Laserfiche WebLink
Hydrological system and available data DRAFT 5 <br />Most groundwater flow within the system is within the coal units, which range in <br />hydraulic conductivity from approximately 0.38 to 0.0004 ft/day. The highest hydraulic <br />conductivities were observed in the Fab, Ga, and Gb seams, with average values of <br />approximately 0.19 ft/day. The hydraulic conductivity of the interburden layers is <br />typically two or three orders of magnitude lower. The modest storativity of the coals <br />(-5 x 10-4) indicates that only small volumes of groundwater need to be removed to <br />create a measurable head change. <br />The bottom of the groundwater system relevant to the Collom Mine appears to be <br />defined by the KM marker bed within the Williams Fork Formation. This layer is about <br />2- to 4-ft thick and comprises clay with very low vertical hydraulic conductivity. The KM <br />is likely an aquiclude, effectively separating the units above it from the K and lower units <br />of the Williams Fork Formation and Trout Creek Sandstone. <br />Valley fill in the stream channels is derived from the Williams Fork Formation. The <br />valley fill is confined to the incised stream channels, which tend to widen downstream. <br />The thickness of the valley fill also tends to increase downstream. In general, the valley <br />fill is composed of heterogeneous, poorly-sorted clayey silts and sands. <br />Slug tests of the valley fill show a wide range of hydraulic conductivities from <br />196 to 0.0014 ft/day. The geometric mean of the available data is 3.3 ft/day. This <br />variability in hydraulic conductivity is normal in valley fill groundwater systems, where the <br />lithology can shift from a clayey sand to a sandy gravel over a very short distance. <br />2.2.2 Structure <br />The major geological structures in the area are the Danforth Hills Anticline to the south <br />and the Collom Syncline to the north. The axis of the asymmetric Collom Syncline <br />trends northwest-southeast and sub-parallels the Danforth Hills Anticline. The Morgan <br />Syncline, trends north-south across the Collom Syncline and influences the dip on the <br />western edge of the Collom study area. Structural deformation at the axis of the <br />syncline may create pathways for groundwater to migrate into the upper units and then <br />into the valley fill. East of the proposed Collom Mine at the Colowyo Mine, beds also dip <br />northeastward. Directly east of the Colowyo Mine, the strata dip steeply east into the <br />axis of the Elkhorn Syncline. These structures influence the dip of the geological units <br />in the model area. <br />There are no major faults in the area that are known to influence groundwater flow. <br />There are two vertical fracture sets, oriented approximately N66°W and N25°E that may <br />have some effect on groundwater flow. These fractures may result in some horizontal <br />and vertical anisotropy. <br />2.2.3 Recharge <br />The primary source of groundwater recharge to the model area is precipitation. In <br />general, precipitation occurs year-round, with a slight increase in the spring. During the <br />winter months, precipitation falls as snow, averaging approximately 70 inches of snow <br />per year in nearby Craig and Meeker. The estimated annual precipitation at the <br />proposed mine is approximately 17.5 inches. Local variations in precipitation are <br />expected based on elevation. The local groundwater basin and model area ranges in <br />elevation from 6,500 to 8,500 ft amsl. <br />2572-R3 <br />Colowyo Coal Company, L.P. <br />Water Management Consultants