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GeoHWrologic Consultants, Inc. Based on the drilling logs, sediments beneath the Site consist mostly of clay, silt, sandy silt, silty clay,sandy clay, and clayey silt from the surface to approximately 65-70 feet bgs. At this depth layers of sand consisting of silty sand, fine to coarse sand,and gravelly sand were encountered interbedded with finer grained soils to a total depth of approximately 115 feet bgs. The Central Valley is a large groundwater basin which is bordered by the Klamath Mountains to the north, by the Tehachapi Mountains to the South, by the Coast Ranges to the west, and by the Sierra Nevada Mountains to the east. Groundwater was encountered at a depth of approximately 50 feet bgs in 1988 within the silty clay sediments. Well MW-3 was installed to 50 feet bgs and wells MW-I and MW-2 were installed to a depth of approximately 60 feet bgs. Because only two wells contained significant quantities of water,the hydraulic gradient could not be measured at that time. During drilling in November 1992,groundwater was encountered 80 feet bgs, indicating a significant drop in the local water table had occurred. Because of this decrease in groundwater elevation, none of the shallow monitoring wells contained groundwater and only wells MW-3R,MW-4, MW-5,and MW-6 were available for groundwater monitoring. The static groundwater level has continued to fluctuate between 1992 and present. Results of quarterly monitoring show a steady rise of the water table through 1993 and 1994. The hydraulic gradient has generally been 0.001-0.003 feet per foot to the east through this period. During drilling in 1995,groundwater was encountered at a depth of approximately 60 feet bgs. The hydraulic gradient was measured to be about 0.003 feet per foot to the northeast. During the 41h Quarter 2010,depth to groundwater beneath the Site ranged from 42.65 feet bgs in monitoring well TB-10 to 46.13 feet bgs in monitoring well MW-6. Groundwater was estimated to be flowing towards the east-northeast with an approximate gradient of 0.016 feet per foot. 3.0 WORKPLAN FOR SOIL VAPOR SURVEY AND HEALTH RISK ASSESSMENT Soil vapor samples will be obtained from ten locations(SV-1 through SV-10) in areas shown on Figure 2 located along the southern property line and eastern property line and next to the station building. The soil vapor probe locations were selected in order to evaluate the vapor intrusion risk to the surrounding area to the south and east of the Site, and near the station building. A soil probe survey permit will be obtained from the County if required to conduct the vapor survey. Soil vapor samples will be collected at a depth of 5 feet bgs from each of the sampling locations by a state of California certified mobile laboratory. A truck- Best 171 Workplan for SVS-HRA Page 2