2017-01-04_PERMIT FILE - C1981044A

Home Browse Search

The calculated inflows are presented on Table 76, Summary of Mine Inflow Input Data - No. 6 Mine. The input data is presented on Table 75, Estimated Mine Inflows. The condition in the last stages of mining would most closely fit the steady-state conditions. Summary. The Jacob -Lohman and Theim methods produced similar predicted mine inflows values. They both predict the highest inflow to occur after year 5 of mining and to range from 1,300 to 1,800 gpm combined inflow from the No. 5 and No. 6 Mines. To be conservative, the highest seepage inflow value for the No. 6 Mine was selected as the prediction to be used in the estimate of the maximum probable inflow. This value, 700 gpm in year 20, was doubled to take into account the unpredictable hydrologic conditions. Only the inflow from the No. 6 Mine north of the No. 5 Mine was doubled. The inflow from the No. 5 Mine has been fairly constant for a number of years and significant incremental inflows are not expected. No unanticipated incremental inflows should occur in the portion of the No. 6 Mine that is under the No. 5 Mine, as no faults were encountered in the overlying mine. The total worst- case maximum inflow is estimated to be 2,490 gpm. Impacts on Aquifers Mining activities at the No. 5 and No. 6 Mines are affecting piezometric levels in at least two sandstone aquifers as well as the coal beds ("E" and "F" coal seams). Declines in piezometric levels have been recorded in observation wells in the Middle Sandstone and the Trout Creek Sandstone. No declines in piezometric levels have been observed in the Twentymile Sandstone. The Trout Creek Sandstone piezometric levels were affected by continuous use of the aquifer for mine water supply. Water level drawdowns in the vicinity of both Trout Creek Sandstone wells have been evaluated using the Theis, non - steady solution for drawdown around a pumping well. The one -foot drawdown caused by the historic pumping at the No. 5 Mine Wells is estimated to occur at a distance of about one mile from the pumping well, which is approximately the distance to the Okie Plaza Well. In estimating the drawdown due to pumping at the No. 5 Mine Wells near the No. 5 Mine portal, the following data and assumptions were used: • The aquifer is confined, and assumed to be semi -infinite, flat -lying, and homogeneous • The sandstone aquifer thickness ranges from 62 to 91 feet over the property; the hydraulic conductivity lies between 0.1 and 1 ft/day; the transmissivity thus lies between 6 and 91 sq. ft/day • The storativity is assumed to lie between 0.001 and 0.0001 • The pumping rate at the No. 5 Mine Wells is 2.5 gallons per minute, which was the average for 1992 • Estimated drawdowns were first calculated for the Okie Plaza located at a distance of 5,200 feet from the No. 5 Mine Wells • The drawdown was calculated for constant pumping for 500 days Table 76A shows that the best estimate for drawdown at the Okie Plaza Well, using the Theis non -steady solution for 500 days of pumping, is on the order of 1 foot. The bounding range for this estimate is 0.2 to 7 feet. The high estimate of 7 feet drawdown would apply only in a uniformly poor -yielding siltstone. Gross aquifer impacts are also difficult to estimate for the coals because there is no potentiometric basis. There are no monitoring wells screened in the mined seams to determine drawdown outside mined areas. There is typically a sharp discontinuity in potentiometric head outside a mined panel, the de -stressed perimeter zone being dewatered and unsaturated, so that computing drawdown as if due to a very large well is not very practical. McWhorter (1981) proposed empirical methods for computing flow into coal faces, and distance/drawdown relations for leaky coal seams, from sparse data. McWhorter gave the following equation relating drawdown sX, at distance .3 from a mined face where the drawdown at the face is so, and where 3 is the "leakage factor". TR14-36 2.05-39 Revised 03/20/14