bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation of Ground‐Water Interaction with Lakes Allowing for Fluctuating Lake Levels
Authors: Cheng Xiangxue , Anderson Mary P.
Published: August 2005

Most numerical models of ground‐water flow incorporate lakes as constant head nodes. We developed a computer package for the widely used MODFLOW code to simulate three‐dimensional steady‐state and transient ground‐water flow with fluctuating lake levels. This package considers precipitation to and evaporation from the lake surface, stream flow, and ground‐water inflow and outflow for multiple lakes. We demonstrate the use of the package through the results of steady‐state and transient simulations for a hypothetical ground‐water flow problem involving a lake and two streams.
A Program of Frequency Analysis Using Gumbel's Method
Authors: El‐Naqa Ali , Zeid Nasser Abu
Published: August 2005

A program (GUMBEL) has been written in QBASIC language to carry out the mathematical procedure for preparing frequency curves using Gumbel's extreme value distribution. The program can be used to analyze hydrologie and meteorologie events so as to establish a relationship between the magnitude of extreme events and their frequency through the use of probabilistic distributions.

Six different plotting positions formulae are included in the program so as to select the most reliable formula of these graphical fitting methods in analyzing the observed hydrologie data. A sample input data file of annual rainfall data from Jordan has been tested, in order to show the applicability and usefulness of the program.
A Stochastic Method for Characterizing Ground‐Water Contamination
Authors: McLaughlin Dennis , Reid Lynn B. , Li Shu‐Guang , Hyman Jennifer
Published: August 2005

It is becoming widely recognized that field‐scale ground‐water contaminant plumes are irregular and difficult to predict. Factors which complicate the characterization of such plumes include geological variability, data limitations, and uncertainties about the source of contamination. This paper describes a new approach to site characterization which accounts for variability and uncertainty in a systematic way. The site characterization procedure extracts more information from limited data by combining field measurements with predictions from a stochastic ground‐water model. The model provides prior estimates of the mean and standard deviation of solute concentration throughout a contaminated site. These estimates are updated whenever new measurements of hydraulic conductivity, head, and/or concentration become available. The updated concentration standard deviation estimates may be used to guide the placement of sampling wells and to evaluate the accuracy of the site characterization. If updating and data collection are carried out sequentially, over a series of discrete sampling rounds, the sampling network can evolve in response to new information. A case study described in the paper illustrates how the characterization procedure can be applied to a typical field site.
Model Development and Simulation of Bioremediation in Soil Beds with Aggregates
Authors: Dhawan S. , Erickson L. E. , Fan L. T.
Published: August 2005

One method of remediating contaminated soil and ground water is through management of the subsurface environment so that indigenous microorganisms can biodegrade organic contaminants. Modeling and simulation offer promising means of assessing the migration and attenuation of such contaminants being treated in situ in the subsurface. In this paper, a macropore flow model has been developed to account for bioremediation in the interstitial spaces among soil aggregates. This model has been combined with another bioremediation model which accounts for diffusion and Biodegradation in the micropores and soil particles in the aggregates. The combined model comprises a system of six coupled equations, of which three are nonlinear ordinary differential equations and three are nonlinear partial differential equations. Dimensional analysis of these equations has yielded useful dimensionless parameters for evaluating the relative significance of each mechanism in remediation. Numerical experiments have been conducted to evaluate the effects of initial contaminant concentration, aggregate size, and soil‐water partition factor. The total time of remediation has been found to depend on the rate at which contaminants are consumed within the large aggregates which, in turn, depends on the Biodegradation kinetics and the rates of diffusion of substrate and oxygen in the aggregates. For soil with aggregates whose radius is 1 cm or larger, the results of simulation indicate that the remediation time for the aggregates is significantly longer compared to the corresponding result for homogeneous soil. For contaminants with large partition coefficients, the estimated time for remediation achieved through microbial oxidation in situ is much shorter than that achieved through purely diffusional transport of the contaminants out of the soil bed.
United Nations Ground Water Software
Authors: Karanjac Jasminko
Published: August 2005

A newly developed ground‐water software package, known as the United Nations Ground Water Software, is a dedicated hydrogeological software, which helps the user create ground‐water data bases on lithology, ground‐water chemistry, water levels, and pumping tests. The emphasis is on graphics screen display and printing or plotting of graphics. The software is in colors, with impermeable formations displayed or printed in red and permeable formations in blue. The software is user friendly and transparent to the user. It combines the functions of ground‐water data base management and digital mapping. The software is in public domain, although the ownership of the United Nations is acknowledged. It is intended primarily for developing countries which do not have easy access to advanced CAD or GIS systems.
Risk‐Based Evaluation of Ground‐Water Contamination by Agricultural Pesticides
Authors: Varshney Peeyush , Tim U. Sunday , Anderson Carl E.
Published: August 2005

