bluuebunny/crossref_metadata_2025_split huggingface dataset:
Development and Testing of a Kinetic Model for Oxygen Transport in Porous Media in the Presence of Trapped Gas
Authors: Donaldson J. H. , Istok J. D. , Humphrey M. D. , O'Reilly K. T. , Hawelka C. A. , Mohr D. H. 
Published: August 2005

AbstractThe ability to predict the transport of dissolved gases in the presence of small amounts of trapped gas in an otherwise water‐saturated porous medium is needed for a variety of applications. However, an existing model based on equilibrium partitioning of dissolved gas between aqueous and trapped gas phases does not accurately predict the shape of experimentally observed breakthrough and elution curves in column experiments. The objective of this study was to develop and test a kinetic model for dissolved gas transport that combines the advection‐dispersion equation with diffusion controlled mass transfer of dissolved gas between the aqueous and trapped gas phases. The model assumes one‐dimensional, steady‐state ground‐water flow, a single dissolved gas component, and a stationary trapped gas phase with constant volume. The model contains three independent parameters: the Peclet number, P, retardation factor, R, and dimensionless mass transfer parameter, ω. The model accurately described the shape of breakthrough and elution curves for dissolved oxygen in column experiments performed with a poorly graded sand and varying amount and composition of trapped gas. Estimated values of P for the bromide tracer increased from 5.92 to 174, corresponding to a decrease in dispersivity from 5.02 to 0.17 cm, as the trapped gas volume increased from 0 to 30% of the pore space. It is speculated that this effect is due to a narrower pore size distribution (and hence more uniform pore scale velocity distribution) caused by trapped gas bubbles selectively occupying the largest pores. Estimated values of R increased from 1 to 13.6 as the trapped gas volume increased and confirmed earlier observations that even small amounts of trapped gas can significantly reduce rates of dissolved oxygen transport. Estimated values of ω ranged from 0.3 to 12.1. Although it was not possible to independently measure mass transfer coefficients or interfacial areas, values computed from flow rates and estimated w values are consistent with values computed by assuming (1) that interfacial area is proportional to trapped gas volume, (2) that trapped gas bubbles are spheres with diameters the same size as soil particles, and (3) that mass transfer is limited by diffusion of dissolved oxygen through water films surrounding trapped gas bubbles.
Site‐Specific Variability in BTEX Biodegradation Under Denitrifying Conditions
Authors: Kao Chih‐Ming , Borden Robert C. 
Published: August 2005

AbstractLaboratory microcosm experiments were conducted to evaluate the feasibility of benzene, toluene, ethylbenzene, m‐xylene, and o‐xylene (BTEX) biodegradation under denitrifying conditions. Nine different sources of inocula, including contaminated and uncont animated soil cores from four different sites and activated sludge, were used to establish microcosms. BTEX was not degraded under denitrifying conditions in microcosms inoculated with aquifer material from Rocky Point and Traverse City. However, rapid depletion of glucose under denitrifying conditions was observed in microcosms containing Rocky Point aquifer material. TEX degradation was observed in microcosms containing Rocky Point aquifer material. TEX degradation was observed in microcosms containing aquifer material from Fort Bragg and Sleeping Bear Dunes and sewage sludge. Benzene was recalcitrant in all microcosms tested. The degradation of o‐xylene ceased after toluene, ethylbenzene, and m‐xylene were depleted in the Fort Bragg and sludge microcosms, but o‐xylene continued to degrade in microcosms with contaminated Sleeping Bear Dunes soil. The most probable number (MPN) of denitrifiers in these nine different inocula were measured using a microtiter technique. There was no correlation between the MPN of denitrifiers and the TEX degradation rate under denitrifying conditions. Experimental results indicate that the degradation sequence and TEX degradation rate under denitrifying conditions may differ among sites. Results also indicate that denitrification alone may not be a suitable bioremediation technology for gasoline‐contaminated aquifers because of the inability of denitrifiers to degrade benzene.
Correcting for Diffusion in Carbon‐14 Dating of Ground Water
Authors: Sanford Ward E. 
Published: August 2005

