SITE INFORMATION AND CORRESPONDENCE

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25775 PATTERSON PASS A - 3 - � 14 March 2000 intersecting the aqueduct are routed under or over the aqueduct, exclusively, and the aqueduct in this area is concrete lined with panels extending 30 feet in depth. In addition, the aqueduct is east of Highway 580, and away from steep slopes. During the 8 March 2000 site visit, I documented that the aqueduct is located east of Highway 580 on level ground. The potential for impacts from the ARCO site is unlikely. The ARCO site overlies Tertiary age sandstone and siltstone deposits of the Great Valley Sequence. The deposits have been upturned in the area of the subject site, and depositional bedding planes dip eastward toward the San Joaquin Valley. Groundwater recharge occurs west of the site in an unnamed valley that is bounded to the west by the San Joaquin fault zone (USGS Map MF-1761, Bartow, 1985), The hydrogeology of the area is therefore structurally controlled, and groundwater flow is toward the San Joaquin Valley. Based on the lack of MtBE data, the concentration, extent, and volume of TPHg and BTEX constituents is soil, and the proximity of contamination to an unsealed drinking water well, ARCO's request for site closure is not appropriate at this time. ARCO has not shown that site geology prevents the migration of contaminants, or that further vertical and lateral migration of hydrocarbons will not occur. Based on the well construction data from the Sparks well, first water may be encountered at 150 to 190 feet bgs, or at approximately 150 feet above MSL. Therefore, ARCO's soil contamination may be within 30 feet of groundwater. Although ARCO's on-site water supply well is screened below the identified site contamination, the well does not have a sanitary seal, and hydrocarbons could also contaminate groundwater by entering the well above the screened zone. Further work is also needed to determine whether MTBE is beneath the site. ARCO estimates 32,537 gallons of TPHg remain in soils, but the amount of MTBE in this mass has not been determined. ARCO must complete an investigation to determine the lateral and vertical extent of petroleum hydrocarbons and fuel oxygenates in groundwater. The groundwater investigation must include sampling groundwater from the uppermost-saturated unit and analyzing the sample for petroleum hydrocarbons and fuel oxygenates. ARCO should utilize their existing vapor wells for sample collection prior to completing additional drilling work. All work is.to be completed according to the requirements of Appendix A of the Tri-Regional Recommendations for Preliminary Investigation and Evaluation of Underground Tank Sites.