in the chromatographic separation of the tracer signals observed at the extraction well(s). The design of a PITT requires careful consideration of the hydrostratigraphic, hydraulic, and certain geochemical properties of the alluvium being tested. A three‐dimensional, numerical model of a heterogeneous alluvial aquifer containing DNAPL has been developed for use with the UTCHEM simulator to demonstrate partitioning tracer testing and to address questions that are frequently raised in its application. The simulations include (1) the estimation of DNAPL volume for the simple case where only residual DNAPL is present in heterogeneous alluvium, (2) sensitivity studies to demonstrate the effect of increasingly low residual DNAPL saturation on the tracer signal, and (3) the effect of free‐phase DNAPL on the estimation of the volume of DNAPL present. Furthermore, the potential interference of sedimentary organic carbon as a DNAPL surrogate on the tracer signal is considered and shown to be readily resolved by the careful choice of tracers. Finally, a protocol for the use of PITTs in alluvial aquifers is presented.
Interaction of Lake Michigan with a Layered Aquifer Stressed by Drainage
Authors: Cherkauer Douglas S. , Carlson Douglas A.
Published: August 2005

Ground‐water interaction with lakes is often difficult to document because of both its diffusiveness and its spatial and temporal variability; reproducible measurements can be difficult to obtain. This paper takes advantage of the construction of a major drain stress in an aquifer system proximal to Lake Michigan to examine ground‐water/lake interactions in a complex hydrogeological setting.

At the time of the measurements, the tunnel (4 to 8 m in diameter, 13 km in length, 1 to 3 km inland, and constructed in a low conductivity unit within a layered, fractured dolomite aquifer) had produced drawdowns of up to 50 m in the aquifer. In turn, the resulting trough of depression was inducing over 1600 m3/day (425,000 gpd) of water to flow from the lake to the tunnel. Seepage meters have been deployed over 40 km2 of the lake bed to define the extent of the tunnel's impact.

The results show that seepage through the lake bed is a combination of downward flow toward the tunnel and lateral inflow generated by recharge events in shallow unconsolidated sediments. Downward seepage is detectable to more than 2 km offshore, so an average of over 20 m of clay‐rich glacial deposits and 10 m of dolomitic shale (both with hydraulic conductivities < 3 × 10‐8 m/sec) does not isolate the lake from the aquifer. Seepage meters have proven to be a viable and cost‐effective means to provide an accurate spatial distribution of ground‐water/lake exchanges over a large area.
A New Time Domain Random Walk Method for Solute Transport in 1–D Heterogeneous Media
Authors: Banton Olivier , Delay Frédérick , Porel Gilles
Published: August 2005

A new method to simulate solute transport in 1–D heterogeneous media is presented. This time domain random walk method (TDRW), similar in concept to the classical random walk method, calculates the arrival time of a particle cloud at a given location (directly providing the solute breakthrough curve). The main advantage of the method is that the restrictions on the space increments and the time steps which exist with the finite differences and random walk methods are avoided. In a homogeneous zone, the breakthrough curve (BTC) can be calculated directly at a given distance using a few hundred particles or directly at the boundary of the zone. Comparisons with analytical solutions and with the classical random walk method show the reliability of this method. The velocity and dispersivity calculated from the simulated results agree within two percent with the values used as input in the model. For contrasted heterogeneous media, the random walk can generate high numerical dispersion, while the time domain approach does not.
Predicting the Probability of Elevated Nitrate Concentrations in the Puget Sound Basin: Implications for Aquifer Susceptibility and Vulnerability
Authors: Tesoriero Anthony J. , Voss Frank D.
Published: August 2005

The occurrence and distribution of elevated nitrate concentrations (≥ 3 mg/l) in ground water in the Puget Sound Basin, Washington, were determined by examining existing data from more than 3000 wells. Models that estimate the probability that a well has an elevated nitrate concentration were constructed by relating the occurrence of elevated nitrate concentrations to both natural and anthropogenic variables using logistic regression. The variables that best explain the occurrence of elevated nitrate concentrations were well depth, surficial geology, and the percentage of urban and agricultural land within a radius of 3.2 kilometers of the well. From these relations, logistic regression models were developed to assess aquifer susceptibility (relative ease with which contaminants will reach aquifer) and ground‐water vulnerability (relative ease with which contaminants will reach aquifer for a given set of land‐use practices). Both models performed well at predicting the probability of elevated nitrate concentrations in an independent data set. This approach to assessing aquifer susceptibility and ground‐water vulnerability has the advantages of having both model variables and coefficient values determined on the basis of existing water quality information and does not depend on the assignment of variables and weighting factors based on qualitative criteria.
Capture Zone Geometry in a Fractured Carbonate Aquifer
Authors: Podgorney Robert K. , Ritzi Robert W.
Published: August 2005

This study examined a fractured carbonate aquifer that has a transition from porous media type (continuum) flow near the bedrock surface to discrete fracture (non continuum) flow at depth. Three depth zones were delineated using a borehole flowmeter, borehole video logs, and pumping tests. The upper zone is fractured to the degree where it behaves hydraulically as a continuum, the middle zone is less fractured and behaves as a discretely fractured aquifer, and the lower zone is least fractured, has no measurable fracture interconnection, and behaves as an aquitard. These zones were not related to lithologic boundaries, showing that monitoring well design based solely on lithology may be inappropriate in some fractured systems.

