Showing papers on "Vadose zone published in 1992"

Electrical resistivity tomography of vadose water movement

Abstract: Cross borehole electrical resistivity tomography (ERT) was used to image the resistivity distribution before and during two infiltration experiments. In both cases water was introduced into the vadose zone, and the change in resistivity associated with the plume of wetted soil was imaged as a function of time. The primary purpose of this work was to study the capabilities and limitations of ERT to image underground structure and ground water movement in the vadose zone. A secondary goal was to learn specifics of unsaturated flow in a complex geologic setting. Tomographs of electrical resistivity taken before infiltration image coarser, well-drained soils (sands and gravels) as more resistive zones, whereas finer grained soils (silts and clays), which hold more water by capillarity, are imaged as more conductive. Images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology. In the ERT images we see the effects of capillary barriers and infer differences between capillary-driven flow through fine sediments and gravity-driven flow through very permeable sediments. Images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs. ERT can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements.

Effects of Atmospheric Pressures on Gas Transport in the Vadose Zone

Abstract: Temporal variations in barometric pressure due to weather patterns may induce air intrusion into the subsurface. This air intrusion can affect monitoring activities aimed at characterizing the composition and movement of gases in the vadose zone. Expressions are presented to estimate gas fluxes due to the combined effects of Knudsen diffusion, multicomponent molecular diffusion, and viscous flow. These expressions are used to evaluate the validity of the single-component advection-dispersion equation for simulating gas transport in the presence of atmospheric pressure variations. The single-component equation provides reasonable results when used to simulate transport in media with relatively high gas permeability. Computer simulations of vertical transport at sites with homogeneous soils indicate that “fresh” air can migrate several meters into the subsurface during a typical barometric pressure cycle. Horizontal pressure gradients can develop at sites with near-surface heterogeneities. These gradients may cause fresh air to intrude meters or tens of meters into the vadose zone during a storm event.

The Application of In Situ Air Sparging as an Innovative Soils and Ground Water Remediation Technology

Abstract: Vapor extraction (soil venting) has been demonstrated to be a successful and cost-effective remediation technology for removing VOCs from the vadose (unsaturated) zone. However, in many cases, seasonal water table fluctuations, drawdown associated with pump-and-treat remediation techniques, and spills involving dense, non-aqueous phase liquids (DNAPLS) create contaminated soil below the water table. Vapor extraction alone is not considered to be an optimal remediation technology to address this type of contamination. An innovative approach to saturated zone remediation is the use of sparging (injection) wells to inject a hydrocarbon-free gaseous medium (typically air) into the saturated zone below the areas of contamination. The contaminants dissolved in the ground water and sorbed onto soil particles partition into the advective air phase, effectively simulating an in situ air-stripping system. The stripped contaminants are transported in the gas phase to the vadose zone, within the radius of influence of a vapor extraction and vapor treatment system. In situ air sparging is a complex multifluid phase process, which has been applied successfully in Europe since the mid-1980s. To date, site-specific pilot tests have been used to design air-sparging systems. Research is currently underway to develop better engineering design methodologies for the process. Major design parameters to be considered include contaminant type, gas injection pressures and flow rates, site geology, bubble size, injection interval (areal and vertical) and the equipment specifications. Correct design and operation of this technology has been demonstrated to achieve ground water cleanup of VOC contamination to low part-per-billion levels.

Estimating paleorecharge and paleoclimate from unsaturated zone profiles

Abstract: Unsaturated zone solute and isotope profiles may be interpreted in terms of past changes in recharge rate in cases where water movement can be approximated by piston flow. A model is presented which describes water movement in terms of piston flow below a surface mixing layer. Below the mixing layer, diffusion and dispersion occur in the liquid phase in the case of solutes, and in both the liquid and gas phases in the case of isotopically labeled water. Climatic events of 4–5 years duration may be preserved in the unsaturated zone for more than 50 years if the recharge rate is greater than approximately 20 mm yr−1. Longer-term fluctuations in climatic conditions (century scale) might be preserved for over 1000 years, provided the recharge rate is greater than approximately 2 mm yr−1, and the unsaturated zone is sufficiently deep. Profiles from Cyprus and northern Senegal appear to record changes in recharge rate over periods up to 400 years. The profiles are well correlated with known variations in rainfall, and fluctuations in lake levels. The model provides limiting conditions to be applied to the interpretation of profiles.

