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

Home Browse Search

No significant dewatering of the Twentymile Sandstone is expected. The Twentymile Sandstone is approximately 500 feet above the "F" coal seam. In addition, the Middle Sandstone lies between the "F" coal and the Twentymile Sandstone. It is unlikely that any open fractures will extend from the longwalled areas up to the Twentymile Sandstone, even in the approximately one-half squaze mile area that will be longwalled in both the "E" and "F" coal seams. The physical effects of the longwall mining are addressed later in this section under Subsidence. The division has expressed concern about the consequences of coincident longwall mining the "E" and "F" coal seams on the overlying Twentymile Sandstone. About one-half square mile of the Twentymile Sandstone will be undermined by longwalling in both coal seams. (Refer [o the No. 5 Mine Plan Map (Map 23). In this area, the Twentymile Sandstone lies at its highest elevation and is near the coal outcrop. Therefore, portions of the Twentymile Sandstone are not saturated and the remainder has very little head. While it is considered very unlikely that the Twentymile Sandstone will be significantly affected, an estimate of worst case effects has been made. The sandstone is approximately 120 feet thick and is estimated to have a permeability similaz to the Middle and White Sandstones (1 ft/day). (Refer to Table 11, Aquifer Characteristics in the Region, in Section 2.04.7, Hydrology Information.) Therefore, [he estimated transmissivity is 900 gpd/ft (120 ft2/day). The storativity is estimated to be 0.0005. The Theis equation for unsteady radial flow to a well was used to estimate the effect. • S = (114.6 Q/T W(u) • u = 2693 r2s/Tt • s = drawdown, ft. • Q = flow rate, gpm • T = transmissivity, gpd/ft. • W(u) = well function • r = distance from pumping point to observation point, ft. • s = storativity, fraction • t = time (minutes) After 15 years of discharge at 100 gpm, the drawdown in the Twentymile Sandstone at a one (1), two (2) and five (5), and ten (10) mile radius from the longwalled area is estimated to be 60, 40, 20, and 7 feet respectively. The neazest domestic well to this area that may be completed in the Twentymile Sandstone is approximately one (1) mile distant, It is the Lux well (refer to Table 12, Wells Registered with the State Engineer (Colorado) Near the Mine Site, Section 2.04.7, Hydrology Information). Sixty feet of additional drawdown should not significantly impair the usefulness of the well, but it may require that the pump be lowered. The continued drawdown in the Middle Sandstone unit is determined to be the most significant impact related to mining operations associated with Mines 5 and 6. The previous Probable Hydrologic Consequences analysis for the previous Permit term predicted drawdown in the Middle Sandstone unit associated with mining the """"and """"coal seams at Mines 5 and 6. Since the extent and magnitude of drawdown were not quantified, a comparison between predicted and actual impacts cannot be performed. However, a comparison of actual drawdown impacts in the Middle Sandstone aquifer with the worst case drawdown projections for the overlying Twentymile Sandstone, presented above, has been performed. The drawdown in Middle Sandstone Well TRH attributable to the longwall mining in the "E" and "F" seams at a distance of approximately one (1) mile is determined to between 50 and 60 fee[. This is comparable to the 60-foot draw downs projected in the PHC for the overlying Twentymile Sandstone at a distance of one mile from the location Permi[ Renewal No. 4 2.05-43 Revised 3/5/03