The recently completed National Pesticide Survey conducted by the United States Environmental Protection Agency (EPA) has identified the presence of various pesticides in ground water. The detection of pesticides in the nation's ground water has raised concerns for public health which has led resource managers in federal and state agencies to recognize that risk assessment is vital for water quality protection. This paper presents a methodology for risk‐based evaluation of ground‐water contamination by agricultural pesticides. The methodology utilizes the Risk of Unsaturated/Saturated Transport and Transformation of Chemical Concentrations (RUSTIC) model to provide simulations that yield the probable risks associated with a given pesticide. Risk is expressed in terms of the probability of predicted pesticide mass exceeding its recommended health standards. Three widely used agricultural herbicides, simazine, atrazine, and alachlor, were evaluated using long‐term (1960–1986) meteorological data for Ames, Iowa. Results indicate that for a well 8 m deep located 200 m horizontally from a pesticide application area in an alluvial sand and gravel setting, the probability of exceeding the maximum contaminant level (MCL) for simazine is about 35%, whereas it is nearly zero for both atrazine and alachlor. Prudent use of simazine in very susceptible areas is recommended.
A Simple Analytical Approach for Predicting Nitrate Concentrations in Pumped Ground Water
Authors: Lerner D. N. , Papatolios K. T.
Published: August 2005

A simple analytical expression is presented which predicts how solute concentrations evolve with time in a pumped well. The basic expression is for uniform recharge and uniform concentration of a conservative solute (e.g. nitrate) in the recharge. It shows that pumped concentrations are independent of pumping rates. The expression can be developed to allow for more complex patterns of recharge and solute loading, and an example is given with three zones of recharge and concentrations, induced river recharge, and a cross‐boundary inflow. Predictions are made for Edgmond Bridge, a new pumping station which lies in a drift‐filled valley in the Triassic sandstone aquifer of Shropshire, UK. The model predicts that concentrations of nitrate will stay below the drinking water limit for 78 years, although this would be substantially reduced if the aquifer were significantly layered or if nitrate loads from agriculture increased. Predictions were reevaluated after two years and found to be consistent with observed patterns once actual, rather than expected, conditions were incorporated in the model.
The Annual Regime of Shallow Ground‐Water Levels in Poland
Authors: Chelmicki Wojciech
Published: August 2005

The annual regime of shallow ground‐water levels across Poland is analyzed according to the dominance of continental and oceanic air masses. Mean monthly ground‐water levels are transformed into relative values and, in order to show the spatial patterns of the annual regime throughout Poland, the contours of such values are derived for each month of the year.

The shallow ground‐water regime in Poland can be correlated with the degree of the climatic continentality. The main difference between the mildly oceanic northwestern part of Poland and the continental eastern part is the pattern of the winter regime which responds differentially to snowmelt. In the northwestern part of Poland, ground‐water levels during late fall and winter are higher than those in the continental eastern part of the country. From April to September, there is no distinct difference between the ground‐water levels in those two parts of Poland.
Urban Ground‐Water Pollution: A Case Study from Coventry, United Kingdom
Authors: Nazari M. M. , Burston M. W. , Bishop P. K. , Lerner D. N.
Published: August 2005

A regional ground‐water quality survey from 28 wells in the Coventry area of the United Kingdom identified widespread ground‐water pollution. Chlorinated hydrocarbon solvents (CHSs) were detected at concentrations exceeding l microgram per liter (μg/I) in all industrial and public water supply wells within the urban area. Of the CHSs analyzed, trichloroethene was most ubiquitous and was also detected at the highest concentrations, reaching a maximum of 547 μg/I at the industrial “Site A” during this study. By comparison with CHS pollution, inorganic and trace element concentrations were elevated relative to assumed baseline levels only in some industrial wells. The limited extent of inorganic contamination may be due to (1) retention of pollutants within the unsaturated zone, (2) ground‐water mixing in wells, (3) ferric hydroxide precipitation resulting in heavy metal coprecipitation, and (4) sorption on sand grains coated with ferric hydroxide. For inorganic determinants, a pollution index was devised which ranked zinc and boron as primary indicators of inorganic contamination. Regionally, concentrations of CHS and inorganic determinants did not reveal any significant correlation with depth.
In Situ Soil Remediation Using Vapor Extraction Wells, Development and Testing of a Three‐Dimensional Finite‐Difference Model
Authors: Sepehr Mansour , Samani Zohrab A.
Published: August 2005

