SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

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Unocal • -2- 14 April 2006' ' 5400-5606 Pacific Ave., Stockton : B. Deletion of the following analytes from the MRP since the monitoring wells "...have no concentrations above screening levels for the groundwater-to-indoor air pathway." l)n-Butylbenzene, 2) sec-Butylbenzene,3) 1,2-Dichloroethane,4)Ethylbenzene, 5)Isopropylbenzene, 6)Methyl tert-Butyl Ether(MtBE), 7)n-Butylbenzene, 8)Toluene, 9) 1,3,5-Trimethylbenzene, and . 10)Xylenes. C. States that "For the remaining wells, it is recommended that the specific constituents showing concentrations above the screening levels be monitored until the concentration has been below the screening level for four consecutive monitoring events... ". D. Requests reducing Summa Canister soil gas monitoring from the vapor extraction wells from quarterlyto annually,during the second quarter of the calendar year. Field instruments would be used for quarterly soil gas monitoring. Report 1 Comment: 1)The Report recommendations and conclusions for groundwater monitoring do not consider Regional Board Water Quality Goals(WQGs)and Beneficial Uses of groundwater at this site. Specifically,TableA-1 screening levels exceed WQGs for petroleum hydrocarbons. Site groundwater has Municipal and Agricultural Beneficial Uses,and the Report does not recommend screening levels that are protective of those Beneficial Uses. Also. there are nearby sensitive receptors that were not considered in the Report(e.g., Delta College Irrigation Well). Therefore, I cannot concur to the Report recommendations for changes to groundwater monitoring at this time. 2)I do concur to reducing the Summa Canister soil gas monitoring to annually, as long as the field instrument monitoring can be used to accurately calculate the mass of petroleum hydrocarbons removed by the Soil Vapor Extraction system. If achieving accuracy with field instruments is problematic, then Summa Canister soil gas monitoring will revert to the current schedule. Report 1 Recommendations: In a phone conversation with ENSR on 3 August 2005, I recommended that UNOCAL resubmit the Report with reduction in sampling recommendations that are also protective of Water Quality and Beneficial Uses. Specifically,I said the Report should take into consideration what analytical data is needed to support a future closure request(groundwater modeling, plume stability, and declining concentration trends)based on the requirements of Appendix A Reports(available from the Internet at http://www.swrcb.ca.gov/rwgcb5/available_documents/ug_tanks/Tri-Regionals_Appendix_A.pdf),which wells are necessary to provide that analytical data, and which wells provide sentinel duty(wam of changes in the groundwater plume)and need to be sampled quarterly. With the proper documentation and rationale,reductions in the frequency of monitoring and deletion of specific analyses for certain wells may be acceptable. Report 2 The requested changes to the Monitoring and Reporting Program (MRP)are stated in the Conclusions/Recommendations Section of Report 2: "It is ENSR's professional opinion that the TPHg and benzene plumes beneath the site are stable and demonstrating an overall decrease. Moreover, ENSR has observed an overall decline in influent vapor concentrations related to the SVE system, and the SVE system has not achieved amass removal rate of at least one pound per day lb/day of TPHg since March of 2005. During winter months the SVE system is prone to unscheduled shutdowns as a result of elevated groundwater overwhelming the water storage capacity at the Site. For these reasons, ENSR requests approval to temporarily cease SVE system operation during the rainy season. ENSR believes that restarting the system is justified for mass removal. ENSSR has restarted the system twice following prolonged shutdown periods. Following both of those periods the SVE system achieved mass removal rates greater than a pound of TPHg per day: 19.02 lbs/day in September of 2004 and 3.651bs/day in March of 2005. The anticipated restart of SVE operation would be during the latter half of the second quarter of 2006 "