bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Sensitivity of Remedial Strategies to Design Criteria
Authors: Ahlfeld David P. , Hill Edward H. 
Published: August 2005

AbstractThe specific criteria for design of systems for the remediation of contaminated ground water can affect the cost, configuration, and feasibility of the remedial system. Two important design criteria are the degree of remediation required, as measured by water concentration, and the time by which compliance must be achieved. In this paper the impact of these criteria on remedial design is studied for a hypothetical pump‐and‐treat remedial system applied to a simplified model of a large organic solvents plume in Tucson, Arizona. The analysis is performed by combining two‐dimensional numerical flow and transport modeling with nonlinear optimization. The use of optimization provides a means of systematically varying design criteria in an experimental framework. Based on the data for this site, variation of the concentration standard over one order of magnitude can affect the optimal cost by as much as 220%. Change in the time to achieve compliance over less than one order of magnitude can affect optimal cost by as much as 175% and in some cases can produce problems which are infeasible due to physical or technical limitations. Under some conditions, a time to achieve compliance can be identified which has minimum cost. Changes in the design criteria are also shown to have significant impact on the distribution of steady pumping rates among the individual wells.
Ground‐Water Management Model for an Extensive Multiaquifer System and an Application
Authors: Gupta A. Das , Nobi N. , Paudyal G. N. 
Published: August 2005

AbstractA ground‐water management model with an economic objective has been developed using the numerically simulated unit response functions representing the physical behavior of the system. A hierarchical approach with multilevel decomposition of the problem has been adopted to arrive at the solution for a large‐scale physical system. The developed model was successfully applied and tested for the multiaquifer system underlying the City of Bangkok.
Use of a Numerical Model for Management of Shallow Ground‐Water Levels in the Yuma, Arizona Area
Authors: Hill Bradley M. 
Published: August 2005

AbstractThe Yuma area has experienced agricultural development since the late 1890s and ground‐water levels have risen over 70 feet due to recharge from heavy application of irrigation water, unlined canals and flooding along the Colorado and Gila Rivers. The resulting shallow water levels have seriously impacted residential areas and prime agricultural land.The Arizona Department of Water Resources in conjunction with Yuma County Flood Control District developed a regional three‐dimensional ground‐water flow model of the Yuma area. The purpose of the model is to assist local agencies in evaluating remedial water management alternatives to mitigate the shallow ground‐water level problems.The model domain incorporates over 900 mi2 of Arizona, California, and Mexico and simulates ground‐water pumpage, deep percolation from agricultural irrigation, evapotranspiration from phreatophytes and flow in 12 canals, 16 drains, and the Colorado and Gila Rivers. The model contains four layers with over 30,000 model cells ranging in size from 40 acres to 640 acres.Different model scenario simulations were conducted to evaluate the effectiveness of proposed water management alternatives on lowering ground‐water levels within the northern portion of Yuma Valley. These scenarios include lining a portion of the East Main canal and pumping two drainage wells, lining the All‐American canal, and simulating a decrease in deep percolation from agricultural irrigation on the Yuma Mesa and northern portion of Yuma Valley.
Spatial Characterization of Hydrogeochemistry Within a Constructed Fen, Greene County, Ohio
Authors: Hite Christopher D. , Cheng Songlin 
Published: August 2005

