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Former Beacon Station No 12419 March 18,2002 Stockton Califomia Problem Assessment Report/Corrective Action Plan 11.3 Remediation To Concentrations Which Protect Beneficial Uses of Groundwater The Central Valley Water Quality Control Plan-Third Edition dated 1994 states "Ground waters shall not contain chemical constituents in concentrations that adversely affect beneficial uses At a minimum, groundwaters designated for use as domestic or municipal supply shall not contain concentrations of chemical constituents in excess of the maximum contaminant levels " Numerical maximum contaminant levels (MCLs) are concentrations that can remain in groundwater without any adverse effect on sensitn a receptors such as organisms using the groundwater aquatic system as habitat, on people consuming those organisms or water, and on other current or potential beneficial uses, assuming that a potential utilization exists for the groundwater The Drinking Water Standards consist of California Department of Health Services (DHS) and U S Environmental Protection Agency (EPA) primary and secondary MCLs are compiled by the RWQCB in A Compilation of Water Quality Goals dated August 2000 The listed Primary Drinking Water Standard MCLs for gasoline constituents are 1 ppb benzene, 150 ppb toluene, 700 ppb ethylbenzene, 1,750 ppb xylenes, 50 ppb TPHg, and 13 ppb MTBE 11.4 Remediation to Concentrations Which Maintain Plume Stability and Allows Natural Attenuation to Progress Quarterly groundwater monitoring data indicate that the distribution of petroleum hydrocarbons in groundwater is relatively stable This condition is protective of human health because the groundwater resource that is encompassed by the groundwater plume is not being used and is not likely to be used in the foreseeable future Additionally, natural attenuation will reduce petroleum hydrocarbon concentrations, especially at the plume margins, and enhance plume stability Remediation of the groundwater to reduce the highest concentrations of petroleum hydrocarbons would further enhance plume stability and allow natural attenuation to progress at an increased rate It is likely that the stability of the plume can be enhanced by moderate levels of remedial action i 419PARCAP3-02 doc 25 Project No 1419 23 HORIZON ENVIRONMENTAL INC.