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Site Conceptual Model October 3, 2002 ARCO Facility No 6100 adjacent property is topographically upgradient of the ARCO facility, petroleum hydrocarbon impact to these wells appears unlikely A water supply well (Map ID #6) was installed in 1987for the Unocal Corporation, southeast of the intersection of Interstate 580 and Patterson Pass Road This appears to place the well cross- gradient of the ARCO facility Hennings Brothers Drilling Company, Inc reported abandoning a well (of unknown ownership to the drilling contractor) with an approximate depth of 300 feet bgs at 25501 Patterson Pass Road in 1997 Given the presence of a Tosco petroleum pipeline northeast of the subject site (possibly formerly owned by Unocal), it appears likely that the water supply well abandoned in 1997 is the same well originally installed for Unocal Because the well was situated cross-gradient of the ARCO facility, and appears to have been abandoned, it is not likely a potential receptor Five wells (Map ID's #4, 5, 7, 8, and 9) were installed northeast of the site in the vicinity of the intersection of Patterson Pass Road and Schulte Road The wells are situated cross-gradient from the ARCO facility, approximately 2,000 to 2,500 feet to the northeast Given the distance of the wells from the ARCO facility, and their cross-gradient relationship to the site, these wells appear unlikely to be impacted with petroleum hydrocarbons originating from the ARCO facility . As discussed above, the U S Geological Survey Topographic Map for the area identifies an artesian well approximately 2,000 feet southeast of the site This places the well downgradient of the ARCO facility Given that the lateral extent of impact in Sand 2 has not been adequately characterized, and the depth to regional water supply aquifer is unknown (although deeper than 200 feet bgs), it is possible that this well is a potential receptor The existence or usage of this well has not been verified 6 2.2 California Aqueduct The California Aqueduct is a concrete-lined canal used to transport water from the Sacramento- San Joaquin River delta to southern California The aqueduct is situated approximately 1,200 feet northeast of the ARCO facility, and trends generally northwest to southeast, parallel to the dip of the strata beneath the ARCO facility The nearest point the aqueduct is downgradient of the site is approximately where it crosses the 2,000-foot radius circle shown on Figure 1 Based on this configuration, the apparent dip of Sand 2 in this direction, the elevations of the base of Sand 2 and the aqueduct, and assuming the aqueduct is approximately 30 feet deep, it appears that the base of Sand 2 is approximately 190 feet beneath the base of the aqueduct It appears unlikely that hydrocarbons from the ARCO facility will impact the California Aqueduct • P IBP-Arco for URS161001Reports16100 SCM doe Page 13 Stratus