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8.0 EVALUATION OF CONTAMINANT PLUME ATC constructed four cross sections to graphically present the previous and new site data. Boring logs, well construction details, and analytical data from the site were incorporated into ' these intersecting site cross sections. The locations of cross sections are shown on Figure 2. Cross Sections A-A'. B-B', and C-C' (Figures 4, 5, and 6, respectively) generally run east-west; ' cross section D-D' (Figure 7) runs generally north-south and ties the others together. The boring logs for SB 1, SB2, and SB3 are included as Appendix B. ' From the surface to a depth of approximately 32 feet bgs, the soils encountered at the site consisted primarily of sands (including silty sand and clayey sand) interbedded with clays ' (including silty clay and sandy clay) and silts (including clayey silt). The sands generally appear to be lenticular and limited in lateral extent. The environment of deposition of these sediments was ' probably that of a meandering stream. The "local upper aquifer" is separated from the "local lower aquifer" by a clayey to silty clay horizon which appears to range in thickness from approximately 2 feet to over 10 feet. Groundwater has been encountered at approximately 7 to ' 17 feet bgs in the upper aquifer and approximately at 8 to 29 feet bgs in the lower aquifer. The horizontal distribution of petroleum hydrocarbons in groundwater at the site is on Figure 3 for the soil borings installed at the site over the last year and a half. The horizontal distribution of ' petroleum hydrocarbons in the site groundwater monitoring wells has been shown in the past Quarterly Groundwater Monitoring Reports for the site. The data shown on Figure 3 and in past ' Quarterly Groundwater Monitoring Reports shows that MTBE is the only significant contaminant of concern at the site. Benzene concentrations in groundwater at the site have generally been very low to non-detectable. MTBE concentrations have declined from a high of 24,000 milligrams per ' liter (ugA) in MW-2 in December 1998, to a recent high of 5,500 ug/l in MW-2 in June 2000. MTBE concentrations at the site generally have been declining. The fact that no significant ' BTEX concentrations have been detected at the site indicates that the release, or releases, responsible for the concentrations detected in groundwater, was not large. ' Because MTBE appears to be the sole significant contaminant of concern at the site, the vertical pp g ' distribution of MTBE concentrations have been shown overlaying Cross Sections A-A', B-B', C- C'and D-D' (Figures 8, 9, 10, and 11, respectively). The most recent MTBE concentrations in groundwater from the current and past investigations are shown on these figures. Figures 8, 9, ' 10, and 11 show that the concentrations of MTBE in groundwater decrease with depth at the site, and are generally well defined laterally. The up-gradient extent of MTBE concentrations has not ]:W Pr*Vm\QU[KSMMS 132(0003WP0R7SU00 KAddDnU M-&c 13 ATC Associates Inc.