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(101) Acid Mine Drainage Impacts in Arid Environments May be Delayed Hundreds of Years.
Atkins, David*,1, Lipton, Josh1, 1 Stratus Consulting Inc., Boulder, CO, USA
ABSTRACT- Mining in arid environments can profoundly influence long-term water quality, but the effects may not be seen for hundreds to thousands of years. For example, many mines generate large quantities of sulfide waste material, and technical and economic limitations often result in reclamation strategies that do not incorporate complete neutralization of the excess acid generating potential of the resulting waste dumps, especially in arid environments where acid drainage is typically not observed during the mine life. The oxidation rate of sulfide minerals in waste-rock dumps can be more rapid in arid environments than in wetter climates, and the infiltration of precipitation through mine dumps can be very slow. These two effects can result in acid and metals taking many years to reach groundwater, with the source and impacts lasting for hundreds to thousands of years. Oxygen consumption rate tests were used to measure in situ sulfide mineral oxidation rates on waste-rock, ore-stockpile, and mine-pit surfaces. Results of these tests indicate that field oxidation rates are controlled more by physical factors (i.e., rock matrix permeability to oxygen, exposure time, and rock fragment size) than chemical factors (i.e. sulfide mineral form and content). Field study results were then used to calibrate a time-variable numerical sulfide oxidation model that was coupled with a simple analytical unsaturated flow and transport model. Model results indicate that groundwater impacts may be delayed hundreds to thousands of years but not diminished in magnitude. Such long-term, delayed impacts present significant technical and regulatory challenges.
Key words: acid, metals, mining, sulfide
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