bluuebunny/crossref_metadata_2025 huggingface dataset:
Significance of Unsaturated Flow and Seepage Faces in the Simulation of Steady‐State Subsurface Flow
Authors: Romanoa Connie G. , Frind Emil O. , Rudolph David L.
Published: August 2005

A fully saturated, free‐surface ground water flow model and a variably saturated model are applied to the solution of the free‐surface ground water flow problem in order to determine the influence of the unsaturated flow component on the accuracy of the saturated flow solution and the water balance for homogeneous systems. The significance of the seepage face that must exist to provide a transition between a phreatic surface and a downstream surface water body is also evaluated. A series of simple generic cross‐sectional test cases are used as the basis for the numerical investigations. The fully saturated and variably saturated models used in this study are two‐dimensional Galerkin finite‐element models that incorporate exit seepage face options. The fully saturated solution is found to be almost identical to the variably saturated solution below the water table, the only significant difference being that the variably saturated model predicts a slightly lower water table due to horizontal flow components in the capillary fringe. This difference is negligible for large‐scale systems, snowing that for homogeneous systems, unsaturated flow can be neglected and steady‐state flow including seepage face discharge can be realistically simulated by a fully saturated free‐surface model. For aquifer‐scale systems where discretization limitations may prevent the explicit representation of seepage faces, the seepage face can be omitted because of its negligible influence on the overall water balance at the larger scale.
Spatial and Temporal Characteristics of Saline Springs: Sea of Galilee, Israel
Authors: Rimmer Alon , Hurwitz Shaul , Gvirtzman Haim
Published: August 2005

Spatial and temporal characteristics of the saline springs that emerge along the western shore of the Sea of Galilee (Lake Kinneret) are analyzed. Three groups of onshore springs (Tiberias, Fuliya, and Tabgha) and two groups of offshore springs (Barbutim and Maagan), contribute saline water to the lake with concentrations in the range of 300 to 18,000 mgCl/L, depending on location and season. It is well accepted that water emerging from these springs is a mixture of two endmembers: deep‐seated saline ground water and shallow, fresh circulating ground water. Temporal trends of discharge rates and of chloride (representing the deep saline aquifer) and nitrate (representing the shallow fresh water aquifer) concentrations within each group of springs are presented. Results show the proportions of the two water bodies while mixing are time dependent. Discharge and concentration peaks in Tabgha springs precede those in Fuliya and Tiberias springs by approximately two months. An analytical solution shows that in Tabgha, variations of these parameters are mainly controlled by recharge variations in the Galilee, and follow an exponential function. In Fuliya and Tiberias, variations of these parameters are mainly dependent on lake level, and follow a sine‐cosine function. The different patterns are attributed to different hydraulic properties of the discharge area.
Characterizing a Faulted Aquifer by Field Testing and Numerical Simulation
Authors: Allenaan Diana M. , Michel Frederick A.
Published: August 2005

Faulted aquifers constitute one of the most complex geological environments for analysis and interpretation of hydraulic test data because of the inherent ability of faults to act not only as highly transmissive zones but also as hydraulic barriers. Previous studies of the fractured carbonate aquifer at Carleton University, Ottawa, Canada, characterized the flow regime as predominantly linear, but with limited radial nature, and undertook to analyze constant discharge test data using both radial and linear flow models. When used as direct input to a numerical model, the hydraulic parameters, calculated directly from hydraulic test data, were inappropriate and resulted in a poorly calibrated model. While our interpretation of the faulted aquifer remains linear‐radial in nature, parameter estimation by numerical simulation highlighted the presence of hydraulic barriers associated with the faults. These barriers are not readily apparent in the constant discharge test data and act to modify the hydraulic test curves at early to mid time, leading to incorrect estimates of the hydraulic parameters. This paper describes the conceptual model and the numerical approach, and demonstrates the importance of using transient simulations for model calibration.
Monitoring the Hydraulic Performance of a Containment System with Significant Barometric Pressure Effects
Authors: Hare Paul W. , Morse Ralph E.
Published: August 2005

Under certain circumstances, wells in unconfined aquifers can display significant water level fluctuations in response to changes in barometric pressure. This is illustrated by Hare and Morse (1997) at a site where a portion of the unconfined aquifer is isolated by a soil‐bentonite cutoff wall and clay cap. A relief well located within the containment system displays water level fluctuations that mirror barometric pressure changes. Water levels fluctuated 0.37 m in response to barometric pressure changes of 2.87 centimeters mercury (cm‐Hg), representing a barometric efficiency of 93.6%. As described in this paper, the short‐term variability in water level elevations inside the containment system had to be considered to develop a reliable post‐enhancement performance monitoring program. The approach that was ultimately selected involves correcting the water level elevations obtained in the relief well within the containment system for the effects of barometric pressure changes prior to comparison with the water level elevations in an observation well in the aquifer outside the system. The reliability of the post‐enhancement performance monitoring program is improved further by simply requiring that any decision to enhance the containment system be based on the water level measurements obtained during two consecutive months. Using this approach, the probability that the containment system's performance will erroneously be deemed unacceptable is low. The post‐enhancement performance monitoring program also requires no extra field work and does not involve any specialized equipment, which helps to keep operation and maintenance costs to a minimum.
Modeling Ground Water Flow and Radioactive Transport in a Fractured Aquifer
Authors: Pohll Greg , Hassan Ahmed E. , Chapman Jenny B. , Papelis Charalambos , Andricevic Roko
Published: August 2005

