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Mr. Paul Supple <br /> Arco Station No. 5469 <br /> 130 S. Wilson Way, Stockton <br /> Page 2 of 2 <br /> • Cross sections showing subsurface geological features, depth to ground water, <br /> man-made conduits, monitoring well construction and the interpreted vertical <br /> extent of soil and ground water contamination. <br /> • Cross-plots of key chemical concentrations vs. time for representative wells that <br /> demonstrate significant contaminant concentration trends. <br /> • Summary tables of contaminant concentrations in the different media. <br /> • Well and boring logs. <br /> • A narrative description of the SCM that describes controls on contaminant <br /> distribution; contaminant migration mechanisms, pathways and rates; plume <br /> disposition over time; and sorbed and dissolved contaminant masses. <br /> While not necessarily showing all data in diagrams and drawings, an adequate SCM can <br /> be utilized to account for most of the known data and to illustrate additional assessment <br /> needs thereby providing the justification for all work proposed in future work plans. All <br /> analytical data should be included in tables. Methods and calculations of contaminant <br /> masses should be included in the SCM, and may be submitted as an appendix. The site <br /> conceptual model should be updated each time new data is acquired that causes a <br /> change of interpretation or expands the model until model modifications are not needed <br /> to explain subsequently acquired data. As the SCM becomes complete, it can be utilized <br /> to reasonably predict additional investigation results and plume response to various <br /> remedial technologies or to changes in groundwater flow regime. The model should <br /> adequately demonstrate all interpretations and opinions. <br /> Although a specific time for the proposed meeting has not been set, it is hoped that such <br /> a meeting could be scheduled for early next year. The SCM should be completed for that <br /> general time fame, and all future work and proposals should be presented in the context <br /> of the SCM. <br /> Questions or comments may be directed to Nuel Henderson by telephone at <br /> (209) 468-3436, or by mail to the address on the letterhead. <br /> Donna Heran, RENS, Director <br /> Environmental Health Division <br /> Nuel C. Henderson, Jr., RG Margaret egorio, REFtS <br /> 9 <br /> Unit IV - Site Mitigation Supervisor, Unit IV <br /> cc: James L. L. Barton —CVRWQCB <br /> Jay R. Johnson - Stratus <br />