Evaluation, reduction, and management of risk at Department of Defense chlorinated solvent contaminated sites pose distinct technological challenges. To reduce containment costs and achieve contaminant mass reduction within acceptable time frames, several alternative treatment technologies have been developed. Despite documented successes in reducing mass within the source zone, no current technology removes all dense nonaqueous phase liquid (DNAPL) contaminants, even under optimum conditions. From a regulatory perspective, the residual contaminant mass may pose significant risks to human health and the environment.
This project developed and assessed a suite of tools to assist site managers (1) quantify the effect of DNAPL source zone treatment on downstream contaminant flux in heterogeneous aquifer systems, (2) evaluate the potential for microbial reductive dechlorination following source zone treatment by surfactant flushing, (3) design monitoring strategies to quantify near source zone contaminant mass flux and estimate flux uncertainty; and (4) conduct cost-benefit analyses for alternative source zone treatment methods.
The research involved a multidisciplinary integration of laboratory, field, and modeling studies. DNAPL recovery and contaminant elution studies were conducted in two-dimensional (2-D) cells designed to simulate heterogeneous saturated formations. Batch and column experiments explored if microbial processes contribute to chlorinated ethene reductive dechlorination in a source zone subsequent to surfactant flushing. Experimental results were used to develop and validate a multiphase remediation simulator for applications to field-scale source zone mass removal estimation and post-treatment contaminant mass flux predictions in a variety of settings. Experimental and modeling data, in conjunction with geostatistical techniques, was used to develop and evaluate sampling protocols for quantification of downstream mass flux and its uncertainty. Cost estimation tools also were developed for comparing five commonly implemented DNAPL source zone treatment technologies and the baseline pump-and-treat case.
Experiments demonstrated that significant reductions in contaminant mass flux (two orders-of-magnitude) can be realized after only moderate mass removal (e.g., 50%) from heterogeneous sandy porous media with surfactant flushing. The relationship between mass flux and mass recovery was found to depend on the DNAPL distribution as characterized by the ganglia-to-pool ratio. Results suggest that the coupling of aggressive mass removal technologies with in situ bioremediation offers considerable promise for source zone remediation and successful long-term plume management. A geostatistical approach for the quantification of mass discharge uncertainty and a multi-stage spatial sampling design algorithm were developed. Numerical simulations suggest that model-directed sampling of 2-3% of a transect area can provide an accurate model of uncertainty for contaminant mass discharge.
This project contributed to an improved understanding of the impacts of DNAPL source zone treatment and the development of reliable methodologies to predict and monitor plume development and attenuation after source zone treatment under heterogeneous conditions. Results provide site managers with tools and protocols to assess the effectiveness and cost-benefit potential of DNAPL source zone treatments. (Project Completed - 2008)