Soil vapor extraction technology is becoming increasingly popular in soil remediation in removing volatile and semivolatile organic compounds from the vadose zone. The rate and extent of removal of volatile organic compounds from the vadose zone by active or passive vapor extraction systems are known to be affected by a number of factors, including soil permeability, soil moisture content, applied suction, air flow rate, temperature, vapor pressure, and external boundary conditions. Some techniques used to describe the dynamics of gas flow are based on simplifying assumptions which limit their application in “real world” situations. This paper describes the development and testing of a three‐dimensional gas flow model “GAS3D” for the design of soil venting systems. The model takes into account the effect of partial penetration and partial screening of vapor extraction wells as well as the nonhomogeneity and anisotropy of the soil. The simulated pressure around a vapor extraction well by the developed GAS3D model was verified and validated by comparing the results of the finite‐difference solution to the actual field measurements and the results of an analytical solution under homogeneous and isotropic conditions. The model was used in a case study to demonstrate the utility of the developed model under heterogeneous and anisotropic conditions for designing a vapor extraction system.
Design and Evaluation of a Mesoscale Model Vadose Zone and Ground‐Water System
Authors: Lawrence J. R. , Zanyk B. N. , Hendry M. J. , Wolfaardt G. M. , Robarts R. D. , Caldwell D. E.
Published: August 2005

A mesoscale model soil and ground‐water system was constructed and instrumented to study physical, chemical, and microbial processes, including transport and degradation of contaminants. The model system was 4.6 m in height by 2.4 m in diameter and contained up to 65 tonnes of soil and geologic materials. The model was filled with Ap (11 cm), B (20 cm), C1 and C2 (369 cm) horizons; the water table was 3.3 m from the soil surface. A rain simulation system applied uniform coverage of rain and/or chloride tracer to the model. Sampling of soil, sediment, pore waters, and gas phase was carried out using a series of in situ collectors and sampling ports. Major anions, metals, pH, alkalinity, CO2, N2, O2, CH4, microbial numbers, microbial biomass and heterotrophic potential (14 C‐amino‐acid uptake) were monitored over time (120 days) and depth. The system approached steady state with respect to solution chemistry and gases after approximately 60–70 days. During this interval, carbon dioxide rose from atmospheric to 3–5% of total gas volume measured at each sampling depth above the water table. Physical, chemical, and biological conditions in the model system after the initial 120 days were comparable to those reported for vadose and saturated zones during field studies on shallow phreatic aquifers. These results indicated that the mesoscale model preserved many of the features of natural systems while providing controlled conditions for studies such as evaluating the fate, and ground‐water contamination potential, of agricultural and industrial chemicals.
A Geographic Information System (GIS) User Interface for Delineating Wellhead Protection Areas
Authors: Rifai H. S. , Hendricks L. A. , Kilborn K. , Bedient P. B.
Published: August 2005

Digital tools such as Geographic Information Systems (GIS) store, analyze, map, and illustrate large amounts of diverse electronic data. This paper develops a GIS modeling user interface for delineating Wellhead Protection Areas (WHPAs) around public water‐supply wells. With this modeling interface, the user can extract the necessary information from the GIS database automatically. The GIS database can then store the delineated WHPAs for further analysis and display. The modeling interface allows local agencies to delineate and manage WHPAs for hundreds of wells more efficiently and effectively than with the traditional paper maps and overlay system.

In order to demonstrate the efficiency of the user interface, this paper presents a wellhead delineation pilot study for the City of Houston. WHPAs for the 202 active public water‐supply wells in the city were delineated using GIS and the modeling interface. Detailed sensitivity analyses were completed to evaluate the effect of uncertainty in model parameters on the delineated WHPAs. Potential sources of contamination within the delineated WHPAs were then identified using the GIS database.

While GIS are perceived as an important tool that can enhance a user's ability for spatial data analyses, they have many problems. The system hardware and software require a substantial investment in financial and human resources. Database development is a tricky process requiring planning and commitment; in most cases, the anticipated outcome from GIS software development is not immediately realized.
Airborne Geophysics: Application to a Ground‐Water Study in Botswana
Authors: Bromley J. , Mannström B. , Nisca D. , Jamtlid A.
Published: August 2005

Summary

A ground‐water study carried out in the Serowe area of eastern Botswana between 1985–1988 has provided the opportunity to evaluate the role of a multiparameter low‐level airborne geophysical survey in a hydrogeological investigation. The survey included magnetic, VLF (very low frequency), and coaxial EM (electromagnetic) measurements. In total, 7,500 line kilometers were flown over an area of 3,300 km2 with a nominal ground clearance of 20 m and a line spacing of 400 m.

The main aquifer, the Ntane Sandstone Formation (Karoo age), is confined between mudstones below and basalt above, and is broken into a series of graben and horst structures by numerous E‐W striking faults. All bedrock, however, is completely masked by a 20–60 m thick overburden of sands, calcretes, silcretes, and sandstones known collectively as the Kalahari beds.

Airborne magnetic and VLF geophysical surveys have been used to penetrate this masking cover. Images and stacked profiles obtained from the survey revealed structural and geological features of major hydrogeological significance. This provided the information necessary for the formulation of a conceptual model.

