Showing papers on "Vadose zone published in 1995"
TL;DR: In this paper, the authors applied solute and isotopic methods for evaluating recharge fluxes, runoff, and spatial and temporal distribution of recharge in the Southern High Plains aquifer of Texas and New Mexico.
Abstract: The High Plains aquifer underlying the semiarid Southern High Plains of Texas and New Mexico, USA was used to illustrate solute and isotopic methods for evaluating recharge fluxes, runoff, and spatial and temporal distribution of recharge. The chloride mass-balance method can provide, under certain conditions, a time-integrated technique for evaluation of recharge flux to regional aquifers that is independent of physical parameters. Applying this method to the High Plains aquifer of the Southern High Plains suggests that recharge flux is approximately 2% of precipitation, or approximately 11 ± 2 mm/y, consistent with previous estimates based on a variety of physically based measurements. The method is useful because long-term average precipitation and chloride concentrations in rain and ground water have less uncertainty and are generally less expensive to acquire than physically based parameters commonly used in analyzing recharge. Spatial and temporal distribution of recharge was evaluated by use of δ2H, δ18O, and tritium concentrations in both ground water and the unsaturated zone. Analyses suggest that nearly half of the recharge to the Southern High Plains occurs as piston flow through playa basin floors that occupy approximately 6% of the area, and that macropore recharge may be important in the remaining recharge. Tritium and chloride concentrations in the unsaturated zone were used in a new equation developed to quantify runoff. Using this equation and data from a representative basin, runoff was found to be 24 ± 3 mm/y; that is in close agreement with values obtained from water-balance measurements on experimental watersheds in the area. Such geochemical estimates are possible because tritium is used to calculate a recharge flux that is independent of precipitation and runoff, whereas recharge flux based on chloride concentration in the unsaturated zone is dependent upon the amount of runoff. The difference between these two estimates yields the amount of runoff to the basin.
323 citations
TL;DR: In this paper, a new alternative septic-system design is presented utilizing reactive porous media barriers for passive in situ attenuation of NO3−, which consists of solid organic carbon (sawdust) which promotes NO3- attenuation by heterotrophic denitrification.
Abstract: 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.
258 citations
Book•
16 Nov 1995
TL;DR: In this article, the authors present an overview of the physical processes relevant to deep soil-water movement, including Hydraulic properties, and their relationship with Soil-Water Budget.
Abstract: Basic Concepts and Theory. Soil-Water Budget. Physical Processes Relevant to Deep Soil-Water Movement. Recharge. Characterizing Hydraulic Properties. Monitoring. Case Studies. References. Index.
239 citations
TL;DR: In this paper, a new theoretical formulation for the analysis of coupled heat, moisture and air transfer is presented, which is applicable to a deformable unsaturated soil, and three isothermal test problems are then considered to model the behaviour of unsaturated soils under varying stress and suction conditions.
Abstract: A new theoretical formulation for the analysis of coupled heat, moisture and air transfer is presented, which is applicable to a deformable unsaturated soil. The approach proposed extends previous analyses of the coupled transport of heat, pore water and pore air to take account of the deformation behaviour of partially saturated soil. A numerical solution of the governing differential equations is achieved by the use of the finite element method as a spatial discretization technique coupled with a finite difference recurrence relationship to describe transient behaviour. Three isothermal test problems are then considered to model the behaviour of unsaturated soil under varying stress and suction conditions. Both heave and collapse due to wetting are simulated. The model is applied to experimental work performed by others for the case of heating of an unsaturated montmorillonite clay and is shown to be capable of producing quantitatively physically correct results. These indicate that strong interactions ...
181 citations
TL;DR: In this paper, the authors show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones.
Abstract: Chlorofluorocarbons and krypton 85 are trace gases whose atmospheric concentrations have been increasing over the past few decades. As they are soluble in water, they have been used as groundwater age indicators over timescales ranging from a few years to a few decades. In this paper we show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones. The apparent time lag is largely dependent on the gas solubility, the gas diffusion coefficient, and the soil water content. In coarse-grained soils the lag time will typically range between 1 and 2 years for a water table depth of 10 m to between 8 and 15 years for a water table depth of 30 m. For the shallower water tables ( 10 m), if this effect is not considered, the use of these gaseous tracers will overestimate the age of the groundwater. In very fine-grained soils where the soil water content in the unsaturated zone may be close to saturation, the effect will be much more pronounced.
