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Workplan for Supplemental Investigation 2005 Navy Drive, Stockton, CA January 2, 2013 Page 4 of 12 2.0 Background There were four groundwater monitoring wells at the site, three of which, MW-1, MW-2, and MW-3, were screened in shallow groundwater from 12 to 25 feet below ground surface (bgs). The fourth well, MW-4, was screened in deep groundwater (from 45 to 50 feet bgs). Therefore, MW-4 is monitoring groundwater from an entirely different zone than the other three onsite wells as supported by pH and conductivity measurements (Figure 2). Well logs are attached (Ramcon, 2004). In the January 2011 groundwater samples (KR Environmental, 2011) the conductivity of water from upgradient well MW-1 was 5980 micro Siemens (mS) from MW-2 3990 mS and down-gradient well MW-3 1740 mS. Deep well MW-04 screened in a separate zone from 45 to 50 feet bgs had much lower conductivity of 190 mS. The higher conductivity in the upgradient wells is likely related to the closer proximity to the deep water channel at the Port of Stockton. Chlorinated solvents have been consistently detected in samples from shallow groundwater monitoring wells, MW-1 and MW-2, as shown on Figure 3 but not in samples from MW-3. When groundwater samples were taken from both MW-1 and MW-2, higher concentrations of cis-1, 2 Dichloroethylene (DCE) and trans-1, 2-DCE were detected in MW-2 than in samples from MW-1. The groundwater samples from the deep groundwater monitoring well, MW-4, had much lower concentrations of cis- and trans- 1,2-DCE than reported in the shallow groundwater monitoring well, MW-2 located about 30 feet from MW-4 as would be expected since MW-4 is screened in deep groundwater (45 to 50 feet bgs) relative to MW-2 (12 to 25 feet bgs). Thus, the surface proximity of MW-2 and MW-4 has no relevance to either their respective contaminant concentrations or to the groundwater flow direction. Risk-Based Decisions, Inc.