bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐Dimensional Modeling of a Two‐Well Tracer Test
Authors: Güven O. , Molz F. J. , Melville J. G. , Didy S. El , Boman G. K.
Published: August 2005

A three‐dimensional (3‐D), advection‐based numerical model has been developed for simulating two‐well injection withdrawal tracer tests in heterogeneous granular aquifers, and the model has been applied to perform various simulations of an actual two‐well tracer test conducted previously in a confined aquifer at a field site near Mobile, Alabama. Information obtained from a series of independent field tests is used as input in the model to account for the 3‐D spatial distribution of the hydraulic conductivity (K) at the test site.

The numerical model is based on a simplified, Lagrangian approach in which the transport of the tracer between the injection and withdrawal wells is modeled taking into account advection only. Processes such as dispersion, sorption, and chemical or biological reactions are neglected. Despite field data limitations and various simplifying assumptions, the model predictions of the withdrawal‐well concentration breakthrough and of the mean tracer arrival times at various levels of two multilevel observation wells are in good overall agreement with the corresponding field observations.

The results of this study indicate that it will be possible to construct realistic, predictive models of contaminant transport in heterogeneous granular aquifers if the necessary effort is made to obtain field measurements of the 3‐D spatial distribution of hydraulic conductivity. The extent and feasibility of such an effort are expected to depend on the particular contamination problem at hand. Future research should be directed toward the problems of field measurement scale selection and the development of K distributions for models from sparse data sets.
Design of Ground‐Water Monitoring Networks Using the Monitoring Efficiency Model (MEMO)
Authors: Wilson Charles R. , Einberger Carl M. , Jackson Ronald L. , Mercer Richard B.
Published: August 2005

An analytical Monitoring Efficiency Model (MEMO) has been developed to assist in the design of monitoring well networks. The method simulates the migration of hypothetical contaminant plumes from a site and quantifies the efficiency of alternative well network designs in detecting the plumes. The computed detection efficiency provides a basis for optimizing the design. Maps of the site showing areas from which releases would or would not be detected by a given well network are produced, providing insight into the benefits of adding, deleting, or moving specific wells.
A Neural‐Network Approach to the Determination of Aquifer Parameters
Authors: Aziz Abd. Rashid Abd. , Wong Kau‐Fui Vincent
Published: August 2005

A new approach to determine aquifer parameter values from aquifer‐test data has been developed that uses the pattern‐matching capability of a neural network. The network is trained to recognize patterns of normalized drawdown data as input and the corresponding aquifer parameters as output. The Theis and Hantush‐Jacob solutions for confined and leaky‐confined aquifer conditions are used to derive the input patterns based on the parameter values selected from predetermined ranges. The trained network produces output of aquifer parameter values when it receives the aquifer‐test data as the input patterns. The results obtained from this new approach are in good agreement with published results using other techniques. The advantages of the present approach are the automated process of obtaining aquifer parameter values and the ability of the network to associate drawdown to the corresponding Theis and Hantush‐Jacob solutions.
A Comparison of Three Functional Forms for Representing Soil Moisture Characteristics
Authors: Bumb Amar C. , Murphy Carolyn L. , Everett Lome G.
Published: August 2005

Three functional forms relating capillary pressure to soil saturation (the Brooks and Corey equation, the Boltzmann distribution, and the Fermi distribution) were evaluated and compared by matching each form to soil moisture characteristic data for 12 different soil types.

Out of 113 data sets, the Boltzmann distribution and the Brooks and Corey equation performed very well–and almost equally well–in matching the soil moisture data. Only 10 data sets were significantly better matched by the Boltzmann distribution, and only nine data sets were significantly better matched by the Brooks and Corey equation. The Fermi distribution did not achieve as high a rate of success, although unlike the other two functional forms, it was able to match data in all regions of the curve, including the capillary fringe.

Considering that the Brooks and Corey equation and the Boltzmann distribution performed almost equally well, the Boltzmann distribution has an additional advantage: it is amenable to an analytic solution to the governing differential equation for flow in unsaturated media. Based on the results of this study, the authors recommend use of the Boltzmann distribution.
Geochemical and Kinetic Evolution of a Karst Flow System: Laurel Creek, West Virginia
Authors: Groves Christopher G.
Published: August 2005

An approach that couples flow rate measurement with mass balance calculations has been used to evaluate the geochemical evolution and kinetic behavior of Laurel Creek, West Virginia as it flows from a siliceous, noncarbonate caprock onto the Greenbrier Group Limestones, into and through the Laurel Creek‐Cross Road Cave System, and out of the spring at the lower end of the system. At a discharge of 1.0 m3 s−1, the stream was calculated to be removing calcite from the aquifer by dissolution at a rate of 4.1 mg 1−1 hr−1, for a total removal of 46 kg CaCO3/hr. Laurel Creek remained undersaturated with respect to calcite throughout the entire reach studied. Comparison of calculated saturation states with laboratory based kinetic mechanism information provided by the work of Plummer et al. (1978) suggests that the elementary reaction that describes attack by carbonic acid dominates calcite dissolution within the main conduit portion of the flow system.
Salt‐Water Upconing in an Aquifer Overlain by a Leaky Confining Bed
Authors: Motz Louis H.
Published: August 2005