Three‐dimensional numerical modeling is used to characterize ground water flow and contaminant transport at the Shoal nuclear test site in north‐central Nevada. The fractured rock aquifer at the site is modeled using an equivalent porous medium approach. Field data are used to characterize the fracture system into classes: large, medium, and no/small fracture zones. Hydraulic conductivities are assigned based on discrete interval measurements. Contaminants from the Shoal test are assumed to all be located within the cavity. Several challenging issues are addressed in this study. Radionuclides are apportioned between surface deposits and volume deposits in nuclear melt glass, based on their volatility and previous observations. Surface‐deposited radionuclides are released hydraulically after equilibration of the cavity with the surrounding ground water system, and as a function of ground water flow through the higher‐porosity cavity into the low‐porosity surrounding aquifer. Processes that are modeled include the release functions, retardation, radioactive decay, prompt injection, and ingrowth of daughter products. Prompt injection of radionuclides away from the cavity is found to increase the arrival of mass at the control plane but is not found to significantly impact calculated concentrations due to increased spreading. Behavior of the other radionuclides is affected by the slow chemical release and retardation behavior. The transport calculations are sensitive to many flow and transport parameters. Most important are the heterogeneity of the flow field and effective porosity. The effect of porosity in radioactive decay is crucial and has not been adequately addressed in the literature. For reactive solutes, retardation and the glass dissolution rate are also critical.
A Field Study of Nonequilibrium and Facilitated Transport of Cd in an Alluvial Gravel Aquifer
Authors: Pang Liping , Close Murray
Published: August 2005

A natural‐gradient tracer experiment was conducted to study Cd transport in an alluvial gravel aquifer. Both a conservative tracer and Cd exhibited tailing in their breakthrough curves (BTCs), indicating the presence of nonequilibrium transport. Solute transport was evaluated using a three‐dimensional nonequilibrium analytical model, and the results were compared with those obtained from a previous laboratory study. At similar flow velocities, the field Cd data gave significantly lower retardation factors (R=7 to 30, median 22), higher fractions of instantaneous sorption sites (β), and greater mass transfer coefficients (ω) than the laboratory data because of a high degree of aquifer heterogeneity, the presence of preferential flow, and the larger transport scale in the field conditions. Multiple peaks in the Cd BTCs were observed due to bacteria‐facilitated and perhaps also colloid‐facilitated transport. The early peaks showed narrower, more symmetric shapes with higher concentrations than the later peaks, and compared well to those of the bacterial BTCs. The multiple peaks of the Cd BTCs imply that a significant fraction of Cd could travel with little, if any, retardation over a 20 to 40 m travel distance when Cd and bacteria coexist in a contamination event.
Modeling In Situ Iron Removal from Ground Water
Authors: Appelo C.A.J. , Drijver B. , Hekkenberg R. , de Jonge M.
Published: August 2005

In situ iron removal is conducted routinely in a number of European countries. A volume of oxygenated water is injected, and subsequently a larger volume of ground water can be pumped with a lower iron concentration than is found in native ground water. The underlying reaction mechanism has not been well described so far, and the process has not been modeled quantitatively. The essential problem is how the electron transfer takes place between the dissolved oxygen in injection water and the dissolved iron in ground water. An intermediate reaction step, involving cation exchange of ferrous iron and subsequent oxidation by oxygen of injection water, explains the efficiency increase during the initial cycles and the absence of clogging by precipitated iron‐oxyhydroxide. A hydrogeochemical transport model has been used to model column experiments with good results. The quantification of the reaction mechanism allows the assessment of operational conditions. For example, it can be shown that increasing the oxidant concentration in the injected water has an insignificant effect when exchangeable ferrous iron is low.
Risk‐Based Decision Analysis of Atmospheric Emission Alternatives to Reduce Ground Water Degradation on the European Scale
Authors: Wladis David , Rosen Lars , Kros Hans
Published: August 2005