176 citations
TL;DR: In this paper, the authors focused on water flow and solute migration through unsaturated fractured chalk in an arid area, where four dry-drilling holes were bored through the vadose zone.
Abstract: This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four dry-drilling holes were bored through the vadose zone. Core and auger samples, collected at 30- to 50-cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6–11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth. Close to land surface, the pore water is strongly evaporated (δ18O = +5.94‰) and highly concentrated (∼29 meq Cl/100 g rock), but changes gradually with depth to amore dilute concentration (∼4 meq Cl/100 g rock) and isotopically depleted composition (δ18O = −4.4‰), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a hydrologic barrier.
153 citations
TL;DR: The significance of ground penetrating radar (GPR) as a tool to detect near-surface contaminants is illustrated at a site in the Midwest representing petroleum product above the water table as discussed by the authors.
147 citations
01 Jan 1995
TL;DR: In this article, the authors interpret the dissolutional record of many carbonate islands as past exposure surfaces, and show that carbonate deposition is not distributed uniformly, because paleosol material is commonly transported into vadose and phreatic voids at depth, and micritized zones similar in appearance to paleosols can develop within existing carbonates.
Abstract: Karst development on carbonate platforms occurs continuously on emergent portions of the platform. Surficial karst processes produce an irregular pitted and etched surface, or epikarst. The karst surface becomes mantled with soil, which may eventually result in the production of a resistant micritic paleosol. The epikarst transmits surface water into vadose pit caves, which in turn deliver water to a diffuse-flow aquifer. These pit caves form within a 100,000 yr time frame. On islands with a relatively thin carbonate cover over insoluble rock, vadose flow perched at the contact of carbonate rock with insoluble rock results in the lateral growth of vadose voids along the contact, creating large collapse chambers that may later stope to the surface. Carbonate islands record successive sequences of paleosols (platform emergence) and carbonate sedimentation (platform submergence). The appropriate interpretation of paleosols as past exposure surfaces is difficult, because carbonate deposition is not distributed uniformly, paleosol material is commonly transported into vadose and phreatic voids at depth, and micritized zones similar in appearance to paleosols can develop within existing carbonates. On carbonate islands, large dissolution voids called flank margin caves form preferentially in the discharging margin of the freshwater lens from the effects that result from freshwater/saltwater mixing. Similarly, smaller dissolution voids also develop at the top of the lens where vadose and phreatic freshwaters mix. Independent of fluid mixing, oxidation of organic carbon and oxidation/reduction reactions involving sulfur can produce acids that play an important role in phreatic dissolution. This enhanced dissolution can produce caves in freshwater lenses of very small size in less than 15,000 yr. Because dissolution voids develop at discrete horizons, they provide evidence of past sea level positions. The glacio-eustatic sea level changes of the Quaternary have overprinted the dissolutional record of many carbonate islands with multiple episodes of vadose, freshwater phreatic, mixing zone, and marine phreatic conditions. This record is further complicated by
145 citations
TL;DR: In this paper, electrical resistance tomograms were used to monitor in-situ remediation processes for removal of volatile organic compounds from subsurface water and soil at the Savannah River Site near Aiken, South Carolina.
132 citations
Book•
01 Jan 1995
TL;DR: In this paper, the authors present a survey of the characteristics of ground water in terms of geology, geophysics, and geology-based solutions for the problem of subsurface contamination.
