bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Joint TEM‐HLEM Geophysical Approach to Borehole Siting in Deeply Weathered Granitic Terrains
Authors: Meju M.A. , Fontes S. L. , Ulugergerli E.U. , La Terra E.F. , Germano C.R. , Carvalho R.M. 
Published: December 2005

AbstractThe accurate location of aquiferous fracture zones in granite beneath a > 50 m thick weathered mantle in semi‐arid regions is a major hydrogeological problem. It is expected that the zone of intensive fracturing will be more susceptible to weathering and thus be characterized by the thickest development of saprolite, a good electrically conductive target for deep‐probing electromagnetic systems. The single‐loop transient electromagnetic (TEM) technique is well known to have the capability for detecting concealed steep mineralized targets in mining environments and can be adapted to this hydrogeological problem. We propose that combining the conventional frequency‐domain horizontal‐loop electromagnetic (HLEM) and single‐loop TEM is an effective practical approach to locating concealed aquiferous fracture zones. In the supporting case studies presented here, we deployed multifre‐quency HLEM profiling (with 50 m transmitter‐receiver separation) and TEM soundings with contiguous 10 or 20 m sided loops along the survey lines in a granitic terrain affected by deep (> 50 m) weathering in northeast Brazil. A somewhat layered structure consisting of resistive hardpan/leached zone, conductive saprolite, and resistive basement is identifiable in the typical TEM depth sounding data. We obtained coincident HLEM and TEM anomalies at all the sites, enabling a relatively straightforward selection of potential drilling positions. Simple resistivity‐depth transformation of the TEM data was done for each site, yielding an approximate section from which drilling depths were estimated. All of the boreholes located were successful. Although our results appear to indicate that the single‐loop TEM method could be used independently for borehole siting in deeply weathered granitic terrains and that the weathering profile over granite can be mapped using TEM depth soundings of appropriate observational bandwidth, we recommend a joint electromagnetic approach for optimal well siting.
Calibration of the DRASTIC Ground Water Vulnerability Mapping Method
Authors: Rupert M.G. 
Published: December 2005

AbstractGround water vulnerability maps developed using the DRASTIC method have been produced in many parts of the world. Comparisons of those maps with actual ground water quality data have shown that the DRASTIC method is typically a poor predictor of ground water contamination. This study significantly improved the effectiveness of a modified DRASTIC ground water vulnerability map by calibrating the point rating schemes to actual ground water quality data by using nonparametric statistical techniques and a geographic information system. Calibration was performed by comparing data on nitrite plus nitrate as nitrogen (NO2+ NO3‐N) concentrations in ground water to land‐use, soils, and depth to first‐encountered ground water data. These comparisons showed clear statistical differences between NO2+ NO3‐N concentrations and the various categories. Ground water probability point ratings for NO2+ NO3‐N contamination were developed from the results of these comparisons, and a probability map was produced. This ground water probability map was then correlated with an independent set of NO2+ NO3‐N data to demonstrate its effectiveness in predicting elevated NO2+ NO3‐N concentrations in ground water. This correlation demonstrated that the probability map was effective, but a vulnerability map produced with the uncalibrated DRASTIC method in the same area and using the same data layers was not effective. Considerable time and expense have been outlaid to develop ground water vulnerability maps with the DRASTIC method. This study demonstrates a cost‐effective method to improve and verify the effectiveness of ground water vulnerability maps.
Potential Artifacts in Interpretation of Differential Breakthrough of Colloids and Dissolved Tracers in the Context of Transport in a Zero‐Valent Iron Permeable Reactive Barrier
Authors: Zhang Pengfei , Johnson William P. , Piana Michael J. , Fuller Christopher C. , Naftz David L. 
Published: December 2005

AbstractMany published studies have used visual comparison of the timing of peak breakthrough of colloids versus conservative dissolved tracers (hereafter referred to as dissolved tracers or tracers) in subsurface media to determine whether they are advected differently, and to elucidate the mechanisms of differential advection. This purely visual approach of determining differential advec‐tion may have artifacts, however, due to the attachment of colloids to subsurface media. The attachment of colloids to subsurface media may shift the colloidal peak breakthrough to earlier times, causing an apparent “faster” peak breakthrough of colloids relative to dissolve tracers even though the transport velocities for the colloids and the dissolved tracers may actually be equivalent. In this paper, a peak shift analysis was presented to illustrate the artifacts associated with the purely visual approach in determining differential advection, and to quantify the peak shift due to colloid attachment. This peak shift analysis was described within the context of microsphere and bromide transport within a zero‐valent iron (ZVI) permeable reactive barrier (PRB) located in Fry Canyon, Utah. Application of the peak shift analysis to the field microsphere and bromide breakthrough data indicated that differential advection of the microspheres relative to the bromide occurred in the monitoring wells closest to the injection well in the PRB. It was hypothesized that the physical heterogeneity at the grain scale, presumably arising from differences in inter‐ versus intra‐particle porosity, contributed to the differential advection of the microspheres versus the bromide in the PRB. The relative breakthrough (RB) of microspheres at different wells was inversely related to the ionic strength of ground water at these wells, in agreement with numerous studies showing that colloid attachment is directly related to solution ionic strength.
Implications of Observed and Simulated Ambient Flow in Monitoring Wells
Authors: Elci Alper , Molz Fred J. , Waldrop William R. 
Published: December 2005

