www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 4 the Waihou River at Paeroa, which then discharges to the Firth of Thames. The northern catchment of the Otahu River drains the four upland sub-catchments with the Wharekirauponga sub-catchment overlying the WUG itself covering 14.5km2. These upland streams follow valleys aligned with the NNE structural trend. The Otahu River occupies a catchment area of around 70km2 draining to the northeast reaching the sea immediately to the south of the town of Whangamatā. The Wharekirauponga subcatchment is flashy following rainfall, with stream flows supported largely by surface runoff and interflow (unsaturated zone subsurface flow). 2.3. HYDROGEOLOGY The regional hydrogeology is typical of an upland volcanic setting with shallow perched aquifers occupying valley fill colluvium underlain by highly fracture-controlled groundwater systems within tight volcanic bedrock at depth. Groundwater flow in the surficial aquifers largely follows topography with short flow paths maintaining stream baseflows, where surface water is in connection with valley-fill deposits and displaying little opportunity for recharge to the deeper systems. Unsaturated zone interflow and surface runoff dominate the maintenance of stream flows as is evident from the flashy nature of the surface water systems. A degree of lithological control to the shallow groundwater is evident where hydrothermal alteration, weathering and volcanogenesis either increase permeability through silicification or decreases this with the presence of clay minerals and fine-grained or welded tuffs. Brecciated volcanics and pumiceous formations may also enhance permeability. The deep groundwater system is controlled by fracture flow in the post-formational veins and faults and follows the structural NNE trend with overall drainage following the topographic trend in a similar direction. Recharge is thought to occur in the upland areas where suitable lithologies allow a greater degree of infiltration (GWS). The structural control of recharge is likely to be dependent on whether these are related to extensional or compressional movement. Deep circulation of groundwater allows discharges to occur as warm and cold springs at intersections with surface water. There is a hydrochemical indication of a general positive hydraulic gradient that connects the warmer, more mineralised, deep groundwater system with the surface via the structural controls. Groundwater discharge to the Wