2008-01-15_PERMIT FILE - M2007044 (2)

Home Browse Search

......what is the potential for increased quantities of ground water inflows that could potentially, in the post-mining situation, build up head behind the double bulkheads and escape the mountain via newly formed springs? Before answering this question, it is important to define the surrounding geology and the current underground conditions at the site. The Top Rim of the Salt Wash unit of the Morrison Formation is the mining zone. The Salt Wash, like the geologic units immediately above and below it, dips gently to the northeast at about 2 degrees. With the exception of shafts and declines, all of the mining occurs within this relatively flat-lying ore zone. The ore zone consists of a tight sandstone that is dry except for water introduced from water-bearing zones located above the mine in the Brushy Basin unit. This ground water enters the mine workings through unplugged drill holes, ungrouted shafts, and declines. Some ground water also exists below the mining zone as evidenced by PR Spring, which seeps from the base of the Top Rim sandstone unit. The mineralized zone is separated from the lower portion of the Top Rim by aloes-permeable mudstone layer (see the March 15, 2001 Memorandum by U.S. Environmental Services, Inc. in Appendix C of the 112d Application). The decline for the Whirlwind Mine descends for 3,200 feet at a 6 percent grade through the Brushy Basin formation to the Salt Wash formation. The Brushy Basin formation consists of about 400 feet of low-permeable mudstones containing only a few sandstone lenses that are from 10 to 40 feet in thickness. Limited volumes of unconfined, perched water are found in the lower portions of some of these sandstone lenses. The sandstone lenses were formed by fluvial processes, are lenticular in shape, and may not be continuous across the mesa. The recharge areas for these sandstone lenses are limited to short, steep outcrops on the southwest (i.e., updip) side of the mesa (see Figures G3 and G4 of the 112d application). The thick mudstone sequences prevent recharge from entering the sandstone lenses from above. Near the bottom of the Whirlwind decline, a sand lens within the Brushy Basin is weeping ground water, which then flows down the decline into the mine workings. This lens historically made about 5 gpm, according to the records kept by the former operator. It currently makes about 1.5 gpm. Additional information and cross- sectional figures describing local ground water conditions and seepage into the decline is provided in Attachment A. The Salt Wash unit is a very fine-grained sandstone with a tested hydraulic conductivity of 2.5 x 10"3 meters per day (m/day) or 2.9 x 10"6 centimeters per second (cm/s). These test results, presented in Attachment B, indicate that the sandstone will transmit water but at a very slow rate. The sandstone tested came from the proposed seal location, which is above the ore zone, but is otherwise representative of the Salt Wash sandstones found in the mine. The water produced in the Whirlwind Decline flows into the lowest portion of the decline and the approximately 700 feet of production drift where it has reached a steady state condition in which the water 1:ne~•~~% fu~;is 12es«t~~~c~a t;c~r~~r:~tic~ri ~?--~~ t..!r~iUn ~orrie~~ard, quite fiQt) 3 L,~l:e~~~oc~d. ~'C~ ~012~ a=;~on~,; 3O3-E)7~?~-2i~0