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No significant dewatering of the Twenrymile Sandstone is expected. The Twenrymile Sandstone is approximateh~ 500 feet above the "F" coal seam. In addition. the Middle Sandstone lies between the "F" coal and the T+ventymile • Sandstone. ]t is unlikely that any open fractures will extend from the longwalled areas up to the Twenrymile Sandstone. even in the approximately one-half square mile area that will be ]ong+valied in both the "E" and "F" coal seams. The physical effects of the longwall minine are addressed later in this section underSubsidence. The division has expressed concern about the consequences of coincident longwall mining the "E" and "F" coal seams on the overlying Twenrymile Sandstone. About one-half square mile of the Twenrymile Sandstone will be undermined by longwalling in both coal seams. (Refer to the No. 5 Mine Plan Map (Map 23). In this ar®, the Twenrymile Sandstone lies at its highest elevation and is near the coal outcrop. Therefore, portions of the Twenrymile Sandstone are not saturated and the remainder has very little head. While it is considered very unlikely that the Twenrymile Sardstone 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 similar to the Middle and White Sandstones (1 fi/day). (Refer to Table 1 ], Aqui~r Characteristics in the Region, in Section 2.04.7, Hydrology Information.) Therefore, the estimated transmissivity is 900 gpd/ft (120 f~/day). The storativity is estimated to be 0.0005. The Theis equation for unsteady radial Clow to a well was used b estimate the effect • u = 2693 rzslTt • s = drawdown, fi. • Q = flow rate, gpm • T = transmissivity, gpd/ft. • W(u)= well function • r = distance from pumping point to observation point, fi. • s = storativity, fraction • t = time (minutes) After ] 5 years of discharge at 100 gpm, the drawdown in the Twenrymile 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 nearest domestic well to this area that may be completed in the Twenrymile Sandstone is approximately one (1) mile distant. II is the Lux well (refer [o Table 12, Wells Registered with the State Engineer (Colorado) Neaz [he Mine Site, Section 2.04.7, Hydrology ]nformation). Sixty feet of additional drawdown should not significantly impair [he 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 Petmil term predicted drawdown in the Middle Sandstone unit associated with mining the"E"and "F"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 ovedying Twenrymile Sandstone, presented above, has been performed. The drawdown in Middle Sandstone Wel] TR-4 attributable [o the longwall mining in the "E" and "F" seams at a distance of approximately one (1) mile is determined to between 50 and 60 feet. This is comparable to the 60-foot drawdowns projected in the PHC for the overlying Twenrymile Sandstone at a distance of one mile from the location Midterm Response 2.05-43 ~ ~ j~ 7/30/01