bluuebunny/crossref_metadata_2025 huggingface dataset:
An Iterative Cokriging‐Like Technique for Ground‐Water Flow Modeling
Authors: Yeh T.‐C. Jim , Gutjahr Allan L. , Jin Minghui
Published: August 2005

The importance of field heterogeneities in ground‐water pollution problems has been widely recognized during the last few decades. To address the impact of field heterogeneities on ground‐water flow and solute transport, many different stochastic methods have been developed. Among all these stochastic methods kriging is the most popular one used by many practitioners to interpolate and extrapolate measured transmissivity data. However, hydraulic head measurements are generally more abundant than transmissivity data. Therefore, the cokriging technique which utilizes both the head and transmissivity measurements to estimate transmissivity and/or hydraulic head distributions has also received much attention in recent years.

Classical cokriging relies on a linear predictor approach and uses covariance and cross covariance functions derived from a first‐order approximation. Consequently, it often results in head and transmissivity fields that can produce unacceptable velocity distributions. In this paper, we develop an iterative method which combines classical cokriging and a numerical flow model to obtain optimum estimates of transmissivity and head distributions and to alleviate the limitations of classical cokriging. Through several numerical examples, we demonstrate that this method is superior to the classical cokriging method in terms of producing mass conservative velocity fields. In addition, results of the study also indicate that hydraulic head measurements can improve for our prediction of ground‐water flow directions and paths in aquifers significantly.
Wellhead Protection Area Delineation Using the Analytic Element Method of Ground‐Water Modeling
Authors: Wuolo Ray W. , Dahlstrom David J. , Fairbrother Mark D.
Published: August 2005

The Analytic Element Method (AEM) of ground‐water modeling was used to delineate wellhead protection areas (WHPAs) for existing and proposed wells in Brooklyn Park, Minnesota by simulating steady‐state flow in two aquifers: the water‐table aquifer (in glacial outwash) and the Franconia‐Ironton‐Galesville bedrock aquifer. Ground‐water time‐of‐travel was used as the delineation criterion. Ground‐water flow is significantly affected by a buried bedrock valley underneath the City. Transmissivity values for the water‐table aquifer are much higher in the buried bedrock valley and the hydraulic connection between the water‐table and the Franconia‐Ironton‐Galesville aquifers is more pronounced. The Analytic Element Method does not utilize a grid mesh and incorporates both regional‐scale and more detailed, local‐scale features in the same solution. This feature of AEM provided for the simulation of the city wells in the context of the regional flow field. Separate models, encompassing an area of approximately 500 square miles, were developed and calibrated for each aquifer using the Single Layer Analytic Element Model (SLAEM) and then linked together using the Multi‐Layer Analytic Element Model (MLAEM). Ground‐water travel‐time zones and WHPAs were delineated for the existing wells using reverse particle tracking.
In Situ Denitrification of Septic‐System Nitrate Using Reactive Porous Media Barriers: Field Trials
Authors: Robertson W. D. , Cherry J. A.
Published: August 2005

A new alternative septic‐system design is presented utilizing reactive porous media barriers for passive in situ attenuation of NO3−. The reactive material consists of solid organic carbon (sawdust) which promotes NO3‐ attenuation by heterotrophic denitrification. Four field trials are discussed demonstrating two barrier configurations: as a horizontal layer positioned in the vadose zone below a conventional septic‐system infiltration bed and as a vertical wall intercepting a horizontally flowing downgradient plume. During one year of operation both barrier configurations have been successful in substantial attenuation (60 to 100%) of input NO3‐ levels of up to 125 mg/1 as N. The horizontal layer configuration can be readily installed during the construction of new infiltration beds, whereas the vertical wall configuration may be more appropriate for retrofitting existing septic systems where NO3‐ contamination has already occurred. The layer configuration allows the flexibility of constructing the barrier in the vadose zone by using coarse silt or fine sand matrix material that has the ability to remain tension‐saturated, and thus anaerobic, even when positioned above the water table.

