Release of volatile chlorinated solvents in the form of dense nonaqueous phase liquids (DNAPL) to the environment has resulted in soil and groundwater contamination at many government and private sites. The presence of DNAPL is particularly prevalent at Department of Defense (DoD) installations and Department of Energy (DOE) facilities. The low aqueous solubility and complexity of movement and entrapment resulting from aquifer heterogeneity cause DNAPLs to persist as long-term, sources of contamination. Because of the long-term costs associated with pump-and-treat schemes, interest has increased in the application of aggressive free-phase removal technologies. However, properly validated tools are not available to make decisions on which technology is most effective at a specific site and how long such treatment methods should be implemented to reduce risk to human health and the environment. There is a need for improved knowledge on the effects of treatment on mass transfer from entrapped DNAPL sources undergoing remediation.

The objectives of this project were to understand, quantify, and model mass transfer from source zones in heterogeneous aquifers where DNAPLs are undergoing physical, chemical and biological transformation during remediation.

Aqueous Phase Bypassing in DNAPL Source Zone Due to Reduced Relative Permeability

As the methodologies to investigate characterization and prediction involve DNAPL sources that undergo physical, chemical and biological transformation, it is necessary to understand the tracer interaction and mass transfer during remediation under controlled conditions. Hence, the approach combined batch and bench-scale experimentation and controlled experiments in intermediate-scale laboratory soil tanks. First, the hypothesis that the tracer partitioning and mass transfer coefficients change during chemical and biological transformations was tested. Transformations associated with bio-treatment, surfactant enhanced dissolution, in situ chemical oxidation, and thermal treatment were studied and quantified. The data generated in columns and in two-dimensional test cells were used to develop methods to upscale the "point-scale" mass transfer coefficients to multi-dimensional flow conditions encountered in the field. Experiments conducted in large soil tanks generated an accurate data set to obtain insight into tracer behavior and mass transfer in heterogeneous systems with complex DNAPL entrapment architecture. In the final phase, data from the tank experiments was used to validate numerical modeling tools and upscaling methodologies.

The basic scientific knowledge gained in this research and the developed prediction tools and site characterization methods will help the DoD to make decisions on managing sites and to conduct cost/benefit analysis on the selection and implementation of different treatment technologies. (Project Completed - 2007)