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1 SITE CLOSURE EVALUATION REPORT Ultramar Station No. 696 2448 West Kettlennan Lane Lodi, Califomia Delta Project No. D093-916 Page 6 7.0 RATIONALE FOR CLOSURE The Water Quality Control Plan, Central Valley Region, Sacramento River and San Joaquin River Basins (WQCP) dated December 1994, by the CRWQCB contains guidelines for evaluating sites for regulatory closure. The WQCP states in Chapter 4, Section 9 (g) that ground water cleanup levels are to be based ' on background concentrations, water quality objectives, risk to human health and the environment, and the technologic and economic feasibility of attaining background concentrations. Only two of the ground ' water monitoring wells (MW-24 and MW-25) at the site have contained detectable concentrations of petroleum hydrocarbons since November 1992. No ground water samples collected at the site since ' August 1994 have contained detectable concentrations of petroleum hydrocarbons, indicating that background concentrations have been attained and no further ground water remediation is needed. The WQCP states in Chapter 4, Section 9 0) that soil cleanup levels are to be based on background concentrations of the individual constituents unless background levels are technologically or economically infeasible. It further states that where background concentrations are infeasible to achieve, soil cleanup ' levels are to be established to ensure that remaining constituents of concern will not cause ground water to exceed the applicable ground water cleanup standards or pose a threat to human health or the environment. It would appear that Ultramar and the previous site owners have expended considerable ' resources and have utilized the best available technology to remove significant concentrations of petroleum hydrocarbons from the soil underlying the site; however,reducing concentrations of petroleum ' hydrocarbons in soil to background concentrations does not appear to be economically feasible. Because the site is capped by asphalt pavement, the petroleum hydrocarbons which remain in soil are present at ' depths of approximately 20 feet or greater, and the concentrations of benzene and toluene in soil are less than analytical detection limit levels, it is not likely that the petroleum hydrocarbons which remain would ' pose a significant threat to human health. Since naturally occurring biological degradation of petroleum will continue to reduce the cbncentration of petroleum hydrocarbons in soil, it is not likely that significant ' impacts to ground water quality will.occur in the future.. The Porter-Cologne Water Quality Control Act in Section 13267(b) provides that the Regional Water Quality Control Boards may require dischargers to submit technical or monitoring reports "provided that RPT134.S!S