Assessing the Evolution of Mine Water Quality With Empirical 'First-Flush' Models
Abstract
Coal mining activities can leave an extensive network of abandoned underground workings that gradually flood after operations cease. This rising mine water, with low pH and high sulfate, acidity, and metals can lead to uncontrolled releases of harmful acid mine drainage to the environment. Treatment plants are used to extract and treat the mine water to maintain its elevations below suspected discharge zones. Accurate predictions of long-term water quality is highly challenging due to the complexity and volume of the underground workings. As numerical models have difficulty recreating complex mine pool geometry and hydrogeochemical processes, empirical models that are based on Younger’s ‘first-flush’ phenomenon, where mine water concentrations peak shortly after flooding and then exponentially decline, may provide better long-term predictive modeling. The objective of this study was to assess the robustness of ‘first-flush’ empirical models for describing and predicting mine water behavior at large, complex mine pools in The Sydney Coalfield (Nova Scotia, Canada). Numerous mine pools in the coalfield flooded at various times over 100+ years, allowing long-term mine water evolution to be studied in various pools of different ages. Analysis of extensive historical data from the older pools demonstrated that the evolving mine water quality, both overall and within each stratified layer, followed the ‘first-flush’ phenomenon. ‘First-flush’ trends were consistent across differing depths, water quality parameters (acidity, sulfate, iron), and concentration ranges. Two newer mine pools, which recently flooded in 2012, rely on a new active treatment plant to manage mine water levels below discharge points. Using behavioral conditions observed in the older mines, such as decay rate, ‘first-flush’ based empirical models were calibrated and validated to early mine water quality data collected at the treatment plant bi-weekly between 2012 and 2021. They were then used to predict future mine water quality and estimates of long-term treatment requirements and related expenses.