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0 0 <br /> remaining concentrations approaching water quality objectives in a <br /> reasonable amount of time. The EHD indicated the possible need for <br /> concentration reduction in the vicinity of U-3, the source area for the EHD <br /> LOP case #1181. <br /> The EHD feels that the plume has not been fully defined, due to the presence <br /> of detectable concentrations of petroleum-hydrocarbons both laterally and <br /> vertically at the boundaries or the proposed plume limits The EHD further <br /> feels that an assessment of the effectiveness of natural attenuation or plume <br /> stability can not be completed until the plume has been defined both laterally <br /> and vertically and feels that plume definition is defined by non-detectable <br /> concentrations of petroleum-hydrocarbon constituents. <br /> Chevron and ENSR will reassess the data in consideration of the County's <br /> comments. <br /> 5. ENSR requested that if active remediation at the site is required, EHD <br /> provide shut-down criteria (concentration goals) at which the system <br /> can be shut down and proceed with Monitored Natural Attenuation. <br /> The EHD referenced declining concentration trends approaching water quality <br /> objectives as shut-down criteria for active remediation. <br /> 6. EHD inquired about the status of the soil vapor probes used to collect <br /> soil vapor samples for the Site Assessment Report. EHD also requested <br /> a more complete interpretation of the results of the soil vapor analysis. <br /> The soil vapor probes used to collect soil vapor samples are still in place and <br /> have not been abandoned. Due to the identified problems using isopropyl <br /> alcohol for leak detection during vapor sampling, which was observed in the <br /> vapor samples for this site, Chevron's vapor sampling protocol now <br /> recommends the use of helium for leak detection. Additionally, two rounds of <br /> vapor sampling are recommended to establish a suitable database for <br /> assessing the risk from vapor intrusion. Chevron and ENSR will reassess the <br /> data in consideration of the County's comments. <br /> 7. Chevron and ENSR suggest that the EHD, RWQCB, Chevron, and ENSR <br /> schedule a meeting to further discuss current site conditions and to <br /> outline a path toward site closure. <br />