Advantages of the barrier system are that it is simple to construct, no surface structures or additional plumbing are necessary, and treatment is passive requiring no energy consumption and little or no maintenance. Mass balance calculations and preliminary results suggest that conveniently sized barriers have the potential to last for decades without replenishment of the reactive material.
Evidence for Enhanced Mineral Dissolution in Organic Acid‐Rich Shallow Ground Water
Authors: McMahon Peter B. , Vroblesky Don A. , Bradley Paul M. , Chapelle Francis H. , Gullett Cheryl D.
Published: August 2005

Total concentrations of formate, acetate, and isobutyrate varied from less than 5 to greater than 9,000 μmol/l over distances of < 3 m in ground water from a shallow hydrocarbon contaminated aquifer. Laboratory incubations of aquifer material indicate that organic acid concentrations were dependent on the amount of hydrocarbon loading in the sediment and the relative rates of microbial organic acid production and consumption. In heavily contaminated sediments, production greatly exceeded consumption and organic acid concentrations increased. In lightly contaminated sediments rates were essentially equal and organic acid concentrations remained low. Concentrations of dissolved calcium, magnesium, and iron generally were one to two orders of magnitude higher in organic acid‐rich ground water than in ground water having low organic acid concentrations. Carbonate and Fe(III)‐oxyhydroxide minerals were the likely sources of these elements. Similarly, concentrations of dissolved silica, derived from quartz and k‐feldspar, were higher in organic acid‐rich ground water than in other waters. The positive relation (r = 0.60, p < .05, n = 16) between concentrations of silica and organic acids suggests that the microbially mediated buildup of organic acids in ground water enhanced quartz/k‐feldspar dissolution in the aquifer, although it was not the only factor influencing their dissolution. A model that included organic acid microequivalents normalized by cation microequivalents significantly strengthened the correlation (r = 0.79, p < .001, n = 16) between dissolved silica and organic acid concentrations, indicating that competition between silica and cations for complexation sites on organic acids also influenced quartz/k‐feldspar dissolution. Physical evidence for enhanced mineral dissolution in organic acid‐rich waters included scanning electron microscopy images of highly corroded quartz and k‐feldspar grains from portions of the aquifer containing organic acid‐rich ground water. Microporosity generated in hydrocarbon contaminated sediments may adversely affect remediation efforts that depend on the efficient injection of electron acceptors into an aquifer or on the recovery of solutes from an aquifer.
Effects of Agriculture on Ground‐Water Quality in Five Regions of the United States
Authors: Hamilton Pixie A. , Helsel Dennis R.
Published: August 2005

Water‐quality conditions in surficial unconsolidated aquifers were assessed in five agricultural regions in the United States. The assessment covers the Delmarva Peninsula, and parts of Long Island, Connecticut, Kansas, and Nebraska, and is based on water‐quality and ancillary data collected during the 1980s. Concentrations of nitrate in ground water in these areas have increased because of applications of commercial fertilizers and manure. Nitrate concentrations exceed the maximum contaminant level (MCL) for drinking water of 10 milligrams per liter as nitrogen established by the U.S. Environmental Protection Agency in 12 to 46 percent of the wells sampled in the agricultural regions. Concentrations of nitrate are elevated within the upper 100 to 200 feet of the surficial aquifers. Permeable and sandy deposits that generally underlie the agricultural areas provide favorable conditions for vertical leaching of nitrate to relatively deep parts of the aquifers. The persistence of nitrate at such depths is attributed to aerobic conditions along ground‐water‐flow paths. Concentrations of nitrate are greatest in areas that are heavily irrigated or areas that are underlain by well‐drained sediments; more fertilizer is typically applied on land with well‐drained sediments than on poorly drained sediments because well‐drained sediments have a low organic‐matter content and low moisture capacity. Concentrations of other inorganic constituents related to agriculture, such as potassium and chloride from potash fertilizers, and calcium and magnesium from liming, also are significantly elevated in ground water beneath the agricultural areas. These constituents together impart a distinctive agricultural‐chemical trademark to the ground water, different from natural water.
Applying Higher Order DIRK Time Steps to the “Modified Picard” Method
Authors: Baker Donald L.
Published: August 2005

The “modified Picard” iteration method, which offers global mass conservation, can also be described as a form of Newton's iteration with lagged nonlinear coefficients. It converges to a time step with first‐order discretization error. This paper applies second‐ and third‐order diagonally implicit Runge Kutta (DIRK) time steps to the modified Picard method in one example. It demonstrates improvements over the first‐order time step in rms error and error‐times‐effort model quality by factors ranging from two to over two orders of magnitude, showing that the “modified Picard” and DIRK methods are compatible.
Spreadsheet Modeling of Slug Tests Using the van der Kamp Method
Authors: Wylie Allan , Magnuson Swen
Published: August 2005

Slug testing is frequently employed to calculate aquifer transmissivity and hydraulic conductivity. The van der Kamp technique for interpreting slug test data which experience force‐free water level oscillations is not routinely employed because it requires adjusting equations to match the observed well response data. This adjustment can be rapid and convenient when a commercial spreadsheet is employed.
Darcian Weighted Interblock Conductivity Means for Vertical Unsaturated Flow
Authors: Baker Donald L.
Published: August 2005

