2002-03-04_REVISION - M1986015 (8)

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MINING PLAN One of the disadvantages to the system described above is the high amount of maintenance normally required. Because the settling basins receiving the dirty water from the wash plant are relatively small, it would normally be expected that they will fill with sediment rather quickly. This would be especially true of the first basin. This will require the basins be cleaned frequently and that can only be done when there is no washing being done. When the cost of frequently cleaning the sediment basins is combined with the "down time" of the plant, this system can become quite costly. Fortunately, this expensive maintenance problem can be eliminated by one of two methods. In Figure 2 is a second circuit diagram which shows an alternate or optional and essentially maintenance free system. In this optional system, the two sediment basins are eliminated and one large sediment basin is constructed out of overburden within the mining pit itself. This sediment basin must be constructed where backfilling will occur. Depositing sediment in the backfilling area thus reduces the cost of backfilling and eliminates the need to clean out ponds. In this system, water seeps from the sediment basin through the overburden berm surrounding the sediment basin and creates a "filtered outflow" of water into the pit. This then mixes with the dewatering water which is discharged to the vegetation filter/lake discharge system used previously. This actually results in even cleaner water entering the discharge circuit because much of the settling actually occurs in the pit area before the water is even pumped to the discharge system. In this circuit, the wash water continues to be obtained from the lake near the river, although it might be obtained from the mining pit. However, in the latter case, it is not likely sufficient water would be concentrated in one place to supply needs of the washing system and therefore the lake near the river would most likely be the best source for wash water. This alternate system could be installed just as soon as room in the mining pit is available. The sooner it is done the sooner cost savings can be realized and the sooner higher quality water will be discharged to the river. Unfortunately, the location of backfilling and dewatering locations might not make this alternate system feasible until those two operations can be well separated. This is due to logistical problems produced simply by the locations of the various features, locations that cannot easily be changed. A second method, and the one most likely to be used, would be to treat the wash water in the processing itself. In this method much of the sediment is removed before it even reaches the first settling pond. Special equipment can be attached to the processing plant that will remove much of the sediment (silt and clay sized particles) basically through a centrifuge process. This would produce much cleaner wash discharge water and cause the sediment basins to fill much more slowly, thereby reducing maintenance costs. Pueblo East Pit Amendment Exhibit D Page 9