Hydrogeology Of An Unconfined Sand Aquifer And Its Effect On The Behavior Of Nitrogen From A Large-Flux Septic System

Abstract: Detailed monitoring of groundwater in a shallow sand aquifer impacted by a large-flux septic system showed that water quality in the shallow water table zone below the tile bed was similar to that of smaller-flux septic systems in similar hydrologic settings where effluent residency in the unsaturated zone was of similar duration. During residency of about one week in the 4-m thick unsaturated zone, effluent NH+ 4 was largely oxidized to NO− 3, about 75% of DOC was biodegraded, and acidity produced by the above reactions was neutralized by dissolution of calcite. Beneath the tile bed and extending laterally downgradient a distance of 80 m to the Lake Erie shoreline, a distinct plume of impacted groundwater was easily distinguished by elevated levels of electrical conductance, Cl−, NO− 3, HCO− 3, NA+, Ca2+, and K+ and by depressed levels of pH and dissolved oxygen. High NO− 3 levels that occur below the tile bed disappear, however, in the anaerobic plume core zone 10 to 70 m downgradient, apparently as a result of denitrification. The rich reserve of solid-phase organic carbon in the aquifer sediment (2.5%) probably provides much of the organic carbon for heterotrophic denitrification. This condition is in contrast to other septic system plumes in sand aquifers where high NO− 3 levels persist and where aquifer organic carbon values are much lower. Although NO− 3 is attenuated in the plume core, persistence of NO− 3 along the aerobic upper fringe of the plume demonstrates the ability of septic systems to cause significant water-quality degradation of sand aquifers when the conditions favorable for denitrification do not exist. The sharp boundary between the plume water and non-impacted water adjacent to and overlying the plume in the area 50 to 100 m downgradient from the tile bed, and the undiluted nature of non-reactive solutes such as Cl− throughout the core of the plume, demonstrates that dispersion has only a weak influence on the plume. This is consistent with dispersion studies in many other sand aquifers.

Mobility and Degradation of Organic Contaminants in Subsurface Environments

Abstract: . Introduction. Contaminant Vapors as a Component of Soil Gas in the Unsaturated Zone. Liquid Contaminants Adhering to "Water-Dry" Soil Particles in the Unsaturated Zone. COntaminants Dissolved in the Water Film Surrounding Soil Particles in the Unsaturated Zone. Contaminants Sorbed to "Water-Wet" Soil Particles or Rock Surface (After Migrating Through the Water) in Either the Unsaturated or Saturated Zone. Liquid Contaminants in the Pore Spaces between Soil Particles in the Saturated Zone. Liquid Contaminants in the Pore Spaces between Soil Particles in the Unsaturated Zone. Liquid Contaminants Floating upon the Water Tables. Contaminants Dissolved in Groundwater. Contaminants Sorbed onto Colloidal Particles in Water in Either the Unsaturated or Saturated Zone. Contaminants That Have Diffused Into Mineral Grains or Rocks in Either the Unsaturated or Saturated Zone. Contaminants Sorbed onto or into Soil Microbiota in Either the Saturated or Unsaturated Zone. Contaminants Dissolved in the Mobile Pore Water of the Unsaturated Zone. Liquid Contaminants in Fractured Rock or Karstic Limestone in Either the Unsaturated or Saturated Zone. Glossary.

VIRTUS, a model of virus transport in unsaturated soils.

Abstract: As a result of the recently proposed mandatory groundwater disinfection requirements to inactivate viruses in potable water supplies, there has been increasing interest in virus fate and transport in the subsurface Several models have been developed to predict the fate of viruses in groundwater, but few include transport in the unsaturated zone and all require a constant virus inactivation rate These are serious limitations in the models, as it has been well documented that considerable virus removal occurs in the unsaturated zone and that the inactivation rate of viruses is dependent on environmental conditions The purpose of this research was to develop a predictive model of virus fate and transport in unsaturated soils that allows the virus inactivation rate to vary on the basis of changes in soil temperature The model was developed on the basis of the law of mass conservation of a contaminant in porous media and couples the flows of water, viruses, and heat through the soil Model predictions were compared with measured data of virus transport in laboratory column studies and, with the exception of one point, were within the 95% confidence limits of the measured concentrations The model should be a useful tool for anyone wishing to estimate the number of viruses entering groundwater after traveling through the soil from a contamination source In addition, model simulations were performed to identify parameters that have a large effect on the results This information can be used to help design experiments so that important variables are measured accurately

