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GeoStrategies March 31, 1997 Mr. Bob Boost Unocal Corporation P.O. Box 5115 San Ramon, California 94583 Subject: Work Plan to Install An Offsite Groundwater Monitoring Well at Former Unocal Service Station No. 0187, 437 East Miner Avenue, Stockton, California. Mr. Boost: At the request of Unocal Corporation (Unocal), GeoStrategies (GSI) has prepared this Work Plan to install one groundwater monitoring well at the above referenced site (Figure 1). The proposed groundwater monitoring well will be installed to delineate the lateral extent of hydrocarbon-impacted groundwater. The proposed well location is shown on the attached Extended Site Plan (Figure 3). This work is proposed in response to a letter from the San Joaquin County Public Health Services (SJCPHS) dated March 18, 1997. The site is a vacant lot situated on the northwest corner of the intersection of East Miner Avenue and California Street in Stockton, California. The most recent station configuration consisted of a station building with three dispenser islands, one 280-gallon waste oil and two 10,000-gallon gasoline underground storage tanks (USTs). Locations of pertinent site features are shown on the Site Plan (Figure 2). PREVIOUS ENVIRONMENTAL WORK Two 1,000-gallon gasoline and one 550-gallon waste oil USTs were removed from the southeast corner of the subject site in 1967. Data related to soil conditions beneath the USTs at the time of removal is not available. These USTs appear to have been replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs situated in a common excavation in front of the station building. The two 10,000-gallon gasoline and one 280-gallon waste oil USTs, associated product piping, hydraulic lifts, and oil/water separator were removed and the station facilities demolished in January, 1992. Petroleum hydrocarbons were not detected in soil samples collected beneath the USTs, hydraulic lifts, or oil/water separator. Total Petroleum Hydrocarbons as Gasoline (TPHg) and benzene were detected in one soil sample collected from the product piping trench between the dispenser islands at concentrations of 150 and 0.81 parts per million (ppm). This trench area was overexcavated, and a confirming soil sample collected from the base of the excavation. Petroleum hydrocarbons were not detected in this soil sample. A second 130-gallon waste oil UST was removed from the eastern portion of the property in May 1992. Petroleum hydrocarbons were not detected in the soil sample collected from beneath this UST. 7927.214 3164 Gold Camp Drive, Suite 240 • Rancho Cordova, CA 95670 • (916) 631-1314 • Fax (916) 631-1317