Environmental degradation due to emissions of sulfur dioxide, nitrate oxides, and ammonia from diffuse sources amounts to substantial costs to society and so do the alternatives to protect and restore the environment. Damage to ground water includes acidification, aluminum leaching, elevated concentrations of nitrate, and eutrophication. Monetary risk‐based decision analysis (on a national scale) is applied to compare alternative actions designed to protect ground water from further degradation. This decision analysis uses simulations of nitrate and aluminum concentrations over a 15 year period with two reduction scenarios for sulfur dioxide, nitrate oxides, and ammonia, and results in estimates of economic uncertainty. For each alternative, an objective function is estimated including the implementation costs, the economic risk associated with failure according to the selected decision criteria, and the economic benefits related to the implementation. The decision criteria are based on the European Community drinking water quality standards for nitrate and aluminum. The study aims at incorporating the hydrogeologic uncertainty resulting from the propagation of errors from data input to model output. A range of economic values has been applied to the ground water resource to study the sensitivity of the decision analysis to valuing ground water. The results indicate that higher reduction rates of the studied pollutants reduce the economic uncertainty but also lead to larger total costs. The study also indicates that the economic uncertainty may be equal to the total cost provided by the objective function. The contamination level of nitrate is much more responsive to the reduction scenarios than the aluminum concentration. For high, but not unrealistic, ground water valuing, the economic uncertainty makes the decision between the studied alternatives unclear.
Determining Karst Transmissivities with Inverse Modeling and an Equivalent Porous Media
Authors: Larocque M. , Banton O. , Ackerer P. , Razack M.
Published: August 2005

Flow simulation is difficult to implement in heterogeneous media such as karst aquifers, primarily because the structure of the rock is extremely complex and usually unknown. The aim of this study was to verify the possibility of using inverse modeling and an equivalent porous media to identify transmissivities in a slightly karstified aquifer, the La Rochefoucauld karst (Charente, France). Different simulation scenarios were tested: using two spatial discretizations with different finite‐element cell sizes and using measured or interpolated heads. The inverse modeling was performed with the downscaling parameterization procedure, using a finite‐element representation of bidimensional ground water flow. The inverse modeling converged satisfactorily with all scenarios: head residuals were small and spring flow rates and the river/aquifer exchanges were adequately simulated. The scenario using small cells and measured heads generated a highly heterogeneous transmissivity field, indicating an overpa‐rameterization of the problem. The calibrated transmissivities and simulated heads of this scenario proved less reliable overall than those of the other scenarios. The use of interpolated heads generated more uniform transmissivities as a result of the head smoothing. A rotation of the initial parameter mesh showed that the scenarios using interpolated heads generate the most stable and reliable results. The scenarios with interpolated heads could therefore be used when head measurements are limited or are unevenly distributed over the aquifer. Overall, the calibrated transmissivities reproduced the entire range of transmissivities measured in the field using different methods. The results indicate that inverse modeling and an equivalent porous media can be used to determine transmissivities in a moderately karstified aquifer.
Hydrologic Setting of Two Interdunal Valleys in the Central Sand Hills of Nebraska
Authors: Gosselin David C. , Drda Steve , Harvey F. Edwin , Goeke Jim
Published: August 2005

The Sand Hills of Nebraska, one of the largest grass‐stabilized dune regions in the world, has nearly 5000 km2 of wetland environments and is one of the most productive waterfowl regions of the United States. Yet, the hydrology of the region is not well defined. This paper presents the results of a two‐and‐a‐half‐year study to characterize the hydrology of two distinctly different interdunal valleys within the Sand Hills. The valleys, located at the Gudmundsen Sand Hills Laboratory in the central Sand Hills, reflect the two major types of interdunal environments: dry, short grass valleys (the east valley) and subirrigated wet meadows (the west valley). In the west valley, ground water flows from west to east and there are significant, upward vertical gradients (0.005 to 0.045). In contrast, ground water in the east valley predominantly flows from west‐southwest to east‐northeast. Where vertical gradients exist, they are downward (0.0025 to 0.07). Therefore, the east valley is a “recharge” or flow‐through valley. Seasonal ground water table fluctuations indicate that net recharge is greatest between growing seasons. The relatively high topographic relief of the water table on the flanks of the west valley, resulting from higher topographic relief of the adjacent dunes, drives ground water discharge into the valley wetland, developing a local flow system. In contrast, the topography of the water table in the east valley, which is flanked by dunes with lower relief, is insufficient to generate a distinct local ground water flow system.
Sample Concentration Response to Laminar Wellbore Flow: Implications to Ground Water Data Variability
Authors: Martin‐Hayden James M.
Published: August 2005

