bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogeological Investigations in Northwestern Yucatan, Mexico, Using Resistivity Surveys
Authors: Steinich Birgit , Marin Luis E.
Published: August 2005

Eight Schlumberger soundings and four Wenner anisotropy measurements were conducted in the northwestern section of the Yucatan Peninsula for hydrogeological investigations of a karst aquifer. This system is influenced by a circular high permeability zone (Ring of Cenotes) probably related to the Chicxulub Impact Crater. Schlumberger soundings and Wenner anisotropy measurements show that the karst aquifer can be modeled as an electrically anisotropic medium. Anisotropy is related to preferential permeability directions channeling ground‐water flow within the aquifer. Directions of maximum permeability were determined using Wenner anisotropy measurements. Electrical soundings were conducted at different sites near the Ring of Cenotes. Resistivity values decrease toward the Ring of Cenotes supporting the hypothesis that selected segments of the Ring have high permeability. Several soundings were conducted in order to study lateral permeability variations along the Ring. A high permeability section can be identified by low resistivity models and is related to a zone of high cenote density. A low permeability section of the Ring was found showing high resistivity models. This zone overlaps with an area of low cenote density. Electrical soundings were used to determine the depth of the fresh‐water lens; the interface was detected along two profiles perpendicular and parallel to the Ring of Cenotes resulting in a depth that ranged from 18 m near the coast up to 110 m in the southeastern part of the study area. The predicted depths of the interface using electrical methods showed a good correlation with Ghyben‐Herzberg and measured interface depths at some sites. Discrepancies between calculated and interpreted interface depths at two sites may be explained by horizontal‐to‐vertical permeability anisotropy.
Multidimensional Water Flow in a Low‐Level Waste Isolation Barrier
Authors: Nichols William E. , Meyer Philip D.
Published: August 2005

An analysis was conducted of multidimensional subsurface moisture flow in a hypothetical low‐level radioactive waste disposal facility. The key design feature examined in the analysis was a sloping sand/gravel capillary barrier designed to route natural infiltration around a concrete vault. Three barrier slopes (1:25, 1:10, and 1:5) and three sand/gravel property combinations were considered. The slopes and material property combinations were selected to represent a range in barrier effectiveness. The porous media flow code used for this analysis was VAM3DCG, a three‐dimensional, finite‐element code which was able to use nonorthogonal grid discretizations and employed robust, efficient numerical techniques. Three‐dimensional modeling demonstrated that flow in the hypothetical design exhibited cross‐slope flow because of the pressure gradient produced in the third, cross‐slope dimension. Barrier effectiveness was shown to be highly sensitive to the sand/ gravel material properties. Barrier slope was less important, especially for the effective material combinations. The presence of three‐dimensional flow could be important in a performance assessment if the quantity of water predicted to breach the capillary barrier by a two‐dimensional model were different from that predicted by a three‐dimensional model. Comparative modeling demonstrated that a two‐dimensional analysis resulted in underestimation of barrier effectiveness. For the considered design, two‐dimensional modeling is a conservative, yet reasonable, approach in a performance assessment application.
Measuring Rates of Biodegradation in a Contaminated Aquifer Using Field and Laboratory Methods
Authors: Chapelle Francis H. , Bradley Paul M. , Lovley Derek R. , Vroblesky Don A.
Published: August 2005

Rates of biodegradation were measured in a petroleum hydrocarbon‐contaminated aquifer using a combination of field and laboratory methods. These methods are based on tracking concentration changes of substrates (both electron donors and acceptors) or final products of microbial metabolism over time. Ground water at the study site (Hanahan, South Carolina) is anoxic, and sulfate reduction is the predominant terminal electron accepting process. Laboratory studies conducted with sediment cored from the site showed that 14C‐toluene was mineralized to 14CO2 with a first‐order degradation rate constant (ktol) of ‐0.01 d−1 under sulfate‐reducing conditions. Under nitrate‐amended, Fe(III)‐amended, or nonamended (methanogenic) conditions, toluene was not significantly mineralized. 14C‐Benzene was degraded at low but measurable rates (kben= ‐0.003 d−1) under sulfate‐reducing conditions whereas degradation under methanogenic conditions was negligible. These results illustrate the extreme sensitivity of laboratory‐measured biodegradation rates to terminal electron‐accepting conditions, and show the necessity of carefully matching experimental conditions to in situ conditions. Concentration decreases of toluene along aquifer flowpaths, when the uncertainty of ground‐water flow velocities was considered, indicated ktol values ranging from ‐0.0075 to ‐0.03 d−1. Concentration decreases of sulfate and concentration increases of dissolved inorganic carbon (DIC), when normalized for assumed stoichiometric oxidation of toluene coupled to sulfate reduction, yielded a kso4 range of ‐0.005 to ‐0.02 d−1, and a kDIC value range of +0.00075 to +0.003 d−1. Because both laboratory and field methods have numerous sources of uncertainty, a combination of these methods is the most appropriate procedure for evaluating biodegradation rate constants in contaminated ground‐water systems.
Industrial Contamination of a Municipal Water‐Supply Lake by Induced Reversal of Ground‐Water Flow, Managua, Nicaragua
Authors: Bethune David N. , Farvolden R. N. , Ryan M. Cathryn , Guzman Andres Lopez
Published: August 2005