The true Darcian interblock conductivity mean for vertical unsaturated flow, Kv, is derived from Darcy's law, q/Ks = Kv (dH/dx), and the assumption that flow is constant between grid centers. It can be expressed as Kv = (Δx Kx −Δp Kh)/(Δx −Δp), where Kx is the gravity flow mean, Kh is the capillary flow mean, and H =×− p (total head = elevation ‐matric suction). Plotting contours of Kx, Kh, and Kv in the plane of adjacent grid center conductivities, (kr1, kr2), shows that a component of Kx behaves like a simple weighted mean, Kw = w kr2 + (1 − w) krl, where w is a nonlinear, monotonic function of the grid separation, Δx. Results from a mass‐conservative model of infiltration into dry sand are compared for substituting: (1) the arithmetic mean, Ka = (krl + kr2)/2, for Kv, (2) Kw for Kv, (3) Ka for Kx in Kv (called Km), and (4) Kw for Kx in Kv (called Kmw), over a range of the ratio of Δx to the conductivity displacement pressure, pk, from 0.01 to 1.6. The simulation for Δx/pk = 0.002 is taken to be the exact solution. Kw and Kmw reduce the gross cumulative net inflow error generated with Ka and Km from 3.84 to 8.84 times. Kmw performs consistently better in terms of gross water content distribution error by up to a factor of three.
Landfills in New York City: 1844–1994
Authors: Walsh Daniel C. , LaFleur Robert G.
Published: August 2005

Historic topographic maps are reviewed to locate landfills constructed within New York City during four time intervals between 1844 and 1994. A total of 184.75 km2 (45,650 acres) of landfill are identified (approximately 20% of the study area). Data are not available to determine the fill composition at most sites but literature sources indicate that municipal solid waste (MSW) has been an important source of fill since at least 1891. Qualitative temporal trends in the spatial distribution of landfills and the composition and thickness of MSW landfills are observed. The oldest landfills are clustered in the vicinity of the early urban center (southern New York County) but expand in spatial distribution after the turn of this century. Logs of borings through 10 MSW landfills show that waste landfills built prior to the mid‐1950s contain abundant ash (coal ash is common in the oldest landfills) and are relatively thin (3‐7 m) with no topographic mounding. In MSW landfills built since that time, uncombusted organic matter is abundant, thicknesses increase greatly (16–27 m), and pronounced topographic mounding is observed. Most landfills identified in this study were built on tidal wetlands. Fine‐grained wetland deposits underlying the landfills and close proximity to large surface‐water bodies favor lateral transport of leachate from MSW landfills in shallow ground water and local discharge to surface water. The wide distribution of historic landfills and common use of MSW for fill indicates that these sites should be considered in investigations of ground water, surface water, and sediment quality in New York City and other urban areas where extensive historic landfilling has occurred.
An Investigation of Saturation‐Capillary Pressure Relations in Two‐ and Three‐Fluid Systems for Several NAPLS in Different Porous Media
Authors: Busby Robert D. , Lenhard Robert J. , Rolston Dennis E.
Published: August 2005

Lack of constitutive data has impeded efforts to model the subsurface transport of organic‐liquid contaminants. To help fill this need, functional relationships between fluid saturations and capillary pressures were obtained using a unique fluid‐retention cell. The functional relationships between water saturations and NAPL‐water capillary pressures, and between total‐liquid saturations and air‐NAPL capillary presures in two‐ and three‐fluid phase systems were measured directly during monotonic drainage of a fine quartz sand and a nonswelling loam soil. Additionally, measurements were made between water saturations and air‐water capillary pressures for an air‐water fluid system in both porous media. The NAPLs investigated were Soltrol 170®, toluene, and trichloroethylene (TCE). Following the measurements, the two‐ and three‐fluid retention relations were compared to test the validity of extending two‐phase saturation‐pressure (S‐P) relations to three‐fluid systems. Good agreement was observed between the two‐ and three‐fluid data for Soltrol 170®, toluene, and TCE in both porous media. An S‐P scaling format for two‐ and three‐phase systems was also evaluated. Results indicate that a single multiphase retention function is suitable for describing two‐ and three‐phase S‐P relations in similar porous media; however, it is unclear whether the scaling factors can be predicted a priori from ratios of interfacial tensions.
Laboratory Investigation of Buoyancy‐Induced Flow (Plume Sinking) During Two‐Well Tracer Tests
Authors: Istok Jonathan D. , Humphrey Mark D.
Published: August 2005