Salt‐water upconing that occurs in an aquifer overlain by a leaky confining bed is described in terms of an analytical model that assumes the existence of a sharp interface between the fresh water and salt water and the occurrence of a critical rise in the interface, above which only an unstable cone can exist. Drawdown is calculated along the salt‐water/fresh‐water interface due to pumping from a well that partially penetrates the fresh‐water zone, and then the Ghyben‐Herzberg relation is used to calculate the steady‐state rise in the interface, which is assumed to be small compared to the thickness of the aquifer. The interface rise and the critical pumping rate (Qc) are determined in terms of aquifer and confining bed properties and the degree of penetration of the pumping well into the fresh‐water zone. The critical rise is assumed to occur when the interface rise is equal to 0.3 times the distance from the original interface location to the bottom of the well. Based on the analytical model, the nondimensional critical pumping rate increases as the ratio of vertical to horizontal hydraulic conductivity (Kz/ Kr) is decreased, and it decreases as the degree of well penetration (t/b) is increased. As an example application, Qc was calculated using site‐specific data from a test well in Pasco County, Florida. A value of Qc= 4.28 × 105 cubic feet per day was determined by assuming that Kz/Kr= 1.0. If Kz/Kr were subsequently determined to be less than or greater than 1.0, then the estimated value of Qc would be increased or decreased, respectively.
Comparison of Flow Models Used to Delineate Capture Zones of Wells:
Authors: Bair E. Scott , Roadcap George S.
Published: August 2005

Analytical, semianalytical, and numerical flow models of a leaky‐confined fractured‐carbonate aquifer were used with particle‐tracking/stream‐function programs to delineate traveltime‐related capture zones of a municipal wellfield and to assess “model” errors imposed by requisite assumptions associated with each flow model. The analytical flow model uses the Hantush‐Jacob equation describing two‐dimensional transient drawdown surrounding a well in a leaky‐confined aquifer with superposition of a nonuniform regional flow field. The semianalytical flow model employs the Thiem equation describing two‐dimensional steady‐state drawdown surrounding a well in a fully confined aquifer with superposition of a uniform regional flow field. The numerical flow model uses a finite‐difference solution describing three‐dimensional steady‐state flow and incorporates five model layers, specified‐flux boundary conditions, spatially variable recharge, and recharge from a lake. Goodness‐of‐fit statistics show that the semianalytical flow model does not calibrate as well to measured heads as do the analytical and numerical flow models. Model errors caused by the inability of the semianalytical flow model to account for vertical leakage and by the required use of a uniform regional flow field cause the size of the five‐year capture zone to be overestimated and its shape to be too simplistic compared to those of the analytical and numerical flow models. Polar plots of reverse‐tracked pathline endpoints suggest that model error also produces significant differences in pathline trajectories. The results show that in this type of hydrogeologic setting, it is necessary to use a flow model that accounts for vertical leakage and for the nonuniform character of the regional flow field.
Methanogenic Biodégradation of Creosote Contaminants in Natural and Simulated Ground‐Water Ecosystems
Authors: Godsy E. Michael , Goerlitz Donald F. , Grbic‐Galic Dunja
Published: August 2005

Wastes from a wood preserving plant in Pensacola, Florida have contaminated the near‐surface sand‐and‐gravel aquifer with creosote‐derived compounds and pentachlorophenol. Contamination resulted from the discharge of plant waste waters to and subsequent seepage from unlined surface impoundments that were in direct hydraulic contact with the ground water. Two distinct phases resulted when the creosote and water mixed: a denser than water hydrocarbon phase that moved vertically downward, and an organic‐rich aqueous phase that moved laterally with the ground‐water flow. The aqueous phase is enriched in organic acids, phenolic compounds, single‐ and double‐ring nitrogen, sulfur, and oxygen containing compounds, and single‐ and double‐ring aromatic hydrocarbons. The ground water is devoid of dissolved O2, is 60–70% saturated with CH4 and contains H2S. Field analyses document a greater decrease in concentration of organic fatty acids, benzoic acid, phenol, 2‐, 3‐, 4‐methylphenol, quinoline, isoquinoline, l(2H)‐quinolinone, and 2(lH)‐isoquinolinone during downgradient movement in the aquifer than could be explained by dilution and/or dispersion. Laboratory microcosm studies have shown that within the study region, this effect can be attributed to rnicrobial degradation to CH4 and CO2. A small but active methanogenic population was found on sediment materials taken from highly contaminated parts of the aquifer.
The Use of Solute and Isotopic Chemistry to Identify Flow Patterns and Factors Which Limit Acid Mine Drainage in the Wasatch Range, Utah
Authors: Mayo Alan L. , Nielsen Peter J. , Loucks Mark , Brimhall Willis H.
Published: August 2005

In this case study, we have demonstrated how a minimal amount of relatively inexpensive isotopic data can be used to help solve a complex Hydrogeologie problem.

