The purpose of this study is to estimate impacts of MTBE as well as other fuel oxygenates on California groundwater through analysis of existing data. Information on occurrence of MTBE and other oxygenates in groundwater near aboveground and underground storage tanks has been obtained from seven of the nine Regional Water Quality Control Boards (RWQCB) and from drinking water suppliers that deliver groundwater. The RWQCB information are being unified into one data base that includes data on MTBE detections and concentration as well as depth to water when available. The data are being carefully screened to eliminate any errors or ambiguities, and a random sample of the data will be checked against site-specific information to assess meaning and relability. While these data will not provide detailed, site-specific information on MTBE plume characteristics, we anticipate it will provide useful statewide indicators to assess severity of the problem.

Mandatory testing of MTBE was just initiated last year for large and small public water systems. The regulatory oversight of many small water systems, whose wells make up the vast majority of the drinking supply wells in California, is delegated to counties. All of the thirty-eight primacy agencies have been contacted, most of which were unaware of the testing requirement. All primacy agencies have been provided with a copy of the Department of Health Services implementation policy which requires that systems deemed vulnerable be tested for MTBE prior to August 31, 1998. To date, we have received cursory information from three of the primacy agencies.

The information obtained from RWQCBs and other sources including Los Angeles Water and Power, Local Oversight Program Agencies, will be used to estimate the probability of MTBE impacting local groundwater resources due to leaking fuel tanks. Modeling studies will compare potential future impacts of MTBE relative to BTEX compounds and alternative oxygenates under various geological and hydrological settings. The Department of Health Services, primacy agencies, and water purveyor information on MTBE in large and small water systems that deliver groundwater will be used to assess the degree to which MTBE has already impacted groundwater quality in a regional sense. Lastly, we are evaluating MTBE impacts in the Tahoe Basin by sampling groundwater and snow.