The occurrence of buoyancy‐induced vertical flow (sinking) of a bromide (Br−) tracer plume at small injection concentrations is investigated in transport experiments conducted in a large‐scale physical aquifer model containing a homogeneous and isotropic sand pack. Two‐well tracer tests are conducted using Br− at concentrations ranging from 50 to 1000 mg/1, corresponding to relative densities between 7.5 × 10−5 and 1.5 × 10−3. Analysis of three‐dimensional solute concentration data indicates that the center of mass of the Br− plume was displaced downward as the denser tracer solution sank through the sand pack. Plume sinking occurred at all solute concentrations investigated; the magnitude of the vertical displacement increased with increasing Br− concentration of the injected tracer solution. The dynamic collapse of the Br−plume caused by buoyancy forces resulted in increased apparent transverse and longitudinal dispersivities. The results suggest that the possibility of buoyancy‐induced flow must be considered when interpreting tracer tests conducted with anion concentrations as low as 50 mg/1. The occurrence of buoyancy‐induced flows at such low relative densities also suggest that the phenomenon may be more widespread than is generally recognized.
A Modified Gauss‐Newton Method for Aquifer Parameter Identification
Authors: Li Jingsheng , Elsworth Derek
Published: August 2005

The accuracy of parameter estimation procedures is evaluated for a modified Gauss‐Newton method applied to transient ground‐water flow. Three different approaches of evaluating the sensitivity coefficient matrix are examined, including influence coefficient, sensitivity equation, and variational approaches. The performance of each of the techniques is evaluated by applying a common synthetic data set. The latter two techniques are shown to perform with least sensitivity to starting parameters and extraneous sampling noise. Where either random or systematic noise is added to the time‐series data set, the resulting predictions become increasingly more sensitive to the form of the starting transmissivity vector. It is concluded that the modified Gauss‐Newton method is attractive because of its simplicity, high rate of convergence, and modest computational demands, especially when the number of the parameters to be identified is not large.
Cokriging Limited Transmissivity Data Using Widely Sampled Specific Capacity from Pump Tests in an Alluvial Aquifer
Authors: Hughson Lance , Huntley David , Razack M.
Published: August 2005

Use of the specific capacity of a pumping well to predict aquifer transmissivity is desirable due to the cost of pumping tests and the availability of specific capacity measurements. The geostatistical technique of cokriging is a method of incorporating the spatial variability of a correlated variable (e.g., specific capacity) in estimating a related undersampled variable (e.g., transmissivity). This study examines the reliability of cokriging transmissivity estimates using a data set of 215 pairs of transmissivity and specific capacity. Subsets of pairs of transmissivity and specific capacity were selected and cokriged to estimate transmissivity at the remaining well locations. The estimates of transmissivity were then compared to actual measurements of transmissivity. The same subsets of pairs were used to estimate transmissivity with loglinear regression of transmissivity on specific capacity and ordinary kriging of transmissivity alone. Comparison of these three methods indicates the number of wells with both transmissivity and specific capacity data necessary to obtain improvement in transmissivity estimates with cokriging over the simpler regression and kriging methods. The results show that significant improvement in the transmissivity estimate is obtained by cokriging with 50 or more pairs of transmissivity and specific capacity, and that loglinear regression is superior when less than 30 pairs are available. With between 30 and 50 pairs of available data measurements, cokriging does not reliably improve the estimate over loglinear regression.
Evaluation of Simple Methods for Estimating Contaminant Removal by Flushing
Authors: Brusseau Mark L.
Published: August 2005

The purpose of this work was to briefly outline and evaluate simple methods used for estimating the volume of water and the time required to remove contaminants from the subsurface by flushing (e.g., “pump‐and‐treat,”“soil washing”). The methods are based on three general approaches to treating flow and transport: perfectly mixed flow, hydraulic, and advective‐dispersive. Data obtained from a small field experiment designed to evaluate aquifer flushing was used to illustrate the performance of the estimation methods. The methods based on the hydraulic and ideal advective‐dispersive transport approaches will generally provide estimates that underpredict the actual time. The magnitude of the underprediction will depend, in part, on the degree of nonideal behavior influencing contaminant removal (e.g., heterogeneity, rate‐limited mass transfer) and on the ratio of target to initial contaminant concentration. The perfectly mixed flow reactor approach, because of its asymptotic removal curve, may be useful in providing rough approximations of required pore volumes and times. However, it cannot be overemphasized that all estimation techniques are prone to failure as long as the conceptual models upon which they are based do not accurately represent field‐scale contaminant transport.
An Examination of Short‐Term Variations in Water Quality at a Karst Spring in Kentucky
Authors: Ryan Martin , Meiman Joe
Published: August 2005

Water quality at many karst springs undergoes very high amplitude but relatively brief degradation following influxes of runoff. Accurately recording transient variations requires more rigorous sampling strategies than traditional methods. A pilot study to determine the usefulness of high‐frequency, flow‐dependent sampling strategies, combined with coincidental quantitative dye tracer tests, was implemented in the Big Spring Ground‐Water Basin in Mammoth Cave National Park, Kentucky.

