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Workplan for Additional Offsite Assessment,Fate and Transport Evaluation Addendum, 5 Sensitive Receptor Survey,and Fourth Quarter 2002 Monitoring And Sampling 7-Eleven Store No 14117,2725 Country Club Blvd,Stockton CA January 30,2003 ' The sod boring will be back-filled to near surface grade with neat-cement grout after completion, and patched to match surrounding grade surface Findings from additional assessment will be submitted with the next quarterly groundwater monitoring data following drilling or within six weeks of field work completion Field work is scheduled for completion by June 2003 pending SCJPHS approval of the scope of work 50 FATE AND TRANSPORT EVALUATION ADDENDUM At the request of the SJCPHS, an earlier fate and transport model conducted using WinTran for dissolved MTBE migration (Shaw, June 2002)was re-evaluated using different hydraulic conductivity values as ' suggested by the SJCPHS (97 ft/day) (Appendix B) Comparative simulations of models based on conductivities of 2 5 and 97 ft/day were run to 498 and 1828 days (1 17 and 5 years, respectively) Using the original gradient setting and the revised conductivity data, the model protected less stable plume conditions than originally calculated After discussion with Mr Nuel Henderson of SJCPHS (communications from 1/21/03-1123/03), the model was re-calibrated using a gradient value reflective of site flow direction fluctuations Results more closely matched observed concentration distribution patterns, verifying general accuracy of revised modeling assumptions Based on this analysis, it is considered that the dissolved MTBE plume is likely to remain generally stable in position with limited offsite migration potential ' 60 SENSITIVE RECEPTOR SURVEY ' A formal 2,000-foot sensitive receptor investigation (to identify all municipal and/or domestic wells in the area)was implemented in May 2001 by ETIC Engineering, Inc , for the adjacent former Exxon Retail Site 7-3708 at 2705 Country Club Boulevard (Appendix C) The survey included results of an approximately 1,000-foot radius door-to-door well survey A single domestic well was identified at 2740 Bristol Avenue, approximately 900 feet northwest of the 7-Eleven site location An unconfirmed domestic well was reported at 2710 Bristol Avenue (adjacent to the 2740 property) An Environmental Data Resources ' Survey (EDR) report included in the ETIC survey reports no federal, state, or public water supply wells within one mile of the subject site The results of this survey place known private wells well outside the ' area of the northern and western plume edge as currently delineated Previous well search results (Fluor Daniel GTI, Onsite and Offsite Assessment Report, November 30, ' 1998) indicate that there are few active wells within a 2,000-foot radius of the site, and that most wells included in the list are perforated at depths of about 100 feet bg or deeper i ' 14117Q24 SAR doc