Characterization of suspended particles collected in groundwater under natural gradient flow conditions

Abstract: Microscale Eulerian variations in the flux, mineralogical composition and size of suspended particles have been found in a contaminated sandy aquifer under natural gradient flow conditions ( ) during an 8-month study period. Particle variability has been detected along a 16-m saturated section of the aquifer at a scale of centimeters and meters in the vertical and horizontal dimensions, respectively. The average concentration of particles in groundwater varied between 1 and 40 mg/L, but high concentrations of up to 5000 mg/L were determined in specific 3-cm vertical intervals of the aquifer. The particles were primarily composed of CaCO3 (11–57%), quartz (7–39%) and clays (8–43%). Most of the particles were within the 140–3000 nm size range with size modes varying from 310 to 660 nm. The large amounts of suspended particles are considered to be related to high inputs of dissolved organic carbon into groundwater, from sewage effluents which have been used for agricultural irrigation since the early 1960s. As a result of organic matter biodegradation in the saturated zone, anoxic conditions developed and the pCO2 content of groundwater increased dramatically (pCaCO2 = 10−1.8 to 10−1.3 atm). It is postulated that part of the carbonate cement of the rocks dissolved and detrital CaCO3, quartz and clay were released as colloidal particles. Part of the clay particles could have also been transported through the unsaturated zone into groundwater after mobilization from surface layers as a result of the intermittent input of water of different chemical quality. In the prevailing anoxic conditions of groundwater at the study site (dissolved oxygen concentrations of <1 mg/L) colloidal stability is enhanced by organic matter coating of particles.

A helicopter‐borne electromagnetic survey to delineate groundwater recharge rates

Abstract: Groundwater recharge is one of the most difficult components of the water balance to measure. For this reason, electromagnetic methods have been used to infer its variability from measurements of apparent electrical conductivity. In this study, groundwater recharge was estimated at 20 sites using unsaturated zone chloride methods. Interpolation between drill sites was accomplished with the aid of a helicopter-borne electromagnetic survey (DIGHEMIV). Correlations between recharge and apparent electrical conductivity were only significant (R2 = 65%) at the highest frequency (56,000 Hz). Using these single-frequency data, variations in recharge were mapped over an area of 32 km2. Recharge, as inferred from the electromagnetic data, appears to be lognormally distributed, and varies from less than 1 to more than 50 mm yr−1. Within the study region, spatially averaged recharge can be estimated from the electromagnetic data, with an accuracy of approximately −60%, +140% (90% probability). This is comparable to the estimation accuracy when surface electromagnetic methods are used. Aerial electromagnetic methods appear very useful for identifying areas of high and low recharge over large regions.

Site Studies Of Ground Penetrating Radar For Monitoring Petroleum Product Contaminants

Abstract: Ground penetrating radar (GPR) provides a possible means of mapping hydrocarbons in the vadose zone. Results of controlled surveys in a sand test pit at The Ohio State University demonstrate conclusively that there is a clear high amplitude GPR anomaly over plastic containers containing diesel fuel and containers containing the host sand material saturated with diesel fuel. Results of surveys at a site in Northern Indiana show a correlation between the decrease of the GPR signal amplitude in the vicinity of the gasoline concentration and the presence of gasoline. A decreased amplitude of the GPR signal is present in the capillary fringe region above the water table at this site, suggesting that vapor-phase hydrocarbons may affect the propagation of the electromagnetic wave. Possible explanations for the observed high amplitude reflections over the confined hydrocarbons in the test pit and the low amplitudes in the field include: 1) the hydrocarbon product may be contained in small dispersed concentrations in the vadose zone, or 2) the hydrocarbon product may have a high loss tangent. Small dispersed concentrations of hydrocarbon product above the water table cotild cause reflecting and diffracting boundaries, resulting in many small isolated anomalies on a GPR record. A high loss tangent for the hydrocarbon product could help to explain both the observed decrease in amplitude for the electromagnetic wave propagating through the capillary fringe, and the high amplitude reflections from the saturated hydrocarbons.