Laguna Asososca, a large ground‐water‐fed volcanic crater, is an important source of municipal water supply for the city of Managua. In 1990, after 65 years of pumping at increasing rates from the crater, the gradient between the Laguna and the highly contaminated Lake Managua had potentially reversed, leading to a scenario where the Laguna was possibly drawing in contaminated ground water from Lake Managua and/or a highly contaminated aquifer below an industrial area located between the Laguna and Lake Managua. A drilling and sampling program undertaken between 1990 and 1992 found: (1) four synthetic organic chemicals in the Laguna (methylene chloride, chloroform, 1,3‐dichlorobenzene and 1,4‐dichlorobenzene), (2) numerous other synthetic organic chemicals near Laguna Asososca in the ground water below the industrial area, and (3) no evidence of Laguna Asososca drawing water from Lake Managua. It appears that the Laguna Asososca capture zone extended into the industrial area but not as far as Lake Managua. Ground‐water flow modeling of the regional ground‐water flow system was consistent with the field interpretation. Estimates of the relative mobilities of the synthetic organic chemicals indicated that the chemicals found in the water of Laguna Asososca likely represented the mobile leading edge of a contaminant plume emanating from the industrial area. The simplest and most effective solution to mitigate contamination of Laguna Asososca is to maintain its water level above that of Lake Managua by reducing its pumpage to about 50% of the 1990 rate.
Gas Partitioning of Dissolved Volatile Organic Compounds in the Vadose Zone: Principles, Temperature Effects and Literature Review
Authors: Washington John W.
Published: August 2005

Enthalpy and entropy of volatilization from dilute aqueous solutions for 26 volatile organic compounds (VOCs) have been determined using Henry's Law values reported in published literature. Based on the linearity of van't Hoff plots, for the temperature ranges common in soils, the differences in heat capacities of volatilization for reactants and products are very small for the VOCs studied.

When volatile solutes such as VOCs are present in soil water, soil‐gas concentration often nearly is in equilibrium with the dissolved solute. Setchinow salting coefficients are linearly related to dissolved partial molar volumes for halogenated aliphatic compounds. Based in part on approximations from this linear relationship, equilibrium deviations from Henry's Law behavior for dilute VOC concentrations due to capillary tension or the presence of ionic solutes are small for common soil conditions.

Since gas/water partitioning of VOCs if temperature‐sensitive and since annual soil moisture and temperature patterns vary geographically in documented fashion, geographically specific temporal patterns in soil‐gas VOC concentrations are predictable in vadose zones containing dissolved VOCs. A U.S. map depicting these general soil‐moisture and temperature patterns is provided. Gas concentrations in vadose zones containing dissolved VOCs tend to increase with increasing temperature and decreasing moisture content due to equilibrium partitioning effects.

Diagrams useful for understanding the results of soil‐gas surveys and the efficacy of various remediation options are provided. The effect of bubbles in VOC water‐sample vials on aqueous concentrations is shown to be very small. The effect of head‐space volume of soil samples on estimated soil‐gas concentrations can be large.
Optimum Hydraulic Conductivity to Limit Contaminant Flux Through Cutoff Walls
Authors: Devlin J. F. , Parker B. L.
Published: August 2005

Cutoff walls are becoming increasingly attractive options for the control of solute migration from long‐term sources of contamination. The main advantage of low permeability enclosures is that they restrict advective transport of solutes away from the source. However, with high concentration source zones surrounded by cutoff walls, there exists the potential for notable mass fluxes outward due to diffusive transport. This paper shows, through the use of the steady‐state flux equations, that there is an optimal range of hydraulic conductivities for barrier materials which permit the outward diffusive flux to be counter balanced by an inward advective and dispersive flux. This concept of designing optimum contaminant containment using an inward advective flux to counter the outward diffusive flux is valid for sealable joint sheet pile walls, bentonite‐slurry walls and clay liners, but not synthetic membrane materials with extremely low hydraulic conductivities. The effective diffusion coefficient for the common chlorinated organic solvents such as TCE in water‐saturated clayey materials is approximately 1 × 10−6 cm2/sec, resulting in an optimum hydraulic conductivity ranging from 1 × 10−6 to 1 × 10−8 cm/sec. This range in hydraulic conductivity is within the range of common barrier materials but not the lowest achievable. The steady‐state concentration profile in a slurry cutoff wall can result in a substantial amount of contaminant mass stored within the wall which will need to be considered over the long term or dealt with during site remediation. Large inward advective fluxes reduce the total chemical mass stored within the low permeability barrier material.
A Hydrogeologic Model of Samana Cay, Bahamas, and Its Implications for the Columbus Landfall Question
Authors: Valdés José J.
Published: August 2005

