2000-04-12_REVISION - M1977342 (2)

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Mr. Bruce Humphries • • April 12, 2000 s.u., considerably lower than Dam No. I seepwater, which has a pH between 4 and 5. One theory is that the design of the Dam No. 3 pumpback system is recycling low pH seepwater through the dam, which has lowered the pH over time. The existing seepwater collection ponds have filled with eroded tailing or chemical precipitates from the seepwater collection system. This limits the total system capacity and covers the outlets of the existing toe drain system which could result in a reduction of the capacity of the toe drain system and increase the elevation of the phreatic surface within the dam during long periods of sustained tailing deposition. The pumpback pipeline has shown evidence of buildup from chemical precipitation, limiting the capacity of the pumpback system. The collection ponds and return line aze all outside of the tailing pond's natural containment so that loss of power, a failure of the pumps or the pumpback line, or an unexpected seepwater surge would likely result in a release outside of Henderson property. Henderson plans to make significant changes to the existing system. The collection ponds will be cleaned out and all residues from these ponds will be deposited into the lazger tailing basin. A channel will be constructed to direct the seepwater flow into two parallel 10-inch diameter HDPE lines that will flow by gravity to the existing Dam No. seepwater collection system. The collection ponds will be maintained to manage any unusual flow or problem with the delivery lines. Henderson believes that combining the two seepwater sources in the Dam No. 1 seepwater collection system will improve our ability to manage seepwater and reduce the potential for a release of seepwater. As part of our Y2K efforts a[ Henderson, the Ute Park Pump Station, which pumps [he seepwater back into the water pool, has been modified so that a diesel pump can be used to pump seepwater or process water. Henderson leased a 455-hp diesel pump, rated at 5000 gpm, as Y2K coverage, and is currently considering purchasing the pump. Each of the HDPE lines will be capable of handling the normal seepwater flow from Dam No. 3. The lines will be constructed to facilitate pigging or high pressure flushing, allowing the lines to be cleaned regularly to avoid the buildup that plagues the current system. Although these lines will be located outside of the containment of the tailing basin, we believe that they are a dramatic improvement over the current system. These lines will not be subject to the pressures of pumping since the flow will be by gravity. There will be no risk due to power or pump failure. [n addition, Climax Molybdenum Co. has had experience with parallel lines feeding influent to the water treatment plants at the Henderson Mine and at the Mt. Emmons Project and has found that the combination of parallel lines and routine cleaning is very successful in improving the reliability of the delivery system. Henderson proposes to clean these lines twice annually and conduct leak down tests annually to confirm the system's integrity. Preliminary drawing D3-1-O1 is attached.