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Mr Nuel Henderson, Jr, SJCPHS July 1,2003 Remedial Action Plan ARCO Station No 6100 Page 4 approximately 30 feet deep, it appears unlikely that hydrocarbons from the ARCO facility will impact the California Aqueduct SUBSURFACE CONDITIONS Geologic Conditions The site assessment studies conducted to date have explored to a maximum depth of 202 feet bgs A total of 38 soil borings were drilled, 20 of the borings were completed as vapor extraction wells VW-1 through VW-20, and seven groundwater monitoring wells were constructed and installed adjacent to the soil boring locations 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 The native soil encountered during site assessment studies 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) 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 The sandy unit is comprised of 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 is located between approximately 40 and 55 feet bgs The lower sandy stratum (Sand 2) is approximately 10 to 22 feet thick, and is located between approximately 80 and 90 feet bgs The intervening silty stratum appears to be 10 to 18 feet thick As depicted on cross-sections A-A' and B-B', 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 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 discontinuous clayey lens, approximately 7 to 20 feet thick, was encountered in borings B-24 and B-35 Except for a 6-mch 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 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 P 1Bp-Arco For URS161001Reports\Arco 6100 RAP doc