2000-08-15_REVISION - M1977342 - Laserfiche WebLink

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c~ • Mr. Erich J. Bower Climax Molybdenum Company August lO, 2000 Page 2 ~. Initial constnaction of each dam consisted of clearing the foundation, installing a foundation drain system and constructing an earth fill starter dam. Tailing was deposited using header and spigot deposition and water was removed from the resulting pond by a gravity decant system. The basin in which tailing was deposited in Dam No. 2 was bisected by a hill, subdividing the decant pond. Two separate decant systems were required [o remove water. The south decant system is aligned generally east and west direction along the south side of the impoundment. The north decant system and subject of this letter extends from the starter dam along the north side of the pond in an easterly direction for approximately 1600 feet. The decant line then turns to the northeast for approximately 900 feet and then turns northwest and finally due north for a distance of approximately 1 100 feet. The Dam No. 2 north decant system has a total length of almost 3600 linear feet. The decant conduit consists of a 30-inch diameter reinforced concrete pipe founded on and backfilled with compacted sands placed in a trench excavated into the glacial till foundation soils. Six decant towers were constructed at selected location along the north decant line and were used to control the decant pond water elevation. Decanted water historically flowed by gravity west toward Starter Dam No. 2. Tailing Dam No. 2 was intended to be a temporary structure. Future tailing deposition in Tailing Dam No. 1 was expected to inundated the dam and cover the impoundment. A third tailing dam (Tailing Dam No. 3) was constructed at the upper end of Tailing Pond No. 2 in the late 197O's. Tailing Dam No. 3 was required once Tailing Dam No. 2 reached a certain elevation. Tailing Dam No. 3 will eventually combine with Tailing Dam No. I to form a single continuous dam. Tailing Dam No. 2 has been abandoned for about 15 years and recent tailing deposition has almost completely covered the Tailing Dam No. 2 crest and decant pond. Tailing Dam No. 3 is similar in design to Tailing Dams No. 1 and No. 2. The embankment was constructed on a prepared subgrade and foundation drains were excavated into the glacial till subsoils. The starter dam was constructed of glacial till and tailing deposited using a header and spigot system. To our knowledge, no detailed records of Dam No. 3 construction are available. The Tailing Dam No.3 starter dam vas constructed over the Tailing Dam No. 2 North decant system with the decant Tower SN located at the downstream toe of the starter dam. It is our understanding from discussions with the Climax staff that a partial or complete plug was installed in the Tailing Dam No. 2 north decant line. However, no records of the decant plug construction or location are available. An inspection of Tailing Dam No. 3 in Fall of 1999 showed that seepage was discharging from Tower SN at the rate of approximately 5 to 10 gallons per minute. This flow was in reverse of normal gravity flow in the historic system. We recommended to Climax that the decant line Tower SN be plugged. It is generally considered an undesirable practice to have an open conduit through a J 1WPMASTERWROJEOTSddd001b4 doc OBI10'00(3 31 PMT _... .._-~.,.,...~.....wr uw odllyw~J'dl PM)