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 35 it, can be managed through the selection of the backfill materials and chemical stabilization. Geochemical modelling is required to enable a prediction of the endpoint groundwater chemistry based on the aforementioned parameters and is still the subject of further studies at this time. 4.5 Effects on Aquifers The primary aquifer that will be dewatered to access the mineralisation is the Rhyolite rockmass. This aquifer intercepts deep groundwater that flows from the upper reaches of the catchment down to well below sea level where it discharges in a submarine environment. During mining there will be a localised reversal of the groundwater flow regime due to dewatering (refer Figure 29). At a catchment scale, however, this is not expected to result in adverse effects as coastal outflows will be maintained. Modelling and experience elsewhere have shown that while some extent of dewatering of the rockmass will occur, it will be localised and will not affect water levels in any overlying aquifers such as the post mineralisation Andesite units. While it is not expected the overlying aquifers will dewater, there will likely be some adjustment of pressure heads in the aquifers due to steeper vertical hydraulic gradients created by increased underdrainage. The extent that this is likely to occur is the subject of ongoing groundwater assessment by FloSolutions. The vent shafts will be similar to a large diameter bore hole that will be continuously lined to prevent the ingress of groundwater. During construction there will be some localised drawdown of the groundwater system around the shafts. Following construction of the shafts the groundwater system will return to its previous state. The shafts will be constructed entirely within the massive andesite volcanic rocks that constitutes one aquifer system. Construction of the shafts will not, therefore, result in the mixing of previously isolated aquifers. 4.6 Effects on Surface Waters Assuming dewatering of the Rhyolite rockmass in the near surface were occur, there are a number of potential mechanisms by which surface water loses might develop: • Losses through the base of the stream beds • A reduction in spring fed inflows from permeable structures • A reduction in spring fed inflows from springs at the stream headwaters • Cessation of the warm spring flow • Regolith soil water loss The following describes these effects further and assesses whether physical mitigation options could be considered. Stream Bed Losses Where the rockmass is particularly fractured and forms part of the stream bed it is possible surface waters may drain into the fractured ground and be lost to the mine dewatering. The extent to which this can happen is limited spatially to areas of exposed Rhyolite flow shown in Figure 30. Along the main reach, where most of the structures exist, the gradient is flatter and, therefore, more susceptible to stream bed losses. The stream bed conductance will determine
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