Acidic (pH 3 to 4) and neutral (pH 6.0 to 7.7) mine drain waters have been identified and sampled in the mining districts in the central Wasatch Range of Utah. The chemical evolution of acid and neutral mine waters follow fundamentally different chemical and isotopic evolutionary pathways which reflect fundamentally different flow histories. Most neutral mine discharges are either low iron calcium‐bicarbonate waters or iron and sulfate rich waters. Stable isotopic values of δ2 H and δ18 O plot near and parallel to the meteoric water line (MWL), suggesting a meteoric origin for all mine waters. 3H concentrations (20.9 to 40.8 TU) and 14C activities (46.2 to 96.3 pmc) indicate mine waters have a major component of modern recharge water. Shallow circulation of most mine discharges is suggested by low TDS, nonthermal discharge temperatures and the proximity of most mine workings to the surface.

The δ34 S values of mine discharge waters tend to cluster about two nodes. Neutral mine drainages with low SO42‐ contents cluster about δ34 S = 0/o, whereas neutral mine drainages with high Sü42 contents cluster about δ34S =+5.5%o. An acid mine drainage also has a 534 S value of about 0 %o. δ34 S values of ∼ 0 %o are attributed to the oxidation ofpyrite and other sulfide minerals, whereas more positive isotopic values are attributed to isotopic fractionation accompanying the dissolution of gypsum and anhydrite. Only mine discharges with a δ34S δ 0%o have the potential to produce acid drainage. Geologic and δ13C data suggest acid mine waters (mean δ13C =−15.0%o) discharge from ground‐water flow systems generally devoid of carbonate rocks. Neutral mine waters (mean δ13C =− 9.7%o) discharge from aquifers containing abundant carbonate rocks which neutralize excess H+ ions released by the oxidization of sulfide minerals.
Sealing Characteristics of Selected Grouts for Water Wells
Authors: Edil Tuncer B. , Chang Michael M. K. , Lan L. T. , Riewe Thomas V.
Published: August 2005

Sealing characteristics of commonly used water well annular space grouts were investigated using a large‐scale laboratory model. The investigation was carried out in two phases. In the first phase, a bentonite drilling mud, often used as a well sealant, was investigated both in the laboratory well model and in the field. For this purpose, Quik‐Gel® mud with different amounts of formation material (in this case sand) entrained in it was used as a sealant. In the second phase, an evaluation of four grouts: neat cement, bentonite‐cement, powder bentonite (Volclay®), and granular bentonite (Benseal® and bentonite slurry mixed according to the “Ohio” recipe) was undertaken with respect to their effectiveness in sealing the annular space. A well model consisting of a sand‐filled plexiglass container was constructed in this investigation. Four metal well casings were installed in boreholes formed in the sand. The annular space between the well casings and the boreholes formed in the sand were filled with various grouts. The effectiveness of these materials in sealing the annular space was evaluated by observation of their structural stability, infiltration of water placed on top of the sealants, and inspection of the sealants dissected during disassembling of the models. A finite‐element computer program modeling the seepage in these well experiments was used to assist in the interpretation of the experimental results. The results indicate that the final success of a sealant depends on its structural stability as much as its permeability. Benseal®‐bentonite slurry grout behaved best; neat cement and bentonite‐cement grouts also provided good seals. Volclay® and various Quik‐Gel® slurries formed poorer seals compared to these. Among the Quik‐Gel® slurries, best results were obtained in the laboratory when a Marsh Funnel Viscosity of about 70 sec/qt was achieved and a mud weight of about 10 to 11 Ib/gal resulted from mixing of formation soils. However, field observations of drilling mud slurries with entrained cuttings exhibited problems of excessive settling even when the slurries had heavy mud weight, greater than 11 Ib/gal. The extent of settling appears to be positively correlated with the relative permeability of the native formation. This limits the types of well construction where these slurries may be used as annular space seal.
A Code to Generate Random Identifiers and Select QA/QC Samples
Authors: Mehnert Edward
Published: August 2005

SAMPLID is a PC‐based, FORTH AN‐77 code which generates unique numbers for identification of samples, selection of QA/QC samples, and generation of labels. These procedures are tedious, but using a computer code such as SAMPLID can increase efficiency and reduce or eliminate errors and bias. The algorithm, used in SAMPLID, for generation of pseudorandom numbers is free of statistical flaws present in commonly available algorithms.
Vapor Extraction of Volatile Organic Compounds from Clay Soil: A Long‐Term Field Pilot Study
Authors: Gibson Thomas L. , Abdul Abdul S. , Glasson William A. , Ang Carolina C. , Gatlin Dallas W.
Published: August 2005

