2023-04-19_PERMIT FILE - C1981010A (8)

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III IIIIIIIIIIIIIIII 999 2.7.5 Pre-Mining Conditions - Ground Water DocDate:12/11/2001 Ground water investigations were begun at Trapper Mine in 1974. These investi- gations obtained pre-mine baseline conditions for primarily the western portion of the permit area, i.e. the area from Pyeatt Gulch west. This was [he area to be affected by mining and reclamation operations covered under Colorado permit C-81-010. In 1985, pre-mine baseline sampling was expanded to cover `the eastern portion of the permit area, i.e. Pyeatt Gulch and the area to the east. The goal of this sampling was to obtain data for the expansion of future mining and reclamation operations, to be covered by a permit renewal in 1987. These pre-mining conditions are presented in the following subsections. Those subsections numbered without the letters 'a' following the number are outlining the baseline conditions primarily in the west portion of the permit area (i.e., 2.7.5.1). Those with the letters, discuss the east portion of the area (i.e., 2.7.S.la) . In many cases, data collected during the same term period from wells in the west- ern portion of the permit area was used [o assist in describing aquifer charac- teristics of the eastern area. 2.7.5.1 Aquifer Characteristics Ground water at the Trapper Mine site occurs in both bedrock and alluvial aqui- fers. The hydrogeologic units of importance to this study are the Twenty- mile Sandstone, the Upper Unit of the Williams Fork Formation, the Lewis Shale, and the Yampa River alluvial aquifer. The bedrock aquifers of importance to this study are the Twentymile Sandstone and the Upper Unit of the Williams Fork Forma- tion. Fresh ground water probably occurs in the units below the Twentymile Sand- stone, including the Lower Unit of the Williams Fork Formation and the Iles Formation. 2-455 REdISED FEB 13 '87