Influence of Methanogenic Bacterial Degradation of Petroleum Hydrocarbons on Mobile Lnapl

A case study is presented highlighting changes in geochemical conditions induced by the anaerobic degradation of petroleum hydrocarbons (PHCs) that enhanced the coalescence of dissolved phase PHCs into light non-aqueous phase liquid (LNAPL). Forensic analysis detected the presence of weathered light, middle and heavy weight PHCs in groundwater and LNAPL samples that were consistent with straight run gasoline, diesel fuel and crude oil. Dissolved phase concentrations migrated over 1,500 feet through dense silt and clay alluvium and mobile LNAPL was observed in recovery wells bisecting the migration path. The combined concentration of PHCs in soil, groundwater and vapor phases exceeded the Soil Saturation Concentration (Csat) literature values for gasoline and diesel fuel in fine sand and silt, indicating the presence of immobile LNAPL. Combined PHC concentrations, however, were well below the LNAPL Saturation Concentration (Cres) literature values for gasoline, diesel fuel and crude oil PHCs in fine sand, silt and clay soil. The ample storage capacity available for PHCs, particularly in soil, suggests that the presence of LNAPL in the central area of the site did not migrate there as a contiguous plume, but rather formed from the enhanced coalescence of dissolved phase PHCs. The findings of subsurface investigations performed to confirm a conceptual site model (CSM) developed for the long range migration of dissolved phase PHCs and the formation of isolated pockets of LNAPL are presented. Also discussed are the results of a pilot test involving the injection of an anionic surfactant and brine solution that simulated similar geochemical conditions associated with the coalescence of PHCs to LNAPL that resulted in the recovery of approximately 80 gallons of LNAPL downgradient of the treatment area.