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' Boyett Petroleum June 29, 2001 419 South Main Street,Manteca, California Page 8 Final Report of Findings for Lateral Extent of Groundwater Contamination ' 6.1 Local Geology The geology underlying the subject site consists of alternating layers of silt and sand to ' an approximate depth of twenty-five feet(25'). The sand unit from twenty feet(20')to twenty-five feet (25') contains a groundwater aquifer. From twenty-five feet (25') to thirty feet (30') a unit of silty clay is present. The clay unit is underlain by a coarse ' sand and pea gravel unit,which contains a second groundwater aquifer. ' 7.0 Hydrology California and Federal Governments have mandated that virtually all groundwater is beneficial for current or future use, unless it can be demonstrated that no beneficial use is known. This "aquifer" is considered to be of beneficial use and as a result, must be protected from contamination. Those identified uses include agricultural and industrial, as well as tpotential use for animal or human consumption. 7.1 Local Hydrology ' Groundwater at this site is at approximately seventeen feet (17') below ground level (b.g.l). Groundwater sampling is conducted on a quarterly basis, and groundwater is measured at each sampling event. 8.0 Conclusions ' Based on the analytical data from the soil and groundwater collected to date, VVW concludes that the source of the contamination, whether off-site or on-site, has not been ' identified. Given the general groundwater flow direction and the levels of MTBE found in MW-I and MW-7 over the past several monitoring events, it is highly probable that the source of contamination is on-site. MTBE contamination at this site is significantly above the state allowable limits. To determine if the source is on or off-site, WHF believes further investigation is warranted. 1