An unprecedented window of opportunity currently exists for subsurface hydrological studies at the Homestake DUSEL in the Black Hills, South Dakota. Rocks at the site consist primarily of fractured Precambrian metasediments and form a regional groundwater system possessing largely unknown and undefined hydrologic properties. If not instrumented in the near future, this unique opportunity will be lost. The former Homestake Gold Mine operated continuously from 1876 to 2001. Mine workings in the gold deposit followed the Homestake formation along 4 miles of plunge, penetrating through a volume of crust 1.7 x 1.9 x 3.1 miles with >320 miles of drift. The rock sequence was folded several times and tilted toward the southeast during the Precambrian. Laramide tectonism raised the Black Hills to their current configuration and was accompanied by brittle fracture, most likely creating the fractures presently observed in the mine. Groundwater was pumped continuously from the Precambrian aquifer during the operation of the mine and pumps were turned off in 2003 after the mine closed. Post-closure flooding has resulted in more than 3000' of water rise and is currently between the 4850-ft level and the 5000-ft level. Current plans are for pumps to be turned on in mid-December 2007 to reduce the water level to the 5300-ft level. Hydrologic properties of the Precambrian aquifer are relatively unknown. Pre-closure research has shown that about 2/3 of the current 750 gal/min inflow rate originate from flow through the fractured aquifer rocks. Bulk permeability has been estimated, based on inflow rates and the volume of mine openings, at about 10-6 to 10-7 cm/sec. The research described in this proposal seeks to measure falling water levels in the mine as it is dewatered and to determine the hydrologic properties of the Precambrian aquifer. Pressure sensors will be lowered into the water column at the No. 6 shaft, either the No. 3 shaft which extends to the 5000-ft. level or the service shaft which extends to the 6800-ft level (service shaft may not be accessible), the No. 4 shaft which extends to the 7400-ft. level, and the No. 5 air intake shaft which extends to the 6200-ft. level. Sensors will be lowered, as required, into the falling water column as the pumping proceeds. Exact sensor placement will depend on access and will be determined with the cooperation of the SDSTA. Analysis of water-level data will be achieved using various models and tested for appropriateness of hydraulic conductivity determinations, including modifications for fracture flow and other factors. Models designed for flow into tunnels will also be used. The results, in turn, can provide new information about the hydrology of fluid flow within a volume of the crust and in determination of hydrologic properties of the local Precambrian aquifer. Microclimatic conditions (temperature, humidity, pressure, and wet bulb temperature) in the underground spaces will be measured concurrently with water reduction data. Meteorological stations underground will be located on the 400, 1200, 2400, 3600, and 4850-ft. levels (or nearest accessible levels). Two stations will be placed on each level, one near the shaft and one up to 1000 feet in the drift. As the water level in the mine is reduced, and the surface inflow is intercepted, subsurface climatic changes will begin to occur. Further changes will occur as the mine goes from its current non-ventilated state into a fully ventilated sate. These climatic changes will be important to various experiments as the mine is prepared for the long-term science and engineering program. These early monitoring efforts will provide valuable information on the initial conditions of the research facility, while helping to establish a long-term monitoring effort. All collected data pertaining to hydrology and microclimate will be placed on a web server, either at Homestake or SDSM&T, and be made available to all interested parties.
