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 32 AECOM Trace element concentrations vary considerably depending on the assumptions around adsorption sites, whether limestone is present (solution is equilibrated against calcite or not) and the assumed nonequilibrated porewater concentration. Concentrations are expected to be diluted considerably outside of the pit area with the natural groundwater. The degree of dilution will depend on the distance from the pit and it is expected that a high degree of attenuation through additional adsorption will occur. Table 16 Gladstone Open Pit Porewater Quality Parameter Post Closure Long Term Min Median 95 Percentile Min Median 95 Percentile pH 3.83 5.6 7.9 3.83 6.7 7.9 Al 0.003 6.9 22 0.003 0.9 2.1 As 0.0001 0.0016 0.01 0.0001 0.0013 0.1 Ba 0.01 0.015 0.04 0.01 0.01 0.04 Ca 31 312 590 31 540 630 Cd <0.00005 0.007 0.01 <0.00005 0.002 0.01 Co 0.0002 3.4 4.5 0.0002 0.57 4.3 Cr < 0.0001 0.003 0.04 < 0.0001 0.0005 0.003 Cu 0.0005 0.09 2.0 0.0005 0.0005 0.3 Fe 0.02 325 435 0.02 30 430 Hg 0.00008 0.0001 0.0005 0.00008 0.00019 0.0005 K 4 20 30 4 20 46 Mg 15 50 280 15 70 280 Mn 0.3 8.1 61 0.3 8.9 61 Mo 0.0002 0.0003 0.001 0.0002 0.0002 0.001 Na 12 240 750 12 140 750 Ni 0.003 1.3 2.9 0.003 0.26 1.7 Pb 0.0001 0.0001 0.0007 0.0001 0.0001 0.0007 SO4 200 2400 2,800 200 1600 2700 Sb 0.0002 0.003 0.004 0.0002 0.0004 0.004 Se 0.001 0.002 0.02 0.001 0.002 0.01 Zn 0.003 2.9 4.4 0.003 0.05 1.5 All concentrations reported in mg/L. 6.4 Estimated WUG Underground Water Quality A highly conservative worst-case assessment of groundwater quality within voids of the saturated backfill (porewater) in the WUG mine post closure is determined using the following approach: • The backfilled mine is flooded with groundwater of a composition comparable to the existing groundwater quality within the vein system; and, • The total oxidised mass of sulphate and associated trace elements (determined based on their individual molar ratios to the amount of oxidised sulphur) within the waste rock is dissolved within the total volume of the initial porewater within the waste rock backfill of the underground mine. This is calculated from sulphate generation rates from the WUG columns.
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