Samana Cay, Bahamas, is a leading candidate among the sites proposed as the first landfall of Columbus in the New World. One main point of contention against this identification is that Samana lacks a feature matching the laguna observed by Columbus in the middle of his landfall island. However, this argument has failed to consider the paleogeographic aspects of the problem. A hydrogeologic computer model suggests the existence of a sizable surface‐water feature in the medial interior of Samana Cay under conditions theorized to have been present when Columbus arrived in the Bahamas. This feature may correspond to the laguna described by Columbus.
Pentachlorophenol Reductive Dechlorination in an Interceptor Trench: Temperature Effects
Authors: Cole Jason D. , Woods Sandra L. , Bricker Timothy R.
Published: August 2005

Pentachlorophenol‐acclimated methanogenic consortia were incubated at temperatures of 5, 10, 21, and 31° C in physical models of an interceptor trench. Four identical 5.4 cm diameter brass columns packed with a uniform pea gravel were operated with a 2 day hydraulic retention time in a continuous upflow mode. The columns were fed 25 or 50 mg of acetate per liter, 0.32 or 4.0 μM pentachlorophenol, and a dilute vitamin and nutrient mixture. In the columns operated at 31 and 21° C, pentachlorophenol was reductively dechlorinated at the ortho positions to form 2,3,4,5‐tetrachlorophenol and 3,4,5‐trichlorophenol. 3,4,5‐Trichlorophenol was the sole degradation product observed in the effluent. The lag period prior to pentachlorophenol biotransformation increased with decreasing temperature. At 31° C, a lag of 72 hours was observed; at 21° C the lag was 120 hours; and at 10° C the lag was 744 hours. No biodegradation of pentachlorophenol was observed at 5° C within an 888 hour period. Conversion of pentachlorophenol to 3,4,5‐trichlorophenol was complete within the first 7.6 cm of the column at 31 and 21° C. Loss of 90% of the pentachlorophenol was observed over the 30 cm column at 10° C. An interceptor trench inoculated with a temperature‐ and pentachlorophenol‐acclimated methanogenic consortia has the potential to anaerobically biotransform pentachlorophenol at temperatures as low as 10° C.
Investigation of Possible Extra Recharge During Pumping in Nottingham Aquifer
Authors: Jiao Jiu J.
Published: August 2005

Approaches to investigate possible recharge during a pumping test period are demonstrated by analyzing the pumping test data from the Nottingham aquifer, UK. The pumping lasted more than 200 days and the data for different observed periods are used to obtain aquifer parameters. If recharge is ignored, estimated transmissivities and storativities change progressively with the observation time. This indicates that a gradually added new source may be involved. Then the observed data in both pumping and recovery periods at one of the pumping sites are matched by the data calculated from the Theis equation. The match is very good except near the end of the recovery period. It shows that the real drawdown recovered more quickly than predicted by the model, indicating that the aquifer obtained extra recharge from other sources. The match between the observation and the model is improved by including a fixed‐head boundary or leakage in the model. The recharge to the aquifer is estimated to be 19% and 34% of the pumpage at the end of pumping, depending on the method used. The real source of the additional recharge is not clear; it could be from the surface water in the unconfined area, the storage of the aquitard, the compaction of the aquitard, or a combination of the three. This requires further hydrogeological investigation.
Environmental Isotope Characteristics of Landfill Leachates and Gases
Authors: Hackley K. C. , Liu C. L. , Coleman D. D.
Published: August 2005

The isotopic characteristics of municipal landfill leachate and gases (carbon dioxide and methane) are unique relative to the aqueous and gaseous media in most other natural geologic environments. The δ13C of the CO2 in landfills is significantly enriched in 13C, with values as high as +20 ‰ reported. The δ13C and δD values of the methane fall within a range of values representative of microbial methane produced primarily by the acetate‐fermentation process. The δD of landfill leachate is strongly enriched in deuterium, by approximately 30 ‰ to nearly 60 ‰ relative to local average precipitation values. This deuterium enrichment is undoubtedly due to the extensive production of microbial methane within the limited reservoir of a landfill. The concentration of the radiogenic isotopes, 14C and 3H, are significantly elevated in both landfill leachate and methane. The 14C values range between approximately 120 and 170 pMC and can be explained by the input of organic material that was affected by the increased 14C content of atmospheric CO2 caused by atmospheric testing of nuclear devices. The tritium measured in leachate, however, is often too high to be explained by previous atmospheric levels and must come from material buried within the landfill. The unique isotopic characteristics observed in landfill leachates and gases provide a very useful technique for confirming whether contamination is from a municipal landfill or some other local source.
Applicability of Slug Interference Tests for Hydraulic Characterization of Unconfined Aquifers: (2) Field Test Examples
Authors: Spane F. A. , Thorne P. D. , Swanson L. C.
Published: August 2005

Slug interference testing may be particularly useful for characterizing hydraulic properties of aquifer sites where disposal of contaminated ground water makes pumping tests undesirable. The design, performance, and analysis of slug interference tests for two field test examples are presented. Results were compared with standard constant‐rate pumping tests. The comparison indicates that slug interference tests provide estimates comparable to those obtained from short duration pumping tests for the determination of transmissivity, storativity, and vertical anisotropy.

