GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 125 Parameter Existing water quality (RU03 and RU10)* Predicted water quality** Receiving water quality criteria Operational Long-term Pb 0.0001 0.0001 0.0001 0.0004 Sb 0.0002 0.0002 0.0002 0.03 SO4 7.0 7.4 7.4 Zn 0.004 0.004 0.004 0.027 Notes: Receiving water quality criteria values for hardness 20 g/m3 CaCO3 Values in bold indicate an exceedance of the receiving water quality criteria * These values represent the median of all samples from RU3 and RU10 (combined), which have similar water quality. ** The minimum stream flow measured has been considered. # The mercury (Hg) criterion is lower than available laboratory detection limits Denotes a predicted increase from existing surface water quality 5.6.3 Consideration of existing TSFs The influence of the existing tailings storage facilities on the environment provides insight into how TSF3 is likely to impact the environment. Section 2.5 outlines a high-level summary of the performance of existing TSFs. The following information is also reported in the EGL (2021b) TSF3 technical report – The existing TSFs do not have a liner over the base of the impoundment and rely on hydraulic containment of the site and the cutoff and associated drain at the upstream toe of the embankment and subsurface drains to intercept and collect groundwater seepage. – Additional controls in the form of a lined impoundment are proposed for TSF3 as an improvement over the previous facilities to limit seepage into the natural environment, and because the depth to rock for a cutoff is too deep to be practical. The proposed lining of the impoundment while still retaining the upstream cutoff drain provides an additional level of protection during this early stage of deposition and for the long term. The HDPE geomembrane will also limit seepage to the subsurface drains located beneath the liner. The effects to the environment will be cumulative from all three TSFs. As noted previously the reader should refer to the GHD (2022a) Water Management Report, which considers the results from Table 5.14. 5.6.4 Monitoring recommendations Environmental monitoring is likely to be a required condition of consent, as per the existing TSFs. This section provides only a high-level overview of monitoring recommendations. Specific requirements should be provided in a detailed monitoring plan, which should include baseline monitoring (for future effects to be assessed against), trigger levels, and mitigation/contingency measures to manage any potential exceedances. – Construction dewatering: Flow monitoring: Flow gauging of the Ruahorehore Stream should be undertaken prior to and during the dewatering works. This information should be used to assess whether diversion of groundwater into the stream is required to maintain stream flow volumes. Groundwater quality: Water quality monitoring of the dewatered groundwater during the excavation period should be undertaken to determine whether the water needs to be diverted to the WTP, or if it can be diverted to the Ruahorehore Stream. – TSF3 construction through to post-closure: Groundwater quality: It is anticipated that a groundwater well network will be established around TSF3 for frequent long-term monitoring. It is also recommended that the nearest private bore users are included, but this sampling may be at a reduced frequency compared to the site monitoring wells. Water samples are expected to be analysed for a similar suite of parameters as the existing TSFs and included in the annual interpretation of water quality data for the wider site.
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