Resource Recovery from a Flooded Underground Mine

Extensive mining in Butte, Montana has created a system of flooded underground mine workings.  The groundwater in the historic mining district has excessive concentrations of heavy metals and metalloids due to past mining activity and the high sulfide mineral content of the area.  The US EPA determined that it is technically impracticable to remediate this bedrock aquifer.  However, there is still potential for utilizing this water in a beneficial way. The idea of using Belmont Mine water for irrigation was investigated through the Mine Waste Technology Program. As part of the source characterization, a 57-day pumping test was performed in July-August of 2007.  This paper will present: 1) interpretation of the hydrological and geochemical results of the 2008 pumping test; and 2) to perform bench-scale experiments to evaluate treatment of this water for use as irrigation water.   Data will be presented for a suite of major and trace metals, major anions, and field parameters, including pH, temperature, specific conductance, and oxidation-reduction potential. Samples were also collected for radon analysis, as well as stable isotopes of water, dissolved sulfate, and dissolved inorganic carbon.  Samples collected after 100 hours had arsenic, manganese, zinc, and sulfate concentrations that greatly exceeded proposed irrigation standards.  In addition, although the pH of Belmont groundwater stayed between 5.0 and 6.0 during the pumping test, this water turns strongly acidic on exposure to air, due to oxidation and hydrolysis of dissolved ferrous iron.  Based on these observations, it is clear that Belmont mine water must be treated before it can be used for irrigation. The bench-top treatability experiments showed that Belmont water responds well to treatments that pair pH adjustments using lime or caustic and oxidation.