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Groundwater Monitoring ARCADIS Report for the Semiannual Period from January 1 through June 30,2012 Uncoln Center,Stockton, California approximately 50 and 250 feet east of the existing well cluster comprised of wells MW-108A, MW-2058,and MW-301 C.The purpose of the first cluster is to monitor the placement and short-term effects of the edible oil substrate(EOS), injected into the subsurface via the ERD injection wells to induce reductive dechlorination in hydraulically downgradient areas.The second well cluster is used to monitor long-term effects of the ERD remedy in the A-zone and the natural attenuation of contaminants in the deeper B-zone sediments. Phase V IRA EOS injections were completed in January and February 2008 and documented in LFR's"Substrate Injection Completion Report,'dated April 30, 2008 (LFR 2008). Samples for ongoing performance monitoring of the Phase V IRA were collected using PDBs as part of the June 2012 event as described in Section 2.2, and analytical results are included in Table 2. A review of the Phase V IRA monitoring data indicates that concentrations of PCE have remained relatively stable at well MW-122A-U, located within the ERD injection area, although TCE and cis-1,2-DCE concentrations increased significantly at this well (i.e., by one or more orders of magnitude)approximately eight to nine months following the FOS injection event.A significant decrease in PCE and corresponding TCE and cis-1,2-DCE production has been observed at well MW-124A-L beginning approximately 30 months following the EOS injections. However, PCE concentrations in this well have rebounded since the last annual monitoring event,increasing from 190 pg1I in June 2011 to 440 pg/I in June 2012. Well MW-123A-L,located downgradient from the ERD injection area,exhibits a decreasing trend in PCE with minor cis-1,2-DCE production. However, concentrations of PCE at well MW-121A-U have generally increased over the monitoring period. Concentrations of TCE and cis-1,2-DCE have also increased steadily at this well beginning approximately 15 months following the EOS injection event. Concentrations of PCE in the B-zone wells have either decreased(MW-214B) or remained relatively stable(MW-2158). Significant decreases in PCE and corresponding cis-1,2-DCE production have been observed at MW-2148 starting approximately three years after the EOS injection event.However, PCE concentrations in this well have rebounded since the last annual monitoring event, increasing from 120 pg/I in June 2011 to 420 pg/I in June 2012. Note that EOS amendment injections were not administered in the B-zone aquifer. m< wk GW/ 912 EMON750&a 10