SITE INFORMATION AND CORRESPONDENCE

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Antonia Vorster - 3 - 30 July 2010 Chevron Corral Hollow and Grant UN,,,,,,Zoads Nfto" Tracy, Contra Costa County presents the historical groundwater analytical data, and Figure 2 shows the posted data for the May 2010 sampling event. Extent Defined: Yes Residual Concentration: Groundwater pollution is limited to the area around monitoring wells MW-6 and MW-12, and this pollution is below Grant Line Road. Threat to Groundwater: Grant Line Road and a commercial development sit atop the Site. Chevron identified three domestic wells within a one-mile radius of the Site. All three wells are more than 100 feet deep and have sanitary seals down to 50 feet bgs. Threat to Human Health: In September 1998, Chevron submitted a Human Health Risk Assessment (HHRA) to evaluate the release area beneath Grant Line Road and the adjacent property to the north. Potential receptors included on-site underground utility maintenance workers, future on-site construction workers, and future on-site commercial workers. Exposure pathways evaluated for the on-site maintenance worker and on-site construction worker included ingestion of soil, dermal contact with soil, inhalation of vapors from groundwater and dermal contact with groundwater. The HHRA did not evaluate the exposure pathway of soil vapor from soil because all contaminated soil was excavated. The only exposure pathway evaluated for the commercial worker was potential exposure via inhalation of vapors from groundwater. Chevron concluded that the concentrations in soil and groundwater at the Site should not result in unacceptable health risks for any of the receptors evaluated. In January 1999, the Department of Toxic Substances Control (DTSC) reviewed the HHRA and agreed that no significant risk or hazard is expected from Site concentrations. Attachment 1 is a copy of the DTSC's review letter. Summary: Chevron conducted groundwater monitoring at the Site since 1987. Groundwater pollution is limited to the area north of the former spill area. Nine of the 14 monitoring wells were properly abandoned during road construction along Grant Line Road and development of the property north of the Site. Concentrations in all the abandoned wells had been below WQOs for several quarters prior to well destruction. Three wells continue to be sampled semi-annually according to the MRP, and petroleum hydrocarbon concentrations are stable but above the WQOs for TPHg, TPHd, and benzene. In 2010, the City of Tracy will widen Grant Line Road, and the rest of the wells will be destroyed. The remaining groundwater concentrations are below Grant Line Road. In 1998, Chevron prepared an HHRA, which DTSC approved. The HHRA concluded that no significant human health risk is expected from dermal contact with soil, ingestion of soil, inhalation of vapors from groundwater, or dermal contact with groundwater to onsite construction workers, onsite commercial workers, or onsite underground utility maintenance workers. The HHRA did not evaluate the potential drinking water pathway, but the current groundwater concentrations are below Grant Line Road, and it is highly unlikely that a water supply well would be installed in this location. The HHRA also did not evaluate the pathway of soil vapor from soil because all w= contaminated soil was excavated. Based on the above evaluation, Central Valley Water Board staff concurs with Chevron that no further action is needed at this Site.