Data recorded following two separate runoff events showed that the concentrations of two nonpoint source pollutants, fecal coliform bacteria and suspended sediment, greatly exceeded prerunoff event values for very short periods of time. A phreatic conduit segment, calculated at 17 million liters in volume, instantaneously propagated head changes, caused by direct runoff entering the aquifer, from the ground‐water inputs to Big Spring. A significant delay between the initial increases in discharge and the arrival of direct runoff, as indicated by a steady decrease in specific conductance, represented the time required to displace this volume of phreatic water. The delay showed that sampling a karst spring only during peak discharge would be an unreliable sampling method.

Runoff from two different subcatchments was tagged with tracer dye and the timing of the passage of the resultant dye clouds through Big Spring were compared to water quality variations. Distinct lag times between the arrival of direct runoff at Big Spring and the bacteria and suspended sediment waveforms were shown through the concurrent quantitative tracer tests to be related to the areal distribution of land‐cover type within the basin.
Biotransformation of Organics in Soil Columns and an Infiltration Area
Authors: Bosma Tom N. P. , Marlies E. , Ballemans W. , Hoekstra Nanne K. , te Welscher Ruud A. G. , Smeenk Johannes G.M.M. , Schraa Gosse , Zehnder Alexander J. B.
Published: August 2005

Laboratory column experiments were performed to evaluate the fate of a series of chlorinated and nonchlorinated organic contaminants in Rhine sediment and in sediment from the infiltration area of the Municipal Water Works of Amsterdam, near Zandvoort, The Netherlands. Columns were operated under aerobic, denitrifying, and methanogenic conditions. All nonchlorinated and few chlorinated compounds were aerobically transformed. Of the compounds tested under denitrifying conditions, only 1,2‐dichloro‐4‐nitrobenzene was partially transformed. Methanogenic conditions favored the transformation of chlorinated substances by reductive dechlorination. Toluene was the only nonhalogenated compound that was transformed under methanogenic conditions. Steady‐state effluent concentrations after biotransformation were at least 10 times lower than the drinking water limit of 1 μg/l except in the case of 1,2,4‐trichlorobenzene which had a steady‐state effluent concentration of 2.6 μg/l. Steady‐state effluent concentrations did not depend on the influent concentration applied. Most transformations proceeded at the same steady‐state rates at a temperature of 4° C, although the process of reductive dechlorination was slower at 4° C than at 20° C. Hydrological calculations revealed that the combined action of hydrology and sorption to organic matter in the infiltration system can reduce the concentrations of 2 week pulses of polar and nonpolar contaminants by at least 80 and 95%, respectively. There was a good qualitative agreement between removals observed in column experiments and in the dune infiltration area.
Monitoring of Water Flow and Solute Transport Through the Unsaturated Zone Using a Large‐Diameter Borehole
Authors: Zilberbrand Michael , Gvirtzm Haim
Published: August 2005

A method for monitoring water flow and solute transport through the unsaturated zone is of crucial necessity for early detection of threats to ground water. A method developed by Ukrainian hydrogeologists in the 1970s involves a borehole of a large enough diameter for a person to enter, that provides the possibility of installing monitoring equipment, and enables collecting undisturbed sediment blocks and samples of pore water and gas. Practically, tensiometers and vacuum lysimeters are installed at various levels in the borehole walls. They provide high quality sampling and measurements, and enable determining parameters of unsaturated water flow and solute transport and their temporal and spatial variability.

The method was tested at several locations. Selected results from two sites, showing the temporal and spatial distributions of capillary pressure and solute concentrations, are presented. The results exhibit the difference between lateral variation in water flow and solute transport characteristics. The accuracy and representativeness of the measurements were tested against the mutual interaction between the various instruments installed in the borehole walls. Monitoring using the large‐diameter boreholes provides the possibility of early detection of threats to ground water under areal contamination inputs, and of quantitative prediction of its future trends.
Subsurface Injection of Dissolved Ferric Chloride to Form a Chemical Barrier: Laboratory Investigations
Authors: Morrison S. J. , Spangler R. R. , Morris S. A.
Published: August 2005

A chemical barrier is a permeable zone of reactive materials emplaced in the subsurface to remove ground‐water contaminants while allowing clean ground water to pass through. Because dissolved ferric chloride hydrolyzes to amorphous ferric oxyhydroxide when it contacts calcite (CaCO3), it may be viable to emplace a zone of amorphous ferric oxyhydroxide (an absorbent for U, Mo, and other inorganic contaminants) into calcite‐bearing geologic units by injecting ferric chloride through wells. For a chemical barrier to be successful, it must remain permeable and must be immobile.

