SECTION 4 Assessment of Effects C:\D Data\GWS C Drive\GWS\Client Files\1344 OGL - WKP\4 Deliverables\WKP Final Draft Report\June Report\WAI-985-000-REP-LC-0030. Final_Rev_0.docx 33 Figure 29 Extent of Near Surface Drawdown Effects (Flosolutions, April 2022) While the preliminary modelling indicates drainage of the near surface could, potentially, occur, this effect would be limited in extent, and constrained to the area where the Rhyolite host rocks area are exposed at the surface. This is around 1.5 km2 or 2% of the catchment surface. This extent is shown in Figure 30. The remainder of the surface exposures within the Otahu catchment are Andesite rocks that post-date the mineralisation. These overlying Andesite rock units are not expected to drain as a result of dewatering the Rhyolite rockmass. This is primarily because these geologic units are of lower permeability, and there is a weathering layer at the base of the units that act as an aquitard, limiting the amount of vertical drainage. Dewatering effects are not expected to extend into the soil Regolith at the surface which has higher permeability and greater water storage capacity than the rockmass beneath. Ultimately, the soil water balance in the Regolith is driven by rainfall recharge, climatic conditions, plant uptake and soil water storage, resulting in a different response compared to groundwater drainage effects in the rockmass. Experiences of underground mining in the area, (Maratoto, Golden Cross, Karangahake and Waihi) confirm such behaviour. The modelling undertaken does not include replication of the soil water balance.
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