IN-SITU ENHANCED BIOLOGICAL REDUCTIVE DECHLORINATION IN FRACTURED BEDROCK AQUIFER

Implementation of a groundwater remediation program at a former solid waste disposal site located adjacent to environmentally protected public recreation area with topographic limitations can be challenging. The Yucaipa Disposal Site (YDS), located in southwestern San Bernardino County, is sited on the southern shoulder of the Crafton Hills in a horst and graben terrain. Local topography is characterized by ephemeral alluvial channels that incise the graben terraces, and groundwater flow and contaminant transport are controlled by normal faulting and fractured bedrock. The San Bernardino County Solid Waste Management Division initiated a Pilot Study at the YDS to assess the potential for enhanced biological reductive dechlorination as a full-scale in situ alternative to remediate elevated concentrations of chlorinated ethenes in groundwater (specifically, tetrachloroethene [PCE] and trichloroethene [TCE]). The study began in mid-2012 with the injection of a carbon-based electron donor substrate into groundwater. The results were promising but full dechlorination did not occur. In mid-2015, a second dose of electron donor media was injected into the groundwater, and the second dose was supplemented with a bacteria inoculant to boost the population of microbial colonies in the aquifer. Preliminary results from the second dosing event were positive and, eight months after the second injection, concentrations of PCE and TCE (which typically were reported at concentrations that were an order of magnitude greater than the California maximum contaminant level [MCL]), declined to half of the MCL and results for cis-1,2-DCE declined to near or below the MCL. Vinyl chloride concentrations have increased steadily since the second injection event, but these concentrations are expected to decrease rapidly as groundwater moves away from the landfill and into a more aerobic environment. Between April 2012 just prior to the Study, and April 2016, the total concentration of chlorinated ethenes has been reduced by 92%.