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• -8 VI. GROUND WATER HYDROLOGY A detailed discussion of the groundwater }rydrology can be found in Volume III, Tab 7 of the permit application. In the vicinity of the Seneca II mine, water table conditions exist in alluvial aquifers and/or other unconsolidated aquifers. Artesian conditions exist in bedrock aquifers which are overlain by impermeable shales and located down dip of recharge areas. Aquifers under artesian conditions may discharge through subcrops and out- crops exposed in stream valleys where the potentiometric surface is above ground level. The Trout Creek Sandstone and the Twenty Mile Sandstone are major, regional sandstone aquifers in the Mesa Verde Group rocks and have reported yields as high as I00 gpm. Several minor, Local aquifers occur in the Williams Fork and Iles Formations. These aquifers consist primarily of thinly interbedded, very fine to fine grained sandstones, siltstones and coal beds. Yields are generally low and are highly variable depending on saturated thickness, porosity, permeability and artesian pressure. inconsolidated fluvial deposits in the vicinity of the permit area vary from narrow bodies of sandy silt and clay deposits along the upper reaches of intermittent and perennial streams such as Grassy and Fish Creeks, to coarser sand and gravel sized deposits of the Yampa River. Major regional structural features appear to control the overall lateral movement of groundwater in the region. Groundwater generally moves down dip from recharge areas located on topographic and structural highs to discharge areas located in topographic and structural lows. Thus water entering recharge areas on the West flank of the Tow Creek Anticline will likely move westward and northwestward along the plunging axis of the Hayden Syncline. Some of this groundwater will be discharged into the Little Grassy Creek alluvium and will exit the permit area as surface water flow. Groundwater flow on the south- east flank of the anticline will initially move South and southeastward into the Twenty Mile Park Syncline and then will flow primarily westward and northwestward into the Hayden Syncline across the structural divide separating these two syn- clines. A small component of groundwater flow from the southeast flank may move northward and eventually discharge into the Yampa River. Another possible avenue for groundwater discharge in the region is a series of normal faults which exist to the North and South of the permit area. The dry condition of Faults encountered during mining in the northern part of the permit area indicate, however, that faults probably act as barriers rather than conduits for groundwater flow. In general, there arc four basic water-bearing lithotypcs at the Seneca II mine. These consist of: (1) the discordant sandstones and shales which make up the over- burden and strata directly below the coal, (2) the fairly uniform Wadge and Wolf Creek coal seams, (3) the nonindurated deposits of spoil and alluvial material and, finally, (4) the deeper continuous sandstone aquifers. each of these systems can be further subdivided in regard to their position on the stratigraphic column.