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"PomboTroperty Corrective Action Plan ' Page: 3 alluvial fans and interfan basins from a mixed rock source. Soils of the Capay Series are typically fine grained and montmorillonitic in nature. ' 2.3 Site Lithology and Groundwater Characteristics Sediments encountered in this investigation are composed of tan silty plastic clays ' which grade into a tan very fine grained, well sorted, silty sand at 15'to 20'below grade (Figure 4. Lithologic Cross-Sections). A thin shallow sandy unit (upper sand) is present within the upper silty clay. This sandy unit appears to be confined to semi-confined beneath the overlying silty clay in borings B3/9, B 10, S'% and B 11. The shallow sandy unit is not present in borings to the east and south of a° the former underground storage tanks. A coarse sand to pebble gravel was ' encountered in the upper 5 feet in boring B 12. The well sorted sand present at the base of the borings (lower sand) dives to the west with an apparent dip of ' approximately 25 degrees. Groundwater was encountered at depths of 9 to 12 feet below grade in the Geoprobj borings. All of the Geoprobe"'�' borings were terminated in the coarse ' sand unit (lower sand). Boring B-9, installed to determine the vertical extent of the contamination, passed through the thin shallow, confined to semi-confined, ' sandy unit (upper sand) and was terminated in the well-sorted sand present at the base of the borings (lower sand). '• All three monitoring wells also terminate in the lower sand. The thin shallow, confined to semi-confined, sandy unit (upper sand) is not recognizable in boring MW3. Based on depth to water measurements conducted during the July 1999 ' monitoring event, ground water flow was north 23 degrees east with a gradient if 0.0124 feet per feet on July 23, 1999 July 23, 1999 groundwater elevations and flow direction are shown on figure 3. Site Map with Groundwater Flow. ' 3.4 CONTAMINANT CHARACTERISTICS ' 3.1 Contaminants of Concern Contaminants of concern have been identified as; benzene, toluene, ethyl ' benzene, xylenes, MTBE and gasoline range petroleum hydrocarbons probably sourced from the former underground storage tank. Benzene has been identified as a carcinogen and, toluene, ethyl benzene, xylenes, are suspected carcinogens. ' A Department of Health Services action level has been established for MTBE, which is considered a potential carcinogen. Taste and odor thresholds have been established for MTBE and gasoline range petroleum hydrocarbons, although ' water quality goals have not been formally adopted. 3.2 Contaminant Distribution ' Soil contamination appears to be restricted to the immediate vicinity of the former pp Y underground storage tank and the distribution controlled by the thin shallow,