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A French drain and dewatering sump were installed below the building to prevent basement flooding, and the excavation extended into and below the location of the former UST system. Another former gasoline retail site is located across the street to the north at 4001 Portola Drive. That site, Opal Cliffs Automotive (Opal Cliffs), is owned by Robert Rudolph Enterprises and has been the subject of ongoing investigation and remediation for releases of petroleum hydrocarbons since the removal of that station's USTs in 1992. The Opal Cliffs site sold gasoline from approximately 1950 through 1987, and gasoline constituents were detected in soil beneath the USTs when the tanks were removed. Gasoline contaminated groundwater was discovered in the French drain collection sump on the former Chevron site in June 1995. The sump was sampled in response to a Regional Board directive for Opal Cliffs to delineate groundwater contamination emanating from its site. Low concentrations of benzene,toluene, ethylbenzene,and xylene (BTEX)were detected, and additional delineation of the Opal Cliffs plume ensued via borings and monitoring wells installed south and southwest of the Opal Cliffs site. In the spring of 1998, the sump pump in the basement of the structure on the former Chevron site failed, and the basement flooded with water that had a strong gasoline odor. Analysis of the water revealed high concentrations of total petroleum hydrocarbon gasoline (TPH-g) and BTEX. The Regional Board directed termination of the sump discharge, which conveys its contents to storm drain inlets or other surface waters. The Regional Board assigned responsibility for investigation of the source of petroleum hydrocarbon contamination on the two sites to Chevron, Opal Cliffs, and the Walter Eller Company, as current property owner of the former Chevron site. Between 1998 and 2000, Chevron and Opal Cliffs conducted soil and groundwater investigations in order to identify the source of the contamination and to characterize the extent and magnitude of contamination affecting groundwater. In September 2000, Regional Board staff concluded that two separate plumes of petroleum hydrocarbon contamination emanated from the sites of the two former service stations, including a pre-1972 release of gasoline from the USTs at the former Chevron site. The Regional Board issued a letter order signed by the Executive Officer under authority of Water Code section 13267, directing 2.