AbstractAn artificial fen environment was constructed near the municipality of Beavercreek, Ohio. The constructed wetland environment utilizes local ground‐water flow patterns to maintain a high degree of water saturation. Chemical reactions driven by the activity of plant roots and soil microorganisms affect the spatial distribution and magnitude of hydrogeochemical parameters, including alkalinity, pH, redox potential, and concentrations of Ca+2, Mg+2, total iron, NO3−, and SO4−2. The purpose of this study is to investigate the chemical interactions between wetland plants and ground water by means of characterizing the spatial variation in hydrogeochemical parameters. Nested piezometers within the artificial fen are used to monitor ground‐water chemistry within, and immediately adjacent to, the rhizosphere of cultivated plant species. The major reactions at the site include oxidation of organic matter by Fe(OH)3 and dissolution of carbonate minerals driven by high CO2 production in the root zone. The relationship between alkalinity and dissolved concentrations of Ca+2 and Mg+2 suggests that organic anions could also be major alkalinity contributors. Redox potential is buffered by the reduction of Fe (OH)3 and is typically maintained in the range of 100–200 mV. This buffering effect prevents the formation of sulfide and methane commonly found in similar wetland environments. This finding suggests that methane production associated with rice cultivation can be prevented by the addition of Fe(OH)3 to rice fields.
Problem‐Dependence of Ground‐Water Model Identification: Significance, Extent, and Treatment
Authors: Gorokhovski Vincent M. 
Published: August 2005

AbstractBecause geologic settings are not fully known (data are sparse) and because geologic properties vary spatially, flow modelers resort to simplifying conceptualization (simplified geological models) of the settings to perform modeling. But as results of identification of a simplified model depend on identification problem formulation, simplification poses two issues: (1) Since coupled identification and prediction problems are differently formulated problems, successful identification (calibration) of a model does not mean that the model will be good in prediction, so the goal of model identification should be the model's parameters effective in an intended application. (2) Since the identification results are problem‐dependent, they may be deprived of geological meaning. Therefore, they should be interpreted in the terms of their relation to the real world: To use them without understanding their meanings is potentially damaging. The key to both issues is explicit mathematical presentation of the spatially variable geologic properties transformations to the ground‐water model parameters. Canonical forms of the transformations, displaying readily inherent features of those transformations, are introduced. Two‐level modeling, exploiting the canonical forms, is suggested as an approach to address the above‐mentioned issues. The higher level models represent real geologic settings. The lower level model presents a conventional geological model. Comparison of responses to the same impact between the higher level models and the geological model should permit inference of the canonical forms for the given geological model. The forms can be used to choose predictive model effective parameters, interpret observed data, and optimize field investigation. Two simple examples illustrate the main notions of the approach.
Ground‐Water Quality and Flow in a Shallow Glaciofluvial Aquifer Impacted by Agricultural Contamination
Authors: Kehew Alan E. , Straw W. Thomas , Steinman William K. , Barrese Patrick G. , Passarella Giuseppe , Peng Wei‐Shyuan 
Published: August 2005

AbstractThe Prairie Ronde fan, a discrete glaciofluvial deposit in southwestern Michigan, contains a productive but highly vulnerable unconfined aquifer used for irrigation, municipal, and domestic supply. A comprehensive hydrogeological study of the aquifer delineated shallow, local flow systems that interact with ponds and wetlands on the fan surface, overlying a deeper intermediate/regional flow system extending to the base of the glacial drift. Ground water within the shallow flow systems contains tritium concentrations indicative of a post‐bomb age and is heavily impacted by nonpoint source contamination. Nitrate commonly exceeds drinking water standards in the shallow flow system. Although no continuous physical barrier separates the two flow systems, the deeper flow system is generally lacking in tritium as well as nonpoint source contaminants derived from surface land uses. High capacity pumping from the deeper flow system, however, will likely draw contaminants downward from the shallow flow system.Background‐water quality in the aquifer is controlled by equilibrium with calcite and slight undersaturation with respect to dolomite. No spatial trends in major ions were observed, suggesting that carbonate mineral equilibrium is achieved rapidly in the vadose zone and further chemical evolution along ground‐water flow paths is minimal. Iron concentrations are highly variable in the aquifer and not correlated with depth. Recharge from lakes and wetlands is a significant cause of elevated iron concentrations in the shallow flow systems.
A Kernel Estimator for Stochastic Subsurface Characterization
Authors: All A. I. , Lall U. 
Published: August 2005

