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February 1996 - 5- 953-7001002 16 3. NATURE AND EXTENT OF PETROLEUM HYDROCARBONS IN ' SOIL AND GROUNDWATER The following sections summarize the extent of petroleum hydrocarbons (TPHg and BTEX) in soil and groundwater before and after remediation at the subject site 31 Extent of Petroleum Hydrocarbons in Soil Four underground fuel storage tanks and associated piping were the source of releases of petroleum hydrocarbons as gasoline at the subject site The underground tanks were ' removed in 1988 and from 1989 to 1993 the extent of the release(s)have been investigated and remedial activities have been performed The occurrence of TPHg and BTEX in soil was limited to a zone (approximately 8 foot thick) from approximately 7 to 15 feet below ground surface This zone likely represents the depths of fluctuation in the uppermost water bearing zone The lateral extent of TPHg and BTEX in soil was defined by a combination of soil borings and excavation sidewall samples (WET, 1990, 1991, 1992 and 1994) On the basis of ' this information, the lateral extent of TPHg and BTEX from the former underground storage tanks was approximately 20 feet north,95 feet northeast and east,25 feet south and 20 feet west of the original underground storage tank excavation Our review of the information ' indicates that minor concentrations of TPHg and BTEX (25 mg/kg at X-18 located approximately 70 feet northeast of the former tank excavation, 0 33 mg/kg benzene, 0 14 mg/kg toluene at S-13 located at a depth of 12 feet approximately 10 feet east of the former tank excavation and 0 32 xylenes at S-10 located at a depth of 13 feet at the south edge of the former tank excavation)may remain in soil at the site However, these concentrations are below levels of concern and our limited in extent as demonstrated by the groundwater monitoring results The maximum reported concentrations of TPHg, ethyl benzene, toluene and xylenes at the ' site were 170 mg/kg,2 3 mg/kg,0 26 mg/kg and 13 mg/kg respectively at a depth of 12 5 feet in boring BX-3 located approximately 80 feet southeast of the former tank locations The maximum reported concentrations of weathered petroleum hydrocarbons (560 mg/kg ' reported as TPHd)was in the area of the former underground tanks The maximum reported concentration of benzene was 0 9 mg/kg from a soul sample from boring BX-1 at a depth of 7 5 feet BX-1 was located approximately 20 feet south of BX-3 and hydraulically ' downgradient of the former underground storage tanks Soil in these areas of maximum reported concentrations was excavated during the remediation of the site Summary tables of soil sampling results are included as Appendix A ' 3.2 Extent oPetroleumyd ocarbon f Hydrocarbons in Groundwater s ' During removal of the underground tanks in 1988, floating product 4 to 6 inches thick was reported on the surface of the groundwater in the tank excavation Subsequently,between 1989 and 1992, a total of fourteen shallow and deep monitoring wells were installed at the site to assess the extent and nature of TPHg and BTEX in groundwater beneath the site Groundwater sampling activities have been performed since 1989 with three rounds of sampling conducted in 1995 as a condition for site closure A number of monitoring wells 9537001 R01 (3/1196) Golder Associates