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4 Surface drainage at the Site is directed to existing stormwater catch basins strategically located within the Site and discharged to a drainage ditch south of the Site. Manmade culverts or drainages are not present within the Site; however,newer industrial developments and residential areas have upgraded drainage systems to manage stormwater. The Tom Paine Slough is the closest surface water body to the Site and is located approximately 1 mile north(downgradient). Regionally, groundwater is divided into three water bearing zones: a deep zone below the Corcoran Clay; an intermediate semi-confined zone above the Corcoran Clay; and a shallow unconfined aquifer within approximately 25 feet of ground surface (CDWR, 1967). The two lower zones (deep and intermediate) were the primary source of groundwater pumping prior to 1951. Groundwater in the region near the Site has been degraded by overdrafting of production wells,which introduced saline/connate waters into the system. Groundwater at the Site is typically encountered within 8 to 12 feet from ground surface and flows to the north-northeast at a gradient of approximately 0.003 feet per foot (PG&E, 1989). Results from previous investigations show that shallow groundwater may be impacted from former Site operations. 3.0 SUMMARY OF RI SAMPLING ACTIVITIES AND RESULTS 3.1 Background The sampling activities described in this section were completed in close consultation with the DTSC. The research methodology was consistent with the procedures described in the Final Remedial Investigation Workplan (Parsons, 2008). A brief summary of the sampling activities conducted during the RI and the results are discussed in the following sections. 3.2 Rationale for RI Sampling Strategy The purpose of the RI sampling was to further characterize the nature and extent of the lateral and vertical impacts in environmental media at the Site associated with historic MGP and Site operations. Characterization was performed through collection and analysis of soil, soil gas and groundwater samples. This data was used to evaluate the present risk involved from Site impacts, and to determine the best potential mitigation measures for the management of these impacts. To further assess the vertical and lateral extent of Site impacts, analytes included the following: PAHs, TPH, Title 22 Metals and VOCs in soil and groundwater; and VOCs in soil gas. The RI soil borings, soil gas sampling locations and monitoring wells along with the data generated during the RI are shown in Figures 3, 4 and 5, respectively. For the purposes of this technical memorandum,these three figures have been provided as a reference in the discussion of potential data gaps to be filled at this Site. This data will be discussed in detail in the RI Report, which will be submitted subsequent to this Supplemental Site Investigation. 3.3 RI Field Activities Soil Sampling. Because the SI sampling conducted by PG&E at the Site in 1989 was conducted on a sampling grid of approximately 100 feet by 100 feet within the interior of the Site and at 50