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L SUD Boyett Petroleum 419 South Main Street October 27, 1999 Manteca, Californias Pae 7 thirty feet (30') a unit of silty clay is present. The clay unit is underlain by a coarse sand and pee gravel unit,which contains a second groundwater aquifer(Figure 4). 6.0 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 Lpotential use for animal or human consumption. 6.1 Local Hydrology Groundwater was encountered between sixteen(16') and eighteen feet (18') below ground level (b.g.1). A second groundwater aquifer was encountered at thirty feet L (30') b.g.l. Groundwater elevations from the September 24, 1999 sampling event were used to construct a groundwater gradient map. The calculated gradient on September 24, 1999 was 0.002 ft/ft at North 12° West (Figure 5). Table 4 provides a tabular summary of the groundwater elevations in each monitoring well. L 70 Conclusions Based on the data developed during this scope of work the following conclusions have been made: • Soil contamination in the form of MTBE at concentrations ranging from 22 ug/Kg to 200 ug/Kg is present in MW-1, MW-3, and MW-4 located in the shallow subsurface underlying the site. • Groundwater contamination is present in the form of the gasoline oxygenates MTBE, TAME, and TBA. Groundwater contamination is present in MW 1, MW-3, and NM- 4. • Soil contamination does not exist in the soil columns represented by the borings from MW-1, MW-2, MW-3, and MW4 until the soil-water interface (capillary fringe) is encountered. Given this fact, it is believed that the source of the groundwater L