AbstractA nonparametric statistical methodology based on kernel function estimation is developed for assessing the probability that a particular location in the aquifer has high or low conductivity using borehole information. The approach presented is an alternative to Indicator Kriging. Soils are classified through a binary indicator function defined as 0 for low and as 1 for a high conductivity soil. Estimates of the probability of occurrence of a high or low conductivity soil are made on a three‐dimensional grid. Each such estimate is formed as a local weighted average of the indicator function values that lie within an averaging interval or bandwidth of the point of estimate. A different vertical bandwidth is chosen at each borehole log. Horizontal bandwidths are selected independently at each horizontal level. These bandwidths are chosen by cross validation. Observations closer to the point of estimate are weighted higher using a kernel or weight function. Unlike Kriging, the underlying stochastic process is not assumed to be stationary. An application using data from Lake Bonneville deposits in Davis County, Utah is presented.
A Numerical Method for Solving the Water Flow Equation in Unsaturated Porous Media
Authors: Amokrane H. , Villeneuve J. P. 
Published: August 2005

AbstractA numerical procedure based on the MacCormack method was used for the simulation of one‐dimensional isothermal transient movement of liquid phase water in an unsaturated soil including precipitation, evaporation, and moisture removal from soil by the roots of transpiring plants. The numerical model utilized finite‐difference explicit predictor‐corrector equations and it was applied to a few flow problems. Comparison between the numerical results and laboratory or field experimental values was very promising. Our predicted values also agreed reasonably well with previously published numerical solutions. The proposed procedure was found to be easily adapted for solving the nonlinear soil‐water flow equation. However, it required only a knowledge of soil hydraulic conductivity and its water potential or diffusivity as functions of moisture content.
Multiple Tracers of Shallow Ground‐Water Flow and Recharge in Hilly Loess
Authors: O'Brien R. , Keller C. K. , Smith J. L. 
Published: August 2005

AbstractVertical profiles of tritium and nitrate pore‐water concentrations were determined to ∼8 m depth across two loess hillslopes. Mean recharge fluxes, estimated from chloride mass balance, are 5–10 times larger at the mid‐ and toe‐slope positions than at the top‐slope; the magnitudes of the values compare reasonably with results from other methods. The tritium and nitrate profiles exhibit multiple peaks which indicate that piston flow is not the sole flow process in this system. Results of a simple 1‐D mixing model suggest that infiltration‐exfiltration cycles in zones of plant root activity explain the shallow tritium peaks. Deeper peaks result from preferential vertical and/or lateral flow. The importance of these dispersive processes is underlined by great dilution of observed tritium concentrations, relative to expected concentrations assuming piston flow. In this setting, chloride is useful as a recharge estimator while tritium and nitrate serve as tracers of landscape‐scale water movement.
Enhanced Organic Contaminant Sorption on Soil Treated with Cationic Surfactants
Authors: Brown Michael J. , Burris David R. 
Published: August 2005

