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0 <br />2.3 GROUNDWATER CONDITIONS <br />Two aquifers have been identified at the site. An upper aquifer that has been identified in <br />the northwestern corner of landfill, wherein groundwater is found on or within clays of the <br />Corcoran Formation at depths ranging from 12 to 65 feet below the ground surface, and a <br />deep aquifer that occurs at depths of 300 to 350 feet (corresponding to elevations ranging <br />from -45 to -57 feet relative to mean sea level). The deep aquifer is approximately 250 feet <br />deeper than the shallow aquifer. Groundwater flows in the deep aquifer to the northeast. <br />Nine wells and one piezometer have been constructed at the site to monitor the two <br />aquifers, though only six wells are presently used for groundwater monitoring as a result of <br />decreasing water levels. Monitoring well construction information is summarized in the <br />following table: <br />Well <br />Name <br />Date <br />Constructed Depth <br />Zone Wells <br />Total <br />(ft) <br />Screen/ <br />Filter Interval (ft) <br />MW -4 <br />April 1988 <br />26.5 <br />14-26.5 <br />MW -5 <br />April 1988 <br />86.5 <br />60-86.5 <br />MW -8 <br />December 2007 <br />85 <br />62-85 <br />SB -1 <br />April 1988 <br />46.5 <br />31-46.5 <br />PZ-1(dry) <br />July 2003 <br />30 <br />6-30 <br />Deep Zone Wells <br />MW -1 (dry) <br />October 1987 <br />275 <br />240-275 <br />MW -2 (dry) <br />May 1988 <br />330 <br />298.5-330 <br />MW -3 (dry) <br />May 1988 <br />326 <br />299-326 <br />MW -6 <br />July 2003 <br />477 <br />358-390 <br />405-435 <br />450-477 <br />MW -7 <br />July 2003 <br />430 <br />302-333 <br />353-385 <br />402-430 <br />As shown above, five wells were constructed to intercept the shallow groundwater that is <br />perched within the Corcoran Formation, and five were constructed to intercept the deeper <br />aquifer. The deep aquifer is the uppermost groundwater unit beneath the site in areas <br />where the Corcoran Formation is absent. Wells MW -1, MW -2, and MW -3 and piezometer <br />PZ -1 have gone dry since construction. <br />Historical groundwater equipotential contours provided in quarterly monitoring reports <br />suggest that groundwater in the shallow perched aquifer flows to the south at a gradient <br />0.0375 feet per foot (ft/ft). The available data suggest that recharge occurs near well SB -1; <br />however, infiltration from a sand and gravel quarry or leakage from the Delta -Mendota <br />Canal and/or California aqueduct to the northeast of the site may also affect the <br />groundwater flow direction and gradient in the shallow aquifer. <br />3 <br />2011-0115 CHSL EMP Workplan Geo -Logic Associates <br />Revision 2 —January 11, 2012 <br />