1981-08-20_GENERAL DOCUMENTS - C1981038

Home Browse Search

-29- —� Factors which limit the inflows of groundwater into the existing mines are: 1) The low permeability of the coals and their associated roof and floor materials; 2) The steep topography; and 3) The very thick overburden above the mines. The permeability of the coals and associated strata have been previously addressed. The steep topography limits recharge to potential aquifers and to fault and fracture systems. The potential sandstone aquifers are cliff formers in the basin and, as such, have limited surface area exposed for recharge along slopes. The steep topography and steep stream gradients also favor runoff over infiltration, thereby reducing the duration of exposure and the amount of water exposed to sandstones and fractures within the channels of the ephemeral tributaries of the North Fork of the Gunnison River. Higher inflow rates, however, have been measured and mapped in the Somerset mine which has mine workings below 2 perennial tributaries, Bear Creek and Hubbard Creek. This is due to the lower stream gradient and greater quantity of water flowing in these larger drainage basins. The thick overburden along with the lenticular and interlayered nature of strata within the Mesa Verde Formation limits the vertical movement of water down from the surface to the coal seams, and also limits the lateral movement of water along the regional dip of the stratum. This produces small, localized groundwater flow systems within the overburden strata which is controlled more by topography than regional geologic structure. This is evident in the many ephemeral springs located 'throughout the North Fork drainage basin. The factor which is the most significant source of mine inflows is the faulting and fracturing within the Mesa Verde Formation. The largest inflows reported into the Somerset mine, Hawk's Nest mine and Orchard Valley mine have been ( - through fractures in the roof and from faults. Dewaterina of fractures and faults within the mine may deplete spring and stream flows. The mines within the North Fork drainage basin, therefore, have been required to do extensive monitoring of springs, streams and inflows during mining. To date, neither catastrophic mine inflows nor catastrophic depletions of surface flows have occured due to coal mine subsidence within the North Fork drainage basin. Both the U.S. Steel Somerset mine and the Western Slope Hawk's Nest mine have undermined tributaries to the North Fork and have not yet observed marked increases in post-mining inflows due to subsidence beneath these tri- butaries. However, with time, the possibility of subsidence effecting stream flow still exists. Subsidence could reduce stream flow by directly diverting surface flow and alluvial groundwater into the bedrock groundwater system. This effect is undesirable both because of the hydrologic effects and the mine operation problems it could produce. The applicant has not proposed a subsidence protection plan for either Stevens Gulch or East Roatcap Creek to protect stream flow. How- ever, protection plans for these streams are not being required as a part of this permit review. The Division prefers that site-specific monitoring data concerning subsidence and it's hydrologic effects be taken into account in the design of any protection plan. WHEN THE OPERATOR SUBMITS A PERMIT APPLICATION TO UNDERMINE STEVENS GULCH AND/OR EAST ROATCAP CREEK, THE SUBSIDENCE PROTECTION PLAN DEVEL- OPED FOR THESE AREAS SHOULD BE,BASED UPON MONITORING INFORMATION COL- LECTED DURING THE EARLIER PHASES OF MINING.