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Workplan for Supplemental Investigation 2005 Navy Drive, Stockton, CA January 2, 2013 Page 7 of 12 expect to find some TCE even if it were mostly degraded to DCE. In our experience, we have not seen DCE in the absence of TCE. Thus, the data suggest an offsite source for the DCE observed in groundwater beneath the site. Of course, if the DCE detected in groundwater were to have originated from an onsite source such as the wash rack area or the machine shop, evidence of CVOCs should be detected in soil gas samples from the vadose zone above the shallow groundwater. 2.2 Groundwater Flow Direction The regional groundwater flow direction in shallow groundwater is to the south- southeast based on monitoring data from nearby sites with many groundwater monitoring wells monitored over several years as reported on Geotracker and summarized in Figure 4. In particular, the Koppel Stockton Terminal north of the site has 18 monitoring wells and has consistently reported a south-southeast flow direction based on 19 measurements from 2000 through 2010. RBDI believes that the northwest groundwater gradient reported in site reports by KR Environmental is incorrect. Water level measurements need to be made on at least three wells screened in the same aquifer, in this case MW-1, MW-2 and MW-3 which are all screened from 12 to 25 feet bgs. MW-2 appears not to have been surveyed. The groundwater elevations for MW-4 should not have been used to estimate groundwater flow direction as it is screened in a deeper zone (45 to 50 feet bgs). It appears that the groundwater flow direction reported by KR Environmental was based on just two wells, MW-1 and MW-3, which likely explains the discrepancy with the more extensive data from nearby properties. The groundwater levels reported for June 2011 and re-calculated by RBDI for the subject site show that the flow is to the south as predicted. The groundwater level in monitoring well MW-1 was -2.93 or 2.93 feet below sea level and in MW-3 was -3.17 Risk-Based Decisions, Inc.