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' G eologual Tcluus IncPage 7 Groundwater Monitoring Report Project No 724 2 March 29,2005 Our recommendations are based on our knowledge of site conditions, and on the state and limitations of subsurface investigative technology ' 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 The highest MTBE concentrations detected in the intermediate and deep aquifers were in wells MW-106 (23 0 µg/1) and MW-209 (8 8 µg/1), respectively ' MW-7 contains MTBE (3 0 �xg/L) The contaminated shallow, intermediate and deep wells are down gradient from the former UST and dispenser locations areas to the east in the predominant groundwater direction 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 intermediate wells, and MW-208 in the deep ' wells, respectively • 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 from an offsite source The rationale for this conclusion is that the TPH-G contaminant found in this well does not corroborate with the MTBE plume that seems to be migrating down gradient in an east-southeast direction, not in an east-northeasterly direction • 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 1still suggests that the levels 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 ' • Perform the work as outlined in the November 24, 2004 letter from San Joaquin County(in progress) 4.0 LIMITATIONS ' This report was prepared in accordance with the generally accepted standard of care and practice in effect at the time Services were rendered It should be recognized that definition 1