Abstract: PART I: BASIC CONCEPTS GEOLOGY, SOILS, AND GEOMORPHOLOGY Geologic Materials Geologic Processes Stratigraphy and Structure Basic Soil Concepts Geomorphology and Ground Water Geologic Settings of Ground Water Occurrence and Quality Guide to Major References GROUND WATER AND VADOSE ZONE HYDROLOGY Ground Water in the Hydrologic Cycle Ground Water-Atmospheric Relationships Ground Water-Surface Water Relationships Water in the Vadose Zone Water in the Saturated Zone (1): Fundamental Concepts Water in the Saturated Zone (2): Water Storage and Flow Guide to Major References SOIL AND GROUND WATER GEOCHEMISTRY AND MICROBIOLOGY Key Characteristics of Chemical Systems Subsurface Distribution Processes Subsurface Transformation Processes Subsurface Environmental Parameters Microbial Ecology of the Subsurface Guide to Major References SOURCES AND BEHAVIOR OF SUBSURFACE CONTAMINANTS Definitions of Contamination Extent and Sources of Subsurface Contamination General Mechanisms of Ground?Water Contamination Contaminant Transport Processes Contaminant Retardation Phase Partitioning to Assess Presence of DNAPLs in the Subsurface Contaminant Plume Behavior Guide to Major References PART II: ASSESSMENT AND MONITORING PLANNING FIELD INVESTIGATIONS Overview of Investigation Planning Process Existing Information Sources Developing a Sampling and Monitoring Plan Data Measurement and Reliability Analytical and QA/QC Concepts Statistical Techniques Guide to Major References GEOPHYSICAL AND REMOTE SENSING TECHNIQUES Overview of Remote Sensing and Geophysical Techniques Airborne Remote Sensing Surface Geophysical Methods Borehole Geophysics CPT and Other Direct Push Sensing Methods Selection of Geophysical Methods Guide to Major References CHARACTERIZATION OF VADOSE ZONE AND GROUND WATER HYDROLOGY Measurement of Vadose Zone Hydrologic Parameters Preparation and Use of Potentiometric Maps Field and Laboratory Measurement of Aquifer Parameters Estimation of Subsurface Hydrologic Parameters Special Considerations in Hydrogeologic Mapping Guide to Major References SOIL AND GROUND-WATER TRACERS Types and Uses of Tracer Tests Tracer Selection Types of Tracers Tracer Tests in Karst and Fractured Rock Tracer Tests in Porous Media Guide to Major Reference FIELD SAMPLING AND MONITORING OF CONTAMINANTS Types of Monitoring Installations Drilling Methods for Sampling and Well Installation Conventional Monitoring Well Installations Sampling Subsurface Solids and the Vadose Zone Sampling Ground Water Field Screening and Analytical Methods Guide to Major References USE OF MODELS AND COMPUTERS IN CONTAMINANT INVESTIGATIONS Uses of Models and Computers Mathematical Approaches to Modeling Classification of Ground Water Computer Codes General Code Selection Considerations Other Geoenvironmental Computer Applications Guide to Major Information Sources PART III: PREVENTION AND REMEDIATION PREVENTION AND MINIMIZATION OF CONTAMINATION General Approaches Delineation of Wellhead Protection Areas Identification of Potential Contaminant Sources Assessing The Risk From Potential Contaminants Wellhead Protection Area Management Guide to Major References REMEDIATION PLANNING Conceptual Approach to Soil and Ground Water Remediation Methodology Selection of Treatment Methods Measurement and Interpretation of Treatment Effectiveness Guide to Major References REMEDIATION OF CONTAMINATED SOILS General Approaches to Soil Remediation Soil Vacuum Extraction (SVE) Bioremediation Other Treatment Approaches Prepared Bed Reactors Guide to Major References REMEDIATION OF CONTAMINATED GROUND WATER Overview Source Control Containment: Ground Water Barriers and Flow Control Ground Water Collection Ground Water Pump-and-Treat Methods In Situ Treatment Guide to Major References Appendices A Summary of Information on Major Subsurface Characterization and Monitoring Techniques Index to ASTM Field and Laboratory Methods Possibly Pertinent to Environmental Site Characterization Tables and Figures for Estimation of Aquifer Parameters Worksheets and Checklists for Ground-Water and Wellhead Protection Aquifer Properties and Contaminant Fate and Transport Problems and Solutions Master List of Figures and Tables with Credits
130 citations
TL;DR: In this article, seasonal cyclic movement of soil water in the top 60-80 cm active zone tends to produce a consistent stable isotope composition for the soil water below the active zone in different seasons.
