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' San Joaquin County-Public Works
<br /> Second Half 2015(Semi-Annual)Groundwater Monitoring Report
<br /> ' Site Background
<br /> Page 3 of 5
<br /> November 3, 2012 groundwater monitoring event were documented in The Fourth Quarter 2011
<br /> ' Groundwater Monitoring Report, November 2011, dated January 20, 2012.
<br /> Between November 3, 2011 and April 17, 2012, the groundwater surface in the seven monitoring wells
<br /> ' rose an average of 0.96 feet, the groundwater gradient decreased to 0.0014 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 November 3, 2011
<br /> and April 17, 2012 were generally similar in MW-1, MW-2, MW-4, and MW-6. The concentration of
<br /> ' MTBE in down-gradient well MW-4 increased from 48 to 88 µg/1. The results of the April 17, 2012
<br /> groundwater monitoring event were documented in The Second Quarter 2012 Groundwater Monitoring
<br /> Report, April 2012, dated May 23, 2012.
<br /> ' Interim remediation was initiated on April 23, 2012. Interim remediation began with soil vapor extraction
<br /> from MW-1.
<br /> ' Between April 17, 2012 and September 28, 2012, the groundwater surface in the seven monitoring wells
<br /> dropped by an average of 0.10 feet, the groundwater gradient increased to 0.0016 ft/ft, and the direction
<br /> of groundwater flow remained toward the northeast. The groundwater flow direction and gradient appear
<br /> ' to be consistent with recent historical trends.
<br /> Detected concentrations of gasoline compounds in groundwater samples collected on April 17, 2012 and
<br /> ' November 28, 2012 were generally similar in MW-1, MW-2, and MW-4. The concentration of MTBE in
<br /> down-gradient well MW-4 decreased from 88 to 59 gg/l. The results of the September 28, 2012
<br /> groundwater monitoring event were documented in the Third Quarter (Semi-Annual) 2012 Groundwater
<br /> ' Monitoring Report and Interim Remediation Update, September 2012, dated January 14, 2013.
<br /> The interim remediation system was reconfigured on November 5, 2012 to remove soil vapor from soil
<br /> vapor well SVE-213 because hydrocarbon concentrations in MW-1 had dropped to 660 parts per million
<br /> vapor(ppmv). The threshold for cost efficient operation of a SVE unit is generally considered to be 1,000
<br /> ppmv. On January 10, 2013 air injection at air sparge well AS-1 was initiated. Extraction from SVE-2B
<br /> continued until February 27, 2013. The system was shut down on two occasions due to the large quantity
<br /> ' of water being removed with the soil vapor. The entrained water in the soil vapor was caused by high
<br /> groundwater elevations. Continued operation was considered to be not cost effective due to high ground
<br /> water and low soil vapor concentrations at that time. Interim remediation was discontinued on February
<br /> 27, 2013. Interim remediation activities were documented in the Interim Remediation Report, dated June
<br /> 6, 2013.
<br /> ' Between September 28, 2012 and March 20, 2013, the groundwater surface in the monitoring wells rose
<br /> an average of 1.09 feet, the groundwater gradient decreased to 0.0015 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, 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 µg/1. 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.
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