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Fourth Quarter 2001 Groundwater Monitoring and Sampling Report-CAO#5-00-707,Stockton,CA January 15,2002 Unocal Site The property located at 5606 Pacific Avenue contained a gasoline station between approximately 1955 and 1985 (Figure 1) In 1985, the station was closed and the USTs removed After completing investigations of the site, Unocal in 1988 excavated approximately 6,000 cubic yards of soil from the site The excavated soil was aerated, and based on the analytical testing results associated with the aerated soil, 5,800 cubic yards of the aerated soil was reused on-site as fill, and 200 cubic yards transported off-site for disposal On August 25, 1989, the RWQCB issued a letter for the Unocal site confirming that the soil contamination at the site has been adequately cleaned up, but the extent of groundwater impact has yet to be defined Between 1992 and 1999, additional soil and groundwater investigations associated with the Unocal site were conducted Table 1 summarizes the historical groundwater testing results for the Unocal site In February 2001, seven new wells (MW-18D, MW-23S, MW-23D, MW-24S, MW-24D, MW-25S, and MW-25D) were installed to assist in defining the vertical and lateral extent of hydrocarbon impacted groundwater at the Unocal site and around the Robinhood Irrigation Well In September 2001, four additional wells (MW-8D, MW-9, AS-26 and MW-27) were installed to further define vertical and lateral hydrocarbon impacted groundwater at the site Robinhood Irrigation Well RIW San Joaquin Delta College owns and formerly operated a groundwater extraction well identified as "the Robinhood Irrigation Well" (RIW) (a k a Delta College Well) The RIW is located southwest of the intersection of Robinhood Drive and Pacific Avenue The RIW was installed in 1950 and is reported to be 322 feet deep with perforations in multiple zones beginning at depths as shallow as 40 feet below ground surface (bgs) According to the Water Well Driller's Report for the RIW, "no strata were sealed against pollution" Groundwater from the RIW was formerly used to irrigate portions of Delta College's campus Groundwater monitoring data for the Unocal and Wards sites show that historical pumping of the RIW influenced and changed the natural direction of groundwater flow in the area from easterly, to south southwesterly, towards the RIW Sampling of the RIW by Unocal and/or Wards for dissolved petroleum constituents began in 1997 and continued through June 15, 2000 Table 1 summarizes the results of testing groundwater samples from the RIW In August 2000, Delta College students and staff complained of a gasoline odor from the water derived from the RIW As requested by the RWQCB, the RIW was taken out of service as of 6 OOAM August 22, • ni �nn1 •- l ri. r rhra i]TIAl be„tr„gtrnyar� 2000 l he RWQCB in a Letter aatea FebrUdiy cu, GVVi, requesLcu tthat t1hW R •• On April 27, 2001, the San Joaquin County Public Health Services, Environmental Health Division (PHS-EHD) approved the EAI report entitled Amended Work Plan for Destruction of Irrigation Well, San r Joaquin Delta College — Robinhood Irrigation Well, 5151 Pacific Avenue, Stockton, Calefainia, dated ® April 26, 2001 The RIW was abandoned in August 2001 240022 01-001 2