2022-03-10_REVISION - C1981044 - Laserfiche WebLink

Home Browse Search

underlying the No. 9 Mine. This estimate is based on the water level contour map for water levels measured in November 1995 (1995 Annual Hydrology Report, Permit C-81-044, Figure 12). The Twentymile Sandstone lies approximately 350 ft.below the"P"seam. Historic water levels in the Twentymile Sandstone have apparently not been effected by mining in the area, as they have remained relatively constant since 1981 (ibid., Figure 10) indicating a limited hydrologic connection between the aquifer and the mine. The maximum piezometric level of the White Sandstone is estimated to be approximately 6,300 feet where it overlies a location in the No. 9 Mine where measurable flow was encountered. This is based on the mine inflow study(Eagle No. 9 Mine Inflow Study,Map IV-1 la,5/8/83)and the December 1985 water level measurements(1985 Annual Hydrology Report, Permit C 81-044, Figure 17). In addition, the maximum projected piezometric level of the White Sandstone where it overlies any portion of No. 9 Mine is less than 6,320 feet; however,this is at the White Sandstone's outcrop area where it is probably not saturated. The White Sandstone overlies the mine by approximately 125 feet and outcrops approximately 1,000 feet north of the portal. Historic water levels in the White Sandstone were not significantly affected by the mining (refer to 1985 Annual Hydrology Report, Permit C-81-044, Figures 12—14), again indicating that there is not a good hydrologic connection between the aquifer and the mine. The indicated vertical groundwater gradient, based on a comparison of the piezometric surfaces in the overlying and underlying sandstones,is downward in the No. 9 Mine area. Even if there were a good hydrologic connection between the aquifers and the mine, the groundwater gradient indicates that the mine would not fill to the surface. Instead, it would refill to an equilibrium level corresponding to the potentiometric surface of the upper(White Sandstone)aquifer. None of the historic underground mines in the area are known to discharge. This includes the Wise Hill No. 4 Mine, which was accessed by a portal in the "bottom area" and had gradually filled with discharge the No. 5 Mine. It discharged briefly when water from the No.5 Mine was injected;however,when injection stopped,the discharge ceased. The following summarize key hydrologic considerations: • The piezometric surface in the aquifer below the mined seam is lower than the portal elevation • The piezometric surface in the aquifer above the mined seam is lower than the ground surface at the portal site • Where there was measured inflow to the No. 9 Mine,the highest piezometric head in the overlying aquifer was lower than the portal elevation • The vertical component of the gradient in the area is downward • The mine was driven in a down-dip and down-gradient direction • The mine portal is located at the crop line • There are no historic springs in the area None of the historic underground mines in the area are known to discharge, and the No. 9 Mine Portal is at a higher elevation than any of the old mines Effects of Seepage from No. 9 Portal Backfill The No. 9 Mine portal backfill will have surface area of approximately five acres. Using an infiltration rate of three- inches per year,the annual infiltration will be less than 1 gpm. This amount is insignificant and will,therefore,have no measurable effect on nearby aquifers. Also,the backfill area is stratigraphically separated from the nearest aquifer,the Twentymile Sandstone,by 360 feet of very low permeability interbedded claystone, siltstones,and sandstones. No. 5 and No. 6 Mine Water Handling System The No. 5 and No. 6 mine water handling system was removed and associated disturbance areas reclaimed as it is no longer needed to support ongoing or anticipated future operations. The area in which the ponds and dewatering boreholes were located was returned to the hay cropland land use. MT8 2.05-31 Revised 02/28/22