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Wendy Cohen - 3 - 23 October 2007 <br /> Volpi Farm Property, Stockton <br /> The plume is centered around the former 1,000-gallon gasoline AGT near MW-3. The last <br /> monitoring event was in May 2005 and consisted of sampling MW-1 and MW-3. Attachment 4 <br /> is the table of historical groundwater data. A summary of the last two monitoring events <br /> (May 2003 and May 2005) is shown below: <br /> Summary of Groundwater Monitoring <br /> Last two events were 5/03 and 5/05. <br /> Date TPHg TPHd Benzene MTBE <br /> Ng/I Ng/I Ng/I Ng/I <br /> MW-1 5/03 290 <50 <0.5 13 <br /> 5/05 <50 <50 <0.5 <1.0 <br /> MW-2 5/03 <50 <50 <0.5 <1.0 <br /> 5/05 NS NS NS NS <br /> MW-3 5/03 <50 <50 <0.5 52 <br /> 5/05 100 <50 <0.5 100 _ <br /> MW-4 5/03 <50 <50 <0.5 <1.0 <br /> 5/05 NS NS NS NS <br /> MW-5 5/03 <50 <50 <0.5 <1.0 <br /> 5/05 NS NS NS NS <br /> MW-6 5/03 <50 <50 <0.5 <1.0 <br /> 5/05 NS NS NS NS <br /> MW-7 5/03 <50 <50 <0.5 1.6 <br /> 5/05 NS NS NS NS <br /> NS- Not sampled <br /> SENSITIVE RECEPTORS <br /> In 2005, AGE conducted a sensitive receptor survey. Two water wells are within a 2,000 foot <br /> radius of the Site. One domestic well is onsite, but northwest (upgradient) of the ALTs. The <br /> nearest downgradient surface water body is the irrigation ditch along the southern portion of <br /> the Site. <br /> FATE AND TRANSPORT <br /> AGE conducted a 100 year fate and transport analysis using the RISC Workbench 4 (RISC) <br /> commercial risk assessment and fate and transport software initially developed by British <br /> Petroleum Company in 1994. RISC was used to determine potential effects on shallow <br /> downgradient wells 130 feet from the source area (equivalent to the distance to the upgradient <br /> domestic well onsite). The analysis was conducted for benzene, toluene, ethylbenzene, <br /> xylenes (collectively BTEX) and MTBE in soil and for MTBE in groundwater. <br /> The saturated soil to groundwater model predicts that BTEX and MTBE source concentrations <br /> will be below detection limits within one to two years. The dissolved source groundwater <br /> analysis was conducted to determine the effect of the MTBE polluted groundwater on any <br /> shallow downgradient well 130 feet from the source area. The model predicted that MTBE <br /> would not be detected in groundwater at this distance. <br /> SUMMARY <br /> Site structures were removed in 1999 and included one 7,500-gallon diesel AGT, one <br />