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groundwater samples indicated that the wells downgradient of the tank field were impacted by petroleum hydrocarbons. TPH-g and benzene were detected in MW3 at concentrations of 940 micrograms per liter (pg/L) and 9.0 µg/L,respectively. Groundwater analytical results are presented in Table 2 (AEGIS, 13 September 1991). • June 1992: Three 10,000-gallon steel USTs and associated product piping were removed ` from the site. The USTs were in fair condition, with only slight corrosion observed. Soil samples were collected from native soil beneath both ends of each tank and from beneath the product piping (Figure 3). Laboratory analysis determined that soil beneath each tank, one old product line, and one new product line was impacted by petroleum hydrocarbons (Table 1). Approximately 300 cubic yards of soil were excavated and stockpiled onsite (AEGIS, 29 October 1992). • March 1993: The approximately 300 cubic yards of excavated soil from the UST removal +.. was used as backfill in the excavation (AEGIS, 9 September 1993). ' a May 1993 : Seven soil borings were drilled to depths ranging from 15 to 60 feet bgs by L- AEGIS and completed as 4-inch-diameter groundwater monitoring and vapor extraction wells(MW4-MW7 and VEW1—VEW2). Petroleum hydrocarbon concentrations were found only in soil samples collected between 10.5 and 35.5 feet bgs in boring VEW2 within the former UST backfill (Figure 3). TPH-g was detected in groundwater samples collected from MW5—MW7 at concentrations ranging from 71 µg/L to 350 gg/L. Benzene was detected in water samples collected at 17 gg/L in MW5 and 4.2 gg/L in MW6 (Table 2) (AEGIS, 9 September 1993). Quarterly groundwater monitoring and sampling has been performed at the site since October 1991. Groundwater monitoring and sampling was performed by AEGIS from October 1991 to December 1993, by FUGRO West, Inc. (FUGRO) from February 1994 to January 1996, and by �. EA from April 1996 to present. During the most recent sampling event on 15 July 1998, groundwater samples were collected and analyzed for TPH-g, BTEX, and MTBE (EA, 1998). Historical and current groundwater sample analytical results are presented in Table 2. 6W 2.4 DISTRIBUTION OF PETROLEUM HYDROCARBONS IN SOIL The distribution and estimated lateral and vertical extent of petroleum hydrocarbons in soil are r shown in Figure 3. A vertical profile of the subsurface geology and hydrocarbon distribution based on the findings of the previous investigations by AEGIS is shown in Figures 4 and 5. The impacted soil appears to be limited to the former UST backfill and native soils immediately beneath the former tanks. Petroleum hydrocarbons in the native soil extend from approximately 16 to 36 feet bgs. 2.5 DISTRIBUTION OF PETROLEUM HYDROCARBONS IN GROUNDWATER w The distribution of TPH-g, BTEX, and MTBE in groundwater during the most recent sampling event on 15 July 1998 is shown in Figure 6. BTEX and MTBE were not detected at ro 137.WPM.CLAs.WP.WPo 4 r..