The results helped guide the subsequent exploration drilling program in an efficient and effective manner, cutting down the need for extensive ground surveys. The investigation confirmed the availability of a 35,000 m3/day resource, sustainable for a 25‐year period. Highest yields were obtained from fracture zones associated with VLF anomalies. Potential wellfields were identified in confined sections of the aquifer, with production boreholes to be sited, where possible, on fractures associated with VLF anomalies.
Assessment of Ground‐Water Flow and Chemical Transport in a Tidally Influenced Aquifer Using Geostatistical Filtering and Hydrocarbon Fingerprinting
Authors: Marquis Samuel A. , Smith Eric A.
Published: August 2005

Traditional environmental investigations at tidally influenced hazardous waste sites such as marine fuel storage terminals have generally failed to characterize ground‐water flow and chemical transport because they have been based on only a cursory knowledge of plume geometry, chemicals encountered, and hydrogeologic setting and synoptic ground‐water‐ level measurements. Single‐time observations cannot be used to accurately determine flow direction and gradient in tidally fluctuating aquifers since these measurements delineate hydraulic head at only one point in time during a tidal cycle, not the net effect of the fluctuations. In this study, a more rigorous approach was used to characterize flow and chemical transport in a tidally influenced aquifer at a marine fuel storage terminal using: (1) ground‐water‐level monitoring over three tidal cycles (72 hours), (2) geostatistical filtering of ground‐water‐level data using 25‐hour and 71‐hour filtering methods, and (3) hydrocarbon fingerprinting analysis.

The results from the study indicate that naphtha released from one of the on‐site naphtha tanks has been the predominant contributor to the hydrocarbon plume both on‐site and downgradient off‐site and that net ground‐water and hydrocarbon movement has been to the southeast away from the tank since 1989. These conclusions are supported by: (1) the elongate configuration of the hydrocarbon plume parallel to the mean ground‐water flow directions determined using the 25‐hour and 71‐hour filtering methods; and (2) the occurrence of free‐naphtha‐product and elevated levels of BTXE and TPH in the naphtha range (C7 to C14) downgradient, but not upgradient, of the naphtha tank. The results also indicate that ground water containing 2.1 to 3.6 ppm TPH in the motor oil range (C22 to C32) is migrating onto the site from an upgradient source area to the north and/or northwest. Therefore, the background concentration or cleanup level of TPH for the facility is 2.1 ppm or greater. This study provides a more quantitative approach than traditional approaches for characterizing flow, assessing chemical transport and upgradient impacts, determining site‐specific ground‐water cleanup levels, and optimally locating ground‐water monitor wells and extraction wells or trenches at tidally influenced hazardous waste sites.
A Rewetting Approximation for a Simulator of Flow in a Surficial Aquifer Overlain by Seasonally Inundated Wetlands
Authors: Merritt Michael L.
Published: August 2005

An approximation is presented to rectify situations occurring as part of the three‐dimensional simulation of surficial aquifer flows in which surface grid cells become dry (zero saturated thickness) and then cannot readily receive flows that would again partially saturate them. The approximation permits the cells to be rewetted by adjusting dry cell pressures by the same amount as the pressure change computed for the uppermost nondry cells in the vertical column. This is continued in successive timesteps until the dry cells once again become partially or fully saturated.

The rewetting procedure was tested in a recent transient simulation of flows in the surficial Biscayne aquifer of Dade County, Florida, which is partly covered by seasonally inundated wetlands. The uppermost layer of the model (layer 1) was used to represent overland sheetflow and grid cells of high equivalent hydraulic conductivity in this layer became dry and were rewetted seasonally, requiring a relatively robust representation of rewetting. The rewetting procedure made possible a simulation of overland sheetflow stages and aquifer water‐table altitudes between 1945 and 1989 that was generally correct to within about 1 foot in inundated and noninundated regions.

In a 6‐year time period (1962–67) in which the behavior of the rewetting procedure was studied in detail, there were 4,723 simulated cell rewettings, of which 3,172 were in the overland sheetflow layer and 1,551 were in the uppermost aquifer layer (layer 2). The average head adjustment in a cell rewetting was 0.10 foot in layer 1 and 0.15 foot in layer 2. This study demonstrates an empirical approach to the problem of cell rewetting that might prove useful in other models of flow in surficial aquifers.
A General Solution to the Step‐Drawdown Test
Authors: Helweg Otto J.
Published: August 2005

A more general solution to the step‐drawdown test, called the General Well Function, GWF, is proposed to replace the classic Jacob's step‐drawdown equation for situations when the time that wells are continuously pumped varies greatly. Jacob's equation requires a preselected discharge duration and does not incorporate time as an independent variable. Jacob's equation has been criticized for that shortcoming since drawdown is obviously dependent on time as well as discharge. The General Well Function, GWF, not only corrects for this weakness but appears to better predict drawdowns that extend beyond the test data than, for example, the FASTEP code based on Jacob's equation. The GWF assumes that both formation and well losses increase over time.
Assessment of Critical Flow Path for Improved Remediation Management
Authors: Poeter Eileen , Townsend Peter
Published: August 2005