Ground water data variability and sample repsentativeness ness are controlled by internal monitoring‐well flow and mixing, processes that require further investigation. By defining a representative sample using flow weighted average concentrations and modeling laminar wellbore flow, this study uses deviations from those representative concentrations as a measure of data variability. The laminar wellbore flow model relates ground water concentration and permeability distributions to the concentration response at a pump during purging or sampling. Given laminar wellbore flow, even if the concentrations within the well are not chemically altered, transient responses of concentrations to pumping lead to various amounts of variability depending on the screen length and position relative to hydraulic and chemical heterogeneities. In the simplest case, uniform inflow and inflowing concentrations, the concentration response to pumping is the same as the concentration response with thorough mixing. In more complex situations involving ground water concentration and permeability heterogeneities, pumped concentrations are controlled by those heterogeneities and wellbore flow. A model that describes concentration responses in the presence of linear heterogeneities allows a first‐order approximation of variability during pumping. Using field screening of concentration and permeability heterogeneities, this first‐order approximation can be used to design screen lengths that will limit the variability to study‐specific tolerances. In the case of existing wells, measurement of inflows and concentration variability will allow the assessment of variability during pumping. This investigation demonstrates that, in order to decipher sources of data variability, investigations must examine internal wellbore processes in detail as well as the relationships to external conditions characterized in the field.
Quantitative Estimates of the Uncertainty in the Evaluation of Ground Water Remediation Schemes
Authors: Copty Nadim K. , Findikakis Angelos N.
Published: August 2005

Due to the natural heterogeneity of subsurface formations and to the often limited number of data, the definition of the physical and chemical parameters that govern flow and contaminant transport processes is normally characterized by a high level of uncertainty. Uncertainty in the relevant input parameters leads to uncertainty in modeling predictions, which may influence the selection of the optimal remedial alternative. This paper describes a statistical procedure for quantifying the uncertainty in modeling predictions made in support of the evaluation of ground water remediation designs. The hydraulic conductivity field is defined as a random spatial variable whose statistical structure is inferred from the available hydraulic conductivity data. Monte Carlo simulations are used to generate equiprobable multiple realizations of the hydraulic conductivity conditioned on the available hydraulic conductivity measurements. Using Bayes’theorem, probability‐based weights for each of the hydraulic conductivity realizations are estimated from “soft” data consisting of historical concentration data and contamination source information. The probability weights are expressed in terms of the simulated concentration distributions and the observed concentration data. Ground water flow and contaminant transport simulation models are then used to probabilistically evaluate and optimize ground water remediation schemes based on the statistical analysis of key model predictions, such as the probability of a particular part of the aquifer being within the capture zone of an extraction well, or the recovered mass of contaminants as a function of time.
Single‐Well Dipole Flow Tests: Parameter Estimation and Field Testing
Authors: Hvilshøj S. , Jensen K.H. , Madsen B.
Published: August 2005

The single‐well dipole method proposed by Kabala (1993) has been tested at a field site in Denmark representing an alluvial sandy unconfined aquifer. A critical review of the method has been carried out, and both the originally proposed analytical solutions to the problem and a numerical model have been tested. The analysis showed that the hydraulic parameters determined from the test data were correlated. Particularly it was shown that the vertical hydraulic conductivity and the specific storage could not be determined to an acceptable degree of reliability when based on the originally proposed analytical solution and on measurements from only the dipole well. Instead, an inverse numerical multilayer model was proposed for parameter identification using the pressure response data measured not only in the dipole well but also in adjacent piezometers. The vertical profiles of horizontal and vertical hydraulic conductivities obtained at the site using the single‐well dipole method were in accordance with the results from previous investigations that used other methods.
Field‐Scale Migration of Colloidal Tracers in a Fractured Shale Saprolite
Authors: McKay L.D. , Sanford W.E. , Strong J.M.
Published: August 2005

A field‐scale tracer experiment carried out under natural gradient ground water flow conditions showed that colloids can be highly mobile in a fractured and highly weathered shale saprolite. Four colloidal tracers (0.100 μm fluorescent latex microspheres, bacteriophage strains PRD‐1 and MS‐2, and INA, a dead strain of Pseudemonas syringae), were introduced to a 6.4 m deep well, and concentrations of the tracers were monitored in the source well and in downgradient monitoring wells at distances of 2 to 35 m. All of the colloidal tracers were detected to distances of at least 13.5 m and two of the tracers (microspheres and INA) were detected in all of the downgradient wells. In most wells the colloidal tracers appeared as a “pulse”, with rapid first arrival (corresponding to 5 to 200 m/d transport velocity), one to six days of high concentrations, and then a rapid decline to below the detection limit. The colloids were transported at velocities of up to 500 times faster than solute tracers (He, Ne, and rhodamine‐WT) from previous tests at the site. This is believed to be largely due to greater diffusion of the solutes into the relatively immobile pore water of the fine‐grained matrix between fractures. Peak colloid tracer concentrations in the monitoring wells varied substantially, with the microspheres exhibiting the highest relative concentrations and hence the least retention. Rates of concentration decline with distance also varied, indicating that retention is not a uniform process in this heterogeneous material. Two of the tracers, INA and PRD‐1, reappeared in several monitoring wells one to five months after the initial pulse had passed, and the reappearance generally corresponds with increased seasonal precipitation. This is consistent with subsequent laboratory experiments that showed that colloid retention in these materials is sensitive to factors such as flow rate and ionic strength, both of which are expected to vary with the amount of precipitation.
Ground Water Flow in a Fractured Carbonate Aquifer Inferred from Combined Hydrogeological and Geochemical Measurements
Authors: Zanini L. , Novakowski K.S. , Lapcevie P. , Bickerton G.S. , Voralek J. , Talbot C.
Published: August 2005

