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

Home Browse Search

Probable Hydrologic Consequences New Elk Mine 28 increase the currently observed drawdown in the Allen and Apache Coal seams. Water level data from packer testing indicate that steep vertical gradients currently exist in the strata adjacent to coal seams. The steep gradients are related to mine dewatering and /or CBM production and suggest that there is limited potential for propagation of drawdown across bedding. No flow or water quality impacts are expected to occur to surface water including springs and seeps due to mine dewatering. Approximately half of the groundwater wells located within a one -mile radius of the permit boundary are completed in alluvium at depths less than 66 feet. No impacts to water level or water quality are expected to occur in alluvial wells. Well records from CDWR indicate that there are 19 permitted wells in the Raton Formation within a one mile radius of the permit boundary. The wells vary in depth from 30 to 750 feet, and have completion water levels ranging from 5 to 598 feet bgs. A study by Watts (2006b) evaluated the potential for groundwater pumping from coal seams in Las Animas county to impact water levels in wells that are used for water supply. He noted that because the permeability of stratified sedimentary rocks generally is greater parallel to bedding than across bedding, the drawdown of water levels in coal seams would have the greatest potential for interfering with nearby water - supply wells in areas where there is little vertical separation between the coal seam and the well production intervals. The analysis assumed that a vertical separation of 100 feet between coal seam and water supply wells would be protective of water supplies, but it was careful to note that the required separation would depend on local geologic conditions. Mining related subsidence will increase vertical permeability for up to 195 feet above the highest mined coal. Wells that produce from 100 feet or higher above the zone of increased vertical permeability are not likely to be adversely impacted by mining. Two bedrock wells are located near the planned mining area in the Allen Seam. The wells are in the northeast half of Section 24, T33E, R68W. Well number 284213 is a monitoring well owned by NECC and is completed in the Allen Seam. The well is 442 feet deep and is designated as NE -1 -10 for the monitoring program. The other bedrock well (permit number 264440) is 200 feet deep and is completed in an unidentified coal bed. The owner of record for the well is Helen Armstrong. Four shallow alluvial wells (12, 13, 14 and PAW -9 on Map 8) are also are also located in northeast half of Section 24, T33E, R68W. Wells 12, 13, and 14 are on land owned by J.I. Vialpando. Well PAW -9 is a monitoring well owned by NECC. Mining of the Allen Seam may have the potential to reduce water levels and impact water quality in the Armstrong well. In the event that water in the well is impacted by mining, the water supply will be replaced with water from the city of Trinidad (city water). Well NE -1 -10 is designed to monitor potential impacts to groundwater in the mining horizon, and impacts to water levels or water quality in the well are not an issue with respect to water supply. Wells 12, 13, 14 and PAW -9 are situated more than 400 feet above the planned mining horizon, and impacts to water quality and availability are not anticipated to these wells. In the event that unanticipated impacts occur the water supply will be replaced with city water. Dewatering discharge from the mine will be treated before being released to the Purgatoire River, and impacts to water quality in the river from discharged water are expected to be similar to those currently observed (i.e an average increase of about 40 mg /1 TDS downstream from the mine). Water quality in the mined coal seams in the permit area is expected to be impacted by the mining operation. Impacts to water quality will include an increase in TDS, mainly in the form of sodium and bicarbonate. Background TDS concentrations in the Allen and Apache coal seams are estimated to be about 435 mgl/L and 1,105 mg/L respectively based on the electrical conductivity (EC) of groundwater from monitoring wells NE -1 -10 (623 µS /cm – Allen) and NE -6 -10 (1,106 µS /cm – Apache) and the assumption that TDS is equal to about 70% of EC. Observed TDS in the sealed portion of the New Elk Mine has averaged 1,628 mg/L. After mining, the TDS concentration of groundwater in the Apache and Allen coal seams near the underground workings is expected to be similar to water in the sealed mine. Monitoring of impacts during mining and previous studies of probable hydrologic consequences (WWL, 1980 and Greystone, 1994) indicate that underground inflows and mine dewatering have little or no communication with surface water. This conclusion is consistent with the subsidence analysis and 4164A.110421 Whetstone Associates Iry cA QtAn 13— 1 no n MAI i