bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractThe 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

AbstractThe 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

AbstractHerbicide 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

AbstractA 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

AbstractShort‐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

AbstractA 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

AbstractThe 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

AbstractLeakage 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

AbstractStable 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

Abstract 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

AbstractZones 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

AbstractGround‐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

AbstractA 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

AbstractThe 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

AbstractThis 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.