GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 81 . Overall, the exceedance of statistical trigger levels and analysis of groundwater quality trends does not suggest any developing or ongoing failure of containment or encapsulation of rock at the existing rock storage facility. A key aspect of interpretation relates to the concentrations of iron in groundwater around the existing rock storage facility; concentrations of iron in groundwater are greatest nearest to the embankment and within the paleochannel (WRS08/DH08 and DH05a). Compared with predicted rock storage infiltration quality (up to 300 g/m3; Section 6.3.1 of AECOM, 2021a) this is a significant reduction in concentrations within just metres of the facility, with further reduction with distance away from the northern stockpile. This suggests that in-ground conditions continue to promote the precipitation of iron hydroxide minerals (such as ferrihydrite and goethite), which are key to attenuating trace element contaminants released from mineralised rock through adsorption and co-precipitation reactions. The occurrence of such attenuating processes is likewise expected to mitigate contaminant discharge from the NRS, which is expected to have discharge and groundwater conditions generally consistent with the existing rock storage facility. Surface water quality Surface water quality under baseflow conditions indicates little change in chemistry as tributary TB1 flows towards the Ohinemuri River. Under these conditions the chemistry of the tributary was recorded to be relatively similar to NRS groundwater samples, confirming a close hydraulic connection between surface water and groundwater. After large rainfall events, a difference in chemistry is recorded between surface water samples along the stream (TB1a, TB1f and TB1b) in comparison with the groundwater fed headwaters (WRS TB) and the Ohinemuri River (OH3) (Figure 4.1 and Appendix F). This is likely to be a result of surface water runoff and interflow. The surface water quality of the Ohinemuri River at sampling point OH3 appears similar to that of deep piezometer WRS04a which is located approximately 100 m to the east of the river (Figure 4.1). 4.3.5 Water resource users There are no registered bores located within approximately 800 m of the proposed NRS (WRC, 2021c), including the area between the proposed NRS site and the Ohinemuri River (Figure 3.11). The closest surface water take is presented in Table 4.2. This take is from the Ohinemuri River upstream of the proposed NRS site. The closest downstream surface water take from the Ohinemuri River is approximately 10 km downstream of the WNP area and is not shown on the report figures. Table 4.2 Water takes in the vicinity of the proposed NRS Property/user Consented take (year granted) Type Approximate distance from NRS Ohinemuri River take (Waihi water supply) AUTH130392.03.01 (2014) Surface water take 300 m upstream 4.4 NRS assessment methodology Potential impacts on groundwater and surface water associated with the proposed NRS were assessed using a variety of methodologies to predict the rate of seepage infiltration through the Zone A soil liner and subsequent effects to the receiving environment during operation and after closure of the proposed NRS. Consideration has been given to the longer-term impacts, owing to the short timeframe for NRS construction and closure, relative to groundwater travel times. The assessment comprised: 1. Conceptual groundwater model development (Section 4.3): Understanding of the current geological and hydrogeological environment using site investigation data (Appendix A). 2. Groundwater flow: Development of an analytical groundwater model to predict existing groundwater flow across the NRS site and baseflow discharge to the Ohinemuri River. 3. Construction dewatering assessment: Development of an analytical groundwater drawdown model to provide analysis of groundwater inflow and drawdown resulting from the installation and operation of the uphill
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