NATURAL ATTENUATION OF DIESEL FUEL CONTAMINATION IN BEACH SEDIMENTS, PRUDENCE ISLAND T-DOCK, NARRAGANSETT BAY, RHODE ISLAND

The Prudence Island T-Dock at the southeast end of Prudence Island in Narragansett Bay, Rhode Island, is the site of a former Navy refueling depot dating from World War II. Closure operations during the 1980s and 1990s conducted by the Army Corps of Engineers resulted in the removal of at least seven tanks and associated pipelines, above ground treatment of contaminated soils, and an air sparging program intended to vaporize residual volatile hydrocarbons from groundwater flowing into the ocean. Despite these remedial activities, sections of the beach immediately adjacent to the T-dock are still saturated with petroleum hydrocarbons.

Our study examines the long-term changes in the character of these residual hydrocarbons, which currently appear to comprise mostly diesel constituents. Our goal is a better understanding of the roles played by both mineralization and biodegradation processes in the degradation and natural attenuation of hydrocarbons in these sediments. We are also investigating the extent to which residual contamination constitutes a risk to human health and the estuarine environment. Recent studies of historic oil spills suggest that the most toxic components of the less volatile hydrocarbons, the PAHs, degrade to methylated versions that may be equally toxic, but are not quantified in typical EPA test methods. Preliminary screens have identified some of these methylated PAHs, and additional analyses will attempt to quantify them. This data will be used to evaluate the extent to which residual contamination at this site continues to represent a risk to marine organisms and to beachgoers at the site, and contributes to the overall contaminant load in Narragansett Bay. Additional work has identified specific bacteria that are viable in the contaminated sediments; ongoing experiments are examining the role these bacteria play in natural attenuation processes.