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6.. SECTION 2 Site Setting and Background L This section summarizes the site setting and background for the Disco site. This summary is �- not intended to provide a definitive description of what is known about this site,but is included to provide the reader with a brief synopsis of the site as it is currently understood by Del Monte,CH2M HILL,PHS/EHD,and the RWQCB. 2.1 Site Setting The Disco site is located near the intersection of Myrtle Street and Filbert Street in Stockton, California (Figure 1).The site is a former underground storage tank (UST) site that consisted of three 10,000 gallon gasoline tanks that were removed in 1985. Currently the site is vacant, unpaved,and fenced. Access to the site is limited to Del Monte employees. •- 2.2 Site Background The Disco site is currently owned by Del Monte. Parmaceast owned the site from mid-1960s to 1976,and operated the USTs that were associated with a former gasoline station. Del Monte purchased the property from Parmaceast in 1976, and never operated or used the USTs. In October 1985,Del Monte removed the USTs under the supervision of PHS/EHD. The tanks were filled with water at the time of removal. Soil beneath the tanks contained gasoline constituents. The soil was aerated on site to reduce the levels of volatile hydrocarbons below 1,000 mg/kg and was backfilled into the excavation (Exceltech, 1985). In 1991,under the direction of the PHS/EHD,Del Monte conducted soil and groundwater investigation in the vicinity of the Disco site that included a soil-gas survey,seven soil borings and eight monitoring wells. The results indicated gasoline constituents in soil at depths between 55 and 75 feet below ground surface (bgs),and in groundwater. The site is underlain by alluvial sediments.The soil types include clay,silt,sand,and gravel. The predominant soil types are interbedded sandy clay and silty sand. The depth to groundwater is currently approximately 47 feet bgs. w Since 1991,groundwater levels have risen approximately 30 feet (from 77 ft bgs to 47 It bgs), and the direction of groundwater flow has changed directions by almost 180 degrees. Between 1991 and July 1994, the groundwater flow direction was predominantly towards the northwest and an average groundwater flow direction and magnitude of 312°and 0.0006 ft/ft. From October 1995, through today, the groundwater flow direction has been consistently towards the southeast, at an average direction and magnitude of 143°and 0.001 ft/ft. r Prior to the switch in groundwater flow directions, the extent of the plume was defined horizontally by monitoring wells MW-DIS-3,MW-DIS-8S,MW-DIS-9,and MW-DIS-10;and ` vertically by monitoring well MW-DIS-7D and-8D. Since 1991,Del Monte has conducted quarterly groundwater monitoring and reported results to PHS/EHD. In February 1995,Del Monte initiated remediation at the Disco site by installing SFO/CADERNET/GROUPS/EBGIDELMONTE/PLANi_33A40RKPLAN-CPT.DOC 3