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I 2 SUBSURFACE CONDITIONS Subsurface descriptions, including hydrogeologic conditions and characteristics of petroleum hydrocarbon impact are contained in the following sections 1 2.1 Hydrogeology The Earthgrains facility is situated in the north-central portion of the Central Valley geomorphic province, which is bound by the Coast Range to the west and the Sierra Nevada to the east Holocene ( 10,000 years old) Basin deposits underlay the site to a depth that may reach several hundred feet These deposits, which were formed in response to erosional and transportational processes of the San Joaquin River and its tributaries, tend to be laterally and vertically discontinuous ' Soil strata at this facility, to the maximum explored depth of 90 feet, consists of interbedded, discontinuous sequences of primarily silt and clay with some fine-grained sand Lithologic logs, derived from the May 1999 CPT borings, are contained in Appendix B Generalized geologic cross sections are shown as Figures 4 through 5 Groundwater at this site generally does not occur in clean, well-sorted sands Rather, groundwater is more likely found within the finer grained silty and clayey sediments, which are relatively less permeable than sand As shown in Table 1 and Table 2, recent depths to groundwater average about 40 feet below ground surface The lowest depth to groundwater, observed during PSC's historic site monitoring, was about 60 feet in December 1992 EHD suggests that the groundwater table may have been at a depth of about 80 feet in 1978 Groundwater has consistently been evaluated to flow in a direction of l approximately N70E across the site As shown in Figure 3, groundwater flowed in this general direction (on February 5, 2003) at a gradient of about ' 2 2 x 10-3 feet per linear foot, typical of historic trends In 1994, PSC submitted the 55-feet depth soil sample from boring MW-11 for hydraulic conductivity analysis using ASTM Method D 5084 (Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter) The hydraulic conductivity of this soil sample was measured to be 3 2 x 10-5 centimeters per second (cm/s), which is typical for a silty/clayey strata Hydraulic conductivity data is contained in Appendix C 9 Remedial Action Completion Report The Earthgrams Company Facility 2651 South Airport Way Stockton California Prepared for ][it FART11CRAINS COMPANY Prepared by Philip Services Corp 11\ENV162400132\121382EG\Stockton\Stockton\RemedlalComplenonRepon clot April 14 2003