A multiple indicator conditional simulation technique is used to generate realizations of hydrofacies units in a two‐dimensional section of the fluvial, unconfined aquifer on the Hanford Site, Richland, WA. Two conceptual geologic models are considered and evaluated for spatial connectivity. The two conceptual models yielded different probabilities of occurrence for three “families” of simulations with distinct styles of connectivity. Critical ground‐water flow paths of each simulated section are evaluated as the paths with the highest percent of high hydraulic conductivity hydrofacies. The effective hydraulic conductivity of these critical paths is evaluated analytically by computing harmonic averages. Estimated ground‐water travel times are consistent with the observed rate of plume migration in the field. This procedure can be used to evaluate the probability of occurrence of alternative paths for contaminant transport as input to risk assessments, and design of characterization programs and remediation systems.
Numerical Modeling of Axisymmetric Flow
Authors: Pandit Ashok , Abi Aoun Jean M.
Published: August 2005

A Galerkin finite‐element model is developed to solve axisymmetric flow problems in confined aquifers subjected to pumping by a single well. The model can be used to analyze axisymmetric problems involving pumping from multiaquifer systems by wells which are partially screened, fully screened, or have multiple screens. A simplified boundary condition at the well bore produces results that match analytical solutions. The location of the zero‐drawdown boundary is determined by using the concept of effective radius of influence. This concept is also used to generate a mesh that automatically expands radially in time. The expanding mesh feature may lead to savings in computational time since it uses relatively few elements and nodes for simulations at small times and progressively adds elements and nodes to the mesh for simulations at higher times.
Recommendations for Usage of SURFER to Gridding Model Results
Authors: Shan Chao , Stephens Daniel B.
Published: August 2005

Contouring is an important aspect of virtually all hydrogeological and contamination investigations. The popular computer contouring package SURFER saves enormous effort in preparing contour plots, but for some cases its misuse can lead to significant errors. We have demonstrated that where regularly spaced data with uniform grid size are to be input to SURFER for contouring, certain procedures are recommended to obtain improved accuracy. For optimal results, the grid size in SURFER should be exactly the same as that in the input file, and the number of nearest points for searching should be set to 1. If these procedures are not followed, contour lines are often not smooth. Consequently, the plots can introduce bias that appears as though there is numerical dispersion. For cases where calculated data have a nonuniform grid size, we recommend using Quadrant as the search method and setting the number of nearest search points to 1.
A Survey of Charge‐Balance Errors on Published Analyses of Potable Ground and Surface Waters
Authors: Fritz Steven J.
Published: August 2005

Although the charge balance for a single chemical analysis of a water's electrolytes is not a reliable gauge for the accuracy of that analysis, the percent charge‐balance error (%CBE) becomes more credible as a means for evaluating analytical technique when applied to groups of analyses.

Just how good are charge balances for chemical analyses of potable ground and stream waters in mainline geological journals? Starting with each journal's first volume, a search found 68 articles in six journals (Appl. Geochem., Chem. Geol, Geochim. Cos. Acta., Ground Water, J. Hydrol, and Water Resources Res.) that had what was deemed to be “complete” analyses of individual samples' major ions. Analyses of brines and hydrothermal solutions were excluded as were “composite” averages. A total of 1,062 %CBEs were computed and tabulated, and the average was 3.99%$ 6.56 (1σ).

The average %CBE (by journals) ranges from 1.55% to 9.34%. Quality of %CBE is slightly better for articles published after 1970. Not surprisingly, low ionic strength samples are prone to charge‐balance errors > 10%. Of the 1,062 charge balances calculated, 612 were positive and 450 were negative. Eight of the 68 articles had analyses that were evenly balanced between positive and negative CB errors. There are 21 articles in which the majority of analyses had negative charge‐balance errors. Thus a clear majority (39) of articles had analyses in which there were more positive than negative charge‐balance errors.

Systematic error is likely for situations where the analyses in an article are predominantly positive or predominantly negative. The likely reason for the former occurrence is lab determination of alkalinity from the nonacidified field aliquot. Failure to filter samples may cause the latter situation—especially in carbonate terranes. Systematic laboratory errors involving salt standards and dilutions can also yield charge‐balance errors for a suite of analyses that are skewed to either positive or negative errors. The care with which the analyst did his/her job may also impact charge‐balance errors.
Assessment of Methods and Conditions to Locate Boundaries: II. One Straight Recharge Boundary
Authors: Chapuis Robert P.
Published: August 2005

