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• GEOLOGY The site is covered by unconsolidated surficial deposits that overly bedrock formations. Tlic surfic.al deposits consist of the Rocky Flats Alluvium, valley fill alluvium, and colluviunt. The Rocky Flats Alluvium was deposited as a series of alluvial fan materials by streams cntanating from Coal Creek Canyon. The thickness of these deposits is generally greater to the west and thins to the east. The fan consists of cobbles, gravels, sand and fine grained material. The deposits generally fine eastward as the distance Crom the mountains increases. The thickness ranges from 10 to 100 feet. The valley fill alluvium occurs in modern stream valleys and m:ty exceed 10 feet in thickness. Valley fill sediments are characteristically coarse grained but fine grained materials occur in the lower reaches of the valleys. Cllluvium occurs on the sides and base of hills and slopes. These deposits are the product of erosional processes and consist. of soil and rock fragments derived from bedrock, and also sand and gravel derived from the older Rocky Flats Alluvium. The colluvial deposits rarely exceed 2 [0 3 feet in thickness. Bcdrot:k formations occur beneath the overlying surficial deposits in the site arcs and include the Fox Hills Sandstone, Laramie Formation, Arapahoe Formation, and Pierre Shale. These formations are cretaceous age. The Fox Hills Sandstone is approximately 75 feet thick in the site ar~.a and is comprised of sandstones interbedded with beds of siltstone and shale. It dips to the east at approximately 45 to 50 degrees beneath the site area. The Laramie Formation is approximately 700 feet [hick, and is divided into two units consisting of a lower sandstone unit • and an upper shale unit. The lower sandstone unit contains clay deposits. The upper shale unit consists of an organic-rich claystone. The Arapahoe Formation is the uppermost bedrock formation beneath the site and ranges up to 250 feet thick. The Arapahoe formation consists primarily of claystones and sandstones. The sandstones occur in five channel sequences that occur ;tt various depths and a basal sandstone. The Pierre Shale is a lower Cretaceous formation consisting primarily of dense shale with minor intercalated sandstone strata. HYDROGEOLOGY The unconsolidated deposits are hydraulically connected and are frequently referred to as the unconsolidated Quaternary aquifer. The bedrock aquifers are composed of the lower sandstone unit of the Laramie Formation and the Fox Hills Formation referred to as the Laramie-Fox Hills aquifer and the Arapahoe aquifer. The Arapahoe aquifer consists of the sandstones within the Arapahoe Formation. The upper claystone unit of the Laramie Formation provides an upper hydraulic seal for the Laramie-Foxhills aquifer. The lower seal is provided by the Pierre Shale. •