The close agreement in hydraulic property values obtained using the two test methods suggests that slug interference testing, under favorable test conditions (for example, observation well distances ≤ 30 m), can provide representative aquifer characterization results. The quality and extent of test data obtained also indicate the potential use of slug interference testing for three‐dimensional hydrologic characterization investigations, when conducted using multilevel monitoring facilities.
A BEM Code for Ground‐Water Problems in Multizoned Domains with Normal Boundary Flux Discontinuities
Authors: Fidelibus C. , Lenti V.
Published: August 2005

A numerical code which utilizes the boundary element method (BEM) for solving steady‐state ground‐water flow problems is illustrated. The paper concentrates on accuracy in studying the situations which are generally considered to involve some mathematical difficulty, such as zoned domains and flux discontinuities. A numerical BEM code is proposed, the main feature of which involves accurately calculating flux discontinuities using a structure particularly flexible in multizoned domains. Two examples are reported, the results of which are comparable with those available in the literature.
Boundary Condition and Soil Attribute Impacts on Anionic Surfactant Mobility in Unsaturated Soil
Authors: Allred Barry , Brown Glenn O.
Published: August 2005

Surfactant mobility in unsaturated soil will impact the effectiveness and efficiency of using these compounds for in situ environmental remediation above the water table. For this reason, transient unsaturated column tests were used to study the influence of boundary conditions and soil attributes on anionic surfactant transport. In these tests, aqueous surfactant solutions were injected into the inlet of horizontally mounted soil columns. Two commercial anionic surfactants were used, an alkyl ether sulfate (AES) and a linear alkylbenzene sulfonate (LAS).

The overall study was divided into two parts. First, boundary condition effects including injected surfactant solution concentration, initial moisture content, and surfactant application rate were investigated. Increasing the injection solution concentration increased anionic surfactant mobility in the column while changes in the initial soil moisture content and surfactant application rate had no significant impact. Second, the impacts of soil attributes such as texture, dominant exchangeable cation, and resident organic matter were measured. With respect to texture, mobility was found to be greater in a sandy soil as compared with two loamy soils. Both surfactants, especially LAS, were found to be more mobile in a Na+ dominated soil rather than one dominated by Ca−2.The absence of soil organic matter increased LAS mobility.
Sources of Water to Wells for Transient Cyclic Systems
Authors: Reilly Thomas E. , Pollock David W.
Published: August 2005

Many state agencies are currently (1995) developing wellhead protection programs. The thrust of some of these programs is to protect water supplies by determining the areas contributing recharge to water‐supply wells and by specifying regulations to minimize the opportunity for contamination of the recharge water by activities at the land surface. The area contributing recharge to a discharging well is the surface area at the water table through which the water flowing to the well entered the ground‐water system. In the analyses of ground‐water flow systems, steady‐state average conditions are commonly used to simplify the problem and make a solution tractable. However, recharge is usually cyclic in nature, with seasonal cycles and longer term climatic cycles. The effect of these cyclic stresses on the area contributing recharge to wells is quantitatively analyzed for a hypothetical alluvial valley aquifer system that is representative of a large class of ground‐water systems that are extensively developed for water supply. The analysis shows that, in many cases, these cyclic changes in the recharge rates do not significantly affect the location and size of the areas contributing recharge to wells. The ratio of the mean travel time to the length of the cyclic stress period appears to be an indicator of whether the transient effects of the cyclic stress must be explicitly represented in the analysis of contributing areas to wells. For the cases examined, if the ratio of the mean travel time to the period of the cyclic stress was much greater than one, then the transient area contributing recharge to wells was similar to the area calculated using an average steady‐state condition. However, cyclic stresses on systems with ratios less than one do have an effect on the location and size of the areas contributing recharge to wells.
Self‐Potential Method in Hydrogeological Exploration of Volcanic Areas
Authors: Aubert M. , Atangana Q. Yéné
Published: August 2005

In volcanic areas, when two conditions are satisfied, there is a linear correlation between the range of negative self‐potential (SP) anomalies and the thickness of the unsaturated zone measured in borings. The first condition is a high ratio between the resistivity of the unsaturated zone, and the resistivities of the substratum and the water‐saturated zone. The second condition is the homogeneity of the unsaturated zone. From this relation, we develop the concept of a geophysical surface (SPS surface) calculated from SP and topographic data. Under these conditions, the SPS surface is both an equipotential SP surface and the interface between the unsaturated zone and the saturated medium below. Borehole logs for three field sites show that drainage courses and watersheds are located exactly on respectively the valleys and the ridge lines of the SPS surface.
Springs in Spain: Classification According to Their Flows and Lithologies and Their Hydraulic Contributions
Authors: Pérez Eugenio Sanz
Published: August 2005

This paper presents an approach to regional hydrological balance where the number and integrated discharge of springs are used to estimate recharge. This technique is applied to Spain, where approximately two‐thirds of the country has an inventory of spring lithology and discharge (17,000 springs). The data are used to extrapolate results to the entire region and are compared to independent methods of estimating recharge. This is an informative use of commonly available information and it could be applied to other regions with a similar data base.

