MTBE CONTAMINATION OF GROUNDWATER IN THE SAN DIEGO AND RIVERSIDE COUNTY AREAS OF CALIFORNIA

This study focuses on the contamination of groundwater in California by methyl-tertiary-butyl-ether (MTBE). MTBE is a highly flammable and volatile chemical compound (C₅H₁₂O) which has been used as a gasoline additive. MTBE contamination of groundwater results when underground storage tanks leak MTBE-enriched gasoline into surrounding areas. This poses a potential hazard to the environment and to human health, as MTBE has been demonstrated to cause hepatic, kidney and central nervous system toxicity, peripheral neurotoxicity and cancer in animals (US EPA, 1997; World Health Organization, 1998).

In order to better characterize MTBE plume behavior, we collected groundwater data and applied groundwater flow and contaminant transport modeling to estimate MTBE concentration, distribution and plume geometry for selected contamination sites in the San Diego and Riverside County areas of California (California Water Board Region 9). Hydrogeologic data from 80 leaking underground storage tank cases wereas downloaded on the Internet from the State Water Resources Control Board GeoTracker programGeoTracker. contamination levels In order to estimate MTBE plume lengths, MTBE concentrations from the source well and one or more down-gradient monitoring wells (average of 2 down-gradient wells per site) were integrated into an Excel program based on the Domenico analytical model (1987). Adjustments were made for longitudinal dispersivity, groundwater velocity and degradation rate constant for each site. the Results show that the majority of the case sites lie in the more urban areas of San Diego, where the highest concentration of MTBE was found (17,000,000 ppb; the state of California considers levels >5 ppb of MTBE to be a concern). Modeling results show that the minimum MTBE plume length was 109 feet – this plume was located east of San Diego in the El Cajon area. The maximum MTBE plume length was 4350 feet – this plume was located in the Temecula area of Riverside County. The average MTBE plume length from all the Region 9 sites was 897 feet. Since all of these geographic areas have different aquifer systems with different lithologies, our results can be compared with data from companion MTBE studies in order to better delineate MTBE plume dynamics across varying hydrogeological settings.