A straight recharge boundary in an ideal aquifer is easily detected in the drawdown graph of an observation well. Two known and one proposed methods can be used to find the location and orientation of the recharge boundary. This paper assesses their performance. Both the law of times and the method of inflection point overestimate the distance ri to the imaginary well when the ratio β= ri/ rr is smaller than 20. In this case, known methods also give incorrect estimates of aquifer parameters. The proposed method gives directly the β‐value when it is higher than 20. For lower β‐values, a correction factor transforms the apparent β‐value into the correct β‐value. Two examples illustrate the quality of predictions by the known and proposed methods. Two known and one proposed methods relative to impervious boundaries are assessed in a companion paper.
Geostatistical Modeling of Heterogeneity in Glaciofluvial, Buried‐Valley Aquifers
Authors: Ritzi Robert W. , Jayne Dale F. , Zahradnik Arthur J. , Field Adrian A. , Fogg Graham E.
Published: August 2005

In the buried‐valley aquifers of the western and central glaciated plains of North America, assemblages of permeable outwash facies may form large and productive regional aquifer systems. In glacially derived aquifers it is common to have low‐permeability facies (e.g., till or lacustrine clay) juxtaposed with high‐permeability facies (e.g., sand and gravel outwash). The heterogeneity in the Miami buried‐valley aquifer system in southwestern Ohio was examined as an example. Practical approaches for evaluating the lithological uncertainty were explored in the areas lacking data. The facies types were represented by an indicator random function, which exhibited an isotropic, exponential covariance structure. A comparative, quantitative analysis of the facies boundaries was accomplished using three different indicator geostatistical methodologies: (1) hydrostratigraphic analysis with indicator point kriging and probability cutoff, (2) hydrostratigraphic analysis with conditional indicator simulation, and (3) leakance zonation with indicator block kriging and grid‐cell averaging. With the first method, it was shown in a theoretical analysis that the 0.5 probability level computed by indicator point kriging may or may not correspond to the boundary between high‐ and low‐permeability facies, depending upon the level of data support. With the data support for the Miami aquifer system, the 0.65 probability contour gave an areal distribution of low‐permeability facies that honored both the original data and the declustered global mean of the original indicator data. The first and third methods gave rise to a single model, in which the facies distribution was best in some local accuracy sense. The second method gave rise to many alternative models, in which the global texture of the facies distribution took precedence over local estimation of the expected value. The first two methods involved evaluating the geometry of the boundaries between the facies types in advance of creating the grid of the numerical ground‐water model, so that grid cell boundaries could be made to correspond to facies boundaries. The third method was useful when updating an existing ground‐water model that has grid cell boundaries not necessarily corresponding to facies boundaries.
Deep Ground‐Water Investigation by Combined VES/MTS Methods Near Ho Chi Minh City, Viet Nam
Authors: Van Pham Ngoc , Boyer Daniele , Thoa Nguyen Thi Kim , Van Nguyen Giang
Published: August 2005

In the agglomeration of Ho Chi Minh City (HCMC), South Viet Nam, located near the eastern edge of the Mekong Delta, both superficial Quaternary and deep Tertiary Alluvium may contain brackish or saline water from ground surface to great depths. The combination of two complementary geophysical methods, the Vertical Electrical Sounding (VES) for shallow investigation, and the Magnetotelluric Sounding (MTS) for deep investigation, gives a more complete electrical image of the sedimentary series from the surface to the deep crystalline basement. It turned out to be very efficient in delineating the ground‐water contamination in both superficial and deep aquifers. The combined results reveal a close correlation of the extent of the contamination with the paleo‐geomorphology of laterites, and subsequently with the paleorelief during the Tertiary and Quaternary periods. A simple guide for hydrogeological investigation is proposed in connection with the present topography and the Tertiary tectonics.
Spatial Variability in Hydrologic Properties of a Volcanic Tuff
Authors: Istok J. D. , Rautman C. A. , Flint L. E. , Flint A. L.
Published: August 2005

Spatial variability of hydrologic properties was quantified for a nonwelded‐to‐welded ash flow tuff at Yucca Mountain, Nevada, the potential site of a high‐level, nuclear waste repository. Bulk density, porosity, saturated hydraulic conductivity, and sorptivity were measured on core specimens collected from outcrops on a grid that extended vertically through the entire unit thickness and horizontally 1.3 km in the direction of ash transport from the volcanic vent. A strong, geologically determined (deterministic) vertical trend in properties was apparent that correlated with visual trends in degree of welding observed in the outcrop. The trend was accurately described by simple regression models based on stratigraphic elevation (vertical distance from the base of the unit divided by unit thickness). No significant horizontal trends in properties were detected along the length of the transect. The validity of the developed model was tested by comparing model predictions with measured porosity values from additional outcrop sections and boreholes that extended 3000 m north, 1500 m northeast, and 6000 m south of the study area. The model accurately described vertical porosity variations except for locations very close to the source caldera, where the model underpredicted porosity in the upper half of the section. The presence of deterministic geologic trends, such as those demonstrated for an ash flow unit in this study, can simplify the collection of site characterization data and the development of site‐scale models.
Interpretation of Slug and Packer Tests in Anisotropic Aquifers
Authors: Zlotnik Vitaly
Published: August 2005

