136 Mine Type Initial pH Range Metal Concentrations mg/L Predicte d Duration years Treat ment Cost USD per m3 Affect ed Area km2 Ann ual Volu me ML SO4: 300-1500 500 - 2000 0 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 post-closure). Figure 3 shows the pH value and metal leaching evolution for tailings. Figure 3 – Tailings Acidification Process and Timeline 3.3 Monitoring and Detection Limitations
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