On February 29, 2000, detection of methyl tert-butyl ether (MTBE) at a concentration of 3.7 micrograms per liter (µg/L) was detected in the groundwater at production Well 118, operated by Rancho California Water District (RCWD). A data base search on Geotracker located several leaking underground storage tanks (LUST) within a 3000 ft radius of Well 118, and are suspected to be the main cause of the MTBE contamination found in the production well. Four capture zone models were generated using a geohydrology computer model WhAEM2000, based on a high and low hydraulic conductivity and flow rate range. The models were created on a base map to show the extent of each capture zone in relationship to the gas stations. Three leaking underground tanks are within the capture zone area, Former Delta Discount, Chevron, and 76 Station. Site characterizations indicated high concentrations of MTBE at the Former Delta Discount Gas and the 76 Station. Chevron had an unauthorized release in 1984 that contaminated the soil and groundwater. The contamination was cleaned up, and the monitoring wells have remained non-detect since then. Therefore, Chevron was eliminated from this investigation. From capture zone model 4, a contaminant front was delineated around the production well, which indicated it would take approximately thirteen years for the MTBE plume to travel from the 76 Station, and the Former Delta Discount using the parameters in that model.
