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Page 3 Tracy Tire Fire Celeste Temple,Project Officer LOCAL GEOLOGY/HYDROGEOLOGY Based on review of the documents referenced in Attachment A, it is anticipated that the following stratigraphic sequence would be encountered in the vicinity of the site: • silty gravels with some lenses of clayey silt from the ground surface to a depth of approximately 25 feet; • silts and sands with varying amounts of clay and gravel ranging from a depth of approximately 25 feet to approximately 300 feet, with the upper aquifer encountered at a depth of approximately 75 feet; • the top of the Corcoran Clay ranging from a depth of approximately 300 feet to approximately 380 feet; and • the lower aquifer encountered at a depth of approximately 380 feet. It is important to note that a unit of hard silt with varying amounts of clay, sand, and gravel commonly encountered at approximately 50 feet represents the practical limit to which gravel can be mined. The unit is commonly referred to as a clay; however, discrete soil samples were collected and logged as silts by geologists registered in the State of California at two locations within one mile of the site. Harding Lawson Associates collected and logged samples at 29900 South MacArthur Rd., and RESNA collected and logged samples at 29099 South MacArthur Rd. For this reason, the logs produced by the two above-referenced consultants were used as the primary sources for constructing the shallow stratigraphy expected near the Tracy Tire Fire site. SITE TOPOGRAPHY The elevation of natural ground at the site is approximately 150 ft MSL. The tires and the fire residues are situated in the bottom of a former gravel pit at a depth of approximately 50 feet below natural grade (100 ft MSL). Rainwater collects in the pit and infiltrates through the tires/residues into the subsurface soils. No rainwater contacting the tires exits the site as runoff. SITE GEOLOGY/HYDROGEOLOGY On August 20, the START mobilized to the site and collected soil samples with the GeoprobeTM 5400 machine and Macro CoreTM sampling system, which were operated by the U.S. Coast Guard. Soil samples were collected continuously from the surface to a depth of twelve feet at two locations. A START geologist logged the samples using the Unified Soil Classification System (see Attachment B, Borehole Logs). The lithologic characteristics of the logged samples were consistent with the stratigraphic sequence described above. Interbedded gravel, sand, silt, and clay were observed and logged to a depth of 12 feet. Five select samples were submitted to the laboratory of Ninyo & Moore Geotechnical and I:\000610\SF389_0339TTRZXX\JTTzur3O.799.wpd 10/17/99 6:56 PM