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Mr. Dave Maxwell - 3 - July 26, 1989 <br />Subsidence monitoring to verify these predictions will be conducted in the <br />same manner as for the F-seam. Three subsidence monuments wi ll oe installed <br />above the 1S, 2S, and 3S panels in the B-seam. These monuments will be <br />monitored yearly until two successive years of data show no additional <br />movement other than that expected as a result of normal consolidation. It is <br />anticipated that subsidence will occur fairly rapidly following extraction. <br />The subsidence monitoring program should be able to verify that suosidence <br />has occurred and that the stability of the foundational area of the Upper <br />Waste Pile has not been Harmed. Tne Division concurs with WECC in the <br />assertion that it is preferable to subside the area where the pile will be <br />located prior to its construction. <br />Hydrologic Monitoring and Control <br />The prooaole hydrologic consequences discussion of mining at the Mt. Gunnison <br />Mine did not change in substance, and thus, the CHIS did not change. Some new <br />projections were made concerning the magnitude of previously identified <br />impacts. The revision application specifically addressed mine inflows and <br />discharges, impacts to the alluvium of the North Fork of the Gunnison, impacts <br />on spring flow from dewatering and/or subsidence, water consumption <br />requirements for the mine, and discharges from the wastewater treatment plant <br />into the North Fork of the Gunnison. The modifications from previous <br />projections will oe discussed below. <br />Mine inflows from the F-seam were originally projected at 99 gpm on a <br />long-term oasis; actual inflows have averaged 35 gpm initially and 11.5 gpm on <br />a long-term basis. Projections for the B-seam in the five-year permit area <br />are 15 gpm initially and 10 gpm on a long-term basis when mining 1.4 million <br />tons per year. This quantity would triple if mining production should double <br />to 2.8 million tons/year. If treatment of inflow is required, Pond MB-1 can <br />handle as much as 200 gallons/minute of mine inflow discharge in addition to <br />the runoff from the surrounding drainage area. Current inflow treatment is <br />required only for inflows from the active mining areas. <br />Tne addition of ramp and B-seam material to the lower refuse pile will have no <br />impacts on the North Fork of the Gunnison alluvium. The lower refuse pile has <br />two under drains to handle the Derched aquifer which is expected to develop at <br />the vase of the pile. At this time no aquifer has developed. Four out of <br />five monitoring wells next to the pile are dry, suggesting that there is <br />little flow through the pile or the surrounding substrate. <br />Tne operator identified Twelve new springs between the E and the F seam, <br />predominantly in the Sylvester Gulch and Lone Pine drainages. No springs were <br />observed oelow the E-seam. Originally there were concerns that <br />suosidence-related fractures in areas of shallow overburden would result in <br />mine inflows and elimination of spring discharge contrioutions to the <br />watershed. To date, no subsidence impacts to springs have been recognized <br />within the mine; F-seam mining in Lone Pine gulch with 120' of overourden has <br />not resulted in fracture flow. B-seam mining will occur in locations with <br />400' - 1000' of overourden. <br />Water consumption requirements for the Mt. Gunnison mining operation were <br />originally estimated at 150 acre feet per year. This figure was not revised <br />