GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 66 . (OH3) is predicted to reduce by <1 m3/day. However, these changes are likely to be unmeasurable within the river which has a median flow of 63,200 m3/day (Frendrups gauge) and significantly higher peak flows. Stormwater runoff and interflow from the GOP area towards both OH3 and OH6 locations is also predicted to reduce by approximately 20%. This reduction in flow from these sub-catchments (currently an area of approximately 30 ha) is again considered to be very small in the context of the total Ohinemuri River catchment (29,000 ha). – No change is predicted to groundwater levels and gradients within the catchment of the Ohinemuri tributary north of the pit (where TB5 is located). This is because the existing groundwater catchment area is not anticipated to be impacted by GOP excavation. Removal of surface area is predicted to reduce surface water runoff and interflow from the GOP area by approximately 16% for TB5. – Groundwater flow to the Gladstone wetland (TB4 tributary) is predicted to reduce to approximately 0.7 m3/day from the current scenario of 1.0 m3/day. Removal of surface area in the TB4 catchment is also predicted to reduce surface water runoff and interflow to the Gladstone wetland by approximately 26%. Despite these predicted reductions in discharge to surface water receptors, all dewatered groundwater and rainfall runoff captured within the Gladstone Pit during excavation will be directed towards the WTP and subsequently discharge back into the Ohinemuri River. This is expected to result in a net neutral or gain to the Ohinemuri River flow. The effect to tributary TB5 from the reduction in stormwater runoff and interflow is expected to be negligible, given that it is inferred to form a drainage channel, rather than a groundwater fed watercourse. As the effect to the wetland water balance is anticipated to be small but measurable, managing flow to the wetland will reduce the potential effect of the proposed activity. In the event that effects to the wetland are greater than predicted, it is anticipated that stream augmentation or stormwater diversion can be used to mitigate the adverse effects. Further discussion of the Gladstone Wetland is provided in the GHD (2022d) Gladstone Wetland Groundwater Assessment Summary Technical Memorandum, which should be read in conjunction with this report. Groundwater users Within the shallow groundwater system drawdown from the pit dewatering is predicted to produce a zone of influence that extends up to 290 m south of the pit boundary, and 210 m west (Table 3.5). There are no registered groundwater users located within these areas nor in alignment of the veins extending to the southwest of Gladstone pit (Figure 3.11). The nearest bore user is recorded 430 m south of Gladstone and is not expected to be impacted during excavation of the Gladstone Pit (Figure 3.8). The extent of dewatering influence on the deep groundwater system is expected to be constrained to within close proximity of the Gladstone vein system by the low permeability country rock. This inference is supported by the limited extent that Favona mine dewatering has on groundwater levels offset from the alignment of the vein system, as indicated by monitoring at locations P79d and P61. Existing groundwater users located south of the Gladstone Pit are not expected to be in hydraulic connection with the Gladstone vein system due to the low permeability of the andesite country rock and large separation from the alignment of the mineralised veins. For the groundwater take at the Black Hill Orchard bore, this is supported by the lack of response to dewatering of the Martha mine (URS, 2003), pumping test results (GWS, 2018b) and a comparison of groundwater quality with GLD04d (Section 3.3.6). It is acknowledged that there may be a slight loss of recharge to the deep groundwater system as a result of initial dewatering of Gladstone Pit and any small ongoing inflows, which would otherwise provide recharge to the deeper system. However, this reduction is expected to be small in magnitude and expected to have negligible influence on the groundwater users outlined in Section 3.3.6. Surface water users The nearest downstream take from the Ohinemuri River (frost protection; AUTH141637) is located approximately 10 km downstream of the WNP area. It is expected there will be no effects to this user in terms of surface water flow reductions, where there is predicted to be no net decrease in surface water flow due to GOP excavation. Similarly, there are no predicted effects to surface water quality as a result of groundwater dewatering.
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