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San Joaquin County - Public Works
<br />1810 E. Hazelton Avenue, Stockton, CA
<br />Site Background through February 2017
<br />Page 3 of 5
<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 April 29, 2011, and
<br />November 3, 2011, were generally similar in MW-1, MW-2, and MW-4. The concentration of MTBE in
<br />down-gradient well MW-4 increased from 29 to 48 micrograms per liter (i.tg/1). The results of the
<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 j.tg/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 mil. 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-2B 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.
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