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Chevron-Banta Transmix UST page 2 22888 Kasson Road, Banta. EHD is familiar with the ongoing non-UST investigation at this site and that much of the subsurface lithology and petroleum impact from the AST has been, or is currently being investigated. With this in mind, EHD advised Chevron in its August 28, 2003 work plan approval letter that the report of findings for this UST investigation was to be in the context of a Site Conceptual Model (SCM). The cross-sections and iso-concentration maps included in a SCM would have assisted EHD in evaluating the potential impact the UST may have contributed to the AST plume. The report of findings was not in this format. Based on the limited data, conclusions, and recommendations included in this report, EHD was unable to fully evaluate whether or not this UST is a major contributor to the impact of the ground water beneath this site. Chevron's only conclusion that the UST and AST plume appears co-mingled is inadequate and in need of further evaluation. The area (soil and ground water) immediately south of the UST and the vertical extent of the contaminants in the former UST location are undefined. Further investigation is needed. Chevron is to submit a work plan to investigate the area south of the UST and to vertically define the extent of the petroleum at the former UST location by October 15, 2004. Chevron is hereby directed to evaluate the relationship between the UST and the'AST contamination and present the findings in the work plan. Rationale for placing new monitoring wells or using existing AST wells should be included. Source and migration concerns should be addressed and areas where conclusions are not supported by laboratory data from soil and ground water samples should have these areas included in the work plan for additional investigation. In the future, all reports of findings for the UST investigation are to be formatted in the context of a SCM. To assist you in identifying the typical inclusions of a SCM, EHD has enclosed Attachment A for your reference. Please address all UST work plans and reports to Michael Infurna. You may call him at (209) 468-3454 should you have any questions. Donna Heran, R.E.H.S. Director Michael J. Infurna, Senior R.E.H.S Margaret Lagono, R.E.H.S. LOP / Unit IV Supervisor MI/mi c: CVRWQCB —James L. Barton, Sacramento. c: CVRWQCB-SLIC Unit— Devra Lewis, Sacramento. c: SWRCB — CUF — Mark Owens, Sacramento. c: Secor— David Evans, 3017 Kilgore Rd, #100, Rancho Cordova, 95670. Enclosure (Addressee and Secor)