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Site Conceptual Model October 3,2002 ARCO Facility No 6100 4 SUBSURFACE CONDITIONS 4.1 Geologic Conditions Geologic logs for the soil borings are presented in Appendix A Interpreted geologic relationships are illustrated on geologic cross sections A-A' (Figure 3) and B-B' (Figure 4) The surface traces of geologic cross sections A-A' and B-B' are included on Figure 2 Additional geologic cross sections constructed during previous investigations, and site plans illustrating the surface traces of those cross sections, are presented in Appendix C Investigations at this site have explored to a maximum depth of 202 feet below ground surface (bgs) Lithologic conditions encountered during drilling consisted of a predominantly fine- grained sedimentary unit (upper silty unit) underlain by a predominantly sandy unit (sandy unit), underlain in turn by a second fine-grained sedimentary unit (lower silty unit) In the upper silty unit, relatively non-transmissive, fine-grained sediments (predominantly silt) comprise the majority of lithologic material encountered Discontinuous zones of non-native fill material (consisting of clay and silt) were generally encountered from the ground surface to depths of up to 10 feet bgs beneath the site The upper silty unit contains several strata of relatively transmissive, sandy sediments approximately 3 to 5 feet thick that appear to form tabular-shaped lenses of limited lateral extent Minor amounts of gravel-sized clasts were observed in some of the sandy strata The sandy unit comprises two laterally continuous sandy strata with an intervening stratum of silt The sandy unit is approximately 30 to 60 feet thick The upper sandy stratum (Sand 1) is approximately 5 to 15 feet thick, and the lower sandy stratum (Sand 2) is approximately 10 to 22 feet thick Sands between approximately 80 and 90 feet bgs appear consistently cemented The intervening silty stratum appears to be 10 to 18 feet thick As depicted on cross-sections A-A' and B-13% the two sandy strata exhibit a fairly consistent lateral continuity, and appear to dip toward the southeast It should be noted, however, that as observed in the overlying silty unit, a fair degree of heterogeneity exits within the subsurface, and it is likely there is some vertical connection between Sand 1 and Sand 2 through the intervening silt stratum A strike of N171E with a dip of 5° from horizontal toward the east was calculated using the top of the sand stratum encountered at approximately 82 feet bgs in boring B-33, 75 feet bgs in boring B-35, and 77 feet bgs in boring B-38 Due to this dip, the lateral extent of the lower sandy stratum was not encountered by borings VW-18, VW-19, or VW-20, which may not have been drilled deep enough to encounter the stratum The lower silty unit consists predominantly of silt A clayey lens, approximately 7 to 20 feet thick, was encountered in borings B-24 and B-35 This clayey lens does not appear to be laterally continuous Except for a 6-inch thick sand stratum in boring B-35 (at 111 feet bgs) and an 18-inch thick sand stratum in boring B-37 (at 117 feet bgs), sand was not encountered in the P 1Bp-Arco for URS161001Reports16100 SCM doc Page 5 Stratus