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June 1996 -5- 953-7001002 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 3.1 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) has 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 (WHF, 1990, 1991,1992 and 1994) 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 soil 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 in Appendix A On the basis of this information, the lateral extent of TPHg and BTEX from the former 1 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 residual concentrations of TPHg and BTEX remain in soil at the site Table 2 presents a summary of final confirmation soil samples that contained detected concentrations of petroleum hydrocarbons It is expected that some soil remaining in the immediate area of these samples contain petroleum hydrocarbons The mass of petroleum hydrocarbons remaining in soil at the site cannot be reasonably estimated However the concentrations of residual petroleum hydrocarbons are low (less than 25 mg/kg TPHg and less than 0 33 mg/kg benzene) and the data suggests that the extent of the remaining petroleum hydrocarbons is limited and that areas of higher concentrations of petroleum hydrocarbons have been excavated Soil above the depths of these samples was excavated and based on data from boreholes at the site (WHF, 1992), it is unlikely that the petroleum hydrocarbons extend to depths greater than approximately 16 feet bgs Because four of the five final confirmation samples with detected petroleum hydrocarbons were within approximately 20 feet of the former UST sources it is also unlikely that significantly higher concentrations of petroleum hydrocarbons are present in the vicinity of the former USTs A fifth confirmation sample with detected petroleum hydrocarbons, X-18, was collected in the excavation sidewall approximately 60 feet NE of the former USTs However, at this location,benzene, the primary constituent of concern was not detected 9537601 R01(713196) Golder Associates