From this sample, a distribution of the spring flow volume contribution in Spain is estimated (a) according to flow spring discharge, (b) as a limit of a lognormal distribution, which in our sample is simplified in a potential formula a(x) = 24,101.x −0.91. In this way it is calculated that the total flow volume of the springs is 14,877 hm3/yr. The study does not take into account seepage (diffuse discharge) draining to seas, rivers, or lakes but only water at the spring source. The most permeable lithological groups considered are the source of 94.2% of the total flow supplied by the springs. Limestone is especially significant, contributing 66.2%. The net recharge of precipitation to ground water varies widely according to lithologies. It reaches to 20.3% in limestones and drops to less than 1% for quartzites, slates, and plutonic rocks.
Multiport Well Design for Sampling of Ground Water at Closely Spaced Vertical Intervals
Authors: Delin Geoffrey N. , Landon Matthew K.
Published: August 2005

Detailed vertical sampling is useful in aquifers where vertical mixing is limited and steep vertical gradients in chemical concentrations are expected. Samples can be collected at closely spaced vertical intervals from nested wells with short screened intervals. However, this approach may not be appropriate in all situations. An easy‐to‐construct and easy‐to‐install multiport sampling well to collect ground‐water samples from closely spaced vertical intervals was developed and tested. The multiport sampling well was designed to sample ground water from surficial sand‐and‐gravel aquifers. The device consists of multiple stainless‐steel tubes within a polyvinyl chloride (PVC) protective casing. The tubes protrude through the wall of the PVC casing at the desired sampling depths. A peristaltic pump is used to collect ground‐water samples from the sampling ports. The difference in hydraulic head between any two sampling ports can be measured with a vacuum pump and a modified manometer. The usefulness and versatility of this multiport well design was demonstrated at an agricultural research site near Princeton, Minnesota where sampling ports were installed to a maximum depth of about 12 m below land surface. Tracer experiments were conducted using potassium bromide to document the degree to which short‐circuiting occurred between sampling ports. Samples were successfully collected for analysis of major cations and anions, nutrients, selected herbicides, isotopes, dissolved gases, and chlorofluorcarbon concentrations.
Potential for Intrinsic Bioremediation of a DNT‐Contaminated Aquifer
Authors: Bradley P. M. , Chapelle F. H. , Landmeyer J. E. , Schumacher J. G.
Published: August 2005

Microorganisms indigenous to a shallow aquifer, which underlies an explosives‐contaminated site, mineralized 28% of [U‐ring‐14C] 2,4–dinitrotoluene to 14CO2 within 28 days under aerobic conditions. Approximately 20% of added 2,4–dinitrotoluene remained undegraded at the end of the incubation while approximately 22% and 6% were transformed to 4–amino‐2–nitrotoluene and 2–amino‐4–nitrotoluene, respectively. In aquifer microcosms containing 2,6–dinitrotoluene, approximately 67% of the substrate remained undegraded and approximately 14% was transformed to 2–amino‐6–nitrotoluene. The radiolabeled study indicated that about 8% of the 2,6–dinotrotoluene was mineralized to CO2. The demonstrated degradation and subsequent mineralization of dinitrotoluene compounds by aquifer microorganisms are consistent with the decline in dissolved dinitrotoluene concentrations observed along the general ground‐water flowpath at the site. The results indicate that intrinsic bioremediation may be a viable alternative for remediating contaminated ground water at this site.
Contamination from Sand‐Bentonite Seal in Monitoring Wells Installed in Aquitards
Authors: Remenda V. H. , van der Kamp G.
Published: August 2005

A six year field experiment has shown that a sand‐bentonite mixture used to seal monitoring wells in aquitards contributes solutes to the ground water sampled from these wells. Monitoring wells were installed at field sites with hydraulic conductivity (K) ranging from 5 × 10 ‐9 m/s to 3 × 1011 m/s. In most cases the boreholes remained dry during installation which allowed the placement of a dry powdered bentonite/sand mixture tagged with potassium bromide (KBr) to seal and separate sampling points. Over six years, wells were sampled periodically and ground‐water samples were analyzed for Br and Cl and other major ions. Typical Br results ranged from 10 mg/1 to 35 mg/1 in the first 700 days, as compared to an estimated initial concentration in the seal material of about 75 mg/1. After six years the bromide concentrations had decreased to between 3 mg/1 and 5 mg/1. The total mass of Br removed in six years is less than 50% of that placed; therefore the contamination effects, although considerably diminished, persist. The trends of Br, Cl, Na, and SO4 indicate that varying degrees of contamination occur. These data show that the materials used to seal monitoring wells in aquitards can have a significant and long‐lasting impact on the chemistry of the water in the wells.
An Outline of a Guidance Framework for Assessing Hydrogeological Risks at Early Stages
Authors: Rosen Lars , LeGrand Harry E.
Published: August 2005

