Relocation of net-acid-generating waste to improve post-mining water chemistry.

Acidic drainage and metal leaching are long-term environmental liabilities that can persist for many decades to millennia. One technique to improve the water chemistry and ecology of post-mining landscapes is to relocate and submerge net-acid-generating mine materials in a lake or water-retaining impoundment. One example of a carefully executed relocation of waste rock took place at the Eskay Creek Mine in Canada. Pre-relocation studies included an empirical relationship that related (1) the amount of acidity retained by the waste rock during past oxidation to (2) the amount of lime needed in each truckload for neutralization of the acidity and for suppression of metal release. During relocation, thousands of rinse pH measurements indicated net acidity varied significantly over short distances within the waste rock and that acidic rock could not be reliably segregated from near-netural rock. After relocation, water from the watershed continued to be acidic for a few years, then returned to near-neutral pH and near-background concentrations of metals. The chemistry of the lake where the waste rock was submerged remains near background conditions. Therefore, with careful planning and implementation, the relocation and submergence of net-acid-generating materials can greatly improve post-mining water chemistry.