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.Stantec Co sulting Services Inc. 301 TKilgore-Road,Suite 100 RanlNc Cordova CA' 95670-6150 W } } �r�+ Tel: (916)86 17043 q 0400 ` 4t .n#.� Fax: (916)861.-0430 t October 28,2013 Ms.Lisa Medina San Joaquin County Environmental Health Department 1868 East Hazelton Avenue Stockton,CA 95202 Reference:Residual Mass Estimates,7-Eleven Store#14117 2725 Country Club Boulevard Stockton,California Stantec Project Number 185750063 Dear Ms.Medina, This letter was prepared by Stantee Consulting Services Inc.(Stantec)on behalf of 7-Eleven Inc.(7-Eleven), for 7-Eleven store.#14117,located at 2725 Country.Club Boulevard,in Stockton,California(Figures 1 and 2). The residual mass estimates of remaining petroleum hydrocarbons were calculated as requested by the San Joaquin County Environmental Health Department(SJCEHD)in an.email dated August 22,2013 (Attachment A). Residual Mass Estimates.in Soil As requested by the SJCEHD,Stantee estimated the amounts of petroleum hydrocarbons in.the soil prior to remedial activities.Using soil sample data from the underground storage tank(UST)removal in July 1998, soil borings advanced in September and October 1998,and monitoring well installations in May 1999 - _ October 2001,and August 2003,Stantec estimated the extent of petroleum-impacted soils to be the area. directly around the former UST basin(Figure 2),and to be limited to depths of approximately 8 to 23 feet below ground surface(bgs).Soil sample analytical results are summarized in Table 1.To calculate the total mass of the hydrocarbons originally adsorbed to the soil,the geometric mean concentration of total petroleum hydrocarbons as gasoline(TPHg)was calculated using analytical results from soil samples collected between 8 and 23 feet bgs in the estimated impacted area(Table 2).Based on calculations and assumptions resented in Table p p 3,it is estimated that up to approximately 74 gallons of hydrocarbons were potentially present in the soil prior to remedial activities.However,it should be noted that the depth to water at this site is approximately 10 feet bgs;analytical results from saturated soil samples collected from below the water table may not be an accurate representation due to dissolved hydrocarbons present in ! groundwater. s Analytical results of recent soil samples collected since 2005 did not reveal additional petroleum impacts. Soil samples TP-2-9.5 and TP-1-9.5 were collected during the June 2013 UST removal near the historically impacted soil borings GP-14 and GP-17;the more recent soil samples did not contain detectable. concentrations of petroleum hydrocarbons.However,soil samples at these locations were collected at Design with community in mind