The geometries of capture zones in this aquifer were determined by combining the field observations with numerical modeling. The capture zone geometries are very complex, containing thin a really extensive features around fractures in the middle zone which extend over an area 17.5 times greater than the capture area in the upper continuum zone. A capture zone computed with lumped aquifer parameters leads to inaccurate conceptualization of capture zone geometry at this site.

The presence of open core hole monitoring wells affected the flow regime under both ambient and pumping conditions. The wells act as short circuits between otherwise isolated fractures and fracture zones. By connecting the continuum to the non continuum flow regime with the wells, ambient flow in the non continuum regime was increased by a factor of 20. Under pumping conditions, the presence of the monitoring wells alters the capture zone of the pumping well. Discrete fractures provide a connection between the pumping and observation wells at depth that causes separate cones of depression to be formed around observation wells in the upper aquifer, and thus, the capture zone in the near‐surface aquifer may include multiple, isolated areas around monitoring wells.
Factors Influencing Methane Distribution in Texas Ground Water
Authors: Zhang Chuanlun , Grossman Ethan L. , Ammerman James W.
Published: August 2005

To determine the factors that influence the distribution of methane in Texas ground water, water samples were collected from 40 wells in east‐central and central Texas aquifers. Among the chemical parameters examined, sulfate is most important in controlling methane distribution. Methane occurs in high concentration (>10 μM) in east‐central Texas only where sulfate concentration is low (<1 μM), supporting the hypothesis that abundant microbial methane production does not begin until sulfate is depleted. Because water samples from central Texas are high in either oxygen (up to 172 μM) or sulfate (up to 28.7 μM), methane concentrations are low (<5 μM) in these waters. A positive correlation between methane and sulfate in these waters indicates a different, perhaps thermogenic, origin for the trace methane.

The 13C/12C ratios of dissolved methane ranged from ‐80%o to ‐21%o in east‐central Texas and from ‐41.2%e to ‐8.5%e in central Texas. Low values of < −50%e in the east‐central Texas ground water indicate a microbial origin for methane and are consistent with the observed sulfate‐methane relationship; high 13C/12C ratios of > −31%e likely result from bacterial methane oxidation. Similarly, methane with high 13C/12C ratios in central Texas may reflect partial oxidation of the methane pool. Overall, water samples from both regions show a positive correlation between sulfate concentration and the 13C/12C ratio of methane, suggesting that methane oxidation may be associated with sulfate reduction in Texas ground water.
Simulated Effects of Irrigation on Salinity in the Arkansas River Valley in Colorado
Authors: Goff Karin , Lewis Michael E. , Person Mark A. , Konikowd Leonard F.
Published: August 2005

Agricultural irrigation has a substantial impact on water quantity and quality in the lower Arkansas River valley of southeastern Colorado. A two‐dimensional flow and solute transport model was used to evaluate the potential effects of changes in irrigation on the quantity and quality of water in the alluvial aquifer and in the Arkansas River along an 17.7 km reach of the river. The model was calibrated to aquifer water level and dissolved solids concentration data collected throughout the 24 year study period (1971–95). Two categories of irrigation management were simulated with the calibrated model: (1) a decrease in ground water withdrawals for irrigation; and (2) cessation of all irrigation from ground water and surface water sources. In the modeled category of decreased irrigation from ground water pumping, there was a resulting 6.9% decrease in the average monthly ground water salinity, a 0.6% decrease in average monthly river salinity, and an 11.1% increase in ground water return flows to the river. In the modeled category of the cessation of all irrigation, average monthly ground water salinity decreased by 25 %; average monthly river salinity decreased by 4.4 %; and ground water return flows to the river decreased by an average of 64 %. In all scenarios, simulated ground water salinity decreased relative to historical conditions for about 12 years before reaching a new dynamic equilibrium condition. Aquifer water levels were not sensitive to any of the modeled scenarios. These potential changes in salinity could result in improved water quality for irrigation purposes downstream from the affected area.
Absolute Hydraulic Conductivity Estimates from Aquifer Pumping and Tracer Tests in a Stratified Aquifer
Authors: Thorbjarnarson K.W. , Huntley D. , McCarty J.J.
Published: August 2005

