Estimating the Decant Rate from a Rehabilitated Opencast Colliery Using a Water Balance Approach

The long-term average decant rates from rehabilitated opencast collieries in South Africa are often estimated by assuming effective recharge rates through the spoils. However, large uncertainties are associated with these assumed recharge rates. Furthermore, this approach assumes that groundwater inflows and pit water outflows are negligible compared to the volumes of water recharged through the spoils. To obtain an estimate of the decant rate at a particular colliery, I used rainfall figures, pumping rates, and water elevations measured over a period of 6 months as well as estimated evaporation rates to construct a water balance. I then calculated a decant rate, independent of assumed recharge rates, that was significantly higher than a previous long-term estimate, despite the lower than average rainfall experienced during these 6 months. This discrepancy suggested that groundwater inflow was indeed contributing to the decant volumes. The decant rate during years of average rainfall was subsequently calculated by adding the estimated groundwater inflow to the recharge volumes found with the method based on assumed recharge. This decant rate was approximately 49 % greater than a previous estimate obtained by assuming negligible groundwater inflow. This study shows that the decant rates at rehabilitated opencast collieries could be significantly underestimated if the decants are assumed to be recharge-driven without considering the possibility of groundwater inflow. Underestimation of the decant rates will lead to flawed water management strategies, which could result in adverse environmental impacts.