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British Columbia Mine Reclamation Symposium

Prediction of water chemistry in open pits during operation and after closure Morin, Kevin A. (Kevin Andrew), 1955-

Abstract

Environmental monitoring at minesites often focuses on tailings impoundments and minerock piles. While these components may produce the greatest impacts on water chemistry at a minesite, the mine itself, consisting of an open pit or underground workings, can also affect water chemistry if the walls of the mine expose rock that releases metals, nonmetals, and acidity. During mining of an open pit or underground workings, all water is typically pumped or drained from the mine to maintain optimum and safe working conditions. After closure, a mine is usually allowed to fill through time to its equilibrium level, or it may be flooded by directing water into it. The ability to predict the chemistry of the minewater during these phases of operation and closure can be valuable for potential treatment costs, engineering design, and closure bonding. In order to predict minewater chemistry, conceptual models have been developed to address water movement to and from mines during operation and closure. The relevant geochemical aspects of unit-rock-surface reaction rates and total amount of reactive rock surface in a mine are then addressed. The compilation of these models and factors forms the basis of the MINEWALL computer program, which is currently being revised, expanded, and tested as Version 2.0. MINEWALL shows that a key factor in predicting minewater chemistry is the estimation of percentages of reactive surface that are flushed regularly, once a year, or nor during operation. However, direct measurement of these percentages is not currently possible. Consequently, MINEWALL can be first calibrated to operational data to obtain the percentages and then used in predictive mode for the remainder of operation and subsequent closure. Data from three mines in British Columbia show, for example, that (1) total reactive surface areas can range from tens of millions to hundreds of millions of square meters, (2) the percentage of these surfaces not flushed during operation range from 70 to 90%, and (3) unitarea leaching rates of copper range from 0.014 to 2.35 mg Cu/m²/wk. Keywords: acidic drainage, open pit mines, geochemical predictions, hydrogeology

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