A vapor‐extraction method was evaluated for remediation of clay soil contaminated by a paint thinner, consisting of a mixture of volatile aromatic hydrocarbons. Natural clays have low permeability to liquid or gas flow, and remediation by in situ methods is usually considered inappropriate. However, successful vapor flow through the clay was predicted on the basis of laboratory studies showing increased clay porosity and fracturing in the presence of organic solvents. A vapor extraction pilot study was conducted in a test area containing an estimated 1780 kg of paint thinner. The site was instrumented with vapor extraction wells and numerous soil vapor and pressure monitoring wells to delineate the zone of reduced pressure and sample the soil gases within the test site. Monitoring for eight months indicated that the contaminated clay was permeable to air and vapor flow with an effective range of influence extending 20 ft from the vapor extraction well and that 19% of the initial paint thinner was removed.
A Numerical Model for Water Flow and Chemical Transport in Variably Saturated Porous Media
Authors: Yeh T.‐C. Jim , Srivastava Rajesh , Guzman Amado , Harter Thomas
Published: August 2005

A two‐dimensional numerical model is developed for the simulation of water flow and chemical transport through variably saturated porous media. The nonlinear flow equation is solved using the Calerkin finite‐element technique with either the Picard or the Newton iteration scheme. A continuous velocity field is obtained by separate application of the Galerkin technique to the Darcy's equation. A two‐site adsorption‐desorption model with a first‐order loss term is used to describe the chemical behavior of the reactive solute. The advective part of the transport equation is solved with one‐step backward particle tracking while the dispersive part is solved by the regular Galerkin finite‐element technique. A precondi tioned conjugate gradient‐like method is used for the iterative solution of the systems of linear simultaneous equations to save on computer memory and execution time. The model is applied to a few flow and transport problems, and the numerical results are compared with observed and analytic values. The model is found to duplicate the analytic and observed values quite well, even near very sharp fronts.
Leachate Mound Changes in Landfills Due to Changes in Percolation by a Cap
Authors: Oweis Issa S. , Biswas Gopal C.
Published: August 2005

A simple analytical method, adapted to hand calculations, is presented for evaluating the rise and fall of the leachate level in landfills due to possible changes of recharge caused by a landfill cap. The purpose is to provide an analytical tool for assessing the performance of landfill caps. A constant head boundary is assumed around the perimeter of the landfill, and the two‐dimensional problem is solved using the linearized form of the Boussinesq equation. Percolation from rainfall or from other sources is assumed to be at a steady rate and the only source for leachate generation and mound development. The numerical results from the simple model, in terms of mound rise or decline, as a function of time, were in accord with the solution using the MODFLOW numerical model.

The results of the analysis show that the level of the leachate mound is sensitive to changes in the percolation rate, and based on this, it is concluded that the effectiveness of different cap conditions can be assessed by monitoring the leachate level within the leachate mound after cap installation. The analysis is specifically applicable for landfills containing a leachate mound such as landfills in a nonarid environment and where perimeter heads are controlled.
Calculation of Low‐Range Flow Velocities in Fractured Carbonate Media from Borehole Hydrochemical Logging Data Comparison with Thermometric Results
Authors: Bidaux Pascal , Drogue Claude
Published: August 2005

Flow rates and concentrations from fractures intersected by a borehole can be estimated by matching simulated concentration profiles to the observed variations in water chemistry along the borehole. Experiments in karstic rocks prove that hydraulic short circuits between fractures yield a very complex flow pattern, and that flow velocities in the borehole may be higher than the velocity at which the piezometric level varies. Combined temperature and concentration studies characterize flow in both high permeability channels and slightly fissured blocks, as chemical profiles detect low velocity flows that cannot be identified on temperature profiles. Such studies are therefore advisable to characterize natural flow in fractured carbonate rocks, but also in other fractured media.
Data‐Analyses Methods for Determining Two‐Dimensional Dispersive Parameters
Authors: Jiao Jiu J.
Published: August 2005

Two methods for calculating two‐dimensional hydrodynamic dispersion parameters by analyzing experimental data, the dispersive‐plume‐area (DPA) method and the linear‐graphic (LG) method, are proposed in this paper. The DPA method determines the dispersive parameters by analyzing the dispersive plume area after injection of a salt‐water slug into an aquifer. The transverse and longitudinal dispersivity and even the porosity of an aquifer may be derived. The LG method transforms the concentration‐time curve into a straight line. Using the slope of the line, dispersivity and velocity can be obtained. Both methods are examined using theoretical considerations and practical examples.
Ground‐Water Remediation Using “Smart Pump and Treat”
Authors: Hoffman Fredric
Published: August 2005

Established pump‐and‐treat ground‐water remediation is criticized for being too expensive and time‐consuming, especially when cleanup standards are set at very low levels, e.g., < 10 parts per billion (ppb) for volatile organic compounds (VOCs). Although other ground‐water remediation technologies are currently being examined and show intriguing promise, pump and treat remains the only dependable technology for cleanup of deep (i.e., > 50 ft below ground surface) widespread ground‐water contamination. The perceived shortcomings of pump and treat result from (1) the tendency of most contaminants to sorb to formation materials, thus retarding contaminant removal; (2) geologic complexity, which requires detailed characterization for the design of optimal extraction systems within available resources; and (3) the failure to apply dynamic well field management techniques.

