Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Glenn V. Wilson

Committee Members

Bob Luxmoore, Larry McKay

Abstract

Subsurface transport processes of low-level radioactive contaminants at hazardous waste sites are highly complex due to a vast continuum of pore regions in heterogeneous media. Contaminant transport is typified by a multitude of interacting concentration profiles that exist due to the complexity of subsurface media. Two concentration profiles are typically assumed to describe the transport of solutes in heterogeneous subsurface media. These profiles provide a means for subsurface contaminants to move rapidly through preferential flow paths associated with larger pores, or they may move slowly and continuously through smaller pore regions. During transport, however, contaminant mass is continuously transferred between the various sized pore regions via hydraulic and concentration gradients.

The objective of this study was to quantify the diffusive mass transfer of nonreactive solutes between the matrix porosity and the preferential flow paths through a heterogeneous subsurface media. The intent of this study was to provide an improved understanding and predictive capability of contaminant migration for improving remediation strategies at contaminated waste sites.

A large, undisturbed subsurface soil column (17 x 41 cm) of highly weathered fractured shale was acquired from a proposed waste site on the Oak Ridge Reservation in eastern Tennessee. The flow interruption technique was used in nonreactive tracer experiments involving the transport of Br (a ³H surrogate) under saturated conditions. The technique involved the displacement of the nonreactive tracer into the column, inhibiting tracer flow for a designated time period (interruption), and then reinitiating the tracer flow. Experiments considered tracer infusion and displacement, variations of duration of flow interruption (6, 18, 84, and 166 hours) and variations in flux (4, 41, 216, and 475 cm d⁻¹).

Displacement of Br through the undisturbed media was characterized by asymmetric breakthrough curves (BTC) that were indicative of heterogeneous flow phenomena. The rapid initial breakthrough and exaggerated "tailing" in BTCs are indicative of rapid solute transport through preferential flow zones coupled with slower transport into the soil matrix. When flow interruption was initiated during the infusion of Br, a concentration (C/C₀) decline was encountered upon flow reinitiation, indicating diffusion between the preferential flow zones and the soil matrix during the interruption period. Likewise during displacement, flow interruption resulted in a concentration rise due to Br diffusing from the soil matrix into the preferred flow channels. More pronounced concentration declines were also observed with higher fluxes (216 and 475 cm d⁻¹). A longer interrupt time (166 h) resulted in larger tracer concentration perturbations. Flux and interrupt duration had profound effects on the magnitude of concentration perturbation. A direct relationship for each was observed with respect to concentration perturbation. In addition, the interrupt duration approached a value at which the percent change in concentration after interruption became constant.

Experimental data were adequately modeled with a two region flow interrupt model. Observed effluent concentrations were adequately modeled by fitting the transfer coefficient to the experimental ETC. These results suggested that a direct relationship exists between the transfer coefficient and flux. Model simulations also suggest that interrupt duration had a negligible effect on the transfer coefficient for low to moderate fluxes.

The results of these investigations conclusively demonstrated that the flow interruption technique can be used to quantify the diffusion of nonreactive solutes in heterogeneous porous media. Results indicate that preferential flow processes are significantly greater at larger fluxes, and that the molecular diffusion of solutes is a significant transport process over a large range of fluxes. In heterogeneous subsurface systems where traditional pump and treat methods have failed because of slow contaminant diffusion, a modified flow interrupt pump and treat method may enhance the effectiveness and efficiency of contaminant removal.

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