Mobilization of aluminium in the wathering profiles of the African surface in Malawi

Abstract: Leaching of aluminium from the weathering profile of the African surface in Malawi was deduced from the chemistry of a vadose profile. Its loss occurs where congruent kaolinite dissolution results in saprolite collapse, leaving a weakly lateritized, sandy residual mantle. This leaching is a contemporary process, as shown by the precipitation of aluminosilicate evaporites in the dambos (seasonally waterlogged bottomlands). This paper presents the results of determinations, by ICP-OES and AAS-graphite furnace, of aluminium levels in groundwaters, examining the effects of a variety of pretreatments. Filtration (0.45 μm) substantially reduces the levels of total aluminium, showing it to be in particulate form. Subsequent acidification causes precipitation of < 0.45 μm forms of aluminium, further reducing the apparent levels to below ICP-OES detection limit. This is consistent with mobilization in organically bound forms which are destabilized at low pH, and accords with earlier work which showed that indigenous micro-organisms can cause dissolution of kaolinite. Organically bound mobilization provides an explanation for aluminium leaching at near-neutral pH in conjunction with the presence of silica in solution. The identification of this mechanism of kaolinite dissolution and aluminium evacuation from vadose profiles provides a realistic process for the levelling of extensive planation surfaces; the contingent collapse of the kaolinized saprolite would lower the interfluves, progressively narrowing the vadose zone until the landsurface approaches the planar form of the stable, regional water-table.

Vapor extraction of organics from subsurface soils is it effective

Abstract: Vapor extraction is an in situ soil-cleaning process designed to remove volatile organic compounds (VOCs) from the unsaturated (vadose) zone of soil (the zone between the soil surface and groundwater). Since the introduction of the vapor extraction system (VES) in 1984, its use has increased markedly; the VES now comprises 18% of selected remedies at Superfund sites, and this number continues to grow. The VES removes VOCs from the subsurface by providing a moving air stream that volatilizes contaminants and carries them to the surface. A review of the technology at 13 sites found vapor extraction to be very effective at removing large quantities of VOCs from the subsurface environment and significantly reducing soil concentrations. VES is very effective for removing labile fractions located in the vapor and free liquid phases or adsorbed to the external surface of the soil matrix. Studies indicate that VES will not be effective for removing contamination trapped in the interior of the soil matrix, therefore VES cannot be relied upon to return long-contaminated soils to their original condition. 16 refs., 1 fig.

Modeling Chemical Transport in Soils: Natural and Applied Contaminants

Abstract: AN INTRODUCTION TO NONPOINT POLLUTION MODELING (H. Ghadiri and C.W. Rose). Polluting Agents. Model History and Classification. Characteristics Important in Choosing Models. SORBED CHEMICAL TRANSPORT MODELING (H. Ghadiri and C.W. Rose). Hydrological Modeling. Modeling Soil Erosion, Sediment Transport and Deposition. Erosion-Productivity Models. Modeling Sorbed Chemical Transport. Comparing Some of the More Widely Used Models. Modeling Sorbed Chemical Transport. Comparing Some of the More Widely Used Models. MODELING CONTAMINANT TRANSPORT IN SUBSURFACE: THEORY AND COMPUTER PROGRAM (Andrew Barry). Reactions in Aqueous Solutions: Determination of Speciation at Equilibrium. Solute Adsorption and Exchange. Continuum Model of Multicomponent, Aqueous Phase Solute Transport. Geochemical Models. Multicomponent Transport and Reaction Models. Semi-Analytical Transport Models. MODELING SALT TRANSPORT IN THE LANDSCAPE (Peter Thorburn, Roger Shaw, and Ian Gordon). One Dimensional Non-Steady Movement of Solutes in the Vadose Zone. One Dimensional Steady State Salt Transport in the Vadose Zone. The Use of Stream Salt Mass Balance in Catchment Management.

