Adaptive Remediation Approach for PCE DNAPL in Fractured Bedrock

An adaptive remediation approach consisting of shifting from an existing pump and treat system to in-situ biological and abiotic strategies for contaminant degradation is currently being implemented for remediation of a tetrachloroethene (PCE) dense non-aqueous phase liquid (DNAPL) source area and associated plume in fractured bedrock geology. Prior to implementation of the injection program PCE was present in the source area at concentrations greater than 100,000 ug/L with downgradient aqueous PCE concentrations in the 10,000s of ug/L. The impacted aquifer consists of low permeability siltstone and shale of low mobile porosity (1 to 4%).  Impacts extend from the water table (~20 ft bgs) to ~100 ft bgs; however, the majority of the source mass appears to be in the upper 45 ft.

Pump and treat operation began in the late 1980s with the installation of a 2 well extraction network with direct discharge.  Following 11 years of pump and treat operations, a revised remedial strategy was developed that outlined the path to no further active remediation.  This revised remedial strategy incorporated innovative remediation techniques for the purpose of accelerating remedial progress and achieving site closure more quickly while at the same time limiting off-site impacts. Current treatment is completed mainly with the injection of molasses to stimulate the enhanced reductive dechlorination (ERD) of PCE by the native microbial population.  Additional aggressive contaminant source area remediation was completed by enhancing local permeability through pneumatic fracturing and injection of zero-valent iron (ZVI) to accelerate the overall remediation schedule as a complementary technology with the molasses injection program. Current conditions in the source area and downgradient areas show complete degradation to ethene limiting contaminant migration out of the source area to downgradient areas. A review of the site history and evolving treatment will be presented.