2010-12-17_REVISION - C1981019 (153)

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r ii Executive summary • development of a 2D numerical pore pressure model to evaluate slope depressurization requirements; and review of the available information regarding jurisdictional waters, which includes "alluvial valley fills" of potential agricultural use and wetlands. The proposed Collom Mine is located north of the Danforth Hills in the Wyoming Basin physiographic province, which is a high, and plateau ringed by mountains. General elevations range from 6,000 to 8,500 ft above mean sea level. The dominant ecosystem of the area is sagebrush /grassland, with aspen woodland at elevation and small amounts of mountain shrub and riparian environments. The climate in the Collom study area is semiarid, with an average precipitation of 14 to 17 inches per year. Monthly average temperatures normally range from 22 °F in January and 67 °F in July. The proposed operation is located within the watershed of the Yampa River. Individual watersheds within the Collom study area include Straight Gulch, Collom Gulch, Little Collom Gulch, and Jubb Creek. In general, the surface water courses are narrow channels incised into the fill material of steep -sided erosional valleys. The principal geologic units in the area include the upper Cretaceous Iles and Williams Fork Formations of the Mesaverde Group. The uppermost unit of the Iles Formation is the Trout Creek Sandstone member, which is generally considered to be the first aquifer of regional significance. The Williams Fork Formation consists of approximately 1,000 ft of alternating beds of sandstone, sandy shale, mudstone, and coal. At the Collom study area, the Williams Fork Formation contains 18 different coal groups (A through O, TC, X, and Y). The proposed Collom Mine is adjacent to three significant folds: the Danforth Hills Anticline to the south, the Collom Syncline to the north, and the Morgan syncline to the west. Beds of the Collom deposit dip northeast to northwest at 4 to 9 percent. The stream valleys contain a mixture of colluvium and alluvium (i. e., valley fill sediments) derived from the Williams Fork Formation, ranging in thickness from a few feet to more than 70-feet. Surface water flow in the larger drainages of the Collom study area is intermittent, with the greatest flows occurring during the spring melt and in response to summer storms. Flows vary with season, but generally do not exceed a maximum of 0.5 cubic feet per second (cfs). Springs and seeps identified in the study area arise from perched layers within the valley fill material and /or bedrock outcrops. Water quality of the surface water is generally good, with circum - neutral pHs, and total dissolved solids (TDS) ranging from approximately 400 to 1800 mg /I. TDS tends to increase with distance downstream. The groundwater bearing units underlying the proposed Collom Mine are composed of numerous thin coal and sandstone units, with the coal unit generally having the highest permeability. The mudstone interbeds act as confining units. The coals have a range of measured hydraulic conductivities from approximately 0.38 to 0.0004 ft/day. The highest hydraulic conductivities were observed in the Fab, Ga, and Gb seams, with average values of approximately 0.19 ft/day. The hydraulic conductivities in the interburden are one to three orders of magnitude lower, depending on their lithology and degree of connection to a coal bed. The modest storativity of the coals ( -5 x 10 -4 ) indicates that only small volumes of groundwater need to be removed to lower the heads. Recharge occurs during the spring and early summer, principally to the southern outcrops located in the Danforth Hills. Lesser- amounts of recharge may occur from surface infiltration and from the saturated steam channel valley fill. 2572 -R Colowyo Coal Company Water Management Consultants