bluuebunny/crossref_metadata_2025_split huggingface dataset:
Leachate Mound Changes in Landfills Due to Changes in Percolation by a Cap
Authors: Oweis Issa S. , Biswas Gopal C. 
Published: August 2005

AbstractA 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

AbstractFlow 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

AbstractTwo 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

AbstractEstablished 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

AbstractThe 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

AbstractThe 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

AbstractUncertainty 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

AbstractWe 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

AbstractThe 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

AbstractNitrate 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

AbstractOne 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

AbstractDuring 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

AbstractNitrate 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

AbstractThe 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

AbstractExtensive 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

AbstractThe 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

AbstractRecent 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.