TL;DR: In this paper, a Laplace transform solution is presented for flow to a well in a homogeneous, water-table aquifer with noninstanta-neous drainage of water from the zone above the water table.
Abstract: A Laplace transform solution is presented for flow to a well in a homogeneous, water-table aquifer with noninstanta-neous drainage of water from the zone above the water table. The Boulton convolution integral is combined with Darcy's law and used as an upper boundary condition to replace the condition used by Neuman. Boulton's integral derives from the assumption that water drained from the unsaturated zone is released gradually in a manner that varies exponentially with time in response to a unit decline in hydraulic head, whereas the condition used by Newman assumes that the water is released instantaneously. The result is a solution that reduces to the solution obtained by Neuman as the rate of release of water from the zone above the water table increases. A dimensionless fitting parameter, γ, is introduced that incorporates vertical hydraulic conductivity, saturated thickness, specific yield, and an empirical constant α1, similar to Boulton's α. Results show that theoretical drawdown in water-table piezometers is amplified by noninstantaneous drainage from the unsaturated zone to a greater extent than drawdown in piezometers located at depth in the saturated zone. This difference provides a basis for evaluating γ by type-curve matching in addition to the other dimensionless parameters. Analysis of drawdown in selected piezometers from the published results of two aquifer tests conducted in relatively homogeneous glacial outwash deposits but with significantly different hydraulic conductivities reveals improved comparison between the theoretical type curves and the hydraulic head measured in water-table piezometers.
11 Oct 1995
TL;DR: In this article, a geostatistical inverse technique utilizing both primary and secondary information is developed to estimate conditional means of unsaturated hydraulic conductivity parameters in the vadose zone.
Abstract: A geostatistical inverse technique utilizing both primary and secondary information is developed to estimate conditional means of unsaturated hydraulic conductivity parameters (saturated hydraulic conductivity and pore size distribution parameters) in the vadose zone. Measurements of saturated hydraulic conductivity and pore size distribution parameters are considered as the primary information, while measurements of steady state flow processes (soil-water pressure head and degree of saturation) are regarded as the secondary information. This inverse approach relies on the classical linear predictor (cokriging) theory and takes the advantage of the spatial cross correlation between the soil-water pressure head and each of the following: degree of saturation, saturated hydraulic conductivity, and a pore size distribution parameter. Using an approximate perturbation solution for steady, variably saturated flow under general boundary conditions, the cross covariances between the primary and secondary information are derived. The approximate solution is formulated on the basis of a first-order Taylor series expansion of a discretized finite element equation. The sensitivity matrix in the solution is evaluated by an adjoint state sensitivity approach for flow in heterogeneous media under variably saturated conditions. Through several numerical examples the inverse model demonstrates its ability to improve the estimates of the spatial distribution of saturated hydraulic conductivity and pore size distribution parameters using the secondary information.
TL;DR: In this article, a study of profiles of dissolved CH4 concentrations, diffusive flux of CH4, and CH4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota was conducted.
Abstract: A study (August 1990 to July 1991) of profiles of dissolved CH4 concentrations, diffusive flux of CH4, and CH4 production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH4 deep in the peat (> 1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH4 concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH4. At one site (2.5 m depth) the concentration of dissolved CH4 in the peat pore-water was 140 mg L−1. In July 1991, at no site did the concentration of dissolved CH4 in the peat pore water exceed 40 mg L−1 in the peat profile. The average calculated diffusive flux of CH4 decreased from 95 to 45 mg m−2 d−1 between 1990 and 1991. Gaseous CH4 was more in evidence in 1990 than in 1991. In 1990, CH4 at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H2CO3, CO2, HCO3− and CO32− decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L−1 to 38 mg C L−1 in 1991. A dynamic mechanism must exist which traps CH4 deep in the peat column allowing gaseous CH4 to build up, increasing dissolved CH4. Other times, CH4 passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH4 bubbles forms at depth in the peat, trapping gaseous CH4. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the water table elevation.
