REMEDIATION OF PCE DNAPL IN FRACTURED BEDROCK USING MOLASSES AND ZVI

A multi-component remedy consisting of both biological and abiotic contaminant degradation is currently being implemented for remediation of a tetrachloroethene (PCE) dense non-aqueous phase liquid (DNAPL) source area in fractured bedrock geology. The biological component consists of the injection of molasses to stimulate the enhanced reductive dechlorination (ERD) of PCE by the native microbial population. In order to more aggressively address the contaminant source and accelerate the overall remediation schedule, zero-valent iron (ZVI) was evaluated as a complementary technology with the molasses injection program.

The subject site is an active facility located in the northeastern United States. The impacted aquifer consists of low permeability siltstone and shale of low porosity (1 to 4%). PCE is present in the source area as DNAPL, and aqueous PCE concentrations in the 10,000s of ug/L are typical. The impacted zone extends 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. The injection of molasses in the source area created a strongly anaerobic (methanogenic) system and successfully supported complete dechlorination of PCE to ethene.

The full-scale ZVI injection program was implemented in November 2005, and included well installation and geophysical logging, pneumatic fracturing, and pre- and post-ZVI monitoring. Over 800 pounds of ZVI were successfully delivered to the target area via three injection wells. Monitoring of pH and ORP during and after the ZVI injection confirm the radius of influence of each well met or exceeded the design goal (25 ft). A reduction in PCE of greater than 96% was observed in the three ZVI injection wells, and a 77 to 93% reduction in PCE was observed in the four shallow source area monitoring wells. Characterization of the microbial community pre- and post-ZVI injection indicates that the ZVI enhanced anaerobic microbial activity, including that of the dechlorinating organisms. This is a positive indicator for the long term success of the combined abiotic-biotic remedy for effective and accelerated remediation of a DNAPL source area.