Slug and packer tests provide data for determining the local value of hydraulic conductivity in aquifers. However, most techniques commonly used for test interpretation disregard any anisotropy of the tested formation. Using dimensional analysis, an extension of available methods for interpretation of slug and packer tests to anisotropic conditions is developed. The technique is a transformation of the radial dimension via the well radius, and involves dividing the true well radius by the square root of the anisotropy ratio which must be specified a priori. This technique can be applied to formulae, computer programs, graphs, or tabulated data available for confined or unconfined isotropic conditions. It also reduces the number of computer computations. As an example, the technique is used to extend the Bouwer and Rice (1976) method, which was developed for isotropic conditions, to anisotropic conditions. A comparison of the Bouwer and Rice (1976) method corrected for anisotropy, with the Widdowson et al. (1990) method, yields horizontal hydraulic conductivities differing by 8% (in one typical case). Ignoring anisotropy for the same case results in a 44% underestimation of horizontal conductivity.
Applicability of Sharp‐Interface Models for NAPL Transport: 2. Spreading of a LNAPL
Authors: El‐Kadi Aly I.
Published: August 2005

This paper, the second in a two‐part series, examines the applicability of the sharp‐interface approach to modeling initial accumulation and spreading of a light nonaqueous phase liquid (LNAPL) in the ground‐water aquifer. The sharp‐interface approach models NAPL transport by developing a time‐distance profile from Darcy's law by assuming a piston or plug flow. The approach is tested here against an immiscible‐phase model that includes capillarity effects in the formulation. Although significant assumptions are included in the sharp‐interface model, it may be useful for initial screening, in obtaining an order‐of‐magnitude estimate of travel times, and in describing the approximate extent of the free and dissolved NAPL contamination. In the absence of data for calibration, the paper introduces guidelines for estimating the parameters of the model. By integrating the formulation in Part 1 and this part, a practical approach for analysis of NAPL contamination is defined; the underlying assumptions and simplifications have to be recognized, however.
Analysis of Particle Tracking Errors Associated with Spatial Discretization
Authors: Zheng C.
Published: August 2005

This paper addresses two particular problems which arise in particle tracking calculations because of the spatial discretization used in the block‐centered finite‐difference flow model. These two problems are: (1) the presence of a “weak” sink or source (i.e., a sink or source that does not result in complete inward or outward gradients on all interfaces of the cell containing the sink or source); and (2) the use of a model grid with varying vertical cell dimensions within an individual layer. While these problems can be mitigated by using a sufficiently fine spatial discretization, there are always computational and practical limitations on the number of model nodes. Two procedures are presented in this paper which minimize particle tracking errors associated with weak sinks/sources or vertically distorted grids. The first procedure uses an approximate analytical solution to define the velocity distribution inside a model cell containing a weak sink/source so that the capture of the particles entering the cell can be determined accurately. The second procedure establishes an automatic adjustment of the particle vertical coordinate according to the degree of vertical grid distortion. These procedures are shown to be effective, and improve the accuracy of particle tracking results under nonideal spatial discretizations.
Inverse Analysis Implementation of the SUTRA Ground‐Water Model
Authors: Piggott Andrew R. , Bobba A. Ghosh , Xiang Jiannan
Published: August 2005

Numerical simulation of ground‐water flow and transport is often impeded by a lack of information regarding the conceptual model and properties that best represent the water‐bearing formation. Inverse analysis assists in the estimation of the properties of the formation once a defensible form of the conceptual model is defined. In essence, inverse analysis reverses the direction of model application to determine the properties that best match data collected from the formation under investigation. This paper describes the development of an inverse analysis implementation of the well‐known and versatile SUTRA model of ground‐water flow and transport. The inverse analysis algorithm retains the extensive functionality of SUTRA in forward simulation and allows various forms of parameter constraints to be stipulated. Addition of inverse analysis functionality to an existing ground‐water model circumvents the development of redundant models, reduces development time, and ensures the consistent and concurrent development of predictive and interpretive capabilities. Two example analyses are presented to demonstrate a portion of the functionality of the novel inverse analysis algorithm.
A Method of Collecting and Analyzing Lineaments for Regional‐Scale Fractured‐Bedrock Aquifer Studies
Authors: Mabee Stephen B. , Hardcastle Kenneth C. , Wise Donald U.
Published: August 2005

