SITE INFORMATION AND CORRESPONDENCE_CASE 2

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Meeks R V (former Pavfic Auto Center) page 2 25355 North Highway 99 March 27, 2009 Acampo, California 95220 • Local and regional plan view maps showing locations of sources, boring and monitoring well locations, lines of cross section, extent of contaminants in each media (i.e., an interpretive drawing — not merely a plot of laboratory results), direction and rate of groundwater flow, and receptor locations, including water supply wells within 2,000 feet of the site; • Cross sections showing subsurface geological features, depth to groundwater, man- made conduits, monitoring well construction and the interpreted vertical extent of soil and groundwater contamination; • Cross-plots of key chemical concentrations verses time for representative wells that demonstrate significant contaminant concentration trends; • Summary tables of contaminant concentrations in the different media; • Rose diagram showing directions of dominant groundwater flow; • Well and boring logs (including construction/screening); and • Narrative description of the SCM that describes controls on contaminant distribution; contaminant migration mechanisms, pathways and rates; plume disposition over time; and sorbed and dissolved contaminant masses. While not necessarily showing all data in diagrams and drawings, an adequate SCM can be utilized to account for most of the known data, and to evaluate the fate of impacted groundwater with an estimate of when background conditions will be restored. All analytical data should be included in tables. Methods and calculations of contaminant masses should be included in the SCM, and may be submitted as an appendix. The SCM should be updated as new data are acquired that cause a change of interpretation or expands the model until model modifications are not needed to explain subsequently acquired data. As the SCM becomes complete, it can be utilized to reasonably predict additional investigation results and plume response to various remedial technologies or to changes in groundwater flow regime. The model should adequately demonstrate all interpretations and opinions. During the teleconference call, the EHD conveyed the recommendation by the Central Valley Regional Water Quality Control Board to collect soil vapor samples to assess the potential for vapor intrusion inside buildings located onsite. Although ERI has compared soil concentrations to Environmental Screening Levels (ESLs), (San Francisco Bay Regional Water Quality Control Board) and has reported that residual fuel hydrocarbons and 1,2-DCA concentrations in shallow soil do not exceed ESLs for urban area ecotoxicity, direct exposure, and groundwater protection, ERI acknowledged the prudence of collecting such soil vapor samples with respect to liability from potential vapor intrusion. Please submit a work plan to the EHD by June 29, 2009, to address the threat posed by soil vapor intrusion and include the analysis for 1,2-DCA.