A conceptual model for ground water flow is presented for a fractured Silurian dolomite in the Niagara Escarpment area of southern Ontario. Such a model is necessary to facilitate remedial efforts of a PCB‐contaminated site located in Smithville, Ontario. Both physical and chemical hydrogeological observations obtained from field investigations were used to deduce the structure of the ground water flow system in the fracture network. The field study was conducted using observations obtained from six bore‐holes drilled in the vicinity of the town of Smithville. The boreholes were diamond cored through the entire thickness of the dolomite formation (approximately 45 m), hydraulically tested using a 2 m packer spacing and then completed using multipacker casing strings. Measurements of hydraulic head were obtained on a weekly basis over a period of two years, and ground water from each borehole interval was collected for geochemical analyses for inorganic and isotopic composition. Transmissivity measurements indicate that the dolomite is divided into two ground water flow systems separated by an extensive unit of low transmissivity that is pervasive throughout the region. The upper flow system is characterized by water enriched in Mg2+ and SO42‐. Below the low transmissivity zone, ground water increases in salinity and is enriched in Ca2+ and SO42‐. Based on the geochemistry, the rate of ground water migration in the lower flow system is surmised to be much less than that in the upper system. Measurements of hydraulic head in conjunction with the results of the analyses of the environmental isotopes (δ18O and δ2H) suggest that ground water flow is mainly horizontal and likely governed by enlarged bedding plane fractures. The isotope geochemistry and topographical features further suggest that ground water recharge is occurring approximately 2 km to the north of Smithville.
Effect of Intra‐Population Variability on the Long‐Distance Transport of Bacteria
Authors: Bolster Carl H. , Mills Aaron L. , Hornberger George , Herman Janet
Published: August 2005

Recent experimental work has suggested that within a single bacterial strain there may exist two distinet subpopulations, each with its own sticking efficiency. We performed a sensitivity analysis using an advection‐dispersion model to elucidate the effects of an influent suspension of bacteria composed of two subpopulations, each with distinet sticking efficiencies (dual‐alpha population), on the removal and transport of bacteria over distances of tens of meters. In the simulations, we assumed idealized conditions (i.e., one‐dimensional transport through physically and geochemically homogeneous porous media). Results demonstrate that in cases where a small fraction of the influent bacteria have surface characteristics favorable for transport, the prediction of field‐scale transport based on laboratory‐derived parameters from short column experiments will tend to overestimate substantially the amount of bacterial removal. Results demonstrate the importance of a priori knowledge of the presence of intra‐population variability when predicting the field‐scale transport of bacteria.
Analysis of Transient Data from Infiltrometer Tests in Fine‐Grained Soils
Authors: Guyonnet Dominique , Amraoui Nadia , Kara Reda
Published: August 2005

Data collected during ring infiltrometer tests are often analyzed while assuming either that the effect of gravity is negligible (early‐time, transient data) or that it is dominant (late‐time, steady‐state data). In this paper, an equation is proposed for inter‐preting both early‐time and late‐time data measured during infiltration tests under falling head conditions. It is shown that the method used by previous authors for interpreting both early‐time data is a special case of the proposed equation. The equation is applied to data collected during tests performed in fine‐grained soils, and results are discussed. The analysis suggests that to assume a priori values of the soil sorptive number, as indicated in the literature for various soils, may in some cases lead to severely underes‐timated values of the saturated hydraulic conductivity. Conversely, in low permeability soils, to assume steady‐state gravity drainage may lead to order of magnitude overestimates of the saturated hydraulic conductivity. A dimensionless analysis provides characteristic times that correspond either to the duration of the log‐log half slope displayed by early‐time data or to the log‐log unit slope characteristic of late‐time data.
Degradation of TCE with Iron: The Role of Competing Chromate and Nitrate Reduction
Authors: Schlicker Oliver , Ebert Markus , Fruth Margit , Weidner Markus , Wüst Wolfgang , Dahmke Andreas
Published: August 2005

