Fractured rock sites, impacted with chlorinated solvents such as tetrachloroethene (PCE) or trichloroethene (TCE), remain a significant environmental challenge for the DoD. Efforts to apply in situ remedial technologies have often proved challenging and/or unsuccessful with respect to attaining remedial objectives in fractured rock aquifers. Contaminant rebound typically is observed due to processes such as dense non-aqueous phase liquid (DNAPL) dissolution, matrix back-diffusion, and/or release of contaminants from low permeability/bypassed fracture zones. Unfortunately, the ineffectiveness of these remedial technologies is typically not recognized until after substantial time and resources have been expended. This lack of understanding in the conceptual site model hinders effective site management.
Dr. Charles Schaefer from CDM Smith and his team from CB&I, the University of Florida, and the U.S. Geological Survey recently completed an ESTCP project in which they developed a relatively small-scale field testing approach and protocol for assessing the practical extent of remedial effectiveness that might be obtained by implementing in situ remedial technologies in fractured bedrock sites. The rapid assessment (RA) protocol involves identifying hydraulically conductive fracture zones, flushing contaminant from the fracture zones using water, then evaluating contaminant rebound within this zone while hydraulically isolating the zone from the surrounding contaminated aquifer. The rate, composition, and isotopic signature of contaminant rebound is then used to evaluate the limits of remedial effectiveness, identify the local source/cause of any observed rebound, and provide improvement to the site conceptual model.
Application of the RA protocol was demonstrated at a shallow bedrock contaminated primarily with TCE, at Calf Pasture Point in Rhode Island. Matrix back-diffusion modelling showed a significant impact in the months following flushing. In addition to the modelling, compound specific isotope analysis (CSIA) was conducted on carbon for TCE and cis-1,2-dichloroethene (DCE) further confirming the source of the rebound, as well as abiotic dechlorination of both constituents in the rock matrix. The CSIA testing served as a line of evidence demonstrating that the rock matrix was the source of the observed rebound, as well as a useful tool for confirming that abiotic dechlorination of TCE and DCE were occurring within the rock matrix. More detailed results from this demonstration are available in the project’s Final Report.
Browse SERDP and ESTCP's web site to learn more about this and other projects focused on remediation and management of contaminated fractured rock sites.