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Page 2 Tracy Tire Fire Celeste Temple,Project Officer BACKGROUND The Tracy Tire Fire site is located at 29425 S. MacArthur Rd. in Tracy (see Figure 1, Site Location Map). On August 7, 1998, a fire was started at the site, then named the Royster Tire facility. It was estimated that two to seven million tires were stored on the 30-acre site. In order to minimize environmental impacts, no measures were taken to mitigate or suppress the fire. Fires burn now only at a few scattered locations. The volume of tires and strength of the blaze have steadily diminished. The EPA is now preparing to implement the Quality Assurance Sampling Plan (QASP) that was completed in December 1998 to investigate the extent to which soil and water have been affected by the fire. This report of site geology/iiydrogeology will be used to determine potential contaminant migration paths and guide the investigation. The geology/hydrogeology and the groundwater analytical results are described below. REGIONAL GEOLOGY/HYDROGEOLOGY This section was developed from the reference listed in Attachment A. The geology found in the area consists of a series of coalescing alluvial fans and plains. Low topographic relief created by headward erosion of the fan surfaces by active stream channels reaches about 10 feet. The alluvial fans contain about 100 feet of Pleistocene to Holocene age unconsolidated clay, silt, sand and gravel derived from several seasonal east-flowing streams draining the Diablo Range to the west. The alluvial fans are underlain by approximately 800 feet of Pliocene to Pleistocene age unconsolidated alluvium of the Tulare Formation. The contact between the top of the Tulare Formation and the overlying alluvial sediments is not distinguishable, because the lithology is undifferentiated. The Tulare consists of upper and lower members of mainly coarse-grained lenticular alluvium. The Tulare Formation conformably overlies the San Pablo Group marine sediments of Miocene age. Near the top of the Tulare Formation, at a depth of approximately 300 feet (elevation -150 feet above Mean Sea Level [ft MSL]), an extensive clay bed called the Corcoran Clay separates alluvial sediments into upper and lower aquifers. Near the site, the Corcoran Clay is approximately 80 feet thick. The lower aquifer is confined by the Corcoran Clay. The upper aquifer is unconfined. Recharge to the unconfined, upper aquifer in the alluvium and upper Tulare is by infiltration of water from rainfall, streams, and excess irrigation. The infiltrated water cannot permeate the Corcoran Clay and is recovered by wells drawing from the upper aquifer. The water level in the upper aquifer is at a depth of approximately 75 feet (75 ft MSL). Movement of groundwater in the aquifer is in a general northeasterly direction. The water quality is generally poor. The recharge areas for the confined, lower aquifer is along the foot hills of the Diablo Range where the Tulare Formation outcrops, and the Corcoran Clay is absent. The lower aquifer, which bears fresh water, is located at a depth of approximately 400-500 feet (-250 to -350 ft MSL). I:100061 O\.SF3 89_0339TTRZXXUTTzur30.799.wpd 10/18/99 8:28 AM