SITE INFORMATION AND CORRESPONDENCE_2005-2014

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• Review of the Memorandum for Recommendation to Shut Down Extraction Well EWA1, DDJC-Sharpe, Lathrop, California August 2005 GENERAL COMMENTS 1. The Recommendation to Shut Down Extraction Well EWAI makes a clear case for why EWA1 should be shut down, but it does not discuss why a replacement extraction well is not being considered for EWA1. It is noted that the groundwater from EWA1 has concentrations of trichloroethene (TCE), tetrachloroethene (PCE), and 1,1-dichloroethene (1-1-DCE) exceeding their respective aquifer cleanup levels (ACL). If contaminated groundwater that was to be captured by extraction well EWA1 is allowed to migrate 650 feet to extraction well EWCA1 at the facility boundary, this would allow additional ;ontaminafion of the aquifer that is currently not present. Please explain why a. replacement extraction well for EWA1 is not being considered to prevent additional contamination of the aquifer. 2. The screen intervals for extraction wells EWA1 and EWCA1 have about a 10-foot vertical difference. The screen interval for extraction well EWA1 is about 19.35 to 29.35 feet and the screen interval for extraction well EWCA1 is about 30 to 40 feet. Please explain if the groundwater cleanup will be affected by this approximate 10-foot vertical difference. SPECIFIC COMMENT 1. Figure 1, Location of EWA1 and Potentiometric Surface Contours in the A Zone, Second Quarter 2005, DDJC-Sharpe: The TCE ACL concentration isopleth on the figure does not extend to extraction well EWCAl, but this contradicts the most recent published data. Extraction well EWCAI is stated to have TCE above the ACL in the Second Quarter 2005 Quarterly Monitoring Report (URS, 2005). Please explain why Figure 1 does not show the TCE concentration isopleth extending to EWCA1. REFERENCES URS Group, Inc., 2005. DDJC-Sharpe Well Monitoring Program, Quarterly Monitoring Report, Second Quarter 2005 Sampling Round. July.