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Ms.Devra Lewis • June 7,2001 Page 5 discussed below. The locations are proposed based upon our current understanding of the conditions at the site. Monitoring wells will provide water-level data to assess the Site-specific direction and gradient of groundwater flow and water quality data to be used in characterizing the extent of VOC-affected groundwater. Drilling activities will be conducted in accordance with San Joaquin County regulations and permitting requirements. Monitoring well depths will range from approximately 38 feet to 85 feet below ground surface and will be constructed of 2-inch diameter PVC casing and screen. 3.3.1 Proposed Monitoring Well Locations and Rationale The rationales for the monitoring well locations are presented in this section. Proposed locations are based on historical groundwater elevation data, groundwater VOC and lithologic data from the CPT investigation, and accessibility of private property for installation of permanent monitoring wells. The majority of the private property north of the site is agricultural so wells are proposed to be located along unimproved roads or right-of-ways to minimize interference with farm equipment and operations. LOCATION A Two monitoring wells, MW-9 and MW-10, will be sited at Location A, approximately 1,000 feet north- northeast of MW-6. These wells are intended to monitor groundwater elevations and groundwater quality in the Upper and Middle Horizons. See Section 3.3.3 for well depths and screened intervals. This location is downgradient of MW-6, the current well with the highest concentrations of chloroform and carbon tetrachloride in the Upper Horizon. Location A is also positioned between CPT investigation locations CPT-6 and CPT-5 which reported some of the highest concentrations of chloroform and carbon tetrachloride in samples from the Upper and Middle Horizons. Wells at Location A will allow for further evaluation groundwater flow trends north of the site and evaluation of offsite VOC trends. The CPT investigation results did not report detectable concentrations of VOCs in the Lower Horizon at locations CPT-5 and CPT-6,therefore installation of a well in the Lower Horizon is not proposed at Location A. LOCATION B Monitoring wells MW-11 and MW-12 will be sited at Location B, approximately 2,400 feet northeast of MW-6, and are intended to monitor groundwater elevations and groundwater quality in the Upper and Middle Horizons. See Section 3.3.3 for well depths and screened intervals. Location B is east of CPT investigation location CPT-8 and is intended to provide consistent lateral and vertical definition of the VOCs in groundwater in the Upper and Middle Horizons. The CPT investigation results did not report detectable concentrations of carbon tetrachloride in the Lower Horizon at locations CPT-8,therefore installation of a well in the Lower Horizon is not proposed at Location B. All chloroform detections reported for the Lower Horizon in the CPT investigation were less than MCLs. LOCATION C Monitoring wells MW-13, 14, and MW-15 will be sited at Location C, approximately 3,000 feet north- northeast of MW-6 and northeast of CPT investigation location CPT-10, and are intended to provide consistent lateral and vertical definition of the VOCs in groundwater in the Upper, Middle, and Lower Horizons. See Section 3.3.3 for well depths and screened intervals. Concentrations of carbon tetrachloride detected in groundwater samples from all three zones at CPT-10 exceeded the MCL of 0.005. The CPT locations to the north,northeast, and northwest of CPT-10 did not report any detectable concentrations of carbon tetrachloride. Chloroform concentration in groundwater samples from all three zones at CPT-10 were less than MCLs and lower concentrations were reported for samples from the locations downgradient (CPT-A,CPT-B, and CPT-Q. The presence of active agricultural operations prevents the proposed Location C from being sited any farther north. HAPioneer-Tracy\WorkP1an2001\workplan.doc SECOR International Inc.