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LO Owlagical J"e&N,r-s lac Groundwater Monitoring Report page 7 Project No.724.2 May 16,2005 +.rte premise that the data we considered, although incomplete, are representative of actual site conditions. We acknowledge that there may be undiscovered conditions, which would upon their consideration,change our interpretation and thus our conclusions. V Conclusions • The groundwater plume does not display a typical "core" but exhibits wide variations in contaminant concentration in both vertical and lateral planes as shown in Figures 6 - 11. Generally the plume has migrated down gradient of the source and is displaced east of the former UST: �- o The highest MTBE concentrations detected in the intermediate and deep aquifers were in wells MW-106 (19.0 [tg/1), MW-208 (15.0 [.g/1) and MW-209 (13.0 Vg/l), respectively. o MW-7 contains MTBE (3.3 Rg/L). o The contaminated shallow, intermediate and deep wells are down gradient from LN the former UST and dispenser locations areas to the east in the predominant groundwater direction. o In addition, the highest TPH-G concentrations were detected to the east of the site in wells MW-106 and MW-208, and the highest Benzene concentrations were detected to the east and northeast in wells MW-106 and MW-108 in the r intermediate wells, and MW-208 in the deep wells, respectively. V .�.� • In their correspondence dated September 3, 2002, the San Joaquin County Environmental Health Department (SJC/EHD) identified two LUFT sites (701 E. Charter Way and 749 E. Charter Way) as being up gradient of the subject site. The ,,. contamination detected in the MW-108/208 location may be partly or entirely originated from an offsite source. • The SJC/EHD also stated in their above letter that a sandy interval at 70 — 80 bgs �.. could be acting as lateral conduit for contaminant migration. The presence of contamination in deep wells MW-208 and MW-209 support this theory. However GTI still suggests that the low concentrations of TPH-G & MTBE in these wells do *� not warrant an engineered clean up. • The data shows that a relatively stable, wide spread, low concentration plume of petroleum hydrocarbon compounds are present in the groundwater at this site. The deeper TPH-G plume is not defined to the north and the deeper MTBE plume is not defined to the east. #" Recommendations • Maintain the current quarterly monitoring schedule to determine contaminant and gradient trends. • To further define the lateral extent of the groundwater plume, the work as outlined in the April 18, 2005 letter from San Joaquin County should be performed. This work is in progress. ..