iv EXECUTIVE SUMMARY OceanaGold New Zealand Limited (OGNZL) are proposing to develop the Wharekirauponga underground mine (WUG) north east of the township of Waihi. This development is part of the wider Waihi North Project (WNP). In order to mine the resource from underground, dewatering of the orebody will need to be undertaken and, as a result, the potential for effects on the groundwater system has been evaluated. The site is located beneath Department of Conservation land and is in a surface water catchment that is understood to be sensitive to changes in stream flow conditions. Because of this, the assessment undertaken has an emphasis on groundwater-surface water interaction and how that could change if dewatering of the orebody was to occur. Near surface effects are also considered carefully given the potential sensitivity of the environment. Exploration drilling, geophysics and mapping of the Wharekirauponga deposit has provided a significant database of geological and geotechnical information that provides a detailed characterisation of the mineralised structures and host rocks. Extrapolation of the hydrologic and hydrogeologic testing data collected has enabled a Conceptual Groundwater Model (CGM) to be developed that provides the basis for understanding what effects, if any, might occur from the proposed dewatering of the orebody. Preliminary numerical groundwater modelling has been undertaken based on the understanding of the CGM to make predictions of how the groundwater systems will react to dewatering of the veins. At a catchment scale, the CGM concludes that where post mineralisation Andesite is present, a shallow groundwater system is also present and dewatering effects from the underlying rockmass do not propagate to the surface. That is because the Andesite has a naturally low permeability that retains infiltrating water and there is a weathering profile on the surface of the underlying rocks that limits the ability for shallow groundwater to move downward. Where Andesite is present at the surface, streams are perched on these units and are not in contact with the deeper groundwater system, meaning no effects on surface water can take place from deep dewatering in those rocks. It is only where Rhyolite rocks are present in the near surface, where the shallow and deeper groundwater systems are connected, that dewatering effects have the potential to develop. If deep dewatering effect the shallow groundwater system, the potential to affect surface water flows exists. However, at a catchment scale, the area of Rhyolite rocks exposed at the surface is limited (circa 1.5 km2) and, therefore, so too is the spatial extent to which any potential dewatering effects can develop. Where Andesite and Rhyolite rocks are exposed at the surface, they are covered by a soil Regolith some meters in thickness. These materials have a high porosity and store water in the soil profile following rainfall. This water is then transmitted to the stream via interflow (lateral drainage). It is estimated that interflow could contribute 90% of the surface water flows. This water will continue to discharge into the catchment and will be unaffected by mining. Because of the presence of the Regolith, dewatering effects are not expected to affect the near surface
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