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<br />Letter to Richard Fanvo -4- February 20, 1996 <br />One measure of tolerable accumulation of water in the Gold Hill Mill <br />tailings pond, would be accumulation to the elevation where there would <br />still be sufficient freeboard to contain the 100-yr, 24-hr flood at the <br />level of the decant risers, which are 6 inches below the dam crest. The <br />following is a calculation of this tolerable accumulation scenario: <br />Current available storage = Storage in pond + Storage in Hazel-A = <br />.52 ac-ft + .15 ac-ft = .67 ac-ft <br />Tolerable accumulation = Current available storage - Storm inflows <br />_ .67 ac-ft - .45 ac-ft = .22 ac-ft <br />Therefore, it is reasonable to conclude that an increase of ground water <br />inflows to the Hazel-A of 1 gallon per minute, and subsequent pumping of <br />this water to the pond, will result in an unacceptable decrease in the <br />freeboard or storage capacity in the tailing pond to contain a 100-yr, 24- <br />hr storm event. It is also reasonable to conclude, based on the <br />sensitivity analysis table above, that an increase in ground water inflows <br />to the Hazel-A adit of just over 2 gallons per minute, will exceed <br />available storage capacity, and a discharge of process fluids from the <br />adit may result. <br />Conclusions <br />During the eriod from 9/17/95 to 1/17/96, the average inflow rate to the <br />Hazel-A adit estimated to be 1.69 gallons per minute. Increased ground <br />water inflows t the Hazel-A can be anticipated during the period from <br />about mid-March to 'd-July. Such increased inflows were observed during <br />an inspection of the zel-A adit conducted on June 7, 1995. Magnitudes <br />for the increased inflo on the order of 1 to 2 gallons per minute or <br />more could occur. As exam es, inflow rates for two underground mines <br />where inflow rates have been antified are presented here: Prior to <br />1978, inflow rates for the Terr Tunnel in San Juan County have been <br />estimated to vary from 5 gpm in aut n to 100 gpm during spring runoff; <br />after 1978 certain stopes connected to e Terry Tunnel were opened to the <br />surface, and since that time Terry Tunnel flows have ranged from 82 gpm <br />in autumn to at least 1400 gpm during spring Hoff. Inflow rates for the <br />Climax Mine in Summit County during 1984 varied om approximately 600 gpm <br />in January to approximately 4500 gpm during Jun and July. Clearly, <br />neither the Terry Tunnel or the Climax Mine are compa ble to the Hazel-A <br />adit. These examples are provided to demonstrate the sorts of wide <br />fluctuations in rates of ground water inflow that can occur, and that have <br />occurred at two sites where the inflows have been measured. <br />The demonstrations provided in this letter indicate that the Gold Hill <br />Mill water storage capacities may be exceeded as a result of relatively <br />modest increases in the rate of ground water inflows to the Hazel-A adit. <br />It is the Division's position that contingency measures need to be enacted <br />to prevent unpermitted discharges from either the Hazel-A adit or the <br />