Baker, A & Moodley, VOD 2026, 'Geochemical predictions and closure planning implications for reprossessing legacy tailings 
at the old tailings storage facility complex, South Deep gold mine, South Africa', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2615_83
(https://papers.acg.uwa.edu.au/p/2615_83_Baker/)
Abstract: Tailings storage facility (TSF) closure planning requires an understanding of long-term acid generation and metal leaching risks to inform closure options. SLR conducted a comprehensive geochemical characterisation of the South Deep old TSF complex, made up of 4 compartments (TSF 1–4), parts of which is currently being reclaimed. Assessment of the mature residual tailings and associated water chemistries was used to develop a kinetic model to predict the complex’s geochemical behaviour and groundwater contaminant source terms. Water quality results indicate that metal exceedances predominantly occur within the reticulated water system and in boreholes and scavenger wells proximal to the Old TSF complex. Acid-base accounting and mineralogical results infer that the tailings are potentially acid-generating and pose a risk of metal leaching, as indicated by metal exceedances reported from static leaching tests which correlate with modelled source terms. A 20-week humidity cell experiment was conducted on composite samples that showed TSF1 and 2 (currently being reclaimed) have pH and alkalinity profiles indicating acidic conditions over the long term. Low carbonate molar ratios (CMRs) and feldspar molar ratios (FMRs) predict that secondary neutralisation capacity will not keep pace with in situ acid generation. The dormant TSF3 reported fluctuating pH and alkalinity profiles supporting a potential acid-generating classification. In contrast, the dormant TSF4 showed a circumneutral pH profile, suggesting acid generation is unlikely over the longer term. Their CMR and FMR values demonstrate sufficient secondary neutralisation potential, but it is likely to be nullified once the reprocessing begins. Chlorite and Talc were the main minerals controlling the complex’s neutralisation potential. The study predicts that acidic conditions will persist throughout the complex, posing a risk of metal leaching that could affect local water resources. Therefore, a conservative approach should be taken into consideration when advising closure options to manage water quality impacts optimally, post-closure.