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Former Beacon Station No. 12474 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan extent feasible for protection of beneficial uses of groundwater is not necessary 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, the relatively stable plume can be controlled by moderate levels of remedial action. It is Horizon's professional opinion that remediation of the groundwater to maximum extent feasible for protection of beneficial uses of groundwater is not economically reasonable or is it necessary to protect human health and the environment. 9.3 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, ground waters 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 causing adverse effects on sensitive receptors such as organisms in the groundwater aquatic system, 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 MCLS for gasoline constituents are: 1 ppb benzene; 150 ppb toluene; 700 ppb ethylbenzene; 1,750 ppb xylenes; 50 ppb TPHg; and 5.0 ppb MTBE. 9.4 Remediation To Concentrations Which Maintain Plume Stability and Allows Natural Attenuation to Progress Remediation of petroleum hydrocarbons in soil and groundwater to an extent which will allow the plume to remain stable and natural attenuation to progress would likely require in- situ soil vapor and groundwater extraction and air sparging. Remediation of the 474PARCAP3-02.doc 19 Project No. 1474.23 HORIZON ENVIRONMENTAL INC.