The prevalent and straightforward routine of impulsively collecting masses of new data for studies relating to ground‐water contamination can be inefficient and costly as it does not provide optimal value from existing hydrogeologic and other information. A preliminary guidance framework is outlined, for early stage monetary risk assessments prior to any new measurements, considering both the probability and the economical consequences of contamination. The framework is aimed at providing a basis for cost‐effective decision‐making regarding ground‐water protection and management actions. It centers on improved conceptual hydrogeological site descriptions based on existing information and professional judgments prior to any new measurements. A key aspect of the framework is to derive maximum possible quantitative understanding of risks from limited prior available qualitative information. The framework has a dual‐site approach, assuming a situation of a contaminant source site and a receptor site. A two‐step procedure leads to the monetary risk assessment with respect to existing compliance levels: (1) conceptual model development, and (2) quantitative model realization, addressing the probability of contaminant release at the contamination source, the hydraulic connection between the source and the receptor, and the contaminant transport conditions. The purpose of the framework is twofold: (1) to provide a risk assessment framework which optimizes use of professional judgment for studies where data are limited, and (2) to give synergistic interpretive values that complement field measurements and that can be used as prior estimates in more detailed studies.
A Semianalytical Model to Predict Recovery of Light, Nonaqueous Phase Liquids from Unconfined Aquifers
Authors: Waddill Dan W. , Parker Jack C.
Published: August 2005

Contamination of soil and ground water due to spills of light nonaqueous phase liquids (LNAPL or simply oil) is a widespread environmental problem. This paper describes the development and testing of a semianalytical model that may be used to design LNAPL containment and recovery systems at spill sites. The objective of this study was to derive an enhanced semianalytical algorithm for calculating recovery and trapping of free phase oil. The enhancements were derived and evaluated by comparison with an established numerical model that describes transient flow of oil and water. The semianalytical model employs an analytical solution for steady‐state drawdown in an unconfined aquifer due to water pumping. When pumping rates are sufficient to contain the separate phase plume, the model calculates recoverable and residual oil volumes based on the initial free oil distribution. Refinements were implemented to calculate the water‐table drawdown and the maximum unsaturated zone residual saturation (Sog) as functions of soil type. Also the influence of hysteresis on the oil‐water capillary fringe was incorporated into the calculation of oil trapping below a rising oil‐water interface. A method was derived to reduce saturated zone trapping to account for oil recovery that occurs while pumping proceeds. The above enhancements yielded close agreement between the semianalytical model and the transient model predictions of recoverable oil and residual oil in the unsaturated and saturated zones. The models were compared for hypothetical gasoline spills in a sandy and a silt loam soil, using a range of pumping rates and regional water‐table fluctuations. Field data from a pipeline leak were evaluated by the semianalytical model for hypothetical scenarios involving oil recovery from three wells and a falling regional water table. Results suggest that the semianalytical model captures many of the trends of transient oil recovery. Recovery is less accurately predicted when irregular water‐table fluctuations occur.
Saratoga Springs and Early Hydrogeochemistry in the United States
Authors: Davis Stanley N. , Davis Augusta G.
Published: August 2005

The earliest quantitative chemical analysis of ground water from the United States was probably made during 1807. The largest number of early analyses were of water from various mineral springs in and near the towns of Ballston and Saratoga Springs, New York. Although most of the scientific and economic interest at the time centered on the reported health benefits from the water, some of the earliest reports describing the springs contained rudimentary yet scientifically based hydrogeochemical explanations for the dissolved constitutents in the water. One motivation for the early analytical work was the desire to concoct a dry mixture of chemicals that would duplicate the mineral water when mixed with ordinary drinking water. The use of a powder would simplify distribution and bypass dealing with owners of the springs. Early analyses of water from Congress Spring published by different individuals between 1817 and 1856 are similar, and if allowance is made for minor temporal changes in quality, the results would be considered useful even by modern standards.
Borehole Flowmeter Application in Fluvial Sediments: Methodology, Results, and Assessment
Authors: Bomana Gerald K. , Molz Fred J. , Boonec Kelly D.
Published: August 2005

In many situations, inadequate design or performance of ground‐water remediation systems is the result of underestimation of aquifer hydraulic heterogeneity, and in particular, the vertical variation of hydraulic conductivity which plays an important role in contaminant migration. Described herein are applications of the electromagnetic (EM) borehole flowmeter to fluvial sediments in Louisiana and South Carolina. The direction of natural vertical flow in the test aquifers was defined easily, and short pumping tests enabled the calculation of hydraulic conductivity profiles for each test well. The results correlated well with other information obtained independently, including natural gamma logs, driller's logs and a hydraulic conductivity profile based on grain size analysis. Large variations in hydraulic conductivity over short vertical and horizontal distances were documented. Tests in gravel‐packed wells suggested that flowmeters produce misleading data for a variety of reasons in such situations. Among other things, an annulus of high permeability around a well screen allows flow to bypass the meter, and the phenomenon is amplified by high pumping rates. The resulting error is displayed as an erroneous high permeability zone at the top of the well screen. This observation deserves further study. In its present form the EM flowmeter is awkward to handle on a routine basis. However, none of the present design flaws preclude its effective use.
The Theis Solution in Heterogeneous Aquifers
Authors: Serrano Sergio E.
Published: August 2005

A new solution of the unsteady confined radial flow equation subject to aquifer heterogeneity, as characterized by the statistics of the transmissivity, is presented. Simple applications to the direct and inverse problems useful to field hydro geologists are included. The results use the classical Thies solution as an initial condition and the method of decomposition, which does not require many of the restrictions of the traditional small perturbation methods. Given the mean and variance of the transmissivity, the direct problem aims at the prediction of the mean and the variance of the draw downs. Given the mean and the variance of the draw down, the indirect problem aims at the estimation of the mean and the variance of the transmissivity.
Quantification of Ground‐Water Flow in Fractured Rock, Oak Ridge, Tennessee
Authors: Moore Gerald K.
Published: August 2005

