Prediction of salt release from coal mine spoils at meso-scale

Open-cut coal mining operations remove enormous quantities of overburden material inorder to access coal seam. Upon interaction with atmospheric conditions, this overburden material referred to as spoils from which salts are leached, possesses the risk of affecting surface and groundwater quality around the mine sites. Due to a distinct lack of field data on leachate rates and chemistry for full-scale spoil piles, studies have relied on geochemical testing at laboratory-scale experiments such as free-draining funnels and columns. While laboratory leaching techniques under a controlled environment are a general predictor of how spoil behaves upon weathering, there remain gaps in understanding the leachate rates taking into consideration actual particle size, flow rates, water content, temperature, and oxygen supply. This study proposes and assesses a new mesoscale approach for predicting salinity release from spoils that is designed to obtain data more relevant to the closure options under consideration. 5 coal mine spoils from 3 mines located in Queensland, Australia was sampled, characterised (physical, geochemical, and mineralogical), and were subjected to weathering at mesoscale (1-2 tonnes sample volume) leaching for 11 cycles under natural conditions. Results showed that soil-like spoils with significant pockets of less permeable clayey or silty material have the ability to retain and release solute slowly with time while rock-like spoils followed a steady decay rate. The mesoscale tests produced distinctive characteristic decay curves of salt release from typical soil-like and rock-like spoils and have been useful in the calibration of flow and moisture-dependent salt kinetic parameters. The mesocosm leaching approach developed as a part of this study was close to real-sized spoil conditions such that it mimicked the water/rock ratios, preferential flow paths, and governing solute transport processes.