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Mr. Thames Hartley -3- June 25, 1995 e) If at any point it becomes clear that the Hazel-A bulkhead cannot be maintained as a watertight fixture, the Division will require that COM, Inc. instal a concrete or other more effective bulkhead as approved by the Division. 3. In item no. 4 of your June 22, 1995 submittal, it is proposed that a water management plan for the tailing impoundment in its raised configuration could rely on evaporation without the need for the Hazel-A adit or any other auxiliary storm water storage facility. It is further proposed that under this plan, the design storm event for the tailing pond would continue to be the 100-year/24-hour event. This scheme would not be acceptable to the Division for the following reasons: a) The Division estimates that the raised tailing impoundment will provide 450,000 cubic feet of additional tailing storage. Assuming a settled density of 90 pounds per cubic foot, this would result in 20,250 tons additional capacity once the dam is raised. In that during the first three years of the Gold Hill Mill's operation, only 2500 tons of tailing were produced, it is possible that the raised dam could provide an operational life of up to 25 years. Over 25 years, the probability of the 100-year storm event being exceeded is more than 22 percent, whereas an acceptable probability of exceedance would not be more than 1 or 2 percent. As such, if a decant system or some other storm water inflow dissipation system is not proposed, the Division will require a substantially larger storm event be utilized in the design. b) Design capacity to contain the 100-year/24 hour event was previously accepted by the Division because of the capability of the decant system to spill the inflow design flood into an auxiliary storage area. The advantage of allowing spillage is that only one extreme event need be considered, the 100-year flood in this case, and with a decant system capable of passing the design flood, there is little danger of dam overtopping in a sequence of very wet events. In the proposal submitted on June 22, 1995, decantation and auxiliary storage are eliminated from the plan, and the tailings pond becomes a totally closed circuit operation. In a totally closed circuit operation, the inflow design flood must be stored in the tailing impoundment without uncontrolled release. In these circumstances it is usual to assume that the design flood occurs after a sequence of very wet events so that the free water pond level is high prior to the flood. An example of this design philosophy is the U.S. Nuclear Regulatory Commission (NRC) which in cases where floods are retained, requires that sufficient impoundment capacity is available to store the sum of: i. A 100 year flood with the initial pond level at its average elevation.