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r Work Plan for Extended Site Ch terization: 7500 West Eleventh Street, Tracy, CA. Page 3 the stratigraphic, hydrogeologic and geochemical data that permitted an assessment of the. hydrogeologic regime beneath the subject property and adjacent areas and an initial evaluation of the characteristics and the lateral and vertical extent of the plume of affected groundwater emanating from the subsurface zones that had been affected by releases of hydrocarbons. Additional data regarding affected groundwater at the site and changes in the distribution and concentration of analytes of concern in the groundwater have been gathered from three additional rounds of groundwater-quality monitoring. Where applicable, data and findings from the site characterization and groundwater-quality monitoring programs completed to date are summarized in the following sections of this work plan to provide a basis for the selection of depths and locations of the additional borings and wells that are now proposed. 3.0 GEOLOGY AND HYDROGEOLOGY The subject property and the surrounding area are situated on level terrain on the distal, northern slope of an alluvial fan. The underlying alluvial sediments are of Quaternary to - Recent age. The site and the immediately adjacent property along the south side of West Eleventh Street and the west side of Chrisman Road have been extensively excavated and backfilled during prior filling station construction and remodeling, utility installation, and highway expansion. The first several feet beneath the surface of the site are composed of multiple layers of } bituminous macadam, concrete slabs and random fill. Beneath the paving and fill, the soils are composed of interbedded clays, silts and sands. These materials have been deposited in a complex lenticular form composed of relatively low permeability clays and silty clays inter- bedded with permeable silts and sands. The sizes of individual permeable lenses vary from ! relatively large features having considerable areal extent to small, localized lenses of limited extent and thickness. In some instances, these lenses merge into each other to form semi- continuous permeable strata within the less permeable clayey material. The depth to groundwater beneath the site varies seasonally between 7 and 11 ft. Regionally, the general direction of groundwater flow is to the north toward the Old River anastomosis branch of the San Joaquin River, the closest tributary of which, the Tom Paine Slough, is one and one-quarter miles north of the 7500 West Eleventh Street site. The local direction of groundwater flow is affected by the local sedimentary geology,particularly where continuous or semi-continuous sand strata provide channels for subsurface flow through less permeable facies. 3.1 Water Supply Wells The initial site characterization program included a search for the presence of existing on- and off-site agricultural, industrial, domestic and other wells within a 2,000-ft. radius of 7500 SIC