GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 87 4.5.3 Water quality Emplacement of rock at the NRS will result in leachate generation during both operation and after closure that will percolate downwards through the rock stack. Quality of leachate is expected to improve over time (AECOM, 2021a). Where not captured by leachate drains the leachate will seep through the underlying Zone A soil liner until groundwater is encountered. A significant volume of impacted groundwater will be captured by the sub-soil drains and diverted for treatment. The remaining leachate impacted groundwater will migrate to the Ohinemuri River. The potential effects of the leachate seepage on the receiving environment have been predicted for the NRS closure scenario and compared against the existing water quality of the groundwater and the Ohinemuri River (Table 4.7). The water quality assessment provides a highly conservative prediction of potential impacts as the methodology does not allow for attenuation, which would otherwise significantly reduce many of the concentrations provided in Table 4.7. Attenuation is expected to occur through interaction with the soil profile and adsorption reactions with groundwater flow, with only residual soluble components then migrating from the point of discharge with groundwater flow. The predicted concentrations of mercury are greater than the RWQC as the laboratory detection limit of this parameter is greater than the criterion. As discussed previously, mercury is typically immobile in groundwater due to volatilization and/or precipitation processes and has not been reported in significant persistent concentrations by OGNZL (2020). It is not a parameter of concern for these reasons. The following is summarised from the water quality assessment: – Overall, concentrations of several parameters associated with rock storage are predicted to increase within the groundwater in the NRS closure scenario, with iron and manganese exceeding the receiving water quality criteria. – The large catchment of the river results in significant dilution of groundwater discharging from the NRS to the Ohinemuri River, so that predicted increases in Table 4.7, with the exception of sulphate, are expected to be unmeasurable within laboratory error and natural variations in the river. Parameter concentrations within the river are predicted to remain below the adopted receiving water criteria. – The mass flux of NRS leachate and groundwater discharging to Ohinemuri River during operation of the NRS is provided in Table 4.8. These values are presented for inclusion in the Water Management Report for the assessment of cumulative effects to the Ohinemuri River catchment and are not discussed further in this report. It is noted that these results are highly conservative as they are for the 50% rainfall recharge scenario during NRS operation and do not consider geochemical equilibration of the leachate with the shallow groundwater or attenuation. Table 4.7 Predicted receiving water quality in the NRS closure scenario (values in mg/L) Parameter Existing shallow groundwater quality (DH05a median) Predicted values after equilibration of NRS leachate with shallow groundwater (AECOM, 2021c) Existing Ohinemuri River quality (OH3 median) Predicted values after mixing with Ohinemuri River Receiving water quality criteria pH 6.5 7.4 7.2 - Al 0.003 0.011 0.013 0.013 As 0.0096 <0.001 <0.0010 <0.0010 0.19 Ba 0.67 0.02 0.027 0.027 Ca 2.7 116.0 3.1 3.6 Cd <0.00005 0.000010 <0.00005 <0.000050 0.0003 Co 0.041 0.020 <0.00020 0.00028
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