This study evaluates the potential of using granular iron metal for the abiotic removal of the organic ground water pollutant trichloroethene (TCE) in the presence of the common inorganic co‐contaminants chromate and nitrate, respectively. Our long‐term column experiments indicate a competitive process between TCE dechlorination and reductive transformation of chromate and nitrate, which is reflected in a significantly delayed onset of TCE dechlorination. Delay times and therefore the ranges of the nonreactive flowpaths increased with increasing experimental duration, resulting in a migration of the contaminants through the iron metal treatment zone. The present investigation also indicates that the calculated migration rates of TCE and the added cocontaminants chromate and nitrate are linearly related to the initial content of the cocontaminants. With an average pore water velocity of 0.6 m/d and a surface area concentration of 0.55 m2/mL in the column, the calculated migration rates varled between 0.10 cm/d and 5.86 cm/d. The particular similarity between the values of TCE migration and the migration of the strong oxidants chromate and nitrate and the long‐term steady state of the TCE dechlorination in the absence of the chromate and nitrate indicates that these competitive transformations are the driving force for the gradual passivation of the granular iron due to the buildup of an electrically insulating Fe(III)‐oxyhydroxide. Based on these passivation processes, general formulae were developed that allow a simplified approximation of breakthrough times for the contaminants TCE, chromate, and nitrate.
How the Kd Approach Undermines Ground Water Cleanup
Authors: Bethke Craig M. , Brady Pathick V.
Published: August 2005

Environmental scientists have long appreciated that the distribution coefficient (the “Kd” or “constant Kd”) approach predicts the partitioning of heavy metals between sediment and ground water inaccurately; nonetheless, transport models applied to problems of environmental protection and ground water remediation almost invariably employ this technique. To examine the consequences of this practice, we consider transport in one dimension of Pb and other heavy metals through an aquifer containing hydrous ferric oxide, into which many heavy metals sorb strongly. We compare the predictions of models calculated using the Kd approach to those given by surface complexation theory, which is more realistic physically and chemically. The two modeling techniques give qualitatively differing results that lead to divergent cleanup Strategies. The results for surface complexation theory show that water flushing is ineffective at displacing Pb from the sorbing surface. The effluent from such treatment contains a persistent “tall” of small but significant levels of contamination. Subsurface zones of Pb contamination, furthermore, do not migrate rapidly or far in flowing ground water. These results stand in sharp contrast to the predictions of models constructed using the Kd approach, yet are consistent with experience in the laboratory and field.
On Capture Width and Capture Zone Gaps in Multiple‐Well Systems
Authors: Erdmann John B.
Published: August 2005

A barrier well line's capture width along its alignment is shown to depend only on total system flow rate QT, and aquifer back‐ground flow rate per unit width Qo (prepumping gradient times transmissivity) under idealized horizontal flow: Irrespective of number of wells, the capture width is QT/2Q0, as for a hypothetical single well with flow rate QT. This result applies even to barrier lines with irregular well spacings and flow rates–and, in fact, to the capture width through the center of any pumping configuration that is symmetrical respecting a line perpendicular to uniform background flow. A flow‐perpendicular line of regularly spaced, equal wells (“regular barrier line”) has the same capture width along its alignment as an identical well line parallel to background flow has through its center. In the general symmetrical case, the center capture width remains QT/2Q0 whether transverse well spacings are large (allowing contaminated ground water to escape through capture zone gaps), critical (with incipient gaps), or safely small. For design reference, critical spacings derived for regular barrier lines of N wells with N through 100 and N→∞ vary from Q/πQ0 (N = 2, previously known) to Q/2Q0 (N→∞), where Q is each well's flow rate. Incipient gaps at critical spacing occur always between the two center wells, or the center well and two adjacent wells. Spacings substantially smaller than critical are recommended for contaminant capture. Designs are illustrated that preclude capture zone gaps.
A Field Technique for Estimating Aquifer Parameters Using Flow Log Data
Authors: Paillet Frederick L.
Published: August 2005

A numerical model is used to predict flow along intervals between producing zones in open boreholes for comparison with measurements of borehole flow. The model gives flow under quasi‐steady conditions as a function of the transmissivity and hydraulic head in an arbitrary number of zones communicating with each other along open boreholes. The theory shows that the amount of inflow to or outflow from the borehole under any one flow condition may not indicate relative zone transmissivity. A unique inversion for both hydraulic‐head and transmissivity values is possible if flow is measured under two different conditions such as ambient and quasi‐steady pumping, and if the difference in open‐borehole water level between the two flow conditions is measured. The technique is shown to give useful estimates of water levels and transmissivities of two or more water‐producing zones intersecting a single interval of open borehole under typical field conditions. Although the modeling technique involves some approximation, the principle limit on the accuracy of the method under field conditions is the measurement error in the flow log data. Flow measurements and pumping conditions are usually adjusted so that transmissivity estimates are most accurate for the most transmissive zones, and relative measurement error is proportionately larger for less transmissive zones. The most effective general application of the borehole‐flow model results when the data are fit to models that systematically include more production zones of progressively smaller transmissivity values until model results show that all accuracy in the data set is exhausted.
A Probabilistic Method for Estimating Monitoring Point Density for Containment System Leak Detection
Authors: Ross Randall R. , Vieux Baxter E.
Published: August 2005

