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Kennedy/Jenks Consultants r� ATTACHMENT A WORKPLAN FOR REMEDIATION OF EXCAVATED SOILS AT 102 S WILSON WAY STOCKTON, CALIFORNIA Background As reported in a City of Stockton Fire Prevention Bureau Tank Removal Permit, two 7500-gallon gasoline underground storage tanks (USTs) were removed from the site at 102 S Wilson Way, Stockton, CA in December 1978 (See Figure 1) After removal of the tanks, the excavation was backfilled to grade and paved. In April 1988, a site assessment consisting of three soil borings (TB-1, TB-2, and T13-3 as shown on Figure 2) was conducted by Balbi & Chang Associates (BCA) to determine the presence or absence of petroleum hydrocarbons in the soil at this site (BCA, 1988) A maximum concentration of 196 mg/kg of Total Petroleum Hydrocarbons as gasoline (TPH) was found in the soil from TB-1 at a depth of 2 feet near the area of the tank excavation In an attempt to remediate this site, soils were excavated to a depth of about 4 feet and stockpiled on site The stockpile and open excavation stili remain, and are the primary focus of this workplan BCA's first site assessment was performed without regulatory agency oversight, therefore BCA was required to conduct a follow-up investigation (BCA, 1989) in April 1989 that consisted of three additional sail borings which were converted into groundwater • monitoring wells Subsequent to BCA's April 1989 site investigation, two other site investigations were conducted by Le--dshill-Herkenhoff (LH) in January 1990 and September 1990 These investigations resulted in a total of 8 additional soil borings and 3 additional monitoring wells (LH, 1990a and LH, 1990b) Representative analytical data collected from LH's soil borings are shown on Table 1 High levels of hydrocarbons were detected in soils located at a depth of about 15 feet in the northwest corner of the site Drill cuttings from both BCA's and LH's site work were segregated by soil boring/well and are currently labelled and contained in 55-gallon drums on the property BCA drums are labelled BC1 to BC9 LH drums are labelled by the associated soil boring number Subsequent to these investigations, soils from the drums filled by SCA and the stockpile of excavated soils have been analyzed for TPH and/or benzene, toluene, ethyl benzene, and xylenes (BTEX) The analytical results of these samplings indicated -[race to non-detectable levels of TPH and BTEX as shown on Table 1 On 24 September 1992, four sidewall samples were collected from the existing excavation. Samples were obtained by digging into the face of the excavation sidewall with a trowel to a depth of 6 inches The vertical location of the sampling point was approximately 2 feet below grade near the center of the excavation sidewall face The trowel was cleaned with trisodium phosphate and triple-rinsed with potable water between samples Analytical results from the previously drilled and sampled soil borings in the . excavation (TB1 and TB7) delineated the vertical extent of the soils contain.ng residual TPH's to a depth of two feet Because of these results, samples from the floor of the $1 mpURosk wp,wakplan 1 9105557 00