Puntsag, T, Tulganyam, S, Sededpurev, O, Bazardorj, S-E, Bayanzul, B, Myagmar, B, Sarangerel, T, Danzan, A, Natsagdorj, M & Nyambuu, P 2025, 'Water quality and potential sources in alluvial bore, downstream of the 
tailings storage facility, Oyu Tolgoi copper mine, Mongolia ', in S Knutsson, AB Fourie & M Tibbett (eds), Mine Closure 2025: Proceedings of the 18th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2515_96
(https://papers.acg.uwa.edu.au/p/2515_96_Puntsag/)
Abstract: Understanding water pathways and salinity transformations associated with tailings storage facilities (TSFs) is critical for long-term groundwater protection and effective mine closure planning, particularly in arid regions such as Mongolia’s South Gobi region. This study examines increasing total dissolved solids (TDS) observed in shallow alluvial monitoring bore OTMB16-79, located downstream of the Oyu Tolgoi (OT) TSF. Isotopebased (δ¹⁸O, δ²H, δ³⁴S, ⁸⁷Sr/⁸⁶Sr) and elemental analyses (major, trace, and rare earth elements) were applied to distinguish TSF seepage from natural geochemical processes, including evaporative concentration, bedrock weathering, and displaced groundwater. Parameters such as δ³⁴S, ⁸⁷Sr/⁸⁶Sr, Na, and Cl were identified as key geochemical signatures for source differentiation. Results indicate that direct TSF seepage contributes less than 10% to the salinity at OTMB16-79, while approximately 31% originates from water collected and returned by the French drain system. These findings demonstrate the value of multivariate geochemical and isotopic tools in identifying source contributions and support the refinement of OT’s conceptual hydrogeological model. The outcomes will contribute to establishing a risk-informed monitoring system and help prevent seepage migration, thereby supporting more effective and optimal TSF management.

Keywords: total dissolved solids, tailings storage facility seepage, strontium, d-excess value, sulphur isotope