Assessing mine water pollution: Geochemical principles

The relative abundance and weathering rate of minerals that produce and consume acidity dictates whether a mine water discharge will be acidic or alkaline. Because metal ion solubility generally decreases with increasing pH, acidic waters are associated with greater riskfrom dissolved metals loads, while alkaline waters area associated with environmental degradation due to the oxidative precipitation of iron as ochrous deposits in surface water channels. Pyrite weathering produces acidity and dissolved iron loads, while calcite and, to a lesser extent, alumino-silicate minerals, provide neutralising capacity that helps maintain circumneutral pH. Relating the relative abundance of these source minerals in the mining environment, and their relative rates of weathering, provides a conceptual framework within which to develop risk-assessment and risk-management approaches to minewater pollution. In particular the lifetime of contamination, and time scales to reach improved or detereorated mine water discharge quality is critical when assessing environmental impacts and risk, and when choosing treatment technology or selecting other approaches to handling minewater pollution.