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/*lb California ifgional Water Quality Control Board <br />Central Valley Region <br />Katherine Hart, Chair <br />Arnold <br />Schwarzenegger <br />Govemor <br />Linda S. Adams <br />Secretary for <br />Environmental <br />Protection <br />11020 Sun Center Drive #200, Rancho Cordova, California 95670-6114 <br />Phone (916) 464-3291 • FAX (916) 464-4645 <br />http://www.waterboards.ca.gov/centralvalley <br />TO: File: Former Donna Gardner Property FROM: Jim Barton <br />26090 N. Thornton Road. Engineering Geologist <br />Thornton, San Joaquin Co. UST Unit 2 <br />DATE: 12 May 2010 SIGNATURE: <br />SUBJECT: UST CASE CLOSURE, FORMER DONNA GARDNER PROPERTY, <br />26090 N. THORNTON ROAD, THORNTON, SAN JOAQUIN COUNTY (CASE # 390412). <br />Site Background <br />Former Donna Gardner Property (Site) is currently a vacant lot located east of the intersection <br />of N. Thornton Road and Mokelunnne Road in Thornton, San Joaquin County (Attachment 1). <br />The parents of Ms. Donna Gardner operated the service station at the Site prior to <br />September 1989, when three unspecified capacity underground storage tanks (USTs) were <br />removed from the Site. Maximum soil confirmation concentrations were total petroleum <br />hydrocarbons as gasoline (TPHg), 830 milligrams per kilogram (mg/kg); ethylbenzene, <br />9.4 mg/kg; and xylenes, 70 mg/kg. The San Joaquin County Environmental Health <br />Department (SJCEHD) required a soil and groundwater investigation. Three groundwater <br />monitoring wells (MW-1 through MW-3) were installed in November 1990, and MW-4 through <br />MW-6 were installed in October 1991. Maximum groundwater concentrations were TPHg, <br />19,000 micrograms per Liter (ug/L); benzene, 110 ug/L; toluene, 79 ug/L; ethylbenzene, <br />640 ug/L; and xylenes, 1,700 ug/L. In February 1995, approximately 1,400 cubic yards of <br />contaminated soil was over-excavated, stockpiled, and aerated onsite until March 1997. <br />Maximum soil confirmation sample results were TPHg, 3,900 mg/kg; benzene, 0.006 mg/kg; <br />toluene, 3.6 mg/kg; ethylbenzene, 18 mg/kg, and xylenes, 130 mg/kg. Approximately <br />10,000 gallons of water pumped from the excavation was treated with ammonium nitrate, <br />tested for petroleum hydrocarbons, and discharge to land without a Regional Board permit or <br />waiver. Eight soil borings (SB1 through SB8) were advanced in November 1996, after areas <br />of residual soil contamination were identified between MW-3 and MW-4. The October 1997 <br />Problem Assessment Report recommended soil vapor extraction with air sparging (SVE/AS) <br />as the engineered remediation; however, the SVE/AS was not installed. In March 2001, a <br />groundwater pump test was proposed and approved by the SJCEHD; however, all <br />groundwater monitoring ceased in April 2002. By July 2003, the property was purchased at a <br />tax sale by the Barbers, and groundwater monitoring continued from August 2004 to <br />September 2005. In May 2006, the SJCEHD transferred the Site to the Regional Board. On <br />29 September 2006, Regional Board staff issued a directive to resume groundwater <br />monitoring. An August 2007 Additional Information Report demonstrated that the nearby SJC <br />Thornton Service Area Well #2 (Well #2) did not show a measurable hydrologic impact (barely <br />perceptible) on the groundwater plume when the municipal supply well was pumping as the <br />California Environmental Protection Agency <br />"Recycled Paper