Soil and Crop Response to Irrigation with Untreated and Partially Treated Acidic Mine Waters

Coal mines in the Mpumalanga Province of South Africa generate large volumes of acid mine drainage (AMD). The AMD is neutralised in high-density sludge (HDS) plants and then treated to potable standard in reverse osmosis (RO) plants. These processes are very expensive, energy intensive, and produce gypsiferous sludges and brines that need to be managed. Irrigation with partially treated or untreated AMD on soils amended with limestone is considered a potential alternative. To test the feasibility of this, two glasshouse pot experiments were conducted, one in the winter of 2023 with barley, wheat, and stooling rye, and the other with sorghum and soybean in the summer of 2023/24. The aim was to evaluate site-specific predictions of fitness for use of these waters using an irrigation water quality decision support system and to determine crop and soil responses to irrigation with untreated acidic waters (pH 2.5 and 3.5) and partially treated mine waters (pH 7.5, 8, and 9). The model predicted rapid soil accumulation of elements for untreated AMD, which was consistent with the pot experiment findings. Even with additional limestone applied to the soils irrigated with untreated AMD to counter the acidity, the soil pH, above-ground biomass, and yield were markedly reduced. Vegetative biomass remained unaffected when partially treated waters were used for irrigation; however, grain yield was slightly reduced. Grains from crops irrigated with highly acidic water exceeded Codex Alimentarius levels for mercury and lead, suggesting consideration of industrial crops that are not consumed, for irrigation with untreated AMD, if yield penalties and environmental impacts of such irrigation are acceptable.