AbstractBatch sorption and column transport laboratory experiments were performed to investigate the use of cationic surfactants to enhance sorption of dissolved organic contaminants in aquifer material with low organic carbon content. The experiments used sand from the Borden aquifer in Ontario, Canada, treated with hexadecyltrimethylammonium (HDTMA+) surfactant. Test results indicate that organic cations are strongly adsorbed by cation exchange reactions on the solid surface. Further adsorption occurs in admicelles, which form by hydrophobic interactions between adsorbed and mobile HDTMA+ monomers. The nonlinear adsorption isotherm may be described by Langmuir adsorption theory. The retardation factor for HDTMA+ on Borden aquifer material is approximately 460 when aqueous surfactant concentrations are less than 100 mg/l, while HDTMA+ is relatively mobile at higher concentrations. Column experiments suggest that the adsorption reaction is very rapid, and thus independent of ground‐water flow velocity. A column treated with HDTMA+ equivalent to 33% of CEC and flushed with 325 pore volumes of surfactant‐free water showed that although the HDTMA+ modified zone was smeared downgradient, all HDTMA+ mass remained adsorbed within the column. The results indicate that an essentially stationary surfactant‐modified zone can be created.Adsorbed HDTMA+ mass increased foc from 0.02 wt% on natural Borden aquifer material to 0.18 wt% on the treated sediments. Organic matter derived from the adsorbed HDTMA+ is an effective partitioning medium for dissolved HOC, and results indicate that it may sorb HOC more efficiently than natural foc on a unit weight basis. Batch sorption tests showed that sorption coefficients (Kd) for trichloroethylene (TCE), perchloroethylene (PCE), and 1,2,4‐trichlorobenzene (TCB) were 13 times higher on HDTMA+ treated Borden aquifer material compared to the natural sediments. Retardation factors estimated from column transport tests confirmed these results, and suggested that sorption of moderately hydrophobic organic contaminants on the HDTMA+ modified sediments was a rapid and reversible process. Data from these laboratory‐scale experiments indicate that it may be feasible to develop an enhanced sorption zone in situ in a low foc aquifer as part of a ground‐water remediation scheme for dissolved organic pollutants.
A Program for Analyzing Transient and Steady‐State Soil Gas Pump Tests
Authors: Falta Ronald W. 
Published: August 2005

AbstractSoil gas pump tests are commonly performed to identify important unsaturated zone parameters such as the effective gas permeability tensor and the influence of tight or leaky confining units. An interactive FORTRAN computer program called GASSOLVE has been developed for analyzing these tests under both transient and steady‐state conditions. The code uses one of two general analytical solutions for radially symmetric gas flow to a well: the solution for transient compressible gas flow to a partially penetrating well in which the ground surface is open to the atmosphere; and the solution for transient compressible gas flow to a partially penetrating well bounded above by a leaky confining layer. Pump tests involving steady‐state conditions, fully penetrating wells, or tight confining layers are considered as special cases of these solutions. The field gas pressure data from multiple observation wells are inverted using a multidimensional nonlinear optimization routine to provide a best fit of the data with the selected analytical solution.
Physicochemical Controls on Nonconservative Anion Migration in Coarse‐Textured Alluvial Sediments
Authors: Seaman J.C. , Bertsch P.M. , Korom S.F. , Miller W.P. 
Published: August 2005

AbstractThree sandy subsurface materials and a sandy surface soil (Orangeburg Series) from the Upper Coastal Plain, were used to assess the influence of mineralogy and surface chemistry on the determination of physical transport parameters using ionic tracers. The clay mineralogy of the surface soil consisted primarily of kaolinite, hydroxy‐interlayered vermiculite, and gibbsite, while the dominant clay mineralogy of the subsurface materials consisted of kaolinite, goethite, and mica (illite). Repacked columns of the four differing strata were leached with tritiated (∼ 200 pCi mL–1) Bromide solutions, either MgBr2 or KBr, of varying ionic strengths (0.1–0.001 N). Pore‐water velocities estimated by bulk density and mass flux were consistent with those estimated from tritium breakthrough. In contrast, Br– breakthrough differed drastically within the four materials and was altered by experimental conditions (sample drying, carrier cation, etc.). The retardation for 0.001 N KBr varied from 0.94 for the surface soil to 2.15 for the subsurface materials and increased with increasing Fe oxide content. For the subsurface samples, Br– was retarded to a greater degree in the presence of Mg2+ compared to K+. In contrast, oven drying the sample reduced the degree of Br– retardation observed for the subsurface materials. These results indicate that retardation can vary dramatically within materials of similar texture, mineralogy, and origin. The apparent conservative behavior of an ionic species under a given set of conditions (mineralogy, pH, ionic strength, scale size, predominant counter ion) does not automatically ensure that transport will remain conservative as those conditions are altered by changes in experimental design or unforeseen circumstances encountered at the field scale.
Biotransformation at 10° C of Di‐n‐Butyl Phthalate in Subsurface Microcosms
Authors: Chauret Christian , Inniss William E. , Mayfield Colin I. 
Published: August 2005