Modeling long‐term solute transport in drained unsaturated zones

Abstract: Long-term assessment of solute transport in the unsaturated zone is an important consideration for irrigation management, pesticide management, and subsurface contaminant restoration analysis and design. Mathematical models are often used to perform such analyses. Modeling fluid flow and solute transport in the unsaturated zone typically requires solution of the nonlinear Richards equation and an advective-dispersive equation for contaminant transport as a function of time. Such solutions are possible but computationally expensive. A simplified water balance approach to solve fluid flow in shallow, drained unsaturated zones has been developed and refined over the last 15 years. The objectives of this study were to use results from a water balance model to obtain solutions for solute transport in drained, shallow water table soils, and to compare the results with solutions based upon Richards' equation. Transient soil water flux rates computed with a water balance model were used as input to a Petrov-Galerkin advective-dispersive transport model to simulate solute transport in unsaturated soils. The transport model was checked for consistency by comparison with an analytical solution. Sample simulations showed good agreement between a Richards' equation-based transport model and a water balance-based transport model. Simulations were performed to show predicted trends in water quality over 1-year periods.

Catchment planning and the nitrate issue: a U K perspective:

Abstract: The imposition of strict water quality standards by the EC had prompted renewed interest in catchment planning in the UK This article examines nitrogen cycling in rural catchments, placing particular emphasis on the delivery of nitrate to the stream system Nitrate levels in UK freshwaters are reviewed Over the last two decades increases in nitrate concentration have been noted for many rivers, both large and small; in many cases, this increase can be related to the intensification of agricultural practices Evidence is presented which shows that nitrate concentrations in many lowland rivers are close to, and in some cases exceed, EC limits for nitrate in drinking water In southeast England, only the chalk aquifer provides low-nitrate water at present, though evidence suggests that the unsaturated zone of the aquifer is already heavily polluted by nitrate The possibility of controlling nitrate loss by managing land use is discussed, including the implications of recent EC legislation; the UK's pilot N

Vadose Zone Monitoring with the Neutron Moisture Probe

Abstract: The neutron moisture probe is widely applicable to vadose zone monitoring problems which require measuring variable moisture contents. Neutron data are proportional to hydrogen density (modified by local chemistry) and sensitive to wetting fronts as well as changing volumes of hydrocarbon liquids. They cannot, however, be used to confirm contaminant chemistry, nor to detect steady-state flow. Neutron data are amenable to statistical analysis, providing a measure of the significance of data variations. Detection of incipient moisture changes at numerous monitoring locations is more practical using raw neutron data than data calibrated for moisture content because calibrations suffer from uncertainties associated with soil heterogeneities. When properly applied, the neutron probe is an effective monitoring tool as illustrated by three example applications described in this paper: (1) neutron moisture logs are used to detect subtle lithologic changes and identify monitoring horizons; (2) sequential neutron data are used to track induced saturation at a soil flushing pilot study; and (3) neutron logs from a horizontal access tube beneath a waste facility are used to pinpoint moisture anomalies.

Modelling soil water dynamics in a forest ecosystem. II: Evaluation of spatial variation of soil profiles

Abstract: This paper is the second in a series of three papers, dealing with the hydrology of a forested lowland catchment, within the context of soil acidification research. the hydrological behaviour of the unsaturated soil zone is described with the numerical simulation model SWIF. the first paper presents a site specific model calibration and discusses the implications of the hydrological behaviour for soil acidification (Bouten et al., 1992). the present paper deals with the extension of the model results from one specific site to a larger research area. a third paper gives a model description and discusses its numerical behaviour (Tiktak and Bouten, 1992). In order to evaluate the effects of the field variability of soil horizon thicknesses, the model SWIF is validated by using measured groundwater table dynamics of three sites, each having different soil horizon thicknesses. Subsequently, a sensitivity analysis is carried out in order to select the location-dependent model parameters that cause the main variation in hydrological behaviour. Finally, the spatial patterns of model results and of the location-dependent model parameters are compared. The drainage depth and the depth of the boundary between the sandy top soil and the underlying boulder clay appear to be the key parameters that cause large differences in transpiration and in the vertical distribution of root water uptake and soil water fluxes. Spatial patterns of model results, therefore, also show resemblance with the spatial patterns of these location-dependent model parameters. Simulation results for a reference location with averaged soil horizon thicknesses turn out to be beyond the 90 per cent confidence interval of the areal mean model results. This emphasizes the necessity to simulate first and then average.

