Assessing water dynamics and net percolation rates within engineered cover systems for mine waste rock piles: A long-term field monitoring study

Abstract

Mine waste rock piles (WRPs) typically contain trace sulfidic minerals that can react with the atmosphere and produce toxic acid mine drainage (AMD) leachate. Water flow through the waste rock affects both the chemistry and transport of the leachate, and a common remedial solution is the installation of engineered cover systems over the WRP to control net percolation (NP) into the waste rock. As NP is mainly controlled by meteorological conditions and the hydrological properties of cover materials, the water balance (WB) method can be employed to investigate NP and water dynamics within cover systems. However, studies on comprehensive WBs at full-scale WRPs have been limited, and most NP studies have relied on controlled, small-scale laboratory and field tests. This unique study contains the long-term field monitoring data at multiple WRPs needed to comprehensively assess NP rates within cover systems. Field monitoring data with high temporal resolution (hourly) was collected from 2012 to 2018 at four covered WRPs located within the Sydney Coalfield (Nova Scotia, Canada), consisting of one simple soil cover and three multi-layer covers lined with the same geomembrane but differing drainage layer materials. Each WB component – precipitation (PPT), evapotranspiration (ET), water/snow storage, surface runoff, and interflow – was determined, with the residual inferred to be NP into the waste rock. The simple soil cover was not very effective, only reducing NP from the pre-cover 34% PPT to 30% PPT. In contrast, the geomembrane-lined cover systems reduced NP to 4% PPT (no drainage layer), 1.6% PPT (thick gravel drainage), and 1.8% PPT (geocomposite drainage net). While ET and water/snow storage changes at all WRPs were similar, the biggest influence on NP was runoff and interflow. This study provides new field information on the performance of cover systems for controlling NP and water dynamics at full-scale WRPs.