SITE INFORMATION AND CORRESPONDENCE_FILE 1

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Occidental Chemical Cor, ation Summary 2006 Annual Review Meeting Minutes 10 and Semi-Annual Report Lathrop, CA - 2 - 23 October 2006 treatment system's run efficiency at approximately 97%, gradient control being maintained in all three water bearing zones, and discussed the reduction in groundwater remedial influent fumigant and sulfolane concentrations compared to last year's (2004) concentrations. The Regional Water Board concurred with the reported system run efficiency, gradient control and the reduction in system influent fumigant and sulfolane concentrations compared to 2004 operations. 2. Because of the high fumigant and sulfolane groundwater concentrations in monitoring well PW18, the Regional Water Board staff raised the question to whether all of the fumigant source areas in the soils had been adequately addressed. MSRM stated that the source areas should have been addressed by previous excavation and groundwater pumping activities; however there may still be a source area near this well. The Regional Water Board staff requests this potential source area be investigated. 3. During the April 2006 meeting, MSRM stated that the CZA was not approved and instead the Regional Board staff approved a system pumping optimization work plan (SPOWP). The SPOWP specified routine system shutdown periods with corresponding hydraulic and chemical compound monitoring. Implementation of the SPOWP was based on the stipulation that a proposed sentry monitoring well cluster (PW-25) would be installed and sampled to evaluate plume migration toward the City of Lathrop municipal wells. Based on recent email correspondence from CRA, received on 25 September 2006, the well cluster was installed during the week of 18 September 2006 and the SPOWP program was to be initiated on 2 October 2006. Based on this system shutdown period, the Regional Water Board staff would like to remind MRSM that the approved shutdown period is for one hydrogeologic cycle only. Future system shutdowns will be considered following evaluation and analysis of environmental responses to this system optimization test. 4. Regional Water Board staff discussed the need to continue efforts to revise tentative waste discharge requirements (WDRs) dated 6 June 2000 due to the age of the current WDRs (1983) and the revised Water Quality Control Plan (Basin Plan), Central Valley Region, for the Sacramento River and San Joaquin River Basins. The revised Basin Plan designates all groundwaters within the Sacramento River and San Joaquin River basins as having beneficial use for domestic and municipal water supplies. Since beneficial uses of these groundwaters include their use as a potential drinking water supply, the narrative toxicity objectives from the Basin Plan apply in developing discharge limits for 1,2-dibromoethane (EDB), dibromochloropropane (DBCP) and sulfolane in the revised tentative WDRs. As part of the narrative toxicity objectives, Cal/EPA's one-in-a-million incremental cancer risk for drinking water and the Office of Environmental Health Hazard Assessment's (OEHHA) Public Health Goals (PHGs) were used as applicable WDR limits for EDB (0.0097ug/L) and DBCP (0.0017 ug/L), respectively. However, since the current reliable laboratory detection limits for these constituents are reported at 0.01 ug/L, this detection level was decided to be the lowest practical monthly average limit for both constituents in the revised tentative WDRs (a daily maximum of 0.05 ug/L was set for both constituents). The Regional Water Board staff also set the monthly average discharge limit of sulfolane at 20 ug/L with a daily