The alternative, “smart pump and treat” consists of (1) detailed characterization of the geology, hydrology, and chemistry; (2) use of computer‐aided data interpretation, data display, and decision support systems; (3) removal of source terms, if possible; (4) initial design for plume containment and source remediation; (5) phased installation of the well field; (6) detailed monitoring of the remediation; (7) active ongoing reevaluation of the operating well field, including redesign as appropriate (dynamic management); (8) reinjection of treated ground water to speed the flushing of contaminants; and (9) setting of appropriate cleanup levels. Use of some or all of these techniques can dramatically reduce the time required to achieve cleanup goals and thus the cost of ground‐water remediation.
Stability Study of a Longitudinal Dispersivity Identification Method
Authors: Pisani Stefano , Tosi Nicola
Published: August 2005

The knowledge of the value of longitudinal dispersivity is very important in the simulation of solute transport. Therefore, a method for the identification of longitudinal dispersivity in a solute transport laboratory experiment was developed. The solute was assumed to be ideal and the medium isotropic and homogeneous, with a one‐dimensional geometry. The sensitivity of the inversion method to a uniform random noise in porosity, which accounts for possible heterogeneities of the medium, and to concentration measurement errors, which are always present in a laboratory experiment, was analyzed. This analysis is necessary to estimate the reliability of the method. Then, supposing the concentration error distributed according to a Gaussian with zero mean, the probability distribution of the relative identification error was computed as a function of the standard deviation of the Gaussian. The mean value of the relative identification error was found to be positive, due to the nonlinearity of the identification algorithm.
Large‐Scale Aquitard Consolidation Near Mexico City
Authors: Ortega‐Guerrero Adrian , Cherry John A. , Rudolph David L.
Published: August 2005

The extensive lacustrine Chalco Plain in the southeastern part of Mexico City is underlain by an aquitard up to 300 m thick composed of a layered sequence of very porous (80‐90%) fine‐grained, organic‐rich Quaternary deposits, with thin horizontal interbeds of volcanic sand (“Capas Duras”). The aquitard overlies a thick sequence of alluvial‐pyroclastic sediments which form a highly productive regional aquifer. The Chalco Plain was a shallow lake until the 1940s when it was drained for agricultural use and human habitation. Historic information indicates that the Chalco Plain was an area of ground‐water discharge prior to the onset of heavy ground‐water extraction from the semiconfined aquifer. Due to aquifer pumping, however, the hydraulic gradient has reversed throughout the full thickness of the aquitard in areas where the aquitard is thin (< 100m), and recharging conditions now prevail. Where the aquitard is thick, the hydraulic head data show a progressive decline with time even though the hydraulic gradient still indicates upward flow in at least the upper part of the lacustrine sequence. Between the early 1960s, when major ground‐water extraction began beyond the periphery of the aquitard, and the onset in 1982 of heavy pumping from aquifers beneath the aquitard, the land surface subsided approximately 3 m. An additional subsidence of 2 m occurred between 1984 and 1989, causing a shallow lake to form and gradually expand. If the present rate of ground‐water withdrawal from the Chalco Basin continues, total land subsidence in the middle of the plain will probably continue to a rate of about 0.4 m per year for many years, and could eventually reach a total subsidence of tens of meters in the thickest part of the Chalco Plain. Pore water in much of the aquitard is saline; however, release of salts and other chemical constituents to the underlying aquifer has not yet significantly impaired the aquifer water quality. A better understanding of the behavior of the aquitard under the influence of aquifer pumping is needed to assist in long‐term management of ground water and land use in the Chalco Plain.
Uncertainty Associated with Semivariograms Used for Site Simulation
Authors: Wingle William L. , Poeter Eileen P.
Published: August 2005

Uncertainty is associated with interpretation of the subsurface, and stochastic simulation techniques are incapable of accounting for all the uncertainty if only a single deterministic semivariogram model is utilized. Jackknifing the data bounds the limits of model semivariograms but typically indicates that a large number of simulations must be conducted to consider the full distribution of possible semivariograms. Latin‐hypercube sampling, particularly when combined with expert opinion, reduces the number of simulations that must be created and evaluated. For small data sets, where there is significant uncertainty, this process provides for a more complete assessment of the potential variability of the subsurface and of flow paths for contaminants, given the available data. Such assessment can be used to guide the data collection program and the decision‐making process.
Modeling the Free Surface of an Unconfined Aquifer Near a Recirculation Well
Authors: MacDonald Thomas R. , Kitanidis Peter K.
Published: August 2005

