Issues

The DNAPL source zones at five contaminated industrial sites on sand aquifers were investigated using continuous cores with closely spaced (1-5 cm) sampling along these cores. Although each of the source zones is the cause of major dissolved phase contamination down-gradient (trichloroethene and tetrachloroethene), the source zones are comprised only of very thin DNAPL layers that are disconnected vertically and generally not positioned on or in visually distinct textural layers. However, these thin layers are continuous laterally and drain into conventional wells, producing DNAPL thicknesses that are misleading in terms of position and thickness of DNAPL layers in the aquifer, thereby leading to overestimates of the amount of DNAPL in the source zones.

Recent research has shown that partitioning tracers are useful in making “larger than point scale measurements” of immiscible liquid saturation of organic contaminants, soil water content, and fluid-fluid interfacial areas in subsurface systems. This article provides a review of the conceptual basis for the use of partitioning tracers for these measurements for vadose zone conditions, and outlines the procedures for field applications.

To characterize the volume of liquid hydrocarbon contaminant present in the subsurface immediately beneath and adjacent to a former fuel depot, gas-phase partitioning tracer tests were performed, and the resulting volume estimates compared with estimates acquired from core data. The two estimates were found to be similar in mean and range. These field tests demonstrated the potential utility of the gas-phase partitioning method to provide field-scale estimates of liquid volumes of immiscible contaminants in the vadose zone.

A review is presented of experimental data and modeling efforts related to surfactant effects on unsaturated flow and transport. The research cited indicates that surfactant effects on unsaturated flow and solute transport can be significant, and that additional research on surfactant effects holds promise to further advance our fundamental understanding of flow and transport processes in the vadose zone.

Surfactant solutions containing sodium dodecylbenzenesulfonate (SDBS) were applied in soil columns to determine the potential impact of surfactants on residual water and oil saturations in porous media. Test results showed low-concentration surfactant systems could reduce both residual water and oil saturations by up to 50% from their original values. Overall, observations suggested low concentration surfactant solutions have the ability to significantly alter water and oil saturations in the vadose zone, thus promoting the release of previously trapped water and oil and potentially increasing free-product recovery rates and reducing vadose zone contamination.

Natural sulfate reducing bacteria can be stimulated to remediate metals-contaminated groundwater by the injection of reactive C and nutrients. Field and theoretical modeling studies show that primary geochemical controls governing the bioremediation include mineral precipitation or adsorption of metals on the surface of hydrous ferric oxides at the site studied. These processes are strongly pH dependent; therefore, we strongly recommend the need to account for pH when evaluating the fate of metals at contaminated sites.

A time domain transmission method for determining the dependence of the dielectric permittivity of municipal refuse on its volumetric water content is developed and tested. The relationship between the dielectric permittivity and the volumetric water content of the refuse showed significant spatial variability within a small municipal landfill. However, a single calibration based on multiple samples collected throughout the landfill showed a root mean square volumetric water content measurement error of <0.04 cubic centimeters per cubic centimeter.

Numerical experiments of infiltration into layered soil profiles were used to discuss the advantages of a finite-difference, pressure head-based Richards equation that incorporates an efficient algorithm for estimating the unsaturated hydraulic conductivity between the adjacent cells of different soil layers. Comparisons were performed with other forms of the governing flow equation, each of which employed the geometric mean representation for the interlayer hydraulic conductivity. The performances of the proposed finite-difference model and a finite-element model were also analyzed. All evaluations were referred to analytical solutions of the Richards equation for the test problems considered.

We show how hydraulic properties of field soils can be estimated from controlled drainage experiments using inverse methods. Improved estimates are made when both water content and pressure head data are available. Inverse modeling, which estimates retention and conductivity functions from observed changes in pressure head and water content, can help optimize the key parameters. Rational experiment design is one product of the study, since suggested proper timing and spatial sampling of water content or pressure can help optimize data collection required for improved estimates of hydraulic properties.

The exchange of water between a cylindrical macropore and the surrounding soil matrix was investigated numerically as well as experimentally by means of controlled laboratory flow experiments. The novel design of the experimental setup allowed us to discriminate between matrix flow and macropore flow. The dual-permeability model of Gerke and van Genuchten was tested using a slightly modified expression for the mass transfer rate coefficient that depended also on the macropore conductivity.

This paper investigates contributions of periodic barometric pressure fluctuations and water table fluctuations to the natural depletion of volatile chemicals in the unsaturated zone and groundwater by way of vapor transport. Barometric effects are found to be dominant over vapor diffusion for conditions of low air-filled porosity, or for deep vadose zones. Water table effects are found to be significant only for cases of high-frequency fluctuations, such as the fluctuations due to tidal variations. In most cases, natural depletion of contaminants due to volatilization will occur over a period of decades.

Tilted stainless-steel runoff-leaching chambers packed with a gravelly loam and a marl soil were used in the field to quantify the loss of endosulfan isomers in surface runoff and leachates, in response to different rainfall intensities and water table elevations. The major loss mechanism of endosulfan was determined to be through the sediments in surface runoff. Management practices that can reduce surface runoff, and specifically reduce the production of sediment, will minimize the loss of endosulfan from calcareous gravelly or marl soils.

The simulation of heat and gases generated at underground nuclear waste repositories requires the quantification of hydraulic and thermal properties of waste isolation materials. In this study unsaturated hydraulic and thermal properties of bentonite-crushed diorite mixtures were obtained from a pressure cell as well as from heating and gas injection experiments using inverse modeling techniques. Parameter estimates from the various measurement methods were consistent with each other.

First-arriving critical refractions can limit the use of zero-offset profiling borehole ground penetrating radar for shallow water content measurement. A method of analysis is developed that allows for the identification of maximum depth below ground surface at which critically refracted waves are first to arrive on BGPR records. The method is then extended to allow for estimation of the shallow water content from first-arriving refractions.

Variations in soil water content change the chemistry of soil water through reaction between the soil solution and the sorbed phase of a chemical species. Calculations with a constant capacitance model for B indicate that the ratio of sorbed B concentration to soil solution B concentration decreases nonlinearly with decreasing water content. The main practical application of the model is to facilitate in situ measurement of B concentrations in soil water.

The active fracture model (AFM) was developed to simulate flow and transport in unsaturated fractured rock. It is shown that the model is able to mimic the fractal patterns manifested in flows observed to occur at the proposed nuclear waste repository at Yucca Mountain. Favorable comparisons were found between model predictions and measurements of carbon-14 age and fracture coating data collected in the unsaturated zone at Yucca Mountain. The AFM will be improved by incorporating the process of film flow and multifractal concepts.

A new methodology for continuous measurements of water content in deep vadose zone was developed implementing thin flexible TDR waveguides attached to the outer side of a flexible sleeve. Installing the system involves inserting the sleeve assembly into a borehole and filling the assembly with a liquid resin. Before curing, the resin forces the flexible waveguides against the borehole walls, ensuring a close fit of the waveguides to the borehole's irregular shape. Laboratory calibration experiments and full-scale field experiments showed the method to be reliable and capable of providing accurate water content measurements for deep vadose zone conditions.