The use of physical and hydraulic containment systems for the isolation of contaminated ground water and aquifer materials associated with hazardous waste sites has increased during the last decade. The existing methodologies for monitoring and evaluating leakage from hazardous waste containment systems rely primarily on limited hydraulic head data. The number of hydraulic head monitoring points available at most sites employing physical containment systems may be insufficient to identify significant leaks from the systems. A probabilistic approach for evaluating the performance of containment systems, based on estimations of apparent leakage rates, is used to introduce a methodology for determining the minimum number of monitoring points necessary to identify the hydraulic signature of leakage from a containment system. The probabilistic method is based on the principles of geometric probability. The method is demonstrated using three‐dimensional ground water flow modeling results of leakage through a vertical barrier. The results indicate that the monitoring point spacing used at many hazardous waste sites likely is inadequate to detect the hydraulic signatures of all but the largest leaks.
Regulation of Injected Ground Water Tracers
Authors: Holmbeck‐Pelham Skelly A. , Rasmussen Todd C. , Fowler Laurie A.
Published: August 2005

Ground water tracer tests are routinely performed to estimate aquifer flow and transport properties, including the determination of well capture zones, hydrogeologic parameters, and contaminant travel times. Investigators may be unaware of tracer test reporting requirements and may fail to notify their regulatory agency prior to conducting tracer tests. The injection of tracers falls under the jurisdiction of the federal Underground Injection Control (UIC) program, which regulates the introduction of substances into underground sources of drinking water as part of the Safe Drinking Water Act. The UIC program is administered by the U.S. Environmental Protection Agency (EPA) and by states with EPA‐approved programs. The federal UIC program requires that tracer tests must not endanger underground sources of drinking water, and all tracer tests must be reported prior to injection. We contacted the UIC program administrator for every state in early 1997. Some states report having more stringent requirements, while some states do not meet minimum federal requirements. Although the primary responsibility for ground water tracer selection and use rests on the investigator, national guidance is required to assure compliance with the UIC program. To assist investigators, we present acceptable tracers that have been identified by two states, Nevada and South Carolina, that require no further regulatory review.
Bimodal Transport of a Waste Water Plume Injected into Saline Ground Water of the Florida Keys
Authors: Dillon Kevin S. , Corbett D. Reide , Chanton Jeffrey P. , Burnett William C. , Kump Lee
Published: August 2005

Two experiments were conducted on Long Key, Florida, United States, to examine the fate of waste water following sewage disposal in 10 to 30 m deep injection wells. This waste disposal practice introduces extraordinary amounts of nutrients into the ground water of the Florida Keys. In these experiments, artificial ground water tracers, sulfur hexafluoride (SF6) and radioiodine (131I) were used to determine transport rates and directions of soluble nonreactive substances injected into the saline ground water underlying the Keys. Two types of transport were observed: (1) rapid flow (0.20 to 2.20 m/hr) presumably due to the many conduits present in the limestone; and (2) slower flow (less than 0.003 to 0.14 m/hr) associated with the limestone's primary porosity. Vertical flow was comparable to horizontal flow due to either the density‐driven buoyancy of the waste water plume or to preferential flowpaths that allow upward advection or combination of both. These experiments showed that conservative artificial tracers injected into the subsurface reach surface water in a matter of days and can remain in the immediate vicinity of the injection well for several months.
In Situ Ground Water Remediation Using the Trench and Gate System
Authors: Bowles Marc W. , Bentley L. R. , Hoyne B. , Thomas D.A.
Published: August 2005

The trench and gate remediation system is a funnel and gate design modified for installation in tills and other low permeability sediments. Modifications include the addition of high hydraulic conductivity trenches along the upgradient side of the funnel walls and a discharge reinfiltration gallery downgradient from the treatment gate. Preferential ground water flow through the high permeability trenches and reinfiltration gallery prevents mounding, which otherwise forces flow beneath and around funnel walls. The resultant capture zone is larger, both horizontally and vertically, than the cross‐sectional funnel area and is significantly larger than the capture zone of a similar funnel and gate system. The system constitutes an economical, in situ, long‐term contaminant plume capture and treatment method. In effectiveness and cost, the trench and gate system compares favorably with, or outper‐forms, other remediation systems in near‐surface, low permeability dissolved contaminant scenarios. A prototype trench and gate system was installed at the East Garrington Gas Plant, Alberta, Canada, in 1995. The installation demonstrated the effectiveness of the concept, and the system has successfully prevented off‐site migration of dissolved hydrocarbon contaminants.
Design of Soil Vapor Extraction Systems Under Conditions of Uncertainty
Authors: Barnes David L. , McWhorter David B.
Published: August 2005

