GHD | Oceana Gold NZ Ltd | 12552081 | Geochemical Assessment Wharekirauponga Underground Mine (WAI985-000-REP-LC-0013) 26 7. Water Quality Based on the current geochemical dataset outlined in Section 4, the geochemistry of the Willows Access Tunnel is deemed to be not significantly different from what has been previously observed from OceanaGold’s Waihi operations. The WUG Mine data however, does show some variation and therefore a conservative approach to water quality predictions has been applied. As the spoil within the WRS at Willows Farm will be dominated by the Willow Access Tunnel, it is considered that runoff and leachate water quality will therefore likely be in line with what has been observed at the current Waihi operations. Data from the field column tests can be utilised to inform differences in leachate characteristics and inform amendments to the rock management when it is available. It is considered that in general, water (ground and surface) interacting with PAF rock material will require treatment before discharge to the receiving surface water environment. The quality of water requiring treatment has been estimated for representative WRS seepage, WRS active area runoff and dewatering water from the Willows Access Tunnel and WUG Mine. Water quality from these respective sources have been derived from monitoring data from the operational mine at Waihi and other available data. The water qualities derived here are utilised in the Water Balance assessment which takes into account the water treatment capacity and availability, trace element removal rates and consented discharge requirements. This assessment is detailed in Waihi North Water Management Assessment (GHD, 2022). It is considered that the water quality data from the Waihi operations is reflective of water quality associated with the Willows Access Tunnels due to similarities in the whole rock trace element concentrations. However, due to the nature of the host rock and observed geochemical differences associated with the WUG Mine it is possible that the concentrations of some elements may differ. A conservative approach has therefore been applied to account for uncertainty in the contaminant concentrations and acid producing potential from the WUG Mine. Comparative datasets from OceanaGold’s Waihi operations have been utilised and the 95%ile concentrations from monitoring data records have been utilised. This approach allows for likely variability in actual runoff, seepage, and inflow water quality, while also capturing periods where monitoring data suggests the waste stream has a higher trace element load. Ultimately, leachate data from the on-site column tests comprising material from the WUG Mine (when available) can be utilised to refine these numbers. Runoff Water Quality It is considered that existing water quality data from collection ponds associated with embankments at the active tailings facilities at Waihi (TSF1A), temporary spoil storage areas and the water treatment plant (WTP) area is likely to be representative of water quality associated with runoff from the active WRS at the WUG portal site. Seepage Water Quality It is considered that seepage water quality from the TSF1A embankment is likely to be representative of water quality associated with seepage from the active WRS at the WUG portal site owing to inferred similarities between Martha spoil, from which the TSF1A embankment is constructed, and Willows Access Tunnel. The 95%ile from the dataset is utilised to allow for geochemical differences and geological variation. The following design components are assumed: Leachate drains; Naturally or engineered liner; Spoil placed in small, compacted lifts; and
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