2010-01-25_PERMIT FILE - C1981044A (4)

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McWhorter (1981) proposed empirical methods for computing flow into coal faces, and distance/drawdown relations for leaky coal seams, from sparse data. McWhorter gave [he following equation relating drawdown s„ at distance 3 from a mined face where the drawdown at the face is so, and where 3is the "leakage factor". x=3.ln~o); s, 3=(Tr.b) ~h ka T~ is the coal transmissivity, and b, and K, are thickness and hydraulic conductivity for leaky strata or "aquitard". While it is difficult to determine these parameters, the analysis was performed for a leaky range (represented by envelope strata hydraulic conductivity), which is thought to represent bounding conditions. For coal transmissivity, T~ equal to 30 ft2/day (from page 2.04-14), so equal to 700 feet (initial head is approximately 700 feet above the seams), b~ equal to 100 ft and k, equal to 0.01 fdday, a 1 ft. drawdown is estimated to occur at a distance of 3,300 feet from [he mine perimeter, and the 5 ft. drawdown at a distance 2,700 feet from the mine perimeter. If the enveloping strata have conductivity only 0.001 ft/day, the 1 ft. drawdown occurs at 10,400 feet. The extent of drawdown should be neazer the low estimate based on the observed very low distributed mine inflow rates and the locus of inflows in fracture zones. Leakage will dampen uniform drawdown within the mined seams, and fracture discontinuities will locally overwhelm it. Ground water in the William's Fork alluvium will not be directly affected by underground mining under this permit. Mine water discharge to the William's Fork River is not anticipated to have a measurable effect on water levels or water quality in the William's Fork Alluvium. Ground water in the Yampa River alluvium will not be directly affected by the development of underground entries beneath the alluvium under this permit. A summary of water quality data collected from Yampa alluvial wells in Big Bottom is found on pages 2.04.7-23a through 2.04.7-23h. As can be seem from this data, quality of water in the alluvium is variable. Quality and quantity is highly subject to seasonal fluctuations. The water quality of all three sandstone aquifers do not indicate any significant trends or adverse impacts related to mining at Mines 5 and 6. The piezometric declines in the Trout Creek Sandstone are thought to be related primarily to pumping of the three mine water supply wells completed in this unit and not from mining the overlying E and F coal seams. The piezometric levels represented by water levels in the No. 5 Mine Well and the Okie Plaza Well, recovered to the levels measured in 1986 through 1988. The recovery of the two wells is apparently from reduction in pumping for use since both wells are used for water supply as well as monitoring. The recovery would not have occurred if the declines observed in 1988 and 1989 were due to dewatering of the E and F coal seams because mine dewatering has continued. 1991 levels in both the No. 5 Mine Well and the Okie Plaza Well appear to be 25 to 50 feet lover than the levels measured when monitoring was initiated in 1985 and 1987, respectively. Water level fluctuations of up to 250 feet have been observed which are undoubtedly related to pumping for use but no seasonal pattern is evidenced or expected. It is likely that the fluctuation is related to pumping for0 use. Since the base of the E coal seatn in the vicinity of both Trout Creek Sandstone wells is above the current piezometric level in these two wells, it is not likely that the recent fluctuations in piezometric levels in the Trout Creek Sandstone aze related to mine dewatering in the overlying E and F coal seams. Plots of field conductivity measurements for the two Trout Creek Sandstone wells are presented in Figure 58a, and Permit Renewal No. 4 2.05-40 Revised 3/5/03