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Occidental Chemical Corpor-tion Summary 2006 Annual Rev, Meeting Minutes and Semi-Annual Report Lathrop, CA - 5 - 23 October 2006 PW20-500. In addition, isocontour maps of nitrate, sulfate, chloride and TDS need to be provided. As presented earlier in Annual Review Meeting Comment No. 7, these requirements will also be included in the next revised MRP for the site. 2. In addition to added sampling for TDS, nitrate, sulfate and chloride to the MRP as mentioned in comment # 1 above, the Regional Water Board staff also requests that these constituents be added to the sampling protocol of the treatment system influent and effluent (well injection points). These requirements will also be added to the revised WDRs for the site. 3. When showing the location of wells, the Well Location Map (Figure 2.1) should show the locations of all private wells. Please provide an updated map showing all private wells in future report submittals. 4. Appendix E of the Semi-Annual Report presents the Treatment System, Permanent Wells and City of Lathrop Water District Wells Analytical Results; however, it does not provide an actual copy of the laboratory analytical reports from the certified laboratory. As per MRP No. R-5 2004-0800 (MRP), the submittal of these laboratory analytical reports is a requirement. This item was discussed with CRA by email on 29 September 2006. CRA proposed to send the Regional Water Board a compact disc with the laboratory analytical reports. Please ensure that all future Semi-Annual Reports contain copies of these reports either as a hard copy or on a compact disc. 5. Figure 4.1 shows that the injection flow rates decreased in late April 2006 by almost 100 gallons per minute per injection well. This flow loss is confirmed in Figure 4.2 by a loss of injection pressures. Field notes, however, did not note this significant decrease. Did the injection wells need to be rehabilitated? Did some of the extraction wells come off-line and if so, why? Were the decreased flows a result of system maintenance activities? Please make sure that significant events like these are noted in the field notes and discussed in the text of future reports. 6. No isocontour pollutant concentration maps were presented in the Semi-Annual Report. As per Monitoring and Reporting Program No. R-5 2004-0800 (MRP), the submittal of isocontour pollutant concentration maps for all groundwater zones is a requirement. Based on the few useable data points generated from the semi-annual monitoring program, the Regional Water Board staff understands that generating isocontour maps with only a limited amount of reportable data points per groundwater zone may not provide significant value; however, prior to eliminating them from the Semi-Annual Reports, there must be a formal request from MSRM and a concurrence from the Regional Water Board Executive Officer. Until the MRP is revised, the isocontour figures must be included in all future Semi-Annual Reports. 7. The field logs are presented in Appendix C of the Semi-Annual Report from what appear to be a print out from the sampling flow through cell module. This print out does not provide the well purge volumes as required in the MRP. Though these volumes can be calculated from the listed flow rates and times, the volumes may not be totally accurate since initial line and well purging may have occurred prior to activating the