Oceana Gold Waihi North Project Waihi North Project Geochemical Assessment – Geochemistry of Tailings and Overburden, Treatment and Mitigation Revision 0 – 17-Jun-2022 Prepared for – Oceana Gold (New Zealand) Limited – Co No.: 2274246 54 AECOM 7.3.2 TSF Porewater Quality Water chemistry associated with TSF3 and GOP TSF porewater was predicted using the following methods: • Existing underdrain data was factored by the differences in total element composition of tailings that comprise seepage to these underdrains. • Comparison of process water in laboratory leach tests (Table 23) to process water from previous tailings porefluid (Table 24) immediately post processing data, identified differences in leachability of trace elements with respect to trace elements in the GOP and WUG ore. • The geochemical model PHREEQC was used to simulate the complexation of heavy metal ions on hydrous ferric oxides based on the model of Dzombak and Morel2. The chemistry of the starting model solution comprised the equilibrated porewater. The model simulation represents the equilibration of this process water in the TSF and represents a theoretical end point for water chemistry within pore spaces in the tailings storage facility. Existing underdrain data factored by tailings composition changes is therefore checked against this modelled data to validate predictions presented in Table 26. Porewater within the tailings material when discharged to the TSFs is notably different from actual fluids collected in the tailings underdrains and decant. Metal ion adsorption is therefore having a significant effect on dissolved species. Surface complexation modelling predicts that arsenic, calcium, cadmium, cobalt, chromium, copper, iron, mercury, magnesium, nickel, and lead concentrations in solution will be reduced via adsorption to hydrous ferric oxides, while potassium, sodium, and sulphate are not significantly altered by adsorption. Predicted porewater is compared to existing underdrain data from TSF1A in Table 26. Unlike the existing tailings facilities, it is proposed TSF3 and GOP TSF will be lined beneath the tailings pond with a geomembrane to limit the migration of tailings seepage into groundwater in the early years of tailings deposition (for TSF3), prior to tailings consolidation. 2 Dzombak, D.A., and Morel, F.M.M., 1990, Surface complexation modeling—Hydrous ferric oxide: New York, John Wiley, 393 p.
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