SITE INFORMATION AND CORRESPONDENCE_FILE 3

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`. Nue vl" San Joaquin County °P'" Environmental Health Department DIRECTOR Donna Heran,REHS 600 East Main Street PROGRAM COORDINATORS r Stockton, California 95202-3029 Robert McClellon,REHS � � . Jeff Carruesco, REHS,RDI c % Kasey Foley, REHS q�iFOR�\/ Website: www.sjgov.org/ehd Linda Turkatte, REHS Phone: (209) 468-3420 Fax: (209) 464-0138 October 24, 2011 Thrifty Oil Company ConocoPhillips Risk Mgmt/Rem Mr. Chris Panaitescu Ms. Shelby Lathrop 13116 Imperial Highway 76 Broadway Santa Fe Springs, CA 90670 Sacramento, CA 95812 Subject: Thrifty Station #172 LOP Case #: 1227 7647 Pacific Avenue CUF #: 002156 Stockton, CA 95207 Global ID#: T0607700504 The San Joaquin County Environmental Health Department (EHD) has reviewed Monitoring Well Installation Report dated July 22, 2011, Site Conceptual Model and Human Health Risk Assessment dated September 30, 2011, and Quarterly Status Summary Report —Third Quarter 2011 dated October 13, 2011, all submitted by ATC Consultants. On behalf of ConocoPhillips (COP), in October 2010, former consultant, Stantec, under the EHD approval, destroyed monitoring wells MW-4, MW-10, and MW-11 R, vapor wells VW-1, VW-3, VW-4(A,B,C), VW-5(A,B,C), and VW-6(A,B,C), and protected wells MW-3, MW-7, and MW-12R in preparation for new construction onsite. ATC then replaced Stantec consulting and in May 2011, under the EHD approval, installed monitoring wells MW- 4R and MW-13 through MW-16 onsite. Additionally, the three previously protected wells were uncovered and returned to usable condition. In previous correspondence with the EHD and COP, it was determined that after the new construction onsite was completed and the replacement wells were installed, COP would evaluate the site for the need for additional investigation and/or remediation. All three reports reviewed by the EHD concluded that the site soil and groundwater plumes were not completed delineated. The current network of monitoring wells does not adequately characterize the dissolved petroleum plume and additional investigation is warranted. The EHD concurs. Please submit a work plan to collect soil samples in the former underground storage tank (UST) area and to continue the investigation to define the lateral and vertical extents of the dissolved petroleum hydrocarbon groundwater plume. In order to evaluate additional remediation needs at the site and to determine if the residual contamination at the site poses a threat to occupants of the building, future construction workers, or residents on adjacent parcels, the human health risk report recommended that soil vapor sampling activities be performed at the site. The EHD concurs. Please submit a proposal to conduct this investigation.