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7-Eleven Store No. 19976 Page 2 1399 North Main Street February 18, 2011 Manteca, California 95336 equivalent tracer gas, by Environmental Protection Agency (EPA) method TO-15. Although Stantec states that soil gas sampling will not be performed within five days after a significant rainfall, there is no mention as to when samples will be collected following the installation of the soil gas probes. Following the Department of Toxic Substances Control (DTSC) Draft-for- review Advisory — Active Soil Gas Investigation, dated March 2010, the purge volume test, leak test, and soil gas sampling should not occur for at least 48 hours after the vapor probe installations using the hand-auger drilling method to allow equilibration time for subsurface conditions and vapor concentrations to stabilize. The EHD approves the work plan with this condition. Submit a boring permit application, permit/ inspection fees of $976.00, and Master File Record form to the EHD and complete the fieldwork by 13 May 2011. The EHD has reviewed Response to Regulatory Request (letter), dated 04 January 2011, submitted by Stantec. The letter was submitted in response to EHD correspondence dated 08 November 2010. In the letter, clarification is made that Stantec and 7-Eleven recognize that additional groundwater sampling events performed above what is directed by the Local Oversight Program. are costs not eligible for reimbursement from the Underground Storage Tank Cleanup Fund (CUF); however, this does not prohibit responsible parties "from conducting as many additional groundwater sampling events as they choose and/or deem warranted." Stantec further states that 7-Eleven has decided to continue quarterly groundwater monitoring and sampling activities in order to closely observe the effectiveness of the remediation activities and to evaluate whether site closure is appropriate." In the letter, Stantec also demonstrates that oxygen injections initiated in March 2003 have increased the dissolved oxygen concentrations and the concentrations of contaminants of concern in groundwater have decreased at this site. As a result, Stantec proposes to terminate oxygen injections following the first quarter 2011, and check for rebound and/or recharge during the subsequent three quarterly sampling events in 2011. If the analytical data indicate no significant rebound of contaminant concentrations, Stantec proposes to submit a formal site closure request. The EHD concurs that the oxygen injections should cease and groundwater should be checked for rebound during the following three quarters in 2011. If you have any questions concerning this letter, please contact Vicki McCartney at (209) 468- 9852, or by email at vmccartney@sjcehd.com. Sincerely, / �� Nuel C. Henderson, Jr., PG Victoria L. McCartney, REHS Senior Registered Environmental Health Specialist Engineering Geologist c: Mr. Ed Simonis, PG, Stantec Consulting Corporation, 3017 Kilgore Road, Suite 100, Rancho Cordova, California 95670 Mr. James L.L. Barton, PG, California Regional Water Quality Control Board Central Valley Region, 11020 Sun Center Drive, Suite 200, Rancho Cordova, California 95670