SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Russell Chapin 1766 W.Monte Diablo Avenue Page 4 of 5 The letter is in the EHD file copied for your review, but the primary objections centered on: 1) The unexplained apparent disconnect between the dominant groundwater flow direction derived from field data and the contaminant migration direction; 2) Inappropriate model design using one layer 25 feet thick and not differentiating the fine- grained layer from the course-grained layer; 3) Failure to include recently acquired soil data into the sorbed mass estimate calculation; 4) Failure to include dissolved contaminant concentrations from the extraction wells for mapping the plume and calculating the mass of dissolved contaminants; 5) Lack of inclusion of effects of active remediation in the model or in the interpretation of contaminant trends in various wells; 6) Lack of a reasonable flux rate from the submerged sorbed contaminant mass; 7) Lack of quantification of natural attenuation forces and rates; and 8) Overestimation of contaminant concentration reduction in MW-3 under natural attenuation despite the higher concentration actually observed under active remediation combined with natural attenuation. In addition, the EHD commented on several closure criteria from the Tri-Regional Board Appendix A mentioned by ATC, with which the EHD had interpretational differences. The EHD included several recommendations for improving the final model. On November 29, 2004, the EHD received the Response to San Joaquin Environmental Health Department Correspondence Regarding Request for Closure for Chapin Brothers Investments, Inc. Property at 1766 Monte Diablo Avenue, Stockton, California, prepared by ATC and dated November 23, 2004. Without testing or utilizing recommendations made by the EHD to improve the site model, ATC defended the model previously submitted. The EHD carefully reviewed the ATC Response letter of November 23, 2004, and reexamined the model with the supplied reasoning and argument. The EHD still found it lacking for much the same points as previously expressed. In a letter dated January 7, 2005, the EHD commented on the ATC Response letter of November 23, 2004. In this letter, the EHD did explain the model deficiencies in more detail, focusing on: 1) Use of cross sections to infer hydrological properties; 2) Number and characterization of layers in the Modflow model; 3) The apparent disconnect between the groundwater flow direction inferred from groundwater elevation and that selected based on contaminant distribution, and the resulting inferred lack of understanding of the hydrogeological framework of the site; 4) Selection and use of the hydraulic conductivity values —field measured vs. inferred by plume matching in the model; 5) Failure to differentiate causes of contaminant concentration reductions, active vs. natural attenuation, and quantification of the contaminant reduction rate for each cause; 6) Calibration of the model and using the same technique for solving for several variables; 7) The lack of a contaminant reflux estimate for submerged sorbed contaminants for use in the final model; 8) Lack of model testing and verification. In addition, the EHD provided more information on EHD interpretations of the No Further Action Criterion points noted above. This EHD letter is also included in the EHD file copied for further detailed consideration.