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1 <br /> OZONE-SPARGE SYSTEM <br /> ' The ozone system included 17 injection wells (AS-6 through AS-22) connected to an H2O Engineering <br /> Model OSU-20-52 ozone-sparge system control panel. The ozone-sparge system well layout and trench <br /> ' and treatment compound locations are shown on Figure 3. Baseline groundwater samples were <br /> collected during the Second Quarter — 2010 groundwater monitoring event. On June 23, 2010, we <br /> began full-time ozone-sparging within 8 of the 17 sparge wells (AS-7 through AS-13 and AS-17) with <br /> oxygen-sparging occurring at the remaining 9 sparge wells. On June 29, 2010, we began <br /> ozone-sparging in each of the 17 sparge wells. We turned the system off on September 21, 2011, to <br /> perform two months of groundwater rebound monitoring as directed by the CVRWQCB. <br /> ' CONCLUSIONS AND RECOMMENDATIONS <br /> The Fourth Quarter — 2011 groundwater monitoring data represents two consecutive months of <br /> ' rebound monitoring following approximately 15 months of full-time ozone-sparging. After 15 months <br /> of treatment and two months of rebound monitoring, the closest wells downgradient of the former <br /> USTs (MW-1, MW-3 and GWX-1) continue to be reported with TPHg and BTEX, although at one to <br /> ' three orders of magnitude lower concentrations than pre-treatment (except for MW-1 which, although <br /> lower, remain at concentrations similar to baseline). <br /> ' In October and November the TPHg and BTEX concentrations for MW-1 demonstrate a decreasing trend <br /> from the post-treatment concentrations. The rebound monitoring TPHg concentrations for MW-3 remained <br /> essentially the same as the post-treatment concentrations while the BTEX concentrations initially increased <br /> slightly in October then decreased back to near the post-treatment concentrations in November. In October <br /> ' and November, the TPHg and benzene concentrations for GWX-1 again decreased to new historical low <br /> concentrations. Based on the lack of significant contamination rebound for each of the source area <br /> monitoring wells, it is apparent that the former ozone injection system was effective in reducing the <br /> dissolved petroleum hydrocarbon concentrations in the groundwater beneath the Site to a de minimis; <br /> condition. In concert with the demonstrated acceptable human health risk evaluation, the current site <br /> conditions warrant closure as a low-risk groundwater site. We recommend that the CVRWQCB grant the <br /> ' Site no further regulatory action status and that each site monitoring and remediation well be appropriately <br /> destroyed in accordance with the San Joaquin County Environmental Health Department standards. Please <br /> contact the undersigned if there are any questions concerning this document. <br /> Sincerely, <br /> ' GEOCON CONSULTANTS,INC. ONAL GFOC <br /> 4 KEVIN N O �n _ <br /> � BROWN � � �Xn (�(/�/�—/',II__' <br /> ' Kev n J. r wn, PG • NO. 7101 Josh Ewer <br /> At <br /> an ger ;.AI 0i&1/1�SSeniorStaffGeologist <br /> I ' (2) Addressee OF CALF <br /> (1) CVRWQCB, Mr. James Barton <br /> (1) San Joaquin County Environmental Health Department, Mr. Michael Infurna <br /> ' (1) California State Controller's Office, Ms. Misty Polasik <br /> 1 <br /> ' Project No.S9183-06-03 -6- January 30,2012 <br />