The theory of ground‐water flow in fractured rocks has been used to interpret aquifer tests and stream flow data for the Oak Ridge Reservation of U.S. Department of Energy. This interpretation has resulted in a better understanding of the data needed for ground‐water models. The hydraulic conductivity values calculated from most aquifer tests are meaningless. Slug tests and early data from pumping tests measure the total transmissivity of all fractures within the area of influence; this value can be used to calculate rates of ground‐water flow and transport near the well. Larger transmissivity values are calculated from late aquifer‐test data and from an analysis of stream flow hydrographs because most ground‐water flow occurs in a thin layer at the water table. This layer supplies most ground‐water discharge to streams and apparently is a late‐time water source for pump age from deeper wells.

The calibration of distributed flow and transport models will be difficult, if not impossible, because about 90% of the ground‐water flow occurs in only 10% of the area, and the locations of these flow paths are unknown. Also, large changes in transmissivity occur with recharge and discharge cycles that change the saturated thickness of the permeable layer near the water table.
Identifying and Removing Barometric Pressure Effects in Confined and Unconfined Aquifers
Authors: Rasmussen Todd C. , Crawford Leslie A.
Published: August 2005

Failing to account for barometric pressure effects in water level measurements can introduce errors by misestimating the total head and by adding noise to water level measurements. For determining the total head in an aquifer, we assert that the air pressure head at the water surface in the well must be added to measured water levels (equivalent to using an absolute pressure transducer) even though the resulting values may have larger temporal and spatial variability than the original water level measurements. At the Savannah River Site in South Carolina, the average barometric pressure variation is 6 to 7 cm, with a range of over 30 cm. Failure to account for barometric pressure variability could result in misestimation of the direction and magnitude of the hydraulic gradient at the site. We also demonstrate procedures for removing barometric effects, such as to reduce noise during an aquifer pumping test, and to identify mechanisms by which barometric pressure affects water levels. Three mechanisms are summarized including: an instantaneous response for confined aquifers; a delayed response due to borehole storage in confined and unconfined aquifers; and a delayed response in unconfined aquifers due to the passage of barometric pressure changes through the unsaturated zone. Using data from the Savannah River Site, barometric efficiencies are estimated using linear regression and a modification of Clark's Method. Delayed responses are estimated using regression deconvolution. The type of barometric effect provides diagnostic information about whether the aquifer is confined or not, the presence of borehole storage or skin effects, and the air diffusivity coefficient within the unsaturated zone. We also show how removal of barometric pressure effects improves the ability to observe otherwise unnoticeable effects.
Hydrogeology and Paleohydrology of Ancient Machu Picchu
Authors: Wright Kenneth R. , Witt Gary D. , Zegarra Alfredo Valencia
Published: August 2005

The study of the ancient Inca water supply of Machu Picchu was performed by the authors under an archaeological permit issued by the Peruvian Government in 1994. The study included an evaluation of the hydrogeology of the ancient spring water supply situated on the north slope of the mountain of Machu Picchu and its relationship to the Machu Picchu fault.

Based on field measurements, the yield of the primary spring has a seasonal variation of between 25 and 125 liters per minute. This flow adequately provided the domestic water requirements of up to 1000 inhabitants of the mountain top royal retreat of the Inca ruler Pachacuti, occupied primarily between 1450 and 1540 AD. The permanent population between visits of the Inca ruler was about 300. Final abandonment occurred in 1572 AD. The estimated geologic catchment area related to the Machu Picchu fault is roughly 50 percent greater than the topographic basin area. A 749 meter‐long canal delivered the spring flow to the center of the community to serve a total of 16 water supply fountains.

A theory that a water shortage caused Machu Picchu to be abandoned proved to be unsubstantiated based on a study of the Quelccaya Ice Cap corings data that demonstrated that the last decade of the occupation was wetter than the previous eight decades. The builders of Machu Picchu developed a community domestic water supply using the spring and a hydraulic infrastructure in an effective and efficient manner. They utilized a gravity flow canal sized to carry the spring flow with a suitable safety factor against overtopping.
Composite Recovery Type Curves in Normalized Time from Theis’ Exact Solution
Authors: Goode Daniel J.
Published: August 2005

Type curves derived from Theis’ exact nonequilibrium well function solution are proposed for graphical estimation of aquifer hydraulic properties, transmissivity (T), and storage coefficient (S), from water‐level recovery data after cessation of a constant‐rate discharge test. Drawdown (on log scale) is plotted versus the ratio of time since pumping stopped to duration of pumping, a normalized time. Under Theis conditions, individual type curves depend on only the dimensionless pumping duration, which depends in turn on S and radial distance from the pumping well. Type curve matching, in contrast to the Theis procedure for pumping data, is performed by shifting only the drawdown axis; the time axis is fixed because it is a relative or normalized time. The match‐point for the drawdown axis is used to compute T, and S is determined from matching the curve shape, which depends on early dimensionless‐time data. Multiple well data can be plotted and matched simultaneously (a composite plot), with drawdown at different radial distances matching different curves. The ratio of dimensionless pumping durations for any two matched curves is equal to one over the squared ratio of radial distances. Application to two recovery datasets from the literature confirm the utility of these type curves in normalized time for composite estimation of T and S.
Comparison of Models for Delineating Wellhead Protection Areas in Confined to Semiconfined Aquifers in Alluvial Basins
Authors: Forster Craig B. , Lachmar Thomas E. , Oliver Douglas S.
Published: August 2005

