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r i C A M B R I A Ms.Lori Duncan September 9, 2005 • Current closure guidelines from the regulatory agencies, such as the California State Water Resources Control Board criteria for low-risk groundwater cases,or • Application of Best Available Technology based on remediation system operation data that demonstrate asymptotic levels have been achieved for chemical concentrations in soil and/or groundwater. Groundwater Clean-up Levels The site resides within the San Joaquin River Basin. The RWQCB — Central Valley Region's September 1, 1998 Water Quality Control Plan (Basin Plan)for the Sacramento River and San © Joaquin River Basins states, in section II-2.00: Unless otherwise designated by the Regional Water Board, all ground waters in the region are considered as suitable or potentially suitable, at a minimum, for municipal or domestic water supply (MUN), agricultural supply (AGR), industrial service supply (IND), and industrial process supply(PRO). In section III-10.00,the Basin Plan states: Ground waters shall not contain chemical constituents in concentrations that adversely affect beneficial uses. At a minimum, ground waters designated for use as domestic or municipal supply (MUN) shall not contain concentrations of chemical constituents in excess of the maximum contaminant levels (MCLS) specified in Title 22 of the California Code of Regulations. The Basin Plan does not require improvement over naturally occurring background concentrations. Cambria presents below the COCs, background levels, and water quality objectives/California MCLS to protect designated beneficial uses of the water body based on Central Valley RWQCB guidelines for fuel releases. We also present the SSTLs determined by RBCA analysis (Attachment D). The table also contains the maximum detected concentrations for the site. Because the MTBE detected in the former UST area is below the California primary MCL of 5 ppb, MTBE is not evaluated as a COC in the RBCA analysis. 9