Independent estimates of absolute hydraulic conductivity were obtained by a “standard” aquifer pumping test and a forced‐gradient tracer test in a highly heterogeneous aquifer. An aquifer hydraulic test was conducted to evaluate the average hydraulic conductivity (K), and to establish steady‐state flow for the tracer test. An average K of 48 m/day was interpreted from the drawdown data in a fully screened well. Type‐curve matching and simulation with MODFLOW of the hydraulic response in partially screened wells indicates K of 10 to 15 m/day for the upper section and 71 to 73 m/day for the deeper section. Iodide and fluorescent dye tracers were injected at low rates in wells located approximately 8 m upgradient of the production well. Tracer breakthrough was monitored in the production well and at ten depth intervals within the fully screened monitoring well. Interpretation of tracer response in the production well reveals tracer transport is limited to a 3.9 m thick section of the 20 m thick aquifer, with a hydraulic conductivity of 248 m/day. However, the depth distribution of these permeable strata cannot be determined from the production well tracer response. When sampled at 1.5 m depth intervals in the monitoring well, breakthrough was observed in only three intervals along the entire 18.2 m screened well. K estimates from tracer travel time within discrete high‐permeability strata range from 31 to 317 m/day. Inclusion of permeameter K estimates for the lower permeability aquifer sands (4–10 m/day) result in a range in relative K of 0.01 to 1.0. This field site has the highest absolute K estimate for a discrete stratum and the widest range in relative hydraulic conductivity amongst research field sites with K estimates for discrete strata. Within such a highly stratified aquifer, the use of an average K from an aquifer pumping test to predict solute transport results in great underestimation of transport distances for a given time period.
Development of Three‐Dimensional Hydrostratigraphical Architecture of the Unsaturated Zone Based on Soft and Hard Data
Authors: Langsholt Elin , Kitterød Nils‐Otto , Gottschalk Lars
Published: August 2005

Three‐dimensional geological maps that include the hydrostratigraphical heterogeneity and depositional architecture of the natural formation are needed in hydrogeology to understand flow and transport. This paper presents a practical procedure for the generation of such maps. The method includes multiple indicator kriging, where the estimation of the structural architecture is conditioned on ground penetrating radar data. Physical core samples are used as tie points. This study utilizes data obtained from the unsaturated zone of a highly heterogeneous, glaciofluvial delta formation in Norway.
Rhodamine WT and Bacillus subtilis Transport through an Alluvial Gravel Aquifer
Authors: Pang Liping , Close Murray , Noonan Mike
Published: August 2005

A field study was conducted to characterize the transport of rhodamine WT dye and endospores of the bacterium Bacillus subtilis through preferential flowpaths in an alluvial gravel aquifer using natural gradient tracer experiments. Preliminary tracer experiments were conducted with rhodamine WT to determine preferential flowpaths using a resin bag method, because of spatial heterogeneity at the study site. Rhodamine WT, Cl, and B. subtilis endospores were then injected 0.1 to 1.6 m below the water table, and downgradient ground water was monitored.

Rhodamine WT behaved like Cl in the field and was used as a nonreactive tracer in this study. The method of time moments (MTM) and a curve fitting method (CFM) using the AT123D model were applied to estimate transport parameters of rhodamine WT and B. subtilis endospores along preferential flowpaths within the aquifer, assuming each individual flowpath to be uniform. The results from CFM fit the observed data better than did MTM. Ground water velocities through preferential flowpaths ranged from 30 to 85 m/day with a median of 63 m/day, and longitudinal dispersivities ranged from 0.71 to 5.24 m with a median of 2.70 m. These values are within the range reported in the literature for similar hydrogeological conditions.

B. subtilis endospores exhibited slightly faster velocities (median retardation factor 0.86) and lower longitudinal dispersivities (0.23 to 2.4 m, median 0.79 m) compared to rhodamine WT. When multiple concentration peaks were observed in the rhodamine WT breakthrough curves (BTCs), the bacteria always arrived with the first rhodamine WT peak, regardless of whether the first peak was small or large. For those wells with a single peak for both the dye and the bacteria, the bacteria tended to coincide with the front portion of the dye BTC. This suggests that convection is relatively more important for B. subtilis endospores than for the dye, probably due to pore size exclusion of the spores. For this reason, the use of non‐reactive solute tracers to reflect the movement of microorganisms may not be appropriate. The total removal rates of B. subtilis varied from 2.4 to 9.36 day−1 (median 3.36 day−1, probably resulting from filtration, sedimentation, and irreversible adsorption, as die‐off was not observed in the field during the 45 days of the study. The longevity of B. subtilis endospores in ground water makes it a good choice to use as a bacterial tracer in the study of transport processes in the absence of die‐off.
Radon in Quaternary Aquifers Related to Underlying Bedrock Geology
Authors: Morland G. , Strand T. , Furuhaug L. , Skarphagen H. , Banks D.
Published: August 2005

A survey of radon concentrations in water abstracted from 31 of Norway's largest waterworks, using ground water from Quaternary fluvial and glaciofluvial sediments, returned values of between 0.4 Bq/L and 83 Bq/L, with a median of 23 Bq/L. Significantly higher Rn concentrations were present in ground water from Quaternary aquifers underlain by gneissic and granitic lithologies compared with those underlain by metasandstones, phyllites and mica schists. Compared to the recommended national action level of 500 Bq/L and concentrations of up to 19,900 Bq/L, which have been detected in boreholes in Norwegian granite aquifers, the concentrations measured in Quaternary aquifers are regarded as unproblematic for consumers, although a more detailed assessment may be required for workers spending a lot of time in wellhead areas.