This paper proposes a risk‐cost approach for the design of soil vapor extraction (SVE) systems that takes into account uncertainties in the soil properties. The proposed design methodology consists of three separate models: a soil‐gas flow and vapor transport model, a parameter uncertainty model, and a decision model based on a risk‐cost objective function. A description of each of these models is provided and an example using a hypothetical scenario is given to illustrate the use of the methodology. Results from the example show that the methodology is suited for determining the number, location, and spacing of SVE extraction wells. The methodology can also be used to size the vacuum blower to be used and to predict when the SVE system should be shut off.
Simple Analytical Equations for Estimating Ground Water Inflow to a Mine Pit
Authors: Marinelli Fred , Niccoli Walter L.
Published: August 2005

Steady‐state analytical solutions are presented for estimating the ground water inflow rate to a mine pit that may contain a pit lake of finite depth. The solutions consider (1) the effect of decreased saturated thickness near the pit walls; (2) distributed recharge to the water table; and (3) upward flow through the pit bottom. While the solutions are not appropriate for all hydrogeologic situations, they are relevant to conditions encountered at many mine sites. An example calculation is presented for an actual mine pit containing a pit lake. The analytical solutions provide an estimated ground water inflow rate that is similar to the rate determined independently from a detailed water balance study.
Hydraulic Gradient Comparison Method to Estimate Aquifer Hydraulic Parameters Under Steady‐State Conditions
Authors: Guo Xiaoniu , Zhang Chuan‐Mian
Published: August 2005

The hydraulic gradient comparison method is an inverse method for estimation of aquifer hydraulic conductivity (or trans‐missivity) and boundary conductance for a ground water flow model under steady‐state conditions. This method, following formal optimization techniques, defines its objective function to minimize differences between interpreted (observed) and simulated hydraulic gradients, which results in minimization of differences between observed and simulated hydraulic heads. The key features of this method are that (1) the derived optimality conditions have an explicit form with a clear hydrology concept that is con‐sistent with Darcy's law, and (2) the derived optimality conditions are spatially independent as they are a function of only local hydraulic conductivity and local hydraulic gradient. This second feature allows a multidimensional optimization problem to be solved by many one‐dimensional optimization procedures simultaneously, which results in a substantial reduction in computation time. The results of the numerical performance testing on a heterogeneous hypothetical case confirm that minimizing gradient residuals in the entire model domain leads to minimizing head residuals. Application of the method in real‐world projects requires rigorous conceptual model development, use of a global calibration target, and an iterative calibration proess. The conceptual model development includes interpretation of a potentiometric surface and estimation of other hydrologic parameters. This method has been applied to a wide range of real‐world modeling projects, including the Rocky Mountain Arsenal and Rocky Flats sites in Colorado, which demonstrates that the method is efficient and practical.
Gravel‐Corrected Kd Values
Authors: Kaplan Daniel I. , Kutnyakov Igor V. , Gamerdinger Amy P. , Serne R. Jeff , Parker Kent E.
Published: August 2005

Standard measurements of solute sorption to sediments are typically made on the <2 mm sediment fraction. This fraction is used by researchers to standardize the method and to ease experimental protocol so that large labware is not required to accommodate the gravel fraction (>2 mm particles). Since sorption is a phenomenon directly related to surface area, sorption measurements based on the <2 mm fraction would be expected to overestimate actual whole‐sediment values for sediments containing gravel. This inaccuracy is a problem for ground water contaminant transport modelers who use laboratory‐derived sorption values, typically expressed as a distribution coefficients (Kd), to calculate the retardation factor (Rf), a parameter that accounts for solute‐sediment chemical interactions. The objectives of this laboratory study were to quantify the effect of gravel on strontium Kd and Rf values and to develop an empirical method to calculate gravel‐corrected Kdgcvalues for the study site (Hanford Site in Richland, Washington). Three gravel corrections, Kd values, were evaluated: a correction based on the assumption that the gravel simply diluted the Kd<2mmand had no sorption capacity (Kdgc,g=0), a correction based on the assumption that the Kd of the intact sediment (Kdtotwas a composite of the Kd<2mmand the Kd>2mm(Kdgc,g = x), and a correction based on surface area (Kdgc,surf). On average, Kd<2mmtended to overestimate Kdtotby 28% to 47%; Kdgc,g = xoverestimated Kdtotby only 3% to 5%; and Kdgc,g = 0and Kdgc,surfunderestimated Kdtotby 10% to 39%. Kdgc,g = xprovided the best estimate of actual values (Kdtot); however, Kdgc,g = 0was appreciably easier to acquire. Although other contaminants will likely have different gravel‐correction values, these results have important implications regarding the traditional approach to modeling contaminant transport which uses Kd<2mmvalues. Such calculations may overestimate the tendency of gravel‐containing sediments to retard contaminant migration.