CORRESPONDENCE_2007 (1/07 - 6/07)

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t .k since December 2001 as a corrective action(source removal) against VOC impacts, as well as to prevent offsite migration of LFG. 1.4 Site Geology The CHSL is located on the western side of the San Joaquin Valley immediately north of Corral Hollow Creek, an intermittent stream. The site is located within the Central Valley Geomorphic Province although sediments have originated from the Coast Range. The site is located within the Tracy-Patterson groundwater storage unit. The landfill was constructed on top of the alluvial fan of Corral Hollow Creek. Sediments originating from the coast range have deposited onto the valley floor as gravels and designated as Holocene aged alluvial fan deposits (D. L. Wagner, e. J. Bortugno and R. D. McJunkin, 2005). The Corcoran Formation,which underlies a portion of the landfill, consists of a diatomaceous lacustrine clay which extends eastward beneath the San Joaquin Valley. (G. H. Davis, J. H. Green, F. H. Olmsted and D. W. Brown, 1959). Site surface soils typically include sands and gravels to a depth of about 70 feet, underlain by low permeability silts and clays to 150 feet. Sands and gravels predominate to a depth of approximately 430 feet. Highly impermeable sandy clays are present at 430 feet. 1.5 Groundwater Occurrence The hydrogeology at the site is complex. The Corcoran Formation rises from the northeast, apparently intercepting the landfill base. This formation, which is clayey in the area of the landfill, creates a shallow aquifer, such that two aquifers exist along the northeast side of the landfill property. The Corcoran Formation does not exist beneath the southwest side of the property. A single, deep aquifer is found beneath that portion of the property. Beneath the northeast side of the landfill property, the first encountered(shallow aquifer) groundwater varies between 12 and 65 feet below existing grade, at an elevation of approximately 230 feet MSL. This aquifer is unconfined, and mounds near monitoring well SB-1. Work in 2003 indicates that this aquifer is inconsistent in elevation, and discontinuous within the landfill property. Under the entire property(and below the shallow aquifer on the east side of the property) is a deep aquifer. The depth of this aquifer varies between approximately 299 and 357 feet below grade, at an elevation of approximately-45 to -57 feet (MSL). Groundwater flow in this aquifer is to the northeast (see Fourth Quarter and Annual 2006 Groundwater and Surface Water Monitoring Report, Corral Hollow Sanitary Landfill). Corral Hollow EMP WP.DOC 1-2 February 28,2007