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L_(V?i%TC b-4- - A S S O C I A T E S I N C groundwater is present at approximately 35 feet below ground surface(bgs) with an east-northeasterly to easterly gradient. 3.0 BACKGROUND One gasoline UST of unknown size and all associated piping and dispensing equipment were removed and transported from the site. Laboratory analysis of soil samples collected from beneath the former UST location indicated the presence of detectable concentrations of total petroleum hydrocarbons. After the tank was removed, the area of the former tank was backfilled. In December 1996, Twining Laboratories advanced four soil probe borings (SP-1 through SP-4) in the immediate vicinity of the former UST. Analytical results indicated that petroleum hydrocarbons were detected in boring SP-1 only. The lateral extent of impacted soil appeared to be less than five feet from the former UST and extended to a depth of approximately 28 feet below ground surface (bgs). A grab groundwater sample collected from boring SP-1 indicated that groundwater was also impacted with petroleum hydrocarbons. The approximate locations of the former UST and soil borings are shown on Figure 2. On July 20, 1998, Advanced GeoEnvironmental Inc. (AGE) supervised the advancement of one soil boring (MW I)through the former UST excavation to a depth of 65 feet bgs. The boring was completed as a two-inch diameter monitoring well screened from 45 to 65 feet bgs. Analytical results of soil samples indicated the presence of petroleum hydrocarbons at 20 feet bgs but the impacted soil appeared to attenuate rapidly with depth. The monitoring well was developed and a groundwater sample was collected from the well although water was measured to be above the top of the well screen. Analytical results indicated the presence of petroleum hydrocarbons. Results are summarized in AGE's Quarterly Report-July 1998. In March 1999, ATC supervised the advancement of two soil borings to 50 feet bgs at the site and the borings were subsequently completed as groundwater monitoring wells MW2 and MW3. " Very low concentrations of toluene, ethyl benzene, and xylenes were detected in the soil sample from well MW2 at 3 5.5 feet bgs. Analytical data indicated that soil samples collected from well MW2 at 50.5 feet and from well MW3 at 35.5 and 50.5 feet contained no detectable concentrations of petroleum hydrocarbons. The initial groundwater sample collected from well MW2 contained a trace amount of toluene. The March 1999 subsurface investigation results are detailed in ATC's Summary Report of Groundwater Monitoring Well Installations, dated June 3, 1999. ATC performed quarterly monitoring and sampling of wells MWI, MW2, and MW3 in June, September, and December 1999. During this period, groundwater was measured at depths ranging from 33.96 to 37.14 feet bgs. The groundwater flow direction was east-northeast to east at a gradient of 0.002 to 0.003 ft/ft. Copies of the gradient maps generated during the monitoring events are included in Appendix B. Petroleum hydrocarbons were detected only in well MW and generally decreased over the monitoring period. No petroleum hydrocarbons were detected in w:\62574.054cporU\closurc-FS3.doc 2 rr