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3212 North California Street Page 2 of 3 During the process of reviewing historical site data, EHD identified discrepancies between cross sections submitted over time and became concerned that MW-1 may be acting as a vertical conduit since the well's screened interval (45 to 75 feet bgs) may be connecting two sand layers. A site conceptual model (SCM) that incorporates findings from the last seventeen years of site characterization is needed to evaluate present site conditions, You are now directed to prepare and submit an SCM to the EHD. The general purposes for an SCM are to demonstrate where the contaminants came from, where they are at the present time, how they move through the subsurface, how they will respond to changes in the groundwater flow characteristics or to potential remediation efforts, what the contaminants' ultimate environmental fate will be, and to help evaluate the risk posed by the contaminants to groundwater supplies and sensitive receptors. In addition, the SCM may demonstrate data gaps, thereby illustrating where additional investigation is needed. To be sure that the SCM will adequately address the assessment, remediation and, ultimately, final closure requirements for this site, please include the following, as appropriate, in the SCM: • Local and regional plan view maps showing locations of sources, boring and monitoring well locations, lines of cross section, extent of conta3ninants 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 ' f • Narrative description of the SCM that describes controls on contaminant i 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. Residual contaminant concentrations should be compared to Environmental Screening Levels (ESL) and California Human Health Screening Levels (CHHSLs) established by California Environmental Protection Agency San Francisco Bay Regional Water Quality Control Board. Methods and calculations of contaminant masses should be included in the SCM, and may be submitted as an appendix.