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r, I, 4.0 Brief Conceptual Site Model and Subsurface Conditions The project site is located at the Tracy Wastewater Treatment Plant in the north side of Tracy, located near the northwestern edge of the San Joaquin Valley (see Figure 1). Holocene alluvial fan and fluvial deposits eroded and dissected by streams draining from the nearby Coast Range Hills to the northwest underlie the area. Surfical soils in the region are mapped by the U.S. Department of Agriculture as Capay-Stomar-Zacharias, moderately well drained and moderately fine textured with gravel forming on alluvial fans, interfan basins and stream terraces. Capey soils are mapped as moderately slow permeability. The underlying alluvium may generally consist of weakly consolidated and irregularly interbedded sand, silt and clay deposits. These subsurface strata may show variable lateral and vertical continuity and extent (Page, 1986• Bertoldi, et al., 1991). Very large groundwater aquifers underlie the Tracy region. These aquifers can produce very large quantities of agricultural and municipal,.drinking water. Regional groundwater flow .is estimated to .flow to the north in the WWTP region. Paine Slough occurs about one half mile northeast of the site. Depth to groundwater in the plant area is about 12 feet below surface grade on the basis of current boring data. Large surface water impoundments, a new clarifier as well as large buildings and treatment equipment are used by the WWTP, and are in close proximity to the subject project site to the north, east and south. It :is likely that some seepage from these impoundments occurs and may influence the shallow aquifer and groundwater.flow direction at the site. However, there were no local groundwater flow direction data available for the project site. 4.1 Subsurface Conditions Borings were advanced to a depth. of 25 feet into native soil.. Sandy clay underlies the surface road gravel and fill soil to depths of about 10 feet. Interbedded sand, clayey sand, and sandy silt occur from depths of about 1.0 feet to 25 feet and these show vertical and lateral.variability under the site (see Figures 2, 3 and 4 and Exploratory Boring Logs): Groundwater was encountered in the shallow aquifer at depths of about '12 to 13"feet below grade in each boring, and appeared to be unconfined. One large .impoundment occurs just north of the project site. however .its influence on the local groundwater flow direction is not known. Unfortunately due to borehole collapse in some holes and the short open borehole time interval, an estimate of groundwater flow direction could not be plotted. On the basis of the dissolved TPHD distribution from the 2003 PEA, the groundwater flow direction was inferred to be north and northwest from the former underground tank .location on the basis of dissolved TPHD presence. One soil sample in the upper part of the shallow aquifer tested for permeability in 2003 in theupperportion of the shallow aquifer revealed a relatively low hydraulic conductivity(1.56 x 10- cm/sec). Page 4 W'9