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' 2.1 Hydrogeology Shallow groundwater occurs within the silty clay, clayey silt, and silty to clayey sand sediments at depths ranging historically from 23 to 48 ft below ground surface(bgs) Over the past two years, depths to ' groundwater have ranged from 23 to 29 ft bgs Groundwater flow occurs toward east to southeast The average local hydraulic gradient is estimated at 0 003 ft/ft ' 2.2 Soil Quality Soil quality has been established through samples collected following removal of the former USTs, product ' distribution luxes, and during drilling of monitoring/extraction wells and soil borings (see Appendix I) Available data suggest that benzene, toluene, ethylbenzene, and xylenes(BTEX)detections have been limited to the former UST backfill and native soils immediately beneath the former tanks, extending from ' approximately 16 to 36 ft bgs Prior to initiation of the SVE system, historical maximum hydrocarbon concentrations in unsaturated soils ranged from I I mg/kg benzene(TK 1-S @ 16 ft bgs), 11 mg/kg toluene (TKI-S @ 16 ft bgs), 100 mg/kg ethylbenzene (TK2-N at @ ft bgs), 830 mg/kg xylenes (TK2-N @ 16 ft ' bgs), and 5,300 mg/kg total petroleum hydrocarbons (TPH) as gasoline(TPH-g)(TK2-N @ 16 ft bgs) Maximum hydrocarbon concentrations in post-remediation confirmation samples from unsaturated soils in ' the former tank basin area included 0 569 rng/kg xylenes (S137 @ 16 ft bgs) and 832 mg/kg TPH-g(SB7 @ 16 ft bgs) Benzene, toluene, ethylbenzene, and MTBE were not encountered at above detection limits in unsaturated confirmation samples Maximum hydrocarbon concentrations in post-remediation confirmation samples from below the water table occurred in boring S135 at 36-ft bgs These concentrations included 3 81 mg/kg benzene, 241 mg/kg ' toluene, 198 mg/kg ethylbenzene, 1,610 mg/kg xylenes, and 14,300 mg/kg TPH-g 2.3 Groundwater Quality ' Groundwater quality at the site has been established through quarterly monitoring since 1991 (see Appendix I) On-going monitoring occurs at nine on-and offsite locations Maximum hydrocarbon concentrations detected in momtormg wells over the past two years include 13 ug/l benzene(MW-5), 22 ' ug/1 toluene(MW-5), 7 4 ug/1 ethylbenzene(MW-5), 38 ug/1 xylenes (MW-5), 240 ug/I TPH-g(MW-5), and 190 ug/1 MTBE (VEW-2) These detections occur within the former tank basin area (VEW-2) and downgradient(east)of the former tank basin within the site boundary (MW-5) With the exception of a single detection of MTBE (1 3 ug/I)hydrocarbon concentrations in downgradient(east)offsite well MW-7 have remained below detection levels in the last year ' Confirmation(post-remediation) grab-groundwater samples collected from the former UST area contained toluene (maximum concentration of 0 665 ug/1), MTBE (maximum concentration of 19 6 ug/1) and TPH-g (maximum concentration of 3,930 ug/1) No other chemicals were detected in the confirmation grab- groundwater samples 2.4 Liquid Phase Hydrocarbons To date, liquid phase hydrocarbons (LPHs)have not been detected in groundwater monitoring wells at the site ' 4