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Site Conceptual Model October 3, 2002 ARCO Facility No 6100 lower silty unit The lower silty unit extends to at least 202 feet bgs, the total depth explored Soil samples collected from 152 feet and 202 feet bgs in boring B-32/MW-1 were submitted for physical analysis These samples were reported to have relatively low permeability values of 6 5 x IO 'centimeters per second (cm/s) and 2 4 x 10-6 cm/s, respectively Previous cross-sections constructed using data collected through 2000 (Figures 5 and 6, Appendix C) do not depict Sand 1, Sand 2, or most of the discontinuous sand strata in the upper silty unit These cross-sections are generally parallel to Figures 3 and 4 It appears that the sampling method (18-inch split spoon samples collected at 5-foot intervals) employed during drilling of the borings depicted in earlier cross-sections missed most of the sand strata Continuous sampling conducted during the last phase of assessment provided a more thorough understanding of the subsurface It has previously been postulated that bedded sediment beneath the site would likely dip in the same direction as local sedimentary bedrock The nearest sandstone outcrop was found approximately one mile west of the site Bedding in the outcrop was measured to have a strike of N15°E, with a dip of 19° from horizontal towards the east-southeast The regional surface topography in the vicinity of the site also dips toward the cast and southeast (Figure 1) The differences in dip measured beneath the site (N17°E, dipping 5° southeast), and at the bedrock outcrop one mile away, are likely caused by deformation related to formation of the adjacent Coast Range 4.2 Hydrogeology Wells VW-1 through VW-20, MW-2, MW-4, MW-6, and MW-8 are screened into or across Sand 1 Wells MW-1, MW-3, MW-5, MW-7, and MW-9 are screened into Sand 2 Well construction details are summarized in the tables at the front of Appendix A Historical groundwater monitoring data (Appendix D) indicate that wells VW-1 through VW-6, VW-14, MW-1, and MW-2 either contain, or have contained in the past, rechargeable quantities of groundwater The groundwater in these wells appears to be perched Wells VW-1 through VW-6, VW-14, and MW-2 are screened between 15 and 63 feet bgs, within Sand 1 Due to the variations in drilling and soil sampling techniques employed over the course of subsurface investigation at this site, it is not possible to accurately correlate lithologic units between all wells that have rechargeable groundwater Sand intervals identified by continuous soil sampling in borings B-32 through B-38 (and confirmed through electric logs) are not reflected in the lithologic logs prepared earlier in the investigation utilizing conventional hollow- stem augers and split spoon sampling However, wells VW-2, VW-3, and VW-4 are close enough to cross-section B-B' (Figure 4), that when projected onto B-B', it appears that these wells are constructed with the bottom several feet of the well placed below the contact between Sand 1 and the underlying silty stratum Well P 1Bp-Arco for URS16I001Reports16100 SCM doc Page 6 Stratus