We examine flow in an unconfined aquifer near a recirculation well, which consists of a source and a sink of equal discharge, with emphasis on understanding the behavior of the free surface. The well is vertical with a pump located between an inlet above and an outlet below to induce recirculation of ground water and thus enhance mixing. The boundary element method is used to model the flow with the free‐surface boundary condition. Numerical simulations show that for this arrangement there is a critical pumping rate beyond which the free surface becomes unstable and is drawn down to the well. The value of this critical rate as well as the maximum drawdown of the phreatic surface were determined for a range of well‐screen (inlet and outlet) separations. In addition to the numerical model, we developed an analytical approximation that yielded an estimate of the critical pumping rate. The analytical estimate is shown to be in reasonably good agreement with the numerical results. We also examined the effect of approximating the free‐surface boundary by a horizontal confining layer. The radius of influence of a single well source‐sink pair was only slightly affected by making this approximation, as long as the pumping rate remained below the critical value.
Assessment of Yield Characteristics of Granitic Aquifers in South India
Authors: Briz‐Kishore B. H.
Published: August 2005

The yield characteristics in clusters of closely spaced and sparsely spaced wells in a typical granitic environment in South India are analyzed. Contrary to the belief of earlier workers, the analysis shows that the yield of the wells is not directly influenced by depth, topography, number of fractures, or the thickness of weathered zones. In a few cases, reduction of discharges were observed at certain depths, and in general, ground‐water yield was contributed by shallow fractured zones. The data were statistically processed further to quantify the observed relationships among these parameters. The analysis of the results indicated similar trends regardless of whether the wells are closely spaced or otherwise throughout the basin area.
Denitrification and Organic Carbon Availability in Two Aquifers
Authors: Starr Robert C. , Gillham Robert W.
Published: August 2005

Nitrate is one of the most common ground‐water contaminants, and denitrification, the bacterially mediated conversion of nitrate into molecular nitrogen, is the only geochemical process that contributes significantly to the renovation of nitrate‐contaminated aquifers. Previous workers have shown that denitrification occurs in some, but not all, nitratecontaminated unconfined aquifers in southern Ontario, Canada. Denitrification tends to occur in aquifers that have a very shallow water table, but not in aquifers whose water table is deeper than about two to three meters.

This paper describes an investigation of the importance of organic carbon in controlling the occurrence of denitrification in two shallow sandy aquifers in southern Ontario, Canada. In situ and laboratory measurements of denitrification rates using the acetylene block technique show that: (1) organic carbon availability decreases with depth below the ground surface; (2) denitrification occurs in an aquifer with a very shallow water table, but organic carbon availability limits the denitrification rate; and (3) denitrification does not occur in an aquifer with a deeper water table because there is insufficient labile organic carbon present.
Ground‐Water Flow Dynamics and Development Strategies at the Atoll Scale
Authors: Griggs John E. , Peterson Frank L.
Published: August 2005

One of the most difficult tasks in hydrogeology is to develop sustainable supplies of potable ground water from small atoll islands. Fresh ground‐water systems within atolls, if they exist at all, are extremely fragile and sensitive to development stresses. In this study, the mathematical model SUTRA was used to examine development alternatives for a two‐phase miscible ground‐water system in a cross section through the Laura area of Majuro Atoll, Marshall Islands. The geological framework modeled for Laura is a multilayered aquifer system in which the primary members are a sequence of moderately permeable Holocene carbonate sediments overlying highly permeable Pleistocene sediments and reef materials. Ground water in this system consists of a thin layer of fresh water separated from underlying sea water by a relatively thick, transitional layer of mixed saline and fresh water. Sensitivity analysis indicates that the modeled depth to the 50% salinity contour is most sensitive to permeability, and the transition‐zone thickness is most sensitive to transverse dispersivity. Results from the development analysis verify that shallow, linear skimming wells, or galleries, are the most efficient means of extracting fresh ground water.
Analysis of Temporal Variability in Hydrogeochemical Data Used for Multivariate Analyses
Authors: Ritzi Robert W. , Wright Stephen L. , Mann Barbara , Chen Manling
Published: August 2005

During the time over which the data for a regional hydrogeocheimical survey are collected, the physical and chemical attributes of the ground‐water flow regime may exhibit appreciable temporal variation. A case study is presented in which the temporal variance of many physical and chemical attributes of ground water at single locations was close to or even exceeded the spatial variance of concentrations measured over the region at a single point in time. Thus, the results of a multivariate analysis of the spatial variability could have led to incorrect inferences had the regional sampling not been done within a period of time that was short enough so that no appreciable temporal variation occurred. A methodology using both spectral and multi variate analyses was used in determining: (a) the window of time allowable in collecting the water samples so that the temporal variability of the attributes did not affect the spatial analyses, and (b) the dominant factors that explain the spatial variability of physical and chemical attributes at that particular period in time.
Nitrogen Isotopes as Indicators of Nitrate Sources in Minnesota Sand‐Plain Aquifers
Authors: Komor Stephen C. , Anderson Henry W.
Published: August 2005

