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Site Conceptual Model October 3, 2002 ARCO Facility No 6100 Delta has suggested that water present in wells MW-1 and MW-2 may be originating from the water supply well The supply well is gravel packed from 35 feet bgs, and the water level in the well is approximately 45 feet bgs It is possible that water rising up in the gravel pack could flow into sand strata encountered by the well borehole However, the presence of groundwater in well MW-2, but not in well MW-3, suggest a possible problem with this theory Sand 1 and Sand 2 both appear to intersect the supply well boring below the static water level of the well Well MW-2 is screened across Sand 1 and intersects the sand/silt contact at the base, and water is r} observed in well MW-2 The hydrograph for well MW-2 (Figure 5) does suggest a trend_ — different from the other Sand 1 wells, and may be due to influence from the supply well r However, well MW-3, situated approximately 60 feet from supply well and immediately adjacent �r to well MW-2, is screened across Sand 2 and intersects the sand/silt contact at the base, but well MW-3 has been dry since installation The possible presence of water from the supply well observed in well MW-2, but not in well MW-3, might be explained by lateral variations in permeability within the sandy strata /(s .'Q t be '(er V_45 C'.""J Pi(a), 4M � wM ref k 4.2 1 Utilities / +)w ! Electrical, natural gas, and water services are routed underground to the station building An onsite water supply well provides water to the ARCO facility Given the depth to groundwater beneath the site, it appears unlikely that subsurface utility trenches act as preferential pathways for hydrocarbon migration Human waste from the mini-mart facility is discharged to a septic leaching system situated northwest of the station building (Figure 2) Human waste from the restroom building along the eastern property boundary is collected in a holding tank located in the northeastern portion of the property, and is removed periodically by truck for off-site disposal Landscaping at the site is irrigated through 1-inch diameter PVC piping feeding a sprinkler system installed in two planters located along the southern portion of the property A Delta representative observed in September 2001 that the piping was installed immediately below the asphalt surface in the vicinity of boring B-33 However, the sprinkler system does not appear to be currently in use, and past use of the sprinkler system is unknown 4.3 Extent of Petroleum Hydrocarbon Impact 4.3.1 Extent of Residual Petroleum Hydrocarbon Impact Analytical data for soil samples collected during construction and drilling activities are presented in Appendix B Plan views and iso-concentration cross sectional profiles of the known extent of petroleum hydrocarbon impact to soil are also included in Appendix C P 18p-Arco for URS161001Reports16100 SCM doe Page 8 Stratus