SITE CLOSURE OF PETROLEUM HYDROCARBONS IN A FRACTURED FRANCISCAN BEDROCK AQUIFER USING NATURAL ATTENUATION

Natural attenuation, which is an innovative and non-invasive process, has been show to be an effective form of remediation of petroleum hydrocarbons in aquifers comprised of granular sediment. A dissolved phase petroleum hydrocarbon plume was evident from monitoring wells completed within fractured bedrock of the Franciscan Formation. The fractures controlled the groundwater flow and contaminant fate and transport at the site.

Evaluation of specific "lines of evidence" was conducted to determine the viability of natural attenuation as a remediation technique. "Lines of evidence" demonstrating the existence of natural attentuation include stabilization of the contaminant plume, a reduction in the contaminant concentrations, and a positive trend in intrinsic bioremediation indicator parameters. Indicator parameter data were collected for the evaluation and included dissolved oxygen, oxidation-reduction potential, nitrate, sulfate, ferrous iron, alkalinity, methane, hydrogen, and manganese.

Results of the evaluation demonstrated that stabilization of the plume and a reduction of the contaminant concentrations were occurring within the fractured bedrock aquifer. Intrinsic bioremediation was aiding in maintaining the stability of the hydrocarbon plume and reducing the mass of contaminants. This was shown by a progression from aerobic to anaerobic conditions as oxygen was depleted in areas of highest hydrocarbon concentrations, and indicator parameter distributions that closely followed the trend of the hydrocarbon plume. Additionally, indicator parameters measured in unimpacted wells demonstrated a propensity to attenuate the plume, should migration occur.

The evaluation showed natural attenuation as a viable remedial alternative for the petroleum hydrocarbon plume. Consequently, the regulatory community concurred that natural attenuation was a viable means for site closure.