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L Section 3 <br /> ► � <br /> L Subsurface Conditions <br /> L <br /> 3.1 Site Geology <br /> Subsurface sediments encountered at the project site consist primarily of flat-lying stream- <br /> Ldeposited alluvial deposits. The alluvium consist of brownish grey gravel in a silty sand <br /> matrix. At the tank cavity midpoint, pea gravel backfill was observed to a depth of <br /> approximately 9 feet bgs. Blue grey siltstone in a matrix of sand, silt and clay (possibly <br /> weathered bedrock) was encountered at the 9 to 10-foot depth interval. <br /> LBased on a review of local stratigraphic data (logs of nearby monitoring wells), the alluvium is <br /> interpreted to attain a thickness of approximately 20 feet at the project area. The alluvial <br /> Ldeposits unconformably overlie bedrock units of the Neroly Formation. The uppermost <br /> bedrock unit consists of siltstone and claystone and is speculated to attain a thickness of <br /> Los. approximately 10 to 15 feet at the project site. Blue-grey sandstone extends beneath the <br /> siltstone unit to the logged depth of approximately 220 feet bgs. The Neroly bedrock units <br /> strike northwest-southeast and dip 8° to 18° to the south-southwest (LLNL, April 1994). <br /> 3.2 Site Hydrogeology <br /> L <br /> Groundwater at the project site has been documented within the alluvial deposits at a depth of <br /> Lapproximately 9 feet bgs. The aquifer is designated as the Cierbo Formation hydrogeologic <br /> unit, an unconfined to semiconfined aquifer. The underlying siltstone unit acts as a confining <br /> Llayer, thus preventing hydraulic conductivity with deeper aquifers. Groundwater in the <br /> alluvium flows eastward at the site before turning northward at Corral Hollow Creek (LLNL, <br /> LApril 1994). <br /> L <br /> L <br /> L4001 <br /> TETRA TECH:Castle Rock Forest Fire Station,Preliminary Site Assessment Report Page 7 <br /> L <br />