2016-01-27_HYDROLOGY - M1979059 (2)

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2.2.1. Upper Silt and Loess Layer The upper unit is a low permeability silty sand (SM) and silty clay (CL). This layer has higher clay content and becomes thicker as the distance from the Big Thompson River 2.2.2. Lower Gravel Aquifer The lower unit is composed of sand and gravel and is the major water bearing strata in the area. The gravel is approximately 10 feet thick in the area. Local groundwater flows through the gravel and discharges to the Big Thompson River. The flow direction is generally from the west sub -parallel to the river flow direction. The groundwater in the gravel is mostly unconfined in the alluvial bottom lands where the river, ponds and proposed gravel pit are located. To the north and south of the alluvial bottom lands the upper silt and loess layer act as the confining layer. 2.3. Aquifer Characterization In conformance with the Division of Minerals and Geology Rule 6.4.7(2(b), we must characterize the alluvial aquifer to be excavated via mining. The characterization should describe the direction of groundwater movement in this shallow alluvial aquifer in and around the area to be excavated, transmissivity, and storage coefficient of the aquifer in the area of excavation. The direction of flow of groundwater in the alluvial aquifer is shown in Sheets 2,4 and 6 of the modeling report. Groundwater moves perpendicular to the contour lines, generally downstream and sub -parallel to the river in the base and post reclamation stages. During dewatering of each pit, ground water moves locally towards the pit. The hydraulic conductivity of alluvial sediments in the Denver Basin range from 100 to 2000 ft/day. The Office of the State Engineer recommends that we use 600 ft/day in this area. The transmissivity is the product of aquifer thickness and conductivity so where the gravel is 10 feet thick the transmissivity is 6000 sq.ft/day. The specific yield is assumed to be 0.20 and the storage coefficient for confined conditions is 1 E-5. Flying W Pit Groundwater Modeling Report Revised June 28, 2005 4of13