Hydro-chemical performance of copper mine waste-based capillary barriers for acid mine drainage control in humid climates

Capillary barriers offer a cost-effective and environmentally friendly method for acid mine drainage (AMD) control. Existing studies primarily focus on arid and semi-arid climates, whereas few address humid climates with high precipitation, where AMD risks are more severe. This study evaluated the hydro-chemical performance of two anti-seepage-enhanced triple-layer capillary barriers constructed from copper mine wastes, using rainfall simulation, sensor monitoring, and water balance analysis in a model box. An extreme rainfall event (354 mm over 24 h) followed by a 30-d drying period was simulated sequentially. The top two layers of Model Boxes I and II withstood 60- and 10-year return period rainfalls, respectively, while the bottom layers delayed percolation by 39.4 h and 81.4 h, respectively, after lateral drainage in the coarse-grained layer (CGL). Despite a significant increase in equivalent permeability at CGL breakthrough (e.g. from 9.9 × 10 −12 m/s to 9.3 × 10 −10 m/s in Model Box I), stable infiltration and low permeability limited percolation to less than 2.5% of rainfall in both barriers. Based on effective particle size and thickness, two empirical relationships were developed to describe the peak capacity and cumulative release rate of the fine-grained layer. Furthermore, the pH and heavy-metal ion concentrations in the lateral drainage remained within standards throughout the 2184.0 h monitoring period, explained by inhibition of the oxidation reaction. In addition, compared to the untreated waste dump, the predicted runoff removal rates achieved by the barriers were 73.4% and 64.8% for Model Boxes I and II, respectively.