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' Gealo f Tec/uucs Inc. Page 7 Groundwater Motutonng Report Project No 724 2 November 12,2004 ' conditions We acknowledge that there may be undiscovered conditions, which would upon their consideration, change our interpretation and thus our conclusions ' Our recommendations are based on our knowledge of site conditions, and on the state and limitations of subsurface investigative technology ' Conclusions • This report details the third sampling event of the nine new wells installed in December 2003 and the existing ten wells 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 - 12 The highest MTBE concentrations detected in the shallow, intermediate and deep aquifer were in wells 7 s,�6j-"7 MW-7 (2 9 µg/l), MW-106 (15 9 µg11) and MW-209 (7 0 µg/l), respectively These J wells are in line 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, respectively • In its correspondence dated September 3, 2002, the San Joaquin County (P� o Environmental Health Department (SJC/EHD) identified two LUFT sites (701 and � ��, 749 E Charter Way) up gradient of the subject site The contamination detected in yes �xr L the MW-108/208 location may be from an offsite source The rationale for this z v Z conclusion is that the TPH-G contaminant found m 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 ' still suggests that the levels do not warrant an engineered clean up • The trace Toluene previously detected in well MW-11 indicates that an up gradient source has contributed to the plume beneath the subject site Additional monitoring is ' R needed to confirm this possibility • 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 • Reduce the sampling frequency of the stable wells to a semi-annual basis to achieve project cost savings (MW-1, MW-3, MW-4, MW-5, MW-6 and MW-104) � 4 c