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 31 AECOM For illustration, Table 14 and Table 15 show examples where PAF rock material with ABA characteristics as per the mean and 95% UCL of Martha and Gladstone rock respectively are placed and compacted and left uncovered for a period of 210 days. A calculated amendment rate of between 0.2% and 1.7% is conservatively recommended in order to extend the lag period assuming a 2 m oxygen profile is present. Table 14 Martha Compacted Rock - Acidity Neutralisation Requirements Total lag Required (days) Additional lag required Limestone Amendment Rate (Mean NAPP) Limestone Amendment Rate (95% UCL NAPP) 210 70 0.2 % 0.4 % Table 15 Gladstone Compacted Rock - Acidity Neutralisation Requirements Total lag Required (days) Additional lag required Limestone Amendment Rate (Mean NAPP) Limestone Amendment Rate (95% UCL NAPP) 210 140 1.3 % 1.7 % Rock placed within the zone of oxidation for the final proposed landforms (typically within the final 2 m of directly placed materials) should comprise NAF material only. 6.3.4 Estimated Backfill Rock Porewater Quality As discussed, rock will be placed in a manner to limit oxidation (refer Section 6.3.3) and appropriate limestone amendment will be carried out to extend the lag period where deemed necessary. With these control measures in place, a limited volume of oxidation is expected to occur within the backfilled material. Oxidation products will nevertheless be evident in porewater and hence a small volume may become evident in groundwater within the immediate vicinity of the backfilled GOP. Porewater quality associated with rock has been calculated using the following steps: • Leachate concentrations from field columns has been assessed and summarised for likely pore fluid composition and concentration (Appendix D). The limestone blended and compacted columns are used for the assessment of post closure backfill water quality. These columns utilise high sulphur Gladstone material and replicate proposed placement methods (limestone blended/loose placement) and compacted placement. Long term post closure porewater quality also considers the saturated column water data to assess effects of groundwater saturating the backfill. • This summarised leachate data has then been analysed in the geochemical modelling software PHREEQC Interactive version 3.6.2 utilising the Minteq.v4 database in order to equilibrate the predicted chemistry based on the oversaturation and secondary co-precipitation of various trace elements. The steps undertaken in PHREEQC are as follows: 1. Equilibrate leachate from the column tests with a range of minerals that typically influence the solubility of the various contaminants of concern. The minerals considered include sulphides, hydroxides, carbonates and sulphates. 2. Determine the influence of changing groundwater conditions on the adsorption/desorption of trace elements to iron oxy-hydroxides or HFO. The insoluble iron mass from over saturation is assumed to be present as iron hydroxides and is provided for sorption reactions. In this manner a new equilibrium between adsorbed and soluble trace elements can be modelled. 3. Modelled results from the above process are then compared to TSF1A leachate data and where the above modelling indicates changes from the existing data, modelled results are adopted in Table 16. Modelled porewater quality is considered to reflect the range of porewater chemistry that would be expected to occur within the backfilled GOP upon flooding. Localised differences are expected, with these the result of the variability of waste rock material, the degree of oxidation, the presence of neutralising minerals and the availability of iron hydroxide minerals for adsorption of trace elements. The range of predicted contaminant concentrations is summarised in (Table 16).
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