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TETRATECH RMC GEOLOGY Mr. Mike Refer May 12, 2004 Page 2 The mine site is located in the alluvial valley of the South Platte River. Geologic mapping at the site (Smith, et al., 1972)t indicates the mine is on the recent alluvial terrace of the South Platte River. Geologic mapping site by Trimble and Machette (1979)Z indicates this terrace corresponds with the Piney Creek and Post Piney Creek Alluvium. The mapping also indicates that the bedrock underlying the alluvium is most likely claystone and sandstone of the Arapahoe formation. Detailed mapping based on drill logs (VanSlyke, et al, 1988)3 also indicates that the underlying bedrock is the Arapahoe Formation. Data for these analyses was extracted from 77 exploratory borings drilled at the site by ADCO Consultants and 53 exploratory borings drilled at the site by Tetra Tech RMC. The logs indicate that the general subsurface profile consists of approximately one to 10 feet (three feet typical) of silty sand overburden overlying approximately five to 46 feet (23 feet typical) of sand and gravel on top of claystone bedrock. The total thickness of the alluvial deposit encountered in the borings ranges from approximately four to 50 feet (30 feet typical). Groundwater depths in the mine area at the time of drilling ranged from approximately eight to 12 feet below the ground surface, but are expected to be higher during irrigation and periods of higher runoff. Typically, the sand and gravel will form most of the mine slope. These soils aze generally strong and stable, particularly when dewatered. The Tucson Gravel Mine adjacent to this site to the north has nearly vertical walls. The presence of a mud lens within the gravel in parts of the site represents a weaker zone and has been included in the calculations below. STRUCTURES WITH 200 FEET OF DISTURBED AREAS The known, permanent, man-made structures within 200 feet of the proposed mine area are listed below: - Tucson Road, including pavement and utilities - Highway 7 (160th Avenue) ,including pavement and utilities - Buildings at northeast comer of west cell - Gas well, tank, and pipeline at west side of west cell - Power poles along the west side of the west cell - Brighton Ditch along the west side of the west cell - Brighton Retum Ditch along the south end of the southwest cell - Gravel road on west side of southwest cell Smith, Schneider, and Petri, 1967, "Groundwater, South Platte River Basin," USGS Water Supply Paper 1658. Trimble &Machette, 1979, "Geologic Map ojtbe Greater Denver Area, Front Range Urban Corridor, Colorado," USGS Map I-856-H. VanSlyke, Romero, Momveo, and Wacinski, 1988, "Geologic St/vcmre, Sandstone/Sibstone fso[ith, and Location ojNon- Tributary Ground Water for the Arapahoe Aquifer, Denver Basin, Colorado," DBA-3, Colorado Division of Water Resources.