This investigation monitored chemical compositions, hydraulic conductivity, and iron mobility in laboratory columns and in a two‐dimensional tank to determine the viability of injecting ferric chloride to form an amorphous ferric oxyhydroxide chemical barrier. We introduced a ferric chloride solution (1,345 mg/l [0.024 m] Fe) to calcite‐bearing alluvial gravel to form a chemical barrier of amorphous ferric oxyhydroxide, followed by solutions contaminated with U and Mo. The simulated chemical barriers decreased U and Mo concentrations to less than 0.05 mg/1 (2.1×10‐7 m) and 0.01 (1.0×10‐7 m), respectively; however, the breakthrough front is spread out with concentrations increasing to more than regulatory guideline values sooner than predicted. The hydraulic conductivity of calcite‐bearing alluvial gravel decreased substantially during ferric chloride introduction because of the formation of carbon dioxide but increased to within factors of 1 to 5 of the original value as synthetic ground water flowed through the system. Amorphous ferric oxyhydroxide that formed in these experiments remained immobile at flow rates exceeding those typical of ground water. These laboratory results, in conjunction with site‐specific characterization data, can be used to design chemical barriers emplaced by injection of ferric chloride.
Combined Use of Flowmeter and Time‐Drawdown Data to Estimate Hydraulic Conductivities in Layered Aquifer Systems
Authors: Hanson R. T. , Nishikawa Tracy
Published: August 2005

The vertical distribution of hydraulic conductivity in layered aquifer systems commonly is needed for model simulations of ground‐water flow and transport. In previous studies, time‐drawdown data or flowmeter data were used individually, but not in combination, to estimate hydraulic conductivity. In this study, flowmeter data and time‐drawdown data collected from a long‐screened production well and nearby monitoring wells are combined to estimate the vertical distribution of hydraulic conductivity in a complex multilayer coastal aquifer system. Flowmeter measurements recorded as a function of depth delineate nonuniform inflow to the wellbore, and this information is used to better discretize the vertical distribution of hydraulic conductivity using analytical and numerical methods. The time‐drawdown data complement the flowmeter data by giving insight into the hydraulic response of aquitards when flow rates within the wellbore are below the detection limit of the flowmeter. The combination of these field data allows for the testing of alternative conceptual models of radial flow to the wellbore.
Application of Factor Analysis to the Study of Contamination in the Aquifer System of Ayamonte‐Huelva (Spain)
Authors: Grande J. A. , González A. , Beltrán R. , Sánchez‐Rodas D.
Published: August 2005

The growth of cultivation (strawberries and citrus fruits) in the coastal region of the Huelva province of Spain in the last 10 years has meant an increase in the demand of water from the Ayamonte‐Huelva aquifer system. It has also meant the introduction of an excess of nitrogen via fertilizers from farming.

The system now has two problems: salt‐water intrusion in the southern sector (González and Romero, 1991; González, 1992); and a general increase in the nitrate content in the saturated zone. This second problem has led to average concentrations of nitrates in the aquifer above potable limits (50 ppm), reaching local values near to 400 ppm of NO3‐(González and Romero, 1991; Grande et al., 1993).

This paper delineates the sphere of influence of the two factors being considered (salt‐water intrusion and agricultural contamination) on the different variables studied. The proximity between both factors is justified for each of the contaminating processes.
Residual Diesel Measurement in Sand Columns After Surfactant/Alcohol Washing
Authors: Martel Richard , Gélinas Pierre J.
Published: August 2005

A new simple gravimetric technique has been designed to determine residual oil saturation of complex hydrocarbon mixtures (e.g., diesel) in sand column experiments because reliable methods are lacking. The He/N2 technique is based on drying of sand columns by circulating helium gas to drag oil droplets in a cold trap (liquid nitrogen). With this technique, residual diesel measurement can be performed easily immediately after alcohol/surfactant washing and in the same lab. For high residual diesel content in Ottawa sand (25 to 30 g/kg), the technique is much more accurate (± 2% or 600 mg/kg) than the standard analytical methods for the determination of mineral oil and grease. The average relative error on partial diesel dissolution in sand column estimated after alcohol/surfactant flooding (residual saturation of 10 to 15 g/kg) is as low as 5%. The precision of the He/N2 technique is adequate to compare relative efficiency of washing solutions when partial extraction of residual oil in Ottawa sand columns is performed. However, this technique is not adapted for determination of traces of oil in sediment or for environmental control of contaminated soils. Each diesel determination by the He/N2 technique costs less than 8 $US in chemical products (helium and liquid nitrogen). A simple laboratory drying setup can be built for less than 400 $US which makes this technique valuable for diesel analyses when a large number of tests are required.
Transient Capture Zone for a Single Well
Authors: Musa Mohammad , Kemblowski Marian W.
Published: August 2005

