SITE INFORMATION AND CORRESPONDENCE

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John Johnson 15 East Grant Line Street,Tracy Page 2 of 3 Please prepare a site conceptual model (SCM) to assist in this investigation. The work plan for the next phase of work should be presented in the context of the SCM that includes analytical data collected during the recent waste oil tank removal in September 2003. The 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 in the SCM: • Local and regional plan view maps with sources, boring and monitoring well locations, lines of cross section, extent of contaminants in each media, direction and rate of ground water flow, and receptor locations, including water supply wells within 2,000 feet of the site. • Cross sections showing subsurface geological features, depth to ground water, man-made conduits,monitoring well construction and an interpretative drawing of the vertical extent of soil and ground water contamination. Note that the generalized boring log for borings B-8 through B-21 may not be reflective of the true lithological variability of the site and will have limited value for constructing a hydrogeological site model. • Cross-plots of key chemical concentrations vs. time for representative wells that demonstrate significant contaminant concentration trends. • Summary tables of contaminant concentrations in the different media. • Well and boring logs. • A 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 illustrate additional assessment needs thereby providing the justification for all work proposed in the work plan. 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 site conceptual model should be updated each time new data is acquired that causes 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. Submit your site conceptual model to EHD by 5 January 2004. Please note that the traffic-box cover for monitoring well (MW-7) should be replaced immediately. Presently there is no cover for this monitoring well and a number of bolts are missing from the other monitoring well covers. Watertight caps are required by both City of Tracy(No. 348) and San Joaquin County(No. 1862)well ordinances.