Nitrate concentrations in excess of national drinking‐water standards (10 mg/1 as N) are present in certain sand‐plain aquifers in central Minnesota. To investigate nitrate sources in the aquifers, nitrogen‐isotope values of nitrate (δ15NNO3No3) were measured in shallow ground water from 51 wells in five land‐use settings. The land‐use settings and corresponding average nitrate concentrations (as N) and δ15NNO3 values are: livestock feedlots, 12.7 mg/1, 21.3%0; cultivated‐irrigated fields, 13 mg/1, 7.4%0; residential areas with septic systems, 8.3 mg/1, 6.0%0; cultivated‐nonirrigated fields, 15.5 mg/1, 3.4%0; and natural, undeveloped areas, 3.8 mg/1, 3.1%0. Values of δ15NNO3 less than 2%0 suggest that nitrogen from commercial inorganic fertilizers exists in ground water beneath all settings except the feedlots. Values of δ15NNO3 greater than 10%0 suggest that nitrogen from animal waste is present in ground water beneath certain feedlots, cultivated‐irrigated fields that are fertilized with manure, and residential areas with septic systems. Values of δ15NNO3 between 22 and 43%0 in ground water beneath the feedlots probably result from denitrification. Values of δ15NNO3 increase with depth in many locations in the sand‐plain aquifers. These increases may be caused by progressive denitrification with depth or by changes with depth in the proportions of nitrate from different sources. Similarly, variations of δ15NNO3 values from 1986 to 1987 in certain locations may be due to temporal variations in the amounts of denitrification or to changes in the proportions of nitrate from different sources. Ambiguities in the interpretation of changes in δ15NNO3 values could be eliminated by increasing the spatial and temporal frequency of sampling.
LNAPL Retention in Sandy Soil
Authors: Ostendorf David W. , Richards Robin J. , Beck Frank P.
Published: August 2005

The vertical distribution of free and residual aviation gasoline, a light nonaqueous phase liquid (LNAPL), through the capillary fringe of a uniform, medium sand is measured and used to calibrate an existing model. The total saturation profile is governed by a classical moisture retention characteristic, with the matric pressure head scaled by LNAPL/air surface tension in the presence of free LNAPL and water/air surface tension in the absence of free LNAPL. The residual LNAPL is described by an empirical equation reflecting hysteretical trapping due to a fluctuating water table. The existing theory is calibrated with new and published field data at an aviation gasoline spill site in Traverse City, Michigan.
Agrotechnically Induced Salinization in the Unsaturated Zone of Loessial Soils, N.W. Negev, Israela
Authors: Magaritz Mordeckai , Nadler Arie
Published: August 2005

Extensive agricultural activity often increases the salt load of the unsaturated zone. The focus of the study was a field investigation of the effects of saline‐water irrigation and gypsum addition in a semiarid region with loessial soils, where intensive evaporation conditions prevail. Using drilling (∼20 m) to obtain sedimentary deposit samples, it was shown that most of the salts added by agricultural activity accumulate in the unsaturated profile. Seventy to ninety percent of SO4−2, Na+, and Cl− added are retained in the upper 2–4 meters of the profile after 6 to 12 years of cultivation, regardless of the soil type. The most common accumulating ions are Na+ and SO4−2 that minimize gypsum amendment thereby increasing with time the danger of an irreversible loss of the loessial soil as productive land.
Modeling Vapor Extraction and General Transport in the Presence of NAPL Mixtures and Nonideal Conditions
Authors: Benson David A. , Huntley David , Johnson Paul C.
Published: August 2005

The design of vapor‐extraction remedial systems and the analysis of their performance can be improved by using models that can simulate the chemical and physical processes affecting the occurrence and movement of multiple‐compound vapor‐phase chemical mixtures. Previous models fall into two categories: (1) multiple‐compound phase distribution models which are either nondimensional (no transport) or one‐dimensional (column experiments); and (2) multidimensional, single‐compound transport models. In this paper, a model is presented that couples the steady‐state vapor flow equation, the advection‐diffusion transport equation, and a multiple‐compound, multiphase chemical partitioning model. The numerical implementation allows spatially variable fields of permeability, confining layer permeability, and initial contaminant concentrations. Based on the concentrations of each chemical compound, the model calculates whether a nonaqueous phase liquid (NAPL) is present, and calculates the chemical phase distribution by the appropriate equilibrium partitioning formulation (Henry's Law or Raoult's Law).

The user can specify the location and discharge rates of any number of extraction or injection wells, including zero wells, in which case the simulation will solve transport by diffusion only. The remediation, by vapor extraction, of hypothetical fuel hydrocarbon spills was simulated to investigate the error introduced by failing to account for natural (nonideal) conditions. The nonideal conditions include inhomogeneous soil permeability, leakage of atmospheric air into the subsurface (as from a bare ground surface), and irregular contaminant distribution. The model was also run in the pure diffusion mode to simulate the transport of benzene to the ground surface, and to show the limitations of single‐compound vapor flux models when a multicompound NAPL (such as gasoline) represents the source of benzene.
Reproducible Well‐Purging Procedures and VOC Stabilization Criteria for Ground‐Water Sampling
Authors: Barcelona Michael J. , Wehrraann H. Allen , Varljen Mark D.
Published: August 2005

