GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 120 Operational stages and final landform Changes to the overall groundwater flow regime are expected to result from construction of the TSF3 facility (1,155 m RL). While there is expected to be less groundwater recharge beneath the HDPE liner than current conditions, greater recharge across the lower valley floor (beneath the embankment) is predicted, resulting in a net increase in recharge to the groundwater system from the baseline condition. Some local changes in hydraulic gradients of the deeper aquifer are expected as a result. The gradients are expected to vary spatially across the different TSF3 stages as the height of the TSF increases, and as drains become operational and then fail. The changes in gradient are expected to be greatest directly beneath the TSF and become subdued with distance from the facility. The greater recharge is expected to increase groundwater flows, locally increase water levels and increase surface water flow of the Ruahorehore Stream. The changes to the groundwater system are summarised below. – Groundwater recharge: Whilst the drains are predicted to capture and remove 1% of the groundwater from the Waihi Basin (Table 5.9), the proportion of combined TSF3 seepage (tailings seepage and embankment rock infiltration) within the groundwater system is expected to increase over the life of the TSF3, from 5% during operation to 10% in the long-term scenario (Table 5.10). As noted previously, this additional recharge is inferred to discharge to the Ohinemuri River. Therefore the drains are overall not expected to reduce the volume of groundwater in the catchment. For comparison, without an HDPE liner, the TSF3 seepage is expected to comprise 65% of the total groundwater source. – Groundwater levels: Groundwater levels south of the diverted Ruahorehore Stream are predicted to increase up to 0.25 m for the post-closure scenario (Appendix I). This level is below the adopted ZOI influence definition of 0.5 m. However, the area is flood-prone and has a shallow water table, so this increase may be influential for farm drainage and the Ruahorehore Stream. The area is more likely to be impacted during high-rainfall seasons (e.g. winter), when it is already impacted. Periods of seasonal surface flooding may be lengthened. To avoid extended periods of flooding, drains can be installed as needed, as is the current practice in the area. This is expected to be able to occur within OGNZL land and mitigate the potential effect of increased groundwater levels. – Surface water: Overall, the Ruahorehore stream is neutral to losing with a very low net groundwater baseflow of 2 m3/day for the length of stream running adjacent to the facility (1,000 m) (Table 5.11). Over the life of the TSF, the flows are predicted to increase slightly as the TSF is constructed, to 2.5 m3/day for the final landform. The source of the increased stream flow volumes is expected to be due to clean water infiltration and run off from the adjacent collection pond crest. This minor addition of flow is expected to be within the margin of error of model predictions. Maintenance of a general neutral to slightly increased stream flow condition is an expected outcome of the TSF3 operation. Potential impacts to overall stream flow rates are not expected to be measurable at the downstream monitoring location (Ruddocks Gauge). Table 5.10 TSF3 seepage as a proportion of groundwater flow Stage TSF3 total seepage (m3/day) TSF3 seepage in groundwater Starter embankment (1,135 m RL) 0 0% Operations to closure (1,155 m RL) 45 5% Post-closure (1,155 m RL) 95 15% Comparison scenario – no HDPE liner: Starter embankment (1,135 m RL) 540 65%
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