2011-05-18_PERMIT FILE - C1981012 (2)

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Probable Hydrologic Consequences New Elk Mine 27 8. PROBABLE HYDROLOGIC IMPACTS OF MINING IN THE APACHE AND ALLEN COAL SEAMS. Probable hydrologic consequences of the mine are attributable to drawdown of the water table due to mine dewatering, subsidence of strata overlying mined areas, surface disturbances, operation of the RDA, and consumption of water to run the preparation plant and control dust during mining. Groundwater inflow to underground workings at the Golden Eagle and New Elk mines has been studied and documented in analyses by prepared by WWL (1980) and Greystone (1994). Mine inflows were mapped on an annual basis prior to 1994 and compared to the locations of stream channels along the Purgatoire River, Apache Canyon, Ciruela Canyon, Santistevan Canyon, and Zamora Canyon. No correlation was apparent between underground inflows and surface water locations (Greystone, 1994). WWL estimated maximum inflows to the Golden Eagle Mine to be 120 and 73 gpm from the roof and floor of the Maxwell seam respectively (193 gpm total). The estimate was based on a manual calculation of inflow to a room and pillar mine with an aerial extent of six square miles and a perimeter of 4.6 x 10 feet. The actual observed inflow to the mine in 1993 at approximately 20 percent of the calculated area (1.2 square miles) and 76 percent of the perimeter (3.5 x 10 feet) was 208 gpm. Sources of inflow to the mine included 74 gpm from the roof and floor, 34 gpm from drill holes and shafts, and 100 gpm from a fault. Greystone updated the inflow analysis for the golden Eagle Mine in 1994 based on new information and observations of inflow from previous mining. Their analysis assumed that the Maxwell, Blue, and Red coal seams were the dominant sources of inflow to the mine and that contributions from channel sandstones and other sources were minor and localized. Greystone recognized that heads in the overburden varied significantly in different strata at different locations and used a range of water levels above the coal (195.4 to 482.2 feet) to bracket calculations of potential inflow. Based on calibration of calculated inflows to observed inflows, Greystone concluded that the transmissivity of the Maxwell seam at the Golden Eagle mine is about 1.98 ft/day. Packer test data indicate that the transmissivity (T) of the Maxwell seam at New Elk Mine is somewhat lower and is calculated to be 0.03 ft /day (T = Kb) using an average hydraulic conductivity (K) of 0.013 ft /day and a seam thickness (b) of 2.5 ft. Calculated transmissivities of the Allen (2.8 ft'/day, b = 5.5 ft), Apache (2.1 ft /day, b =4.5 ft), and Blue (1.72 ft /day, b = 4 ft) seams at the New Elk Mine are similar in magnitude to the value estimated for the Maxwell seam at the Golden Eagle Mine. Observed inflows to the Golden Eagle Mine ranged from 0.09 gpm/acre to 0.125 gpm /acre during operation. Given the heterogeneous stratigraphy and relatively simple structural setting of the mine, Greystone concluded that the observed inflows provided a direct relationship that could be used to predict future mine inflows. Observed inflows to the New Elk Mine during previous operation are reported to be between 123 and 177 gpm (Greystone 1994). Approximately 109 gpm of the inflow originated from a mined -out area of totaling 2,800 acres (0.039 gpm /acre). Packer test data indicate that the Allan, Apache, and Blue coal seams have similar hydraulic properties in the area of the New Elk Mine (K = 0.43 to 0.51 ft /day and T = 1.72 to 2.8 ft /day). The calculated range of transmissivities is also similar to the Maxwell seam at the Golden Eagle Mine. Based on the previously observed groundwater inflows to the two mines, inflow to the New Elk Mine during mining of the Allen and Apache seems is expected to be between 0.04 and 0.1 gpm/acre. Inflow along faults (to the extent that they are present) could modify the range upward. The maximum observed inflow along a fault is 100 gpm at the Golden Eagle Mine. It is likely, however, that mine inflow will be less than predicted based on historical observations because of extensive dewatering of the coal seams related to CBM production. Mine dewatering has the potential to draw pre- mining water levels down in Allen and Apache seams by 200 or more feet in the permit area. Calculations performed by Greystone suggest that mine dewatering could draw water levels in the coal seams down by one foot at distances of up to 2.1 miles away from the underground workings. Given the heterogeneity and limited lateral continuity of coal beds in the Raton Formation, this estimate is considered to represent a maximal distance. Water levels in the coal seams are currently depressed by groundwater pumping for CBM production. Mine dewatering will incrementally 4164A.110421 Whetstone Associates rrn c1< r_11+ Qrn� D 1 n Q n MA I