Applicability of Sharp-Interface Models for NAPL Transport: 1. Infiltration

Abstract: A sharp-interface, or piston flow model is developed for nonaqueous phase liquids (NAPL) flow through the unsaturated zone. This model, which assumes saturated plug flow downward from the surface, is verified through comparison with an immiscible-phase model that includes capillarity effects in the formulation. Although significant assumptions are included in the sharp-interface model, it may be useful for initial screening and in obtaining an order-of-magnitude estimate of the time of travel of NAPL in the unsaturated zone. In the absence of data for calibration, guidelines are introduced for estimating the model's parameters.

Vadose zone monitoring system having wick layer enhancement

Abstract: A vadose zone monitoring system for detecting leaks from a contaminant storage facility or the like is disclosed in which leaks are detected in a confined radial area around at least one generally horizontal, neutron access tube disposed under the landfill. A wicking layer of material increases the leakage detection coverage area by laterally transmitting leakage to the access tube via capillary action. A clay liner may be disposed between the landfill and wicking layer for controlling the rate of leakage to prevent build-up of a pressure head large enough to force liquid through the wicking layer before it is conducted laterally to the access tube. Leak detection may be carried out by neutron moderation techniques or other moisture detection schemes for developing logs indicative of the leakage from the landfill. The wicking layer permits an entire network of access tubes or other leak detection means to be installed under substantially the entire landfill for monitoring leaks therefrom.

A Model For the Effects Of Point-Source Emission Of Aerosols On Groundwater Systems

Abstract: The effect of natural aerosols on groundwater chemistry is widely accepted, but the effects of anthropogenic aerosol emissions have received relatively little study. This sensitivity analysis examines the factors most significantly influencing the concentration of aerosols in groundwater. Processes include aerosol dispersion from a point source, deposition on the land surface , transport through the unsaturated zone, and mixing with underlying groundwater. The composite simulation model integrates well-documented models for the atmospheric, unsaturated-zone, and saturated-zone systems; models were chosen which could use the most readily obtainable or estimated input parameters. The analysis indicates that point-source aerosol contaminants possess a significant potential to affect aquifer water quality adversely. Furthermore, aerosols may have an impact on background concentrations on a regional scale. The key parameters are the source concentration, net infiltration, transport-velocity distributions in the unsaturated zone, and depth of the water table. We surmise that instances where groundwater contaminant dispersal greatly exceeds predicted travel times could be the result of aerosol contamination and not underestimations of seepage velocities or hydrodynamic dispersion. Aerosol contamination of groundwater should not be dismissed a priori.

Experimental examination of integrated soil vapor extraction techniques

Abstract: Soil vapor extraction (SVE) has been shown to be effective at removing hydrocarbons from the unsaturated zone. However, at many spill sites significant fractions of the mass are at or below the water table, in which case SVE is far less effective. To improve its efficiency in cases where gasoline is trapped below the water table, SVE can be used in conjunction with other techniques to get at that trapped mass. In the last few years the direct injection of air into the formation below the water table (i.e., in situ sparging) has become a popular technique. Another approach is to lower the water table to improve air flow in the vicinity of the trapped product. This can be accomplished either in the localized area of a groundwater drawndown cone or as the result of larger scale dewatering. In experiments conducted at the Oregon Graduate Institute (OGI), hydrocarbon spills into a large three-dimentional physical model filled with sand are being used to study the efficiencies of SVE combined with other techniques. Experiments to date have examined SVE operating as a stand-alone technique, as well as in conjunction with air sparging below the water table, dewatering of the smear zone (i.e.,more » where product is trapped as residual below the water table), and air injection into the dewatered smear zone.« less

Removal of Semivolatiles from Soils by Steam Stripping. I. A Local Equilibrium Model

Abstract: A mathematical model for the in-situ steam stripping of volatile and semivolatile organics from contaminated vadose zone soils at hazardous waste sites is developed. A single steam injection well is modeled. The model assumes that the pneumatic permeability of the soil is spatially constant and isotropic, that the adsorption isotherm of the contaminant is linear, and that the local equilibrium approximation is adequate. The model is used to explore the streamlines and transit times of the injected steam as well as the effects of injection well depth and contaminant distribution on the time required for remediation.