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`./ N%.i' <br /> San Joaquin County <br /> Environmental Health Department DIRECTOR <br /> Donna Heran,REHS <br /> z !-� ? 600 East Main Street <br /> a. '' PROGRAM COORDINATORS <br /> N " :� Stockton, California 95202-3029 Robert McClellon,RENS <br /> Jeff Carruesco,REHS,RDI <br /> . �q •. `:.�;�P Website: www.sjgov.org/ehd Kasey Foley,REHS <br /> �IFOR <br /> Phone: (209)468-3420 <br /> Fax: (209)464-0138 <br /> May 6, 2010 <br /> Ms. Stephanie A. McKenna <br /> Chevron Environmental Management Company <br /> 6111 Bollinger Canyon Road, Room 3652 <br /> San Ramon, CA 94583 <br /> Subject: Former Chevron Service Station 9-0557 <br /> 139 South Center Street <br /> Stockton, California <br /> Dear Ms. McKenna: <br /> The San Joaquin County Environmental Health Department (EHD) has received and reviewed <br /> Site Conceptual Model (SCM), dated September 30, 2009, and Feasibility Study (FS), dated <br /> September 30, 2009, both prepared by your consultant, Holguin, Fahan & Associates, Inc. <br /> (HFA). In the SCM, HFA recounts the subsurface investigations of the unauthorized release <br /> from the underground storage tank (UST) system removed from this site in 1990, and <br /> describes the hydrogeological framework and the contaminant distribution for use to predict <br /> the plume behavior and potential plume response to various remedial technologies. <br /> By letter dated April 29, 2009, the EHD had recommended the submittal of a preliminary SCM <br /> prior to proceeding with any additional site investigation, since the EHD believed that <br /> petroleum hydrocarbon contaminants had not been delineated vertically and laterally in <br /> groundwater at this site, and that contaminants of concern had not been delineated vertically <br /> in soil near the former USTs removed in 1990. The EHD had also directed the submittal of an <br /> FS that included methods and calculations for estimating the contaminant masses in <br /> groundwater and soil as part of the FS. <br /> In the SCM, HFA states that "the lateral extent of the contaminant plume has decreased over <br /> time, and concentrations in the source area are stable. The vertical extent of the plume is <br /> delineated, and no additional delineation is required." The EHD concurs that the lateral extent <br /> of the contaminant groundwater plume has decreased locally based on groundwater analytical <br /> data collected from monitoring wells screened within the smear zone, described by HFA as <br /> extending from approximately twenty-one feet below surface grade (bsg) to approximately <br /> forty-five feet bsg, including wells screened as deep as sixty feet bsg down-gradient of the <br /> source area; however, the EHD has concern regarding the completeness of the delineation of <br /> impacted soil and groundwater based on the following observations: <br /> 1. Concentrations of contaminants of concern increase significantly between groundwater <br /> samples collected from monitoring well MW-15, screened between twenty and thirty <br />