2008-10-07_PERMIT FILE - C1981047 (4)

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. 2.05.6(3) McWhorter, et al (1975), estimated that the normalized annual salt pickup rate varies from 0.23 metric tons per acre per inch in a surface and subsurface runoff regime of 16 inches where approximately 50 percent of the precipitation occurs as snow, to 0.79 tons per inch in a surface and subsurface runoff regime of leas than 1 inch. These are order-of-magnitude estimates that serve only as indicators of potential impact. Assuming the upper limit (0.79 metric tons/acre/inch) of salt pickup rate, 3.2 inches of annual runoff, of which 0.4 is surface and 2.8 is subsurface flow and deep seepage, salt pickup during the short-term lease period would be: 20 acres disturbed by mine x 0.8 Tlacre/in x 2.8 in/year (subsurface runoff) = 44.8 T/year or 2.24 T/Acre/Year The rough calculations above are by no means complete, but they do serve to put the situation in perspective, considering that the average annual salt loading from irrigation return flow in the Uncompahgre and Lower Gunnison River Valleys is perhaps 6 T/acre (McWhorter, et al, 1975). Thus, estimated salt uptake from the spoils is roughly one-third (2.24 / 6) that from irrigation return flow. puring 1984 and 1985 the mine bench was stabilized by • rebuilding the mine bench with an engineered fill of underground coal mine waste. Rock underdrains were placed in and under the coal mine waste to channel spring water and seepage from the coal mine waste fill. During 1986 it is estimated that the underdrains discharged 1.37 acre feet. in subsequent years the flow has generally decreased. During 1991 the underdrains discharged only 0.035 acre feet. The conductivity of the discharge from the north underdrain has ranged from 1600 to 3100 umhos/cm. The conductivity of the discharge from the south underdrain has ranged from 3300 to 5300 umhos/cm. The primary concern with regard to the underground coal mine waste is the impact on Hubbard Creek from water that seeps through the pile. The pile encompasses an area of about 5 acres. A worst case scenario would be in the case the average rainfall during a year (16.94" ref. page 47) all percolated through the pile. in this case 7 acre feet, at a conductivity of 5300 umhos/cm would reach Hubbard Creek and the downstream receiving waters. The data on the conversion between conductivity and TDS for this site is limited. if it assumed that 5300 umhos/cm equates to a TDS of 5300 mg/1, then the 7 acre feet that percolates through the pile would contribute approximately 51 tons of salt to Hubbard Creek during the year. The Colorado Department of Health considers mine discharges of less that 350 tons per year (1 ton per day) to be no salt discharge. Therefore this worst case estimate of 51 tons per year is considered to be an insignificant contribution of salt to the downstream receiving waters. • 121 (Rev. 12-16-92)