TL;DR: Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years as discussed by the authors.They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries.
Abstract: Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which they are named, is the result of their great depth, and they may lead to cave systems below sea level. Blue holes are polygenetic in origin, having formed: by drowning of dissolutional sinkholes and shafts developed in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and dissolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.
TL;DR: In this article, the coupled transport process through an integrated soil-groundwater system is quantified for kinetically sorbing solute that originates from a time dependent source at the soil surface and is transported by steady random velocity.
Abstract: The coupled transport process through an integrated soil-groundwater system is quantified for kinetically sorbing solute that originates from a time dependent source at the soil surface and is transported by steady random velocity. The derived expressions of ensemble mean solute breakthrough at some arbitrary control plane normal to the mean flow direction involve probability density functions (pdfs) of advective solute travel time through the unsaturated and the saturated zone of the transport domain. A nonstationary travel time pdf is derived for the saturated zone, to account for possible effects of flow nonuniformity due to recharge of water from the unsaturated zone. Nonuniform mean flow in the saturated zone decreases the relative influence of spatial variability within that zone on the ensemble mean solute breakthrough curve. Factors such as the longitudinal extent of the solute source and the unsaturated zone variability become more important for the spreading of the expected solute breakthrough as the degree of flow nonuniformity in the saturated zone increases. This implies that possible far-field simplifications based on the assumption that the transport process in an integrated soil-groundwater system is dominated by the transport conditions in the saturated zone may not be valid in cases with significant groundwater recharge from the unsaturated zone.
TL;DR: In this paper, the results of field studies in Tucson, Arizona, to estimate the soil aquifer treatment (SAT) effectiveness of a 5.7-ha water spreading facility, the Sweetwater Underground Storage and Recovery Facility, were analyzed for pathogens.
Abstract: This paper summarizes the results of field studies in Tucson, Arizona, to estimate the soil aquifer treatment (SAT) effectiveness of a 5.7-ha (14-acre) water spreading facility, the Sweetwater Underground Storage and Recovery Facility. Groundwater samples collected from the facility during 1989 to 1990 were analyzed for pathogens. A specific basin was selected during the 1990 to 1991, 1991 to 1992, and 1992 to 1993 recharge seasons for sampling source water (tertiary effluent), pore-liquid samples from the vadose zone, and groundwater during recharge. These samples were analyzed for the nitrogen species, dissolved organic carbon (DOC), and total organic halide (TOX). The results showed that the site provides effective SAT. Enteroviruses were completely removed during travel in the 37-m (120-ft) thick vadose zone. No Giardia ware detected in any of the groundwater samples. The DOC and TOX were reduced by 92% and 85%, respectively. After leaching of indigenous nitrogen from the vadose zone, total nitrogen was reduced by approximately 47% during recharge. Near-surface anaerobic conditions promote denitrification
TL;DR: In this article, analytical solutions for determining the vertical air permeability of unsaturated soils using observed air pressure data at the land surface and at depth in the vadose zone are presented.
Abstract: The study of flow and transport in unsaturated soils requires a good knowledge of the soil properties. When advective gas phase transport is of interest, air permeability is the key parameter of soil characterization. Although an air permeability test can easily be done in a laboratory, the reported values are frequently found to be not very representative of the air permeability in situ. Air permeability determined by a field test is useful both in the study of gas flow through unsaturated soils and in the prediction of the saturated hydraulic conductivity. This paper presents analytical solutions for determining the vertical air permeability of unsaturated soils using observed air pressure data at the land surface and at depth in the vadose zone. One solution is developed for a single-layer soil, and the other is developed for layered soils. Example calculations using field data have demonstrated the significant potential of the solution applications in field tests.
TL;DR: In this paper, the van Genuchten model was used to fit mathematical functions to the laboratory-measured moisture retention data for predicting unsaturated hydraulic conductivities for field soils.