AbstractA recent paper by Hutchins and Acree (2000) has called attention to ground water sampling bias due to ambient (natural gradient‐induced) flows in monitoring wells. Data collected with borehole flowmeters have shown that such ambient flows are ubiquitous in both confined and unconfined aquifers. Developed herein is a detailed three‐dimensional model of flow and transport in the vicinity of a fully penetrating monitoring well. The model was used to simulate a measured ambient flow distribution around a test well in a heterogeneous aquifer at the Savannah River Site (SRS) near Aiken, South Carolina. Simulated ambient flows agreed well with measurements. Natural flow was upward, so water entered the well mainly through high K layers in the lower portion of the aquifer and exited through similar layers in the upper portion. The maximum upward discharge in the well was about 0.28 L/min, which implied an induced exchange of 12 m3/month from the bottom half of the aquifer to the upper half. Tracer transport simulations then illustrated how a contaminant located initially in a lower portion of the aquifer was continuously transported into the upper portion and diluted throughout the entire well by in‐flowing water. Even after full purging or micropurging, samples from such a well will yield misleading and ambiguous data concerning solute concentrations, location of a contaminant source, and plume geometry. For all of these reasons, use of long‐screened monitoring wells should be phased out, unless an appropriate multilevel sampling device prevents vertical flow.
Viral Transport in a Sand and Gravel Aquifer Under Field Pumping Conditions
Authors: Woessner William W. , Ball Patrick N. , DeBorde Dan C. , Troy Thomas L. 
Published: December 2005

AbstractGround water supplies contaminated with microbes cause more than 50% of the water‐borne disease outbreaks in the United States. Proposed regulations suggest natural disinfection as a possible mechanism to treat microbe‐impacted ground water under favorable conditions. However, the usefulness of current models employed to predict viral transport and natural attenuation rates is limited by the absence of field scale calibration data. At a remote floodplain aquifer in western Montana, the bac‐teriophages MS2, φX174, and PRD1; attenuated poliovirus type‐1 (CHAT strain); and bromide were seeded as a slug 21.5 m from a well pumping at a steady rate of 408 L/min. Over the 47‐hour duration of the test, resulting in the exchange of 12 to 13 pore volumes, 77% of the bromide, 55% of the PRD1,17% of the MS2,7% of the φX174, and 0.12% of the poliovirus masses were recovered at the pumping well. Virus transport behavior was controlled by mechanical dispersion, preferential flow, time‐dependent nonreversible and reversible attachment, and apparent mass transfer to immobile domains within the sand and gravel dominated aquifer. The percentage of virus recovery appears correlated with reported viral isoelectric point (pI) values. Successful modeling of viral transport in coarse‐grained aquifers will require separation of viral specific properties from reported lumped viraltransport system parameters.
Development and Evaluation of an Ultrasonic Ground Water Seepage Meter
Authors: Paulsen Ronald J. , Smith Christopher F. , O'Rourke Daniel , Wong Teng‐Fong 
Published: December 2005

AbstractSubmarine ground water discharge can influence significantly the near‐shore transport and flux of chemicals into the oceans. Quantification of the sources and rates of such discharge requires a ground water seepage meter that provides continuous measurements at high resolution over an extended period of time. An ultrasonic flowmeter has been adapted for such measurements in the submarine environment. Connected to a steel collection funnel, the meter houses two piezoelectric transducers mounted at opposite ends of a cylindrical flow tube. By monitoring the perturbations of fluid flow on the propagation of sound waves inside the flow tube, the ultrasonic meter can measure both forward and reverse fluid flows in real time. Laboratory and field calibrations show that the ultrasonic meter can resolve ground water discharges on the order of 0.1 μm/sec, and it is sufficiently robust for deployment in the field for several days. Data from West Neck Bay, Shelter Island, New York, elucidate the temporal and spatial heterogeneity of submarine ground water discharge and its interplay with tidal loading. A negative correlation between the discharge and tidal elevation was generally observed. A methodology was also developed whereby data for the sound velocity as a function of temperature can be used to infer the salinity and source of the submarine discharge. Independent measurements of electrical conductance were performed to validate this methodology.
Selecting MODFLOW Cell Sizes for Accurate Flow Fields
Authors: Haitjema Henk , Kelson Vic , de Lange Wim 
Published: December 2005

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

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

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

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

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

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

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

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