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San Joaquin County - Public Works 1810 E. Hazelton Avenue, Stockton, CA Site Background through February 2017 Page 5 of 5 Between November 7, 2014 and May 15, 2015, the groundwater surface in the monitoring wells increased an average of 0.40 feet, the groundwater gradient remained stable at 0.0026 ft/ft, and the direction of groundwater flow remained toward the northeast. Detected concentrations of gasoline compounds in groundwater samples collected on May 15, 2015 were generally similar to results from the past three years in MW-1, MW-2, and MW-4. The concentration of MTBE in down-gradient well MW-4 decreased from 35 to 12 !Aga, the lowest result since December 2005. Analytical results indicate that concentrations of hydrocarbons are generally decreasing in well MW-1 and have increased in well MW-2 since mid- 2011. The results of the May 15, 2014 groundwater monitoring event were documented in the First Quarter 2015 (Biennial) Groundwater Monitoring Report-May 2015, dated June 8, 2015. Between May 15, 2015 and November 16, 2015, the groundwater surface in the monitoring wells decreased an average of 2.50 feet, the groundwater gradient remained stable at 0.0026 ft/ft, and the direction of groundwater flow remained toward the northeast. The concentration of MTBE in down- gradient well MW-4 decreased from 12 to 11 ig/1 since November 2015. Analytical results indicate that concentrations of hydrocarbons are generally decreasing in well MW-1 and have increased in well MW-2 since mid-2011. The results of the November 16, 2015 groundwater monitoring event were documented in the Fourth Quarter 2015 (Semi-Annual) Groundwater Monitoring Report-November 2015, dated December 29, 2015. Between November 16, 2015 and May 31, 2016, the groundwater elevation beneath the Site increased an average of 0.97 feet, the groundwater gradient remained stable at 0.0026 ft/ft toward the northeast. The concentration of MTBE in down-gradient well MW-4 decreased to 4.3 41. Analytical results indicate that concentrations of hydrocarbons are generally decreasing in well MW-1 but have increased in well MW-2 since mid-2011. The results of the May 31, 2016 groundwater monitoring event were documented in the Semi-Annual Groundwater Monitoring Report-January 1 through June 30, 2016, dated July 28, 2016. Condor submitted the Ozone Remediation Pilot Test Work Plan dated June 15, 2015, which outlined proposed methodology to install ozone injection wells and an ozone injection remediation system at the Site. Six ozone injection wells (OW-1 through OW-6) were installed between May 16 and May 18, 2016. The ozone injection wells were installed to a depth of approximately 45 feet bgs and equipped with 18- inch long Kynar sparge points from 43.5 to 45 feet bgs. Two additional proposed ozone injection wells were not installed as planned due to observance of kerosene odors in shallow soil between the former kerosene UST (UST #5) and the existing diesel UST (UST #2). Condor was directed to suspend installation of proposed ozone injection wells OW-7 and OW-8, and collected a shallow soil sample where odors were noted (OW-7-2). The shallow soil sample contained TPH as kerosene at a concentration of 280 mg/kg, but no detectable benzene. Ozone well installation activities were summarized in Condor's August 29, 2016 report entitled Ozone Remediation Pilot Test Well Installation Report. Between May 31, 2016 and December 29, 2016, the groundwater elevation beneath the Site decreased an average of 1.86 feet, but the groundwater gradient remained stable at 0.0027 ft/ft toward the northeast. The concentration of MTBE in down-gradient well MW-4 decreased to 3.0 Analytical results indicate that concentrations of hydrocarbons are generally decreasing in well MW-1 but have increased in well MW-2 since mid-2011 (although concentrations in well MW-2 fell slightly between May and December 2016). The results of the December 29, 2016 groundwater monitoring event were documented in the Semi-Annual Groundwater Monitoring Report-July 1 through December 31, 2016, dated February 22, 2017. X:\Project\5000_prj\5960 SJC UST RFQ\5960C County of San Joaquin - SJC Public Worlcs\5960C6\Reports\ Work Plan\App B Background\20170601_5960C6 Site Background.docx