AbstractDi‐n‐butyl phthalate (DBP) was found to be transformed by microorganisms under aerobic and anaerobic conditions at 10° C in microcosms simulating the Canadian Forces Base (CFB) Borden subsurface environment. Biotransformation of DBP was observed under aerobic, nitrate‐reducing, Fe(III)‐reducing, and sulfate‐reducing conditions. The biotransformation of DBP in the microcosms was significantly decreased as the redox potential was lowered, especially under sulfate‐reducing conditions. However, other factors such as nutrient depletion and buildup of toxic intermediates could have affected the biotransformation rates. The highest DBP biotransformation rate (0.57 μg DBP.g sediment−1.day−1) was observed under aerobic conditions whereas the lowest rate (0.05 μg DBP.g sediment−1.day−1) was under sulfate‐reducing conditions. Biotransformation of DBP at 10° C was significantly enhanced by the addition of 10 mM NaNO3 suggesting that both the addition of nitrate and high redox conditions favor its biotransformation in subsurface environments.
Biogeochemical Evolution of Domestic Waste Water in Septic Systems: 2. Application of Conceptual Model in Sandy Aquifers
Authors: Wilhelm Sheryl R. , Schiff Sherry L. , Robertson William D. 
Published: August 2005

AbstractAqueous geochemical data from unconfined sand aquifers beneath two operating domestic septic systems are used to illustrate and evaluate a conceptual model of septic‐system geochemistry. This model emphasizes the changing redox and alkalinity conditions in the septic system and the subsurface. The septic‐tank effluents flow as distinct plumes downward through the unsaturated zones and then primarily laterally in the ground‐water zones. The composition of the effluent was measured at several points in each system. At each site, the septic‐tank effluent underwent aerobic oxidation in the unsaturated zone, which caused conversion of NH4+ to NO3−, organic C to CO2 and organic S to SCh42‐. At the first site, calcium carbonate dissolution in the unsaturated zone buffered the acidity released by the redox reactions. In contrast, the second system was poorly buffered and the pH dropped from 6.7 to 4.9 as aerobic oxidation occurred. Below the water table a small amount of aerobic oxidation occurred at each site. Nitrate‐N concentrations in the cores of both plumes were above 25 mg/1 as the plumes traveled from the septic systems. At the second site, the ground‐water plume discharges to a river at the edge of the property. As the effluent flowed through the organic C‐rich sediments of the river bed, NO3− disappeared and alkalinity increased, presumably due to denitrification. Differences in sediment composition at the two sites also led to different behaviors of Fe, Al, and possibly PO43‐. The conceptual model offers an organized approach to interpreting the major geochemical trends observed in the two systems, which are determined mostly by the well‐aerated unsaturated zones below the drain fields and the amount of buffering material present in the sediments.
Uranium Contamination in the Great Miami Aquifer at the Fernald Environmental Management Project, Fernald, Ohio
Authors: Sidle W. C. , Lee P. Y. 
Published: August 2005

AbstractGround‐water investigations at a former U.S. Department of Energy nuclear weapons complex near Fernald, in southwestern Ohio, included the delineation of uranium contamination above the USEPA proposed drinking water standard of 20 μg/l. Contamination occurs in a buried valley and has migrated > 1.5 km south‐southeast of the facility boundary. Flooring of the plume(s) appears to be > 32 m below the water table of the Great Miami Aquifer. U6+ predominates in the modeled U‐O2‐CO2‐H2O system and U retardation decreases at depth. U234/U238 disequilibria analyses complement hydrogeologic studies which suggest that U leakage through the clayey till cap is less significant than the predominant transport pathway of infiltration via drainage channels incised into the aquifer.