Poregas distributions in waste-rock piles affected by climate seasonality and physicochemical heterogeneity

Abstract

Abstract The weathering of sulfide minerals in waste rock controls drainage water quality and water treatment requirements at mine sites. The availability of oxygen strongly affects the weathering rate of sulfidic waste rock but poregas distributions in heterogeneous, unsaturated waste-rock piles and the governing physical transport processes remain scarcely studied on practice-relevant scales. Here, we investigate the spatiotemporal variations in poregas in five large-scale, instrumented waste-rock piles (10,000\u202f±\u202f2000\u202fm3) with different types of waste rock. Three piles with coarse, low-S waste rock exhibited atmosphere-like poregas compositions throughout, but two experimental piles with fine-grained, sulfide- and carbonate-rich waste rock contained localized hotspots with elevated temperatures, O2 levels 2%. Oxygen depletion and CO2 production from carbonate dissolution under acidic conditions was most pronounced during the wet season, when infiltrating precipitation strongly increased moisture content and repressed atmospheric gas exchange. Thus, spatial and temporal fluctuations in poregas were related to chemical reactivity and permeability of the waste rock as well as to seasonal climatic variations in hydrological transport. This study demonstrates that a quantitative understanding of these factors can lead to improved assessments of waste-rock weathering and drainage rates and therefore optimized design and management of waste-rock piles.