AbstractIt has generally been recognized that molecular diffusion can be a significant process affecting the transport of carbon‐14 in the subsurface when occurring either from a permeable aquifer into a confining layer or from a fracture into a rock matrix. An analytical solution that is valid for steady‐state radionuclide transport through fractured rock is shown to be applicable to many multilayered aquifer systems. By plotting the ratio of the rate of diffusion to the rate of decay of carbon‐14 over the length scales representative of several common hydrogeologic settings, it is demonstrated that diffusion of carbon‐14 should often be not only a significant process, but a dominant one relative to decay. An age‐correction formula is developed and applied to the Bangkok Basin of Thailand, where a mean carbon‐14‐based age of 21,000 years was adjusted to 11,000 years to account for diffusion. This formula and its graphical representation should prove useful for many studies, for they can be used first to estimate the potential role of diffusion and then to make a simple first‐order age correction if necessary.
Design of a Ground‐Water Extraction/Reinjection System at a Superfund Site Using MODFLOW
Authors: Bumb Amar C. , Mitchell Judson T. , Gifford Seth K. 
Published: August 2005

AbstractA sand and gravel quarry was simultaneously operated as a landfill. The underlying contaminated aquifer is unconfined at the landfill and becomes confined as it dips to the north. A ground‐water model based on the MODFLOW program was developed to evaluate remediation alternatives and provide a basis for modifying the Record of Decision for the site. Hydraulic conductivities were contoured and continuously varied across the modeled area instead of defining zones of constant hydraulic conductivities. The model was calibrated by matching monthly water levels and travel times for nonadsorbing contaminants to migrate from the landfill to the extent of the contaminant plume. Drawdown data from a 72‐hour pump test were used to confirm the model's ability to predict an accurate response to pumping stresses and to reduce the uncertainty of the model.Modeling indicated that ground‐water recharge was required to prevent soil desaturation, minimize recontamination of ground water due to rebounding water levels, and to provide continuous flushing of the soils. The ground‐water extraction system designed for the site, consisting of eight extraction wells and one recharge trench, was predicted to reverse the natural ground‐water gradients observed at the edge of the contaminant plume, minimize travel distance for contaminants, and reduce contaminated ground‐water flow through clean aquifer soils. Recharged water was predicted to flow through the most heavily contaminated soils and be captured primarily by two extraction wells closest to the landfill. Model results were used to successfully modify the existing Record of Decision design.
Mobile NAPL Recovery: Conceptual, Field, and Mathematical Considerations
Authors: Sale Tom , Applegate David 
Published: August 2005

AbstractRecovery of mobile Nonaqueous Phase Liquids (NAPLs), referred to as “oil recovery,” is one of the most common remedial technologies currently being implemented at sites where NAPLs have been released. The rationale for oil recovery typically includes resource recovery, mitigation of further NAPL migration, and compliance with regulatory mandates for source reduction. Efficient oil recovery can be achieved by optimizing conditions within the oil flow path. This concept is referred to as flow path management. Building on this concept, a waterflood oil recovery technique utilizing dual recovery and parallel delivery drain lines has been developed for recovery of creosote‐based wood‐treating oil, a Dense Nonaqueous Phase Liquid (DNAPL). Full‐scale application of this technique at a contaminated site has yielded 1.5 million gallons of DNAPL. Furthermore, an operational endpoint of 95 percent recovery of the mobile oil is being achieved. Building on the concept of flow path management and the observed performance of the waterflood oil recovery system, a first‐order analytical solution for DNAPL flow to a drain line has been derived and validated using field data. This solution leads to a set of useful design equations and further insight into the factors that control oil recovery.
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

AbstractDefinition 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

AbstractA 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

AbstractUpward 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

AbstractA 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

AbstractExperiments 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

AbstractDetail 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

AbstractA 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

AbstractBedrock 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.