2010-12-17_REVISION - C1981019 (140)

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Model development DRAFT 14 • The hydraulic conductivity of the KM (layer 13) was set at 2.8 x 10-7 ft/d based on laboratory testing results. • The transmissivity for the interval from the bottom of the KM through the Trout Creek Sandstone (model layers 14 through 16) was set to 3 ft2/d based on measured values for the Trout Creek Sandstone. • In most bedrock units, excluding the Fab and KM, the vertical hydraulic conductivity was set as one order of magnitude less than the maximum horizontal value for the bedrock layers. This vertical anisotropy is based on experience in bedded sedimentary rocks where a 10 to 1 or higher vertical anisotropy is typical. • Initially, no horizontal anisotropy was applied to any of the bedrock model layers. During the calibration process, some horizontal anisotropy was included to simulate observed conditions during the pilot dewatering test. The hydraulic conductivity of the valley fill is variable, depending on the relative amount of coarse and fine sediments. Existing data indicate a wide range of hydraulic conductivity from less than 0.01 to 200 ft/d. The lower end of these values represents fine sediments that are probably not characteristic of the valley fill deposits as a whole. A reasonable range of values representing the average valley fill alluvium is considered to be 10 to 100 ft/d. Two hydraulic conductivity zones were represented in the valley fill to simplify the variability. A value of 100 ft/d was assigned to the valley fill in the upper reaches of the streams where sediments are relatively coarse and a value of 20 ft/d was assigned to the valley fill in the mid to lower portions of the stream where sediments are finer (Figure 3.9) No anisotropy was assigned to the valley fill layers. 3.4 Storage Prior to calibration, the drainable porosity of all bedrock units was estimated to be 5% based on similar rock types with porosity due to fractures. Also, initially the confined specific storage of all bedrock units was estimated to be about 5 x 10-7 1/m based on estimates from pilot test analysis of the response in the F/G sequence. This value was refined during the calibration process. The drainable porosity of all valley fill deposits was estimated to be 20% based on typical porosities for unconsolidated alluvial and fan deposit sediments. These deposits are not under confined conditions so a confined storage coefficient was not defined. 3.5 Structures The anticlinal and synclinal structures that are present in the Collom area are incorporated into the model as part of the model surface elevation contours. There are no major faults in the model area that are known to influence groundwater flow. Therefore, faults are not specifically represented in the regional model. 2572-R3 Colowyo Coal Company, L.P. Water Management Consultants