Abstract: Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, θ, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(θ) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratory-measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten–Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, Ks, were compared to those obtained by a scaled-predictive method that uses a single K(θ) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten–Mualem relationships showed considerable disagreement. This suggests that the use of laboratory-measured Ks results in an inadequate characterization of K(θ) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low θ. However, use of the same moisture retention curve-fitting parameters and a single steady state head control-based K(θ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse-textured soils considered in this study, results indicate that a K∥θ) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low θ. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.
TL;DR: In this paper, electrical resistivity tomography was used to detect and delineate the extent of the zone influenced by air sparging in a vadose or saturated soil. But the results showed that the electrical images could not detect the hydrocarbons in either the vadiose or the saturated zone.
TL;DR: In this article, velocity covariances and the resultant macrodispersion coefficient tensor are applied for unsaturated flow conditions, employing the assumption that for a given mean capillary pressure head, water saturation is a deterministic constant and log conductivity is a multivariate normal, stationary random space function.
Abstract: Velocity covariances, and the resultant macrodispersion coefficient tensor, derived by Russo (this issue) for saturated flow conditions, are applied for unsaturated flow conditions, employing the assumption that for a given mean capillary pressure head, water saturation is a deterministic constant and log conductivity is a multivariate normal, stationary random space function. The applicability of the approach for modeling flow and transport in the vadose zone was evaluated by the use of the stochastic theory of Yeh et al. (1985a, b) for steady, unsaturated flow. Results of the analyses suggest that the approach may be applicable to vadose zone flow and transport, as long as the scale of heterogeneity in the direction of the mean flow is smaller than approximately one tenth of the characteristic length of unsaturated flow. For porous formation of given statistics, the magnitude of macrodispersion in unsaturated flow is larger than that in saturated flow, and increases as water saturation decreases. For a given water saturation, transport in unsaturated flow may approach asymptotic Fickian behavior more slowly than in saturated flow, when the two formation properties log Ks and α are positively cross-correlated and when the correlation scale of α is relatively large as compared with the correlation scale of log Ks.
TL;DR: In this paper, the authors used the WAVE-model to simulate the behavior of water and nitrogen in soils cropped with winter wheat and sugar beet, and evaluated the performance of the integrated model using field data for a three-year period.
01 Oct 1995
TL;DR: In this paper, the authors identify, compile, and interpret all available data for the principal soil types in the 200 Areas plateau, including particle-size distribution, moisture retention, and saturated hydraulic conductivity.
Abstract: Over the years, data have been obtained on soil hydraulic properties at the Hanford Site. Much of these data have been obtained as part of recent site characterization activities for the Environmental Restoration Program. The existing data on vadose zone soil properties are, however, fragmented and documented in reports that have not been formally reviewed and released. This study helps to identify, compile, and interpret all available data for the principal soil types in the 200 Areas plateau. Information on particle-size distribution, moisture retention, and saturated hydraulic conductivity (K{sub s}) is available for 183 samples from 12 sites in the 200 Areas. Data on moisture retention and K{sub s} are corrected for gravel content. After the data are corrected and cataloged, hydraulic parameters are determined by fitting the van Genuchten soil-moisture retention model to the data. A nonlinear parameter estimation code, RETC, is used. The unsaturated hydraulic conductivity relationship can subsequently be predicted using the van Genuchten parameters, Mualem`s model, and laboratory-measured saturated hydraulic conductivity estimates. Alternatively, provided unsaturated conductivity measurements are available, the moisture retention curve-fitting parameters, Mualem`s model, and a single unsaturated conductivity measurement can be used to predict unsaturated conductivities for the desired range of field moisturemore » regime.« less
TL;DR: In this article, the relative importance of different recharge mechanisms has been investigated for a karstic area in a subhumid to semi-arid region of South Australia, where the chemical and isotopic signature (δ2H and δ18O) of regional groundwater, sampled from shallow bores in the study area, as well as water from sinkholes, swamps, drainage bores and water extracted from soil in the unsaturated zone has been measured.
TL;DR: In this paper, a transport model was developed to predict liquid-phase tortuosity as a function of readily obtained soil parameters, including density, moisture content, particle size distribution and pore size distribution.
