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San Joaquin County - Public Works
<br />1810 E. Hazelton Avenue, Stockton, CA
<br />Site Background through February 2017
<br />Page 4 of 5
<br />Detected concentrations of gasoline compounds in groundwater samples collected on September 28, 2012
<br />and March 20 and 21, 2013, were generally similar in MW-1, MW-2, and MW-4. The concentration of
<br />MTBE in down-gradient well MW-4 decreased from 59 to 34 p,g/l. The results of the March 20, 2013
<br />groundwater monitoring event were documented in the First Quarter 2013 (Biennial) Groundwater
<br />Monitoring Report-March 2013, dated June 5, 2013.
<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 ps/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 6/23/2014, 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 />ilg/1 on April 4 to 25,000 gig/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.
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