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' San Joaquin County-Public Works
<br /> Second Half 2015(Semi-Annual)Groundwater Monitoring Report
<br /> ' Site Background
<br /> Page 4 of 5
<br /> ' Between March 20, 2013 and December 9, 2013, the groundwater surface in the monitoring wells
<br /> dropped an average of 1.20 feet, the groundwater gradient increased slightly to 0.0016 ft/ft, and the
<br /> direction of groundwater flow remained toward the northeast. The groundwater flow direction and
<br /> ' gradient appear to be consistent with recent historical trends.
<br /> Detected concentrations of gasoline compounds in groundwater samples collected on September 28,
<br /> ' 2012, March 21, 2013, and December 9, 2013 were generally similar in MW-1, MW-2, and MW-4. The
<br /> concentration of MTBE in down-gradient well MW-4 decreased from 34 to 22 µg/1, the lowest result
<br /> since December 2005. The results of the December 9, 2013 groundwater monitoring event were
<br /> documented in the Fourth Quarter 2013 (Semi-annual) Groundwater Monitoring Report-December 2013,
<br /> dated March 12, 2014.
<br /> ' The State Water Resources Control Board reviewed the site status including the Interim Remediation
<br /> Report. They updated the Path to Closure Plan and Low Threat Closure Policy working documents to
<br /> indicate that they concurred that vadose zone contamination had been reduced, groundwater remediation
<br /> was needed. The Path to Closure Plan FY 12/13 as of 612312014, indicated that a work plan should be
<br /> ' submitted to pilot test a technology to reduce benzene and MTBE concentrations; then the pilot test
<br /> should be implemented; and results should be reported with recommendations and a corrective action
<br /> plan; public participation for the corrective action plan should occur; and the approved corrective action
<br /> ' should be implemented.
<br /> Between December 9, 2013 and April 4, 2014, the groundwater surface in the monitoring wells rose an
<br /> ' average of 0.64 feet, the groundwater gradient increased slightly to 0.002 ft/ft, and the direction of
<br /> groundwater flow remained toward the northeast. The groundwater flow direction and gradient appear to
<br /> be consistent with recent historical trends.
<br /> ' Detected concentrations of gasoline compounds in groundwater samples collected on September 28,
<br /> 2012,March 21, 2013, December 9, 2013 and April 4, 2014 were generally similar in MW-1, MW-2, and
<br /> ' MW-4. The concentration of MTBE in down-gradient well MW-2 increased from 71 to 1,300 µg/1, the
<br /> highest result since May 2010. The results of the April 4, 2014 groundwater monitoring event were
<br /> documented in the Second Quarter 2014 (Annual) Groundwater Monitoring Report-April 2014, dated
<br /> July 15, 2014.
<br /> Between April 4 and November 7, 2014, the groundwater surface in the monitoring wells decreased an
<br /> average of 1.78 feet and the groundwater gradient remained at 0.002 ft/ft toward the northeast.
<br /> ' Concentrations of gasoline compounds in MW-2 increased relative to concentrations detected in this well
<br /> during 2013 and 2014. The concentration of MTBE in downgradient well MW-2 increased from 1,300
<br /> µg/1 on April 4 to 25,000 µg/1 on November 7, 2014, the highest detected concentration in this well since
<br /> ' October 2009. The results of the November 7, 2014 groundwater monitoring event were documented in
<br /> the Fourth Quarter 2014 (Semi-Annual) Groundwater Monitoring Report-November 2014 dated June 5,
<br /> 2015.
<br /> ' Between November 7,2014 and May 15, 2015,the groundwater surface in the monitoring wells increased
<br /> an average of 0.40 feet, the groundwater gradient remained stable at 0.0026 ft/ft, and the direction of
<br /> ' groundwater flow remained toward the northeast. The groundwater flow direction and gradient appear to
<br /> be consistent with recent historical trends.
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