The 1986 Safe Drinking Water Act Amendments include provisions for state wellhead protection (WHP) programs that address wellhead protection areas (WHPAs). In many states, WHPAs are delineated based on time‐of‐travel (TOT) criteria. This study compares 250‐day and 15‐year TOT capture zones computed in a confined to semiconfined aquifer system in an alluvial basin using semianalytical and two‐ and three‐dimensional numerical ground‐water flow models, and evaluates the relative importance of several sources of uncertainty, such as aquifer hydraulic conductivity, aquitard leakance, vertical transit time, hydraulic gradients, transient pumping effects, well interference, and three‐dimensional aquifer geometries.

A numerical model should be used to delineate 15‐year TOT capture zones for wells in confined to semiconfined aquifers in alluvial basins. A semianalytical program may be acceptable for computing the 250‐day TOT capture zones; however, such codes can be applied only under a very narrow range of conditions.

Hydraulic conductivity plays a critical role in controlling the sizes and shapes of capture zones computed in confined to semiconfined aquifers. Small, circular capture zones are computed in low hydraulic conductivity areas. More complex geometries should be expected where hydraulic conductivities are higher and pumping wells are in close proximity to each other. Aquifers with horizontal hydraulic conductivities that are greater than 1,000 times the vertical hydraulic conductivity of the overlying aquitard are effectively fully confined, and larger capture zones would be computed for these aquifers than for semiconfined aquifers where significant leakage is induced by pumping. In addition, relatively large drawdowns are computed in low hydraulic conductivity areas, resulting in short vertical transit times. Vertical transit times are longer where aquifer hydraulic conductivities are higher.
Experimental Evaluation of a Drive‐Point Ground‐Water Sampler for Hydraulic Conductivity Measurement
Authors: Scaturo David M. , Widdowson Mark A.
Published: August 2005

Drive‐point ground‐water samplers, designed to enable delineation of the vertical and areal extent of a contaminant plume, have been modified to obtain an in situ measurement of hydraulic conductivity in aquifers. The multlevel drive‐point sampler (MLDPS) consists of a 30‐cm long screen pushed to a desired depth within a retractable outer sleeve and a pressure transducer to monitor the rate of fill within a sample chamber. The accuracy and variability of a MLDPS to measure hydraulic conductivity was evaluated in a 50‐m3 laboratory aquifer comprised of a uniform medium sand. Twelve measurements were performed using the MLDPS followed by duplicate slug tests performed at identical locations and depths within the aquifer. MLDPS and slug test data were analyzed using the method of Widdowson et al. (1990). The MLDPS provided an estimation of hydraulic conductivity at least one order of magnitude below slug test‐derived hydraulic conductivity and the range of hydraulic conductivity associated with a medium sand. MLDPS‐derived hydraulic conductivity values also show greater variability and range relative to the slug test‐derived values. Hydraulic conductivity ranges from 0.072 to 2.3 m/d using the MLDPS (coefficient of variation = Cv= 1.3) compared to a range of 10 to 16 m/d (Cv= 0.14) for the slug test values. Variability and magnitude of the MLDPS‐derived hydraulic conductivity is attributed to disturbance of aquifer material and experimental factors inherent to the design of the instrument.
NCF: A Finite‐Element Computer Program to Simulate Ground‐Water Flow Within the U.S. Geological Survey Modular Ground‐Water Flow Model (MODFLOW)
Authors: Jones Michael A.
Published: August 2005

A finite‐element package has been developed for the U.S. Geological Survey Modular Ground‐Water Flow Model, MODFLOW (McDonald and Harbaugh, 1988). The package, entitled the Node Centered Flow (NCF) Package, offers alternatives for constructing and solving ground‐water flow problems. As with MODFLOW's Block Centered Flow (BCF) package, NCF uses a finite‐difference solution for vertical flow, but unlike BCF it uses a 2d Galerkin finite‐element solution for flow within a layer. The algorithm used in NCF is described in Neuman et al. (1982). This technique permits simulation within a layer of off‐diagonal elements in the (2 × 2) transmissivity or hydraulic conductivity tensor. The most advantageous feature of the NCF Package is the capability for designing a nonrectangular grid. User specification of the horizontal (x, y) coordinates of individual nodes allows model features such as meandering stream channels, faults, and wells to be more precisely represented. NCF uses the same input style as BCF and will be immediately familiar to MODFLOW users; BCF input files are also easily converted to NCF input files. NCF parameters are input by layer, row, and column, allowing NCF and BCF to work with all of the same MODFLOW packages with the exception of matrix solving packages.