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DRAFT <br /> <br />2 HYDROLOGICAL SYSTEM AND AVAILABLE DATA <br />3 <br />2.1 Hydrogeologic units <br />Regional groundwater flow in the vicinity of the proposed Collom Project occurs <br />principally in the water-bearing units of the Williams Fork Formation and Trout Creek <br />Sandstone (i.e., bedrock units). The groundwater flow regime also includes the valley <br />fill material in the stream channels. The model incorporates groundwater flow between <br />bedrock and the valley fill deposits in the main stream channels, as well as the <br />interactions between streams and groundwater. The regional model area is shown in <br />Plan 2.1. <br />The hydrogeologic units of the Collom study area can be broadly divided into bedrock <br />and valley fill sediments. The water-bearing bedrock units consist of the Cretaceous <br />Williams Fork Formation and the Trout Creek Sandstone Member of the underlying Iles <br />Formation. A generalized stratigraphic column is presented in Figure 2.1. <br />At the Collom site, the Williams Fork Formation is approximately 1,000 ft thick and <br />contains 18 different coal groups (A through O, TC, X, and Y) but includes up to 77 <br />individual seams. The targeted coal beds of the Collom Mine are the D through Gb. <br />Coal beds are separated by discontinuous interbeds composed of sandstone, siltstone <br />and mudstone. A key hydrologic unit within the Williams Fork Formation is the KM <br />marker, which is an ash bed that has weathered to clay and has a very low hydraulic <br />conductivity based on laboratory testing. It is considered to hydraulically isolate the <br />units above it from the units below it. <br />Local occurrences of Quaternary alluvium, colluvium, alluvial fan deposits, and landslide <br />deposits are present along the major stream channels and are collectively referred to as <br />valley fill deposits. These deposits are thin (0 to 40 ft) and coarse-grained in the upper <br />reaches of the stream valleys and thicken (40 to over 70 ft) and become finer-grained in <br />the lower reaches. <br />Hydraulic characteristics of the water-bearing units have been measured in single well <br />tests (both slug and pumping), and the pilot dewatering test. A summary of the <br />measured hydraulic conductivities from all data sources is provided in Table 2.1, which <br />is taken from the Pre-feasibility Hydrology Report. <br />• <br />2572-R3 <br />Colowyo Coal Company, L.P. <br />Water Management Consultants