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... San Joaquin County <br /> Environmental Health Department DIRECTOR <br /> p?• . c Donna Heran, REHS <br /> Z.• ,,--c,�r—v ..y 600 East Main Street PROGRAM COORDINATORS <br /> :< Stockton, California 95202-3029 <br /> N: —— — Robert McClellon,REHS <br /> Jeff Carruescruesco, REHS,RDI <br /> Kasey Foley, REHS <br /> Website: www.sjgov.org/ehd <br /> F p `P <br /> Phone: (209)468-3420 <br /> Fax: (209) 464-0138 <br /> September 2, 2010 <br /> Thrifty Oil Company ConocoPhillips Risk Mgmt/Rem <br /> Mr. Chris Panaitescu Ms. Shelby Lathrop <br /> 13116 Imperial Highway 76 Broadway <br /> Santa Fe Springs, CA 90670 Sacramento, CA 95812 <br /> Subject: Thrifty Station #172 LOP Case #: 1227 <br /> 7647 Pacific Avenue CUF #: 002156 <br /> Stockton, CA 95207 Global ID#: T0607700504 <br /> The San Joaquin County Environmental Health Department (EHD) has reviewed Interim <br /> Remedial Action Plan (TRAP), dated August 27, 2010, submitted by Stantec on your behalf. <br /> Stantec proposes in the IRAP to conduct groundwater extraction (GWE) on monitoring well <br /> MW-11 R during remodeling of the subject site. <br /> The IRAP indicates that GWE had been in operation nearly concurrently with the soil vapor <br /> extraction (SVE) remediation that addressed petroleum hydrocarbon-impacted soil in the <br /> vadose zone during much of the time period between early 2001 and early 2009. By the end of <br /> the SVE operational period, recovered soil gas had very low hydrocarbon vapor concentrations <br /> and Stantec estimated 210,156 pounds of hydrocarbon had been recovered from the vadose <br /> zone soil, yet groundwater sampled from monitoring well MW-11 R remains significantly <br /> impacted. <br /> The EHD believes that interim groundwater extraction from an existing monitoring well during <br /> the site redevelopment will be a relatively low-cost operation as you presumably already have a <br /> means of treatment and disposal of the extracted groundwater. The proposed interim <br /> remediation will continue contaminant mass removal from impacted groundwater and provide <br /> some degree of hydrological control of the plume of impacted groundwater, therefore the EHD <br /> approves the proposed scope of work as adequate and necessary [H&S 25296.10 (c)(3)] during <br /> the site reconstruction; however, the EHD has some concerns regarding the remedial <br /> technology adequacy for the long term as detailed below. <br /> Site Conceptual Model (SCM), dated October 23, 2009, notes that the vertical extent of <br /> impacted soil has not yet been delineated and that soil as deep as 55 feet below surface grade <br /> (bsg) in MW-11 was impacted by total petroleum hydrocarbons quantified as gasoline (TPHg) at <br /> a concentration of 1,500 milligrams per kilogram (mg/Kg) and benzene at 17 mg/Kg. Soil <br /> collected from the boring for MW-12 at a depth of 65 feet bsg contained 240 mg/Kg TPHg and <br /> 2.9 mg/Kg benzene. During the early 1990s when the site investigation began, depth to water <br /> (DTW) ranged from 46 to 58 feet bsg (MW-1), during the period of SVE operation, the DTW <br /> ranged from approximately 29 feet to 39 feet bsg (MW-3), potentially leaving 25 vertical feet of <br /> impacted soil in the saturated zone unaffected by the SVE operation. If the plume of impacted <br /> soil in the saturated zone has significant mass, it may continue to flux to groundwater and <br /> IRAP Approval Letter 0910 <br />