Ground‐water wells are frequently used to intercept contaminant plumes. The conventional design procedure is to plot the steady‐state capture zone and check whether the contaminant plume lies inside it. This approach ignores the pollutant's movement during the transient development of the cone of depression. In this paper we present a new method to analyze the transient performance of a single well that combines the analytical solutions for the steady‐state capture zones and transient velocity field with particle tracking. Using the particle trajectories, an effective capture zone can be estimated. No contaminant which is outside of this zone at the start of pumping can be captured by the well. The use of this method is illustrated by a hypothetical example.
Effect of Trapped Gas on Dissolved Oxygen Transport — Implications for In Situ Bioremediation
Authors: Fry Virginia A. , Istok Jonathan D. , O'Reilly Kirk T.
Published: August 2005

In situ bioremediation of contaminated aquifers is often limited by the concentration of dissolved oxygen in the ground water. Various methods have been used to increase dissolved oxygen concentrations in ground water, but the effect of a trapped gas phase on the distribution and transport of dissolved oxygen needs to be understood. The two‐dimensional transport of dissolved oxygen is investigated in experiments conducted in a large‐scale physical aquifer model (2 m × 4 m × 0.2 m) where a gas phase is trapped in the pore spaces of an otherwise‐saturated porous medium. The transport of dissolved oxygen is shown to be retarded up to 11.2 times the transport of the bulk water due to the mass transfer of oxygen between the aqueous phase and the trapped gas phase. The theoretical model for dissolved gas transport in the presence of a trapped gas phase is evaluated in a two‐dimensional ground‐water flow field using the U.S.G.S. numerical model MOC. The results show that dissolved oxygen transport can be modeled with the advection‐dispersion equation with linear equilibrium mass transfer but only when the longitudinal dispersion is increased compared to the value determined using a bromide tracer of the water flow. Increased longitudinal dispersion of the dissolved oxygen plume may be due to a temporally or spatially varying retardation factor or rate‐limited mass transfer. The presence of even a small amount of a trapped gas phase in an aquifer will significantly affect the distribution and transport of dissolved oxygen (trapped gas filling only 5% of the pore space will cause a retardation factor for oxygen of 2.6 at T = 15°C) and thus should be considered when designing ways to increase the dissolved oxygen concentration in ground water for in situ bioremediation.
Locating VOC Contamination in a Fractured‐Rock Aquifer at the Ground‐Water/Surface‐Water Interface Using Passive Vapor Collectors
Authors: Vroblesky D. A. , Rhodes L. C. , Robertson J. F. , Harrigan J. A.
Published: August 2005

Chlorinated organic solvents introduced to unlined lagoons at an industrial waste‐water treatment plant in the Inner Piedmont of South Carolina resulted in ground‐water contamination of a fractured‐rock aquifer. Part of the ground‐water contamination discharges to Little Rocky Creek, downgradient from the waste‐water treatment plant. Passive vapor collectors were buried in the bottom sediment of the creek to locate areas where ground water contaminated with volatile organic compounds was discharging to the creek. High concentrations of volatile organic compounds (VOCs) were found in passive vapor collectors in an area where VOCs were known to be discharging from ground water to surface water. This area was also a site where very low frequency electromagnetic anomalies (interpreted as fracture zones) intersected the creek or converged near the creek. The data show that passive vapor collectors in bottom sediment of Little Rocky Creek provided information on the location of fractures that were discharging contaminated ground water to surface water.
SF6 as a Conservative Tracer in Saturated Media with High Intragranular Porosity or High Organic Carbon Content
Authors: Wilson Ryan D. , Mackay Douglas M.
Published: August 2005

