2010-06-25_HYDROLOGY - M1977300 - Laserfiche WebLink

Home Browse Search

June 25, 2010 7 of 15 observed under near-neutral pH conditions. Following peak concentrations, a marked decrease of Zn, Cd, and Hg, and to a lesser extent Pb, occurred. After seven years of rebound, the concentrations of these metals are relatively low at most mine sites, although the levels are generally still higher than in unmined areas (Cidu, et al., 2005). At the Lokken Mine in Norway, underground mine water quality improved considerably when the mine was flooded after partial closure (Amsen and Iversen, 1997). At the Lincang mine in China, researchers determined that the most effective remediation method was to plug and seal the underground mine by curtain grouting and /or backfilling. This method was used to ensure that adits, open raises and the inclined shaft do not release radon or waste water after remediation (Xu, et al., 2008) The Sleeper Mine in Nevada is an example of an open pit mine that was deliberately flooded as a mitigation strategy. After completion of mining, the pit was rapidly filled with water pumped from the alluvial aquifer to reduce the exposure time of sulfide wall rocks and waste rocks in the ultimate pit. The rising lake waters were amended with lime to buffer excess acidity contributed by reactive pit wall rocks during submergence. In 2004, water quality in the flooded mine pit met all Nevada primary drinking water standards with the exception of sulfate, TDS, and manganese, which were slightly elevated, as predicted (Dowling, et al., 2004). In Colorado, the selected mitigation strategy for the Dinero mine tunnel near Leadville was to bulkhead the mine. This strategy was identified by the Bureau of Land Management (BLM), Fish and Wildlife Service (FWS), Colorado DRMS and Colorado Department of Public Health and Environment (CDPHE) as the most effective means of managing underground mine water that was flowing from the tunnel. As stated by the BLM, "Construction of a bulkhead will significantly decrease the volume of acidic mine drainage discharging from the Dinero Tunnel. This action will decrease the off-site migration of acidic mine drainage and reduce risk associated with exposure of humans and wildlife to the acidic mine drainage." The draft and final Environmental Assessment (Stratus Consulting, 2009, 2010) stated that after the tunnel was sealed, mine drainage would back up into the mine workings to near pre-mining groundwater levels. The bulkhead was successfully installed in August, 2008. BLM's website for the project states that: The bulkhead is a permanent acidic mine drainage fix or source control method much preferable and lower cost than the long term operation and maintenance commitment required for water treatment alternatives. Properly designed and constructed, bulkheads will last indefinitely, and allow groundwater to return to its original condition prior to being disrupted by mining. (BLM, 2010) Dewatering poses substantial physical feasibility challenges in installing a pump, pipe and other equipment into the mine. Setting a pump at 500 feet below the Steve Level involves significant technical challenges. Basically, there are two means of installing the pump and piping. The first is to carry the equipment into the mine through the Sunshine raise and set the pump using hand equipment. The second is to excavate a drill pad on the surface above the mine and use a drilling rig to drill a vertical hole to intercept the #2 Shaft which is 74 square feet in area. Problems with the first method occur because the mine has been sealed, and the Sunshine raise was not designed to allow access of people and equipment. A 15-stage submersible pump and 15 horsepower motor would be approximately 6 feet long and weigh 175 pounds. The pump would have to be set on