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construction reported observing the Corcoran Clay on the northeastern edge of the <br /> landfill. The Corcoran Formation is characterized as moderately to highly plastic clays, <br /> and mixtures of clay with silts, sands, and gravels. Regionally, the presence of significant <br /> clays in the Corcoran Formation allows water to perch on it, though historical <br /> groundwater investigations at the site indicate that the shallow groundwater is locally <br /> perched on or entrained within these clay units at the site. <br /> 3.3 HYDROGEOLOGIC SETTING <br /> Two aquifers have been identified at the site. An upper aquifer is present in the <br /> 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 <br /> a deep aquifer that occurs at depths of 300 to 350 feet (corresponding to elevations <br /> ranging from -45 to -57 feet relative to mean sea level). The deep aquifer is <br /> approximately 250 feet deeper than the shallow aquifer. Historical groundwater <br /> elevation data suggest that groundwater in the shallow aquifer flows to the northwest <br /> and northeast, while groundwater in the deep aquifer flows 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 <br /> of decreasing water levels (Figure 2). Monitoring well construction information is <br /> 1 summarized in Table 1. <br /> Table 1 <br /> Existing Well Construction Summary <br /> Well Date Total Screen/ <br /> Name Constructed Depth(ft) Filter interval (ft) <br /> Shallow Zone Wells <br /> MW-4 April 1988 26.5 14-26.5 <br /> MW-5 April 1988 86.5 60-86.5 <br /> MW-8 December 2007 85 62-85 <br /> SB-1 April 1988 46.5 31-46.5 <br /> PZ-1(dry) July 2003 30 6-30 <br /> Deep Zone Wells <br /> 1 MW-1(dry) October 1987 275 240-275 <br /> MW-2 (dry) May 1988 330 298.5-330 <br /> MW-3 (dry) May 1988 326 299-326 <br /> 358-390(l) <br /> MW-6 July 2003 477 405-435 <br /> 450-477 <br /> 302-333(l) <br /> MW-7 July 2003 430 353-385 <br /> 402-430 <br /> (1) Wells MW-6 and-7 are constructed with three sampling intervals. The upper <br /> interval is in use at this time <br /> EMP Report—Phase IIGeo-Loic Page ■ a <br /> Corral Hollow Sanitary Landfill A s s o c I A r e sg <br /> August 21,2012 <br />