Abstract
Waste rock spoils from coal mining result in drainage containing selenium and sulphate, often at concentrations of environmental concern. Additionally, nitrates leach from residual explosives within the waste rock, presenting another significant concern. Small-scale field and laboratory experiments are often used to predict the drainage chemistry of future spoils and to assist in mine planning, development, and mitigation. This work presents a case study of a full-scale unsaturated waste rock spoil in Western Canada with regular downstream drainage chemistry monitoring. The spoil was developed in the late 1990s, culminating in ~9.6 million bank cubic meters of waste rock, with regular flow and drainage chemistry monitoring at a downstream sedimentation pond. This monitoring began before the spoil development and is on-going, providing a continuous record of the spoil drainage chemistry. Increased concentrations of selenium, sulphate and nitrate are observed following development of the spoil, reaching a peak at approximately five years. Subsequently, nitrate concentrations decrease rapidly, as residual explosives are flushed from the spoil. Selenium and sulphate behave similarly, being stable at peak levels of around 10 μg/L and 300 mg/L, respectively. Sixteen years after development, selenium concentrations begin to decline, with average concentrations decreasing to half of the observed peak concentrations within six years. In contrast, sulphate concentrations have remained at peak levels over the same period. While laboratory and small-scale experiments are often used to make larger scale drainage predictions, the use of these data requires many assumptions. This study presents an example of full-scale coal waste rock spoil drainage chemistry over 25 years, includes evidence of concentrations decreasing over time, and discusses potential mechanists of the decreases. The results from this study enable comparisons, and therefore improvements, in drainage chemistry prediction methods from coal mining.