TL;DR: In this article, the authors investigated the ability of surveys to delineate DNAPL chlorinated solvent sources and associated ground-water plumes and found that dissolved-phase plumes from pools or residual located about a meter or more below the water table are unlikely to be directly detected by soil-gas surveys.
Abstract: Field experiments have been conducted at the Borden research site to evaluate the widely applied soil-gas survey method. In particular, the ability of surveys to delineate DNAPL chlorinated solvent sources and associated ground-water plumes has been investigated. Field experiments indicated that dissolved-phase plumes from DNAPL pools or residual located about a meter or more below the water table are unlikely to be directly detected by soil-gas surveys. Soil-gas plumes observed at real sites are attributed to volatilization of source material in the vadose zone and consequent formation of very shallow interface zone ground-water contamination that is able to partition to the soil gas as it transports downgradient. The distribution of DNAPL sources and dissolved-phase plumes deeper in the ground-water zone may often bear little resemblance to the shallow interface zone ground-water plume above, and hence the location of this deeper contamination will remain highly difficult to identify by soil-gas surveys. Field observations are confirmed with simple analytical 1-D diffusion modeling and the implications of the results to the practice of soil-gas surveys at contaminated sites indicated.
TL;DR: In this paper, a flexible numerical model that simulates transient fluxes of heat, liquid water, water vapor, and isotopic species is presented to simulate both infiltration and evaporation under fluctuating meteorological conditions and thus should be useful in reproducing changes in field isotope profiles.
TL;DR: In this paper, a hydrogeological study was conducted, during the 1991-1992 water year, in the clay-soil mantled portion of a limestone terrain in southern Indiana to investigate the modes of soil-water infiltration contributing to rapid transport of nitrate to the saturated zone.
TL;DR: In this article, a simulation of pre-acid rain weathering in a non-calcareous sandy aquifer at Klosterhede, Denmark, is presented, showing that pH ranges from 4.2 to 4.9 with Al concentrations of up to 0.8 mmol L{sup {minus}1}.
Abstract: Acidification of groundwater in a noncalcareous sandy aquifer at Klosterhede, Denmark, is the result of acid rain deposition. In the 4- to 5-m-thick unsaturated zone the pH ranges from 4.2 to 4.9 with Al concentrations of up to 0.8 mmol L{sup {minus}1}. The groundwater at the top of the saturated zone still has a pH below 5. Deposition of sea salt affects the solute profiles, and its importance varies both spatially from the forest margin to the inner part of the forest and temporally through seasonal variations in infiltration and dry deposition. As a result, pulses of high solute concentrations travel downward through the unsaturated zone. The cation exchange capacity (CEC) of the sediments ranges between 0.2 and 1 meq 100 g{sup {minus}1}, and in the acidified zone, base saturation is around 17%. The pore waters are close to equilibrium with gibbsite, supersaturated for kaolinite, and strongly undersaturated for other silicate minerals. Mass balance calculations on increases in dissolved silica over depth suggest that the buffering effect of silicate weathering is small. Buffering processes and solute transport were modeled with the code PHREEQM. Simulation of pre-acid rain weathering indicates that this process operates on a timescale of thousands of years,more » yielding minimum pH values near 5.2 and a base saturation of greater than 70%. The present leaching of Al{sup 3+} rich acid water from the soil yields acidification rates of 7 and 10 cm yr{sup {minus}1} for weathering of a naturally weathered and a pristine profile, respectively. Simulation of infiltration of sea-salt pulses indicates that the cation distribution quickly becomes attenuated by the exchanger composition. However, due to coupling of gibbsite equilibrium with ion exchange processes, downward traveling pulses with high solute concentrations will cause pH variations throughout the unsaturated zone by precipitation and dissolution of gibbsite. 67 refs., 16 figs., 3 tabs.« less
TL;DR: In this article, a controlled field experiment was conducted to provide data for a one-dimensional numerical model for simulating transient water flow, isotope transport, and heat transfer in the unsaturated zone (ODWISH).