GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 91 Rock stack leachate and water quality The proposed NRS has the potential to influence the water quality within the receiving environment due to seepage of rock leachate through the base of the Zone A liner. Between 415 – 605 m3/day of leachate is predicted to seep through the liner and discharge into the groundwater beneath the rock stack in the NRS operation scenario, reducing to approximately 260 m3/day following NRS closure. The leachate will mix with the groundwater and any residual soluble mass, following attenuation, will either be captured by the sub-soil drainage system or migrate to the Ohinemuri River with groundwater flow. The assessment results indicate that between 70 – 100 m3/day of leachate is predicted to migrate to the Ohinemuri River in the NRS operational scenario, reducing to approximately 50 m3/day in the NRS closure scenario. The results of the water quality assessment considered dilution of contaminant mass predicted to discharge to groundwater and predicted to migrate to the Ohinemuri River. Adsorption of contaminants to aquifer materials along the groundwater flow path has not been considered and in this regard, the prediction of effects to groundwater and Ohinemuri River quality are considered to be highly conservative. Monitoring of existing storage facilities indicates that following closure longer term impacts should reduce, with these considered more relevant to the Ohinemuri River water quality owing to groundwater travel times and contaminant adsorption. Following dilution and mixing of groundwater with the Ohinemuri River, the assessment indicates given the length of the flow path and expected attenuation, changes to Ohinemuri River water quality are expected to be minimal. Sulphate is predicted to increase between 1 – 2 mg/L which may be measurable within the river, however this increase is considered to be small in the context of the downstream water quality changes. Under baseflow conditions, the Ohinemuri Tributary (TB1) at the NRS site displays neutral results during flow gauging, and a similar chemical signature to nearby piezometers. This indicates that this stream is in close connection with the shallow groundwater system. Placement of sub-soil drainage along the alignment of TB1 therefore has the potential to capture a large proportion of leachate seepage occurring in the eastern portion of the proposed NRS. This presents the opportunity to identify potential issues with discharges and reduce the flux to the receiving environment. The presence of the paleo-channel along the same alignment of TB1 is considered to provide a preferential groundwater flow pathway towards the Ohinemuri River. The greatest permeability is likely to be within the boulder and cobble alluvium in the base of the paleo-channel, therefore there is the potential for leachate seepage from the NRS to by-pass the sub-soil drains, as already appears to be occurring from the northern stockpile. In the event that groundwater flow to the sub-soil drainage system does not eventuate, monitoring within the paleochannel provides an alternative means of identifying and responding to unforeseen conditions. Water users As there are no current or anticipated groundwater users or wetlands identified between the NRS and the Ohinemuri River, any changes to groundwater levels or quality are likely to have a negligible effect on the groundwater receiving environment. Although minor reductions in groundwater levels have been recorded around the existing northern storage and TSF2, the levels remain within historical range, providing supporting evidence for the above interpretation. 4.6.3 Monitoring recommendations Monitoring is recommended as a condition of consent, as per other mining activities at Waihi. This section provides only a high-level overview of monitoring recommendations. Specific requirements should be detailed in an overarching NRS Water Monitoring and Contingency Plan. The plan should include requirements for baseline monitoring (for comparative analysis to provide a degree of confidence in the assessment), trigger levels, and contingency measures to manage any potential exceedances. – Construction dewatering: Monitoring of groundwater inflow rates and groundwater levels should be undertaken during the excavation and dewatering works to assist with understanding groundwater response. – NRS operation and closure:
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