1061 also plays a role since multiple interacting processes involving over 20 different chemical reactions make prediction and control difficult [12]. Legacy impacts are part of the challenge since historical mining operations continue to generate AMD decades after closure, with treatment costs exceeding $1 billion annually in North America alone [12]. Table 3 summarizes a quantitative analysis of AD generation and persistence. Table 3 – AMD Generation and Persistence Mine Type Initial pH Range Metal Concentrations mg/L Predicted Duration years Treatment Cost USD per m3 Affecte d Area km2 Annual Volume ML Coal 2.5-3.5 Fe: 50-200, Al: 10-50, Mn: 525, SO4: 500-2000 20-50 2-5 5-50 100-5000 Copper 2.0-3.0 Cu: 20-100, Fe: 100-500, Zn: 10-80, SO4: 1000-8000 30-80 3-8 10-200 500-15000 Gold 2.8-4.0 As: 5-25, Fe: 20-150, Cu: 530, SO4: 800-3000 25-60 4-10 2-100 50-2000 Iron Ore 3.5-4.5 Fe: 200-800, Mn: 10-40, SO4: 300-1500 15-40 1-4 20-500 100020000 Source: Compiled from Akcil & Koldas, 2006 [12]; Kefeni et al., 2017 [13]; Lottermoser, 2010 [22] 3.2 Legacy Contamination and Groundwater Impacts Historical mining activities have created groundwater contamination that poses ongoing challenges, with an estimated 500,000 abandoned mine sites worldwide requiring water management intervention [22]. These impacts are related to two main conditions, flooded underground workings and abandoned tailings storage facilities. As an example, The Witwatersrand gold/uranium fields case study demonstrates the complexity of managing flooded underground workings, with over 270 km of interconnected underground voids generating 60-100 ML/day of contaminated discharge [7]. Key challenges include interconnected void spaces creating regional contamination (affecting areas up to 1,000 km), variable hydrostatic heads affecting discharge patterns (ranging from 5-150 m head), long-term liability and responsibility issues (treatment costs: $50-200 million over 20-30 years), and high treatment costs for large-volume, low-concentration contamination ($0.50-2.00 per m). As per the tailing’s storage facilities, these represent one of the largest sources of long-term water contamination risk, with global tailings volumes estimated at 230 billion tonnes stored in over 3,500 facilities worldwide [22]. Large volumes of potentially acid-generating material (individual facilities: 10-1,000 million tonnes), complex hydrogeochemical interactions (involving 15-25 different mineral phases), climate vulnerability and extreme weather risks (failure probability: 1 in 1,000 to 1 in 10,000 annually) and Long-term monitoring and maintenance requirements (duration: 50-500 years postclosure). Figure 3 shows the pH value and metal leaching evolution for tailings.
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