Recent research has confirmed that low flow rate purging (i.e., ∼11/min) is a valid technique for 2 (5 cm) diameter monitoring wells with short screened intervals. The use of low flow, dedicated pumping devices for purging and sampling minimizes both the disturbance of stagnant water in the well casing and the potential for mobilization of particulate or colloidal matter which could lead to sampling artifacts. In addition, these techniques allow the use of purging indicator parameters (e.g., dissolved oxygen and specific conductance) to determine when to collect a sample for volatile organic compound (VOC) determinations. Detailed results from a monitoring network in a sand and gravel aquifer contaminated by organochlorine solvents support these findings. Recent publications on monitoring well hydraulic behavior and considerations of mass averaging effects due to pumping also support the use of low flow rate, minimal drawdown purging procedures to achieve reproducible sampling results. The suggested procedure includes documenting purging indicator parameters while purging with dedicated devices at low flow rates with minimal drawdown. This sampling method is less time‐consuming and reduces the need to handle large volumes of purge water since VOC concentrations, 02, and specific conductance values stabilized consistently in less than one bore volume.
An Electronic Minipermeameter for Use in the Field and Laboratory
Authors: Sharp J. M. , Fu L. , Cortez P. , Wheeler E.
Published: August 2005

The quantification of permeability and geostatistical scaling from laboratory to well field to regional systems requires large spatial data sets for which field and laboratory minipermeameters have proven to be effective tools. We developed an electronic gas minipermeameter for use in either the field or the laboratory. This design alleviates several problems common to minipermeameters. In particular, errors in measurement of gas flow rate, calibration of flow rate meters, and countering pressure and temperature effects were reduced by using electronic mass‐flow transducers. Problems of sealing the injector (probe) tips and of measuring the permeability of weathering rinds and fracture “skins” were remedied with a sample chamber capable of handling thin wafers or rock cores of arbitrary dimensions. Calibrations with samples of known permeability were excellent. It is now possible to measure permeability very accurately on weathering rinds, fracture surfaces, utcrops, and large hand samples.
Relation Between Hydraulic Conductivity and Texture in a Carbonate Aquifer: Observations
Authors: Rovey Charles W. , Cherkauer Douglas S.
Published: August 2005

Based on analysis of over 300 injection‐pressure tests and their assignment to logged stratigraphic intervals, it is argued that the Silurian dolomite aquifer in Milwaukee County, Wisconsin, behaves as both a fractured and porous medium. This paradox is resolved by examining the aquifer's individual stratigraphic units and their respective sedimentologic characteristics. Like many carbonates, the aquifer contains shoaling upward sequences and cycling through a variety of depositional environments and textures. Most of the stratigraphic units are fine‐grained mudstones, but interbeds with porous coarsegrained packstones and grainstones are also present.

The packstone and grainstone units have unimodal distributions of hydraulic conductivity with geometric means between 10 4 and 10 3 cm/sec. The high matrix porosity masks any effects of jointing in these deposits, and they behave as porous high conductivity media. In contrast, hydraulic conductivities within the mudstones are bimodal, with one mode of very low matrix conductivities and a second higher mode controlled by jointing. Mudstone conductivities increase logarithmically with joint frequency, but never attain the magnitude of the values for the coarse‐grained units. Because the geometric mean hydraulic conductivities of the porous units are up to 1000 times greater than the thicker mudstones, the aquifer's bulk hydraulic conductivity is controlled by the thinner porous zones. Therefore, as a whole, the aquifer behaves as a porous medium even though most of the section is fracture‐dominated.

Only the large number of hydraulic conductivity measurements isolated within specific stratigraphic units has allowed recognition of the importance of porous medium conditions in this aquifer. Because upward shoaling sequences are common within carbonates, a similar combination of porous and fractured medium conditions may be present, but unnoticed, in other carbonate aquifers. Investigators working in dolomites with minimal tectonic deformation should be particularly alert for porous medium effects.
Ground‐Water Contribution to Stream Salinity in a Shale Catchment, R.S.A.
Authors: Greeff G. J.
Published: August 2005

The salinity in the Breede River in the Robertson area (S. W. Cape, South Africa) has increased at an alarming rate over the past two decades. Water analyses have shown that a few tributary streams which drain Bokkeveld Shale catchments are responsible for the high salinity in this agriculturally important river. Research carried out to investigate the sources of salinity in one such catchment, the Poesjesnels River Valley, found that heavy salt loads were contributed by return flow seepages after irrigation of deeply ripped, thin soils which overlie decomposed shale. A smaller contribution was coming from ground water which had leached out some salt from these shales while moving through bedrock fractures under pressure from the head in the encircling mountains, and eventually seeped into the river; evidence for this was provided by comparison of the oxygen isotope ratios of the various water bodies in the valley with that of the river water. A salt balance model drawn up to differentiate between the sources of salinity gave an indication of the systems which need management in future hydrological and agricultural development in this and similar valleys.