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SECTION 4 FIELD ACTIVITIES AND RESULTS much as 50 feet thick northwest of the site in the vicinity of MW-DIS-8S/8D,and thins to 10 feet in the vicinity of CPT-11 and-12 (Figure 4-3). The lateral extent of the unit south of the site does not extend east to CPT-9 and-13(Figures 4-6 and 4-7). An apparently continuous fine-grained/clay unit underlies this permeable zone. 4.2.2 Analytical Results Groundwater beneath the Disco site contains detectable levels of total petroleum hydrocarbons (TPH)as gasoline, BTEX compounds(benzene,toluene,ethyl benzene,and total xylenes),1,2-dibromoethane,also known as ethylene dibromide(EDB),and 1,2-dichloroethane(1,2-DCA). These compounds are known as lead scavengers and were a common gasoline additive prior to 1991. Tert-butyl alcohol,another gasoline additive used as an octane booster,was also detected in several groundwater samples. The results of the sampling and analysis are summarized below. Soil Soil samples were collected at discrete depth intervals from CPT borings CPT-1,-2,-3,-4, and-5 beneath the contaminant source area. Soil analytical results for petroleum hydrocarbons are summarized in Table 4-1. Petroleum hydrocarbons were not detected in ' any of the soil samples collected within the vadose zone (between 30 and 31 ft bgs)or just below the top of the water table.The soil sample from CPT-4(92-93 ft bgs) did contain petroleum hydrocarbons,however,this depth and location is within the core of the petroleum hydrocarbon groundwater plume. Soil samples from CPT-1 and-2 were also analyzed for the physical parameters wet and dry unit weight,moisture content,specific gravity, total porosity,and total organic carbon. A summary of the physical parameters collected and their corresponding sample intervals are presented in Table 4-3. Groundwater Groundwater samples were collected at discrete depth intervals from CPT borings CPT-1 through-15 within the source area,and downgradient and cross-gradient to the source area in order to assess the extent of groundwater contamination. Two groundwater samples,one shallow and one deep,were collected from each boring (except CPT-3 and CPT-13).Samples were collected from permeable zones,as determined from the CPT logs. Groundwater samples were analyzed for petroleum hydrocarbons,oxygenates,EDB,and 1,2-DCA. Table 4-2 summarizes groundwater analytical data for the CPT investigation. Groundwater plume maps for benzene and 1,2-DCA are shown on Figures 4-8 and 4-9. Cross-sectional plume maps for benzene and 1,2-DCA are shown on Figures 4-10 and 4-11. Groundwater analytical results are also posted on geologic cross-sections A-A' through E-E' (Figures 4-3 1 through 4-7). Groundwater sample results are organized below by boring location:within source area,downgradient,and cross-gradient. 1 . Source Area(Borings CPT-1,-2,-3,-4,and-5) TPH-gas and BTEX compounds were detected in all of the deeper groundwater samples(all below 90 ft bgs)collected within the source area at(Table 42;Figure 4-3). TPH-gas and BTEX compounds were detected at significant concentrations at 75 feet bgs in CPT-4,and at minor concentrations in CPT-1.All other groundwater samples collected from between 64 and 76 ft bgs were non-detect for TPH-gas and BTEX compounds. The maximum TPH-gas SFOICPT REPORT_1.DOC16D3670Q36 4-3 1