The application of dissolved stable gases as injected ground‐water tracers is a relatively new area of study, and the transport behavior of dissolved gases in a comprehensive range of saturated porous media types is poorly understood. Gas tracers, and sulphur hexafluoride (SF6) in particular, possess many of the characteristics of an ideal conservative, nonreactive tracer. SF6 is a volatile, nontoxic, inorganic gas that, when dissolved in water, behaved similarly to bromide in an idealized highly uniform saturated medium, exhibiting little physical nonequilibrium. For sand possessing significant intragranular porosity, SF6 did not show persistent tailing as a result of physical nonequilibrium transport to the same degree as bromide. This is consistent with less mass transfer of SF6 to immobile porosity due to its lower diffusion coefficient. This nonequilibrium is enhanced by the high pore‐water velocities employed. SF6 is slightly hydrophobic, with a measured octanol‐water partitioning coefficient of 13.8. This raises the question of whether SF6 would be slightly retarded in media with significant organic carbon, but no evidence of retardation was seen in sand with foc as high as 2.5 wt%.
Pumping Strategies for Management of a Shallow Water Table: The Value of the Simulation‐Optimization Approach
Authors: Barlow Paul M. , Wagner Brian J. , Belitz Kenneth
Published: August 2005

The simulation‐optimization approach is used to identify ground‐water pumping strategies for control of the shallow water table in the western San Joaquin Valley, California, where shallow ground water threatens continued agricultural productivity. The approach combines the use of ground‐water flow simulation with optimization techniques to build on and refine pumping strategies identified in previous research that used flow simulation alone. Use of the combined simulation‐optimization model resulted in a 20 percent reduction in the area subject to a shallow water table over that identified by use of the simulation model alone. The simulation‐optimization model identifies increasingly more effective pumping strategies for control of the water table as the complexity of the problem increases; that is, as the number of subareas in which pumping is to be managed increases, the simulation‐optimization model is better able to discriminate areally among subareas to determine optimal pumping locations.

The simulation‐optimization approach provides an improved understanding of controls on the ground‐water flow system and management alternatives that can be implemented in the valley. In particular, results of the simulation‐optimization model indicate that optimal pumping strategies are constrained by the existing distribution of wells between the semiconfined and confined zones of the aquifer, by the distribution of sediment types (and associated hydraulic conductivities) in the western valley, and by the historical distribution of pumping throughout the western valley.
Ground‐Water Potential Evaluation of West Godavari District, Andhra Pradesh State, India—A GIS Approach
Authors: Kamaraju M.V.V. , Bhattacharya A. , Reddy G. Sreenivasa , Rao G. Chandrasekhar , Murthy G. S. , Rao T. Ch. Malleswara
Published: August 2005

Voluminous geoscientific data related to various ground‐water parameters provide enormous scope for using the Geographic Information System (GIS) to draw meaningful conclusions on the ground‐water propsects of an area. To demonstrate the efficacy of the GIS for ground‐water studies, information on the parameters controlling ground water such as lithology, geomorphology, structure, and recharge condition of West Godavari district were analyzed using ARC/INFO GIS. Existing maps and records were used as data. An evaluation of ground‐water potential and generation of a map showing three major hydrogeological conditions with distinct ground‐water prospects which would serve as a basic tool in the exploitation of ground‐water resources of the district has resulted. The study has also revealed that the GIS techniques are time/cost‐effective and can be employed successfully in the planning stages of a ground‐water exploration programme. In addition, the GIS data generated for the study of ground‐water prospects can be updated and used for the planning and management of ground‐water resources of the district.
The Sensitivity of Remedial Strategies to Design Criteria
Authors: Ahlfeld David P. , Hill Edward H.
Published: August 2005

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

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

The Yuma area has experienced agricultural development since the late 1890s and ground‐water levels have risen over 70 feet due to recharge from heavy application of irrigation water, unlined canals and flooding along the Colorado and Gila Rivers. The resulting shallow water levels have seriously impacted residential areas and prime agricultural land.

The Arizona Department of Water Resources in conjunction with Yuma County Flood Control District developed a regional three‐dimensional ground‐water flow model of the Yuma area. The purpose of the model is to assist local agencies in evaluating remedial water management alternatives to mitigate the shallow ground‐water level problems.

The model domain incorporates over 900 mi2 of Arizona, California, and Mexico and simulates ground‐water pumpage, deep percolation from agricultural irrigation, evapotranspiration from phreatophytes and flow in 12 canals, 16 drains, and the Colorado and Gila Rivers. The model contains four layers with over 30,000 model cells ranging in size from 40 acres to 640 acres.

Different model scenario simulations were conducted to evaluate the effectiveness of proposed water management alternatives on lowering ground‐water levels within the northern portion of Yuma Valley. These scenarios include lining a portion of the East Main canal and pumping two drainage wells, lining the All‐American canal, and simulating a decrease in deep percolation from agricultural irrigation on the Yuma Mesa and northern portion of Yuma Valley.
Spatial Characterization of Hydrogeochemistry Within a Constructed Fen, Greene County, Ohio
Authors: Hite Christopher D. , Cheng Songlin
Published: August 2005

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

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