GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 90 Only small changes to groundwater levels are predicted to occur due to placement of the NRS, however groundwater discharge to the Ohinemuri River is expected to reduce as a large component of groundwater flow will be captured by the sub-soil drains. Existing rainfall recharge will be intercepted by the rock stack and the perimeter drains, however this is expected to be offset or increased by seepage of leachate through the NRS soil liner. Although there is the potential for groundwater mounding to occur beneath the NRS where the rate of leachate seepage exceeds that of the existing rainfall recharge, groundwater levels are expected to be largely controlled by the elevation of the sub-soil drainage system. The results of the assessment indicates that between 1,090 – 1,245 m3/day of combined leachate and groundwater may be captured by the sub-soil drains during NRS operation, reducing to approximately 965 m3/day following NRS closure. Where mounding of groundwater occurs, the likelihood of saturated conditions being present within the rock increases, driving horizontal flow within the rock towards the leachate collection drains and reducing the proportion of leachate that may seep through the soil liner. This is demonstrated in the results of the sensitivity analysis (Appendix H), where groundwater levels up- and down-gradient of the NRS were both tested within the SEEP/W model. Furthermore, interception of elevated groundwater by the sub-soil drains and diversion for treatment is anticipated to reduce the contaminant flux that may migrate within groundwater beyond the NRS footprint and discharge into the Ohinemuri River. Regardless of the degree to which leachate infiltration generates mounding, groundwater flow to the receiving environment is not expected to be significantly changed as a result of the proposed NRS. This is supported by monitoring of existing storage facilities which indicates that water levels remain similar to existing conditions. Former gullies and permeable material across the site, including the boulder and cobble alluvium and weathered ignimbrite in the paleo-channel beneath the NRS footprint, have the potential to act as preferential pathways for groundwater flow, providing a conduit for groundwater discharge to the Ohinemuri River where not captured by sub-soil drains. Elsewhere, depending on the hydraulic gradients, and the elevations of the NRS liner and sub-soil drains, seepage may infiltrate deeper in the shallow groundwater system and ultimately discharge to the Ohinemuri River via existing groundwater flow paths. Surface water The footprint of the proposed NRS and associated features are located within the surface water catchments of the Ohinemuri River and tributary TB1. At present, surface water runoff and interflow discharges directly to these surface water features. During operation of the rock stack, clean water diversion drains will capture stormwater and interflow which will be discharged directly to the Ohinemuri River. During operation of the NRS, rainfall that is intercepted by the rock stack will either evaporate or generate leachate. After closure of the NRS, a degree of recharge to the rock stack will still occur through the capping layer and continue to generate leachate. Runoff from the rock during operation, or from the capping layer after closure, will be collected within perimeter drains and diverted to the collection pond for treatment, after which it will be discharged to the Ohinemuri River. During both operation and closure of the NRS, the leachate generated within the rock stack has three potential pathways: 1. Capture by leachate collection drains, treatment and discharge to the Ohinemuri River. 2. Seepage through the NRS soil liner into groundwater, capture by sub-soil or toe drains, followed by treatment and discharge to the Ohinemuri River. 3. Seepage through the NRS soil liner into groundwater, followed by migration and discharge to the Ohinemuri River. Although the pathways of runoff, interflow and groundwater will change, and may be intercepted during operation and after closure of the NRS, the Ohinemuri River will remain as the final receiving environment. As no additional water is proposed to be introduced to the NRS system by way of a dam or pond (i.e. as with TSF3), the volume of discharge to the river is therefore expected to remain similar to those under existing conditions. Changes to surface water flow and levels in the Ohinemuri River during operation and after closure of the NRS are therefore expected to be unmeasurable.
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