A new method is proposed for collecting and reducing large collections of lineament data. The method consists of three steps: (1) collection of lineament data using multiple observers, multiple observation trials, and several types of imagery; (2) reproducibility tests; and (3) domain overlap analysis. Collection of lineament data and reproducibility tests are performed by overlaying lineament maps drawn by several observers or by superimposing multiple maps prepared by a single observer and identifying lineaments which are coincident (coincident lineaments = lineaments that have azimuths within 5 ± and separation distances are within 1–2 mm at the scale of drawing). Domain overlap analysis is accomplished by measuring the trends of near‐vertical fractures at outcrops distributed over the study region and comparing the spatial distribution of these trends with similar‐trending coincident lineaments. Lineaments that are not reproducible and are not geographically correlative with fractures are considered unimportant and removed from the data base. The method was applied to a 44 km2 study area in Maine and resulted in a reduction in the lineament data base from 6500 to 217. Transmissivities determined for bedrock wells located within 30 meters of lineaments that are both reproducible and geographically correlative with outcrop‐scale fractures are generally higher than the transmissivities of wells located near lineaments that are not separated on the basis of these criteria. Application of the method serves as an important filter by providing a more manageable lineament data base from which to begin detailed field checking and/or geophysical surveys directed toward specific lineaments.
Specific Yield as Determined by Type‐Curve Analysis of Aquifer‐Test Data
Authors: Moench Allen F.
Published: August 2005

Type‐curve analysis of water‐table aquifer pumping‐test data has often resulted in values of specific yield that are unrealistically low when compared with values obtained by volume‐balance calculations. It has been suggested that such values are the result of inadequate representation of drainage processes in the unsaturated zone. The commonly used analytical solution developed by Neuman for a homogeneous, water‐table aquifer assumes that the drainage of pores in the zone above the water table due to lowering of the water table occurs instantaneously. Published pumping‐test data from three relatively homogeneous, unconsolidated, granular aquifers with significantly different hydraulic conductivities are used to show that type‐curve analysis based on the Neuman model will result in estimates of specific yield that agree with volume‐balance calculations, provided that established procedures are followed. These involve the use of composite plots of drawdown observed at more than one location and inclusion of effects of partial penetration. Noninstantaneous drainage of pores in the unsaturated zone accounts for the finding that both type‐curve analysis and volume‐balance calculations yield values of specific yield that are slightly less than those obtained from laboratory column‐drainage experiments. It may also account for a slight underestimation of vertical hydraulic conductivity as obtained by type‐curve analysis.
Field Study of a Long and Very Narrow Contaminant Plume
Authors: van der Kamp G. , Luba L. D. , Cherry J. A. , Maathuis H.
Published: August 2005

A contaminant plume more than 7,700 meters long and a few hundred meters wide has been delineated using a network of bundle piezometers. The location, concentration, and starting time of the source are well‐defined. The plume occurs in a stratified, highly permeable, sand and gravel aquifer in which the ground‐water flow is unusually steady in direction and magnitude. A ground‐water velocity of 380 meters per annum and a longitudinal dispersivity of 60 to 120 meters are obtained by simulating chloride breakthrough data at 7,700 meters from the source. Simulation of the cross‐sectional distribution of chloride concentrations at 5,500 meters shows the horizontal transverse dispersivity to be at most 0.10 meters. This very small maximum value for horizontal transverse dispersivity is two to four orders of magnitude smaller than values reported in the literature for plumes of similar lengths. It is postulated that the very weak lateral dispersion is due primarily to the steadiness of the ground‐water flow.

An ICP‐MS, equipped with an ultrasonic nebulizer and active‐film multiplier detector, is used to attempt to determine 54 trace elements directly in ground water. Lithium, arsenic, rubidium, strontium, barium, and antimony are found in the microgram‐per‐liter (part‐per‐billion = ppb) range. Most of the other elements are present at nanogram‐per‐liter (part‐per‐trillion = ppt) concentrations. Ion exchange preconcentration is utilized in order to improve the sensitivity for measuring the rare earth elements that exist at concentrations as low as 0.05 ppt for lutetium, thulium, and terbium. The formation of molecular species in the plasma produces false positive results for some of the elements. The presence of silicon or carbon dioxide interferes with the measurement of scandium, strontium interferes with rhodium and palladium, and barium interferes with europium. Correction procedures for these interferences are discussed. All together, the concentrations of the 54 elements in water from four Nevada springs span almost seven orders of magnitude.
Quantification of Uncertainty in Exposure Assessments at Hazardous Waste Sites
Authors: Goodrich Michael T. , McCord James T.
Published: August 2005

Many of the risk assessment approaches being applied at hazardous waste sites fail to adequately account for the ground‐water flow and solute transport processes (and their uncertainties) that move the contaminant from the source to the receptor. For example, one recommended model assumes the contaminant concentration at the source is equal to the contaminant concentration at the receptor. A more realistic way to estimate receptor concentrations for risk assessment would be to use technically accepted ground‐water flow equations coupled with the advection‐dispersion equation and use Monte Carlo techniques to address parameter uncertainty. A complex numerical model and a simple analytical model were used to simulate ground‐water flow and solute transport from a hypothetical waste site to a subsurface drain using both small and larger scale hydraulic properties. Results indicate that the predicted receptor concentration is considerably lower than the source concentration when knowledge of the local hydrogeology is considered in the transport simulations.