GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 121 Table 5.11 Ruahorehore Stream flow rates Stage Modelled groundwater contribution to stream flow (m3/day) Proportion of minimum stream flow* Pre-TSF3 (calibrated base model) 2.0 0.4% Diverted stream (pre-TSF3) 1.0 0.2% Operations to closure (1,155 m RL) 1.5 0.3% Post-closure (1,155 m RL) 2.5 0.5% * 540 m3/day; refer to Section 5.3.5 5.6.2 Water quality Influence on groundwater quality The predicted groundwater concentrations resulting from TSF3 discharges are presented in Table 5.12. Values for the starter embankment are not provided on the basis that the drains are predicted to capture all seepage (Section 5.5.5). The following key comments are made: – Almost all parameter concentrations are predicted to increase in groundwater. – The results are based on the predicted mean tailings and median embankment seepage quality values provided in the AECOM (2021a) report. – The assessment considers mixing and dilution only. No attenuation processes are allowed for in predicting a mixed water quality. As such, the predicted values are considered to be conservative. For this reason, review of TSF1A and TSF2 groundwater quality after tailings seepage mixing has been included in Table 5.12; the influence of these TSFs on groundwater at these locations provides context of likely impact to water quality. – A range of groundwater flows were considered, from limited mixing beneath the TSF to full aquifer mixing prior to discharge to the Ohinemuri River. The larger predicted values in Table 5.12 would be expected in groundwater close to TSF3 and these are generally consistent with the reported values from existing TSF1A and TSF2 long-term monitoring wells. This is with the exception of iron and manganese in a limited number of wells adjacent to TSF21, which have measured in the upper ranges of that predicted for TSF3. Concentrations of these trace elements in groundwater tend to decrease with increasing distance from the TSFs due to hydrogeochemical processes within the subsurface (Section 2.5). – In terms of effects to nearby groundwater users, the long-term predictions of effects to groundwater quality are considered to be more reflective of likely impacts. The nearest downgradient groundwater user is a farm bore located over 1 km away. The concentrations in groundwater that might be seen at this location are the lower ranges provided in Table 5.12, after full aquifer mixing has occurred. These values are likely to be much higher than expected at the bore, as no geochemical processes have been accounted for in the results. Trace elements and metals such as aluminium, iron, manganese, nickel, cobalt and chromium are expected to be significantly reduced along the 1 km flow path by the added combination of processes such as precipitation, co-precipitation reactions, and adsorption. These processes are seen in other mine areas (such as TSF1A and rock stacks, refer Section 2.5) to significantly attenuate contaminant migration in groundwater in relatively close proximity to the facilities. As such, trace element concentrations are not expected to measurable increase at the farm bore. It is possible that some increases in major ions reporting to the farm bore may occur, such as calcium, sodium, magnesium and/or sulphate. Based on the existing TSF groundwater measurements, it is expected that any concentration increases to the bore water are likely to remain well below levels that would cause issues like corrosion or plumbosolvency. The impact to this bore is therefore considered to be minimal. Monitoring of groundwater is proposed (Section 5.6.4) to evaluate the expected changes to groundwater quality over time. 1 Monitoring wells MW1D2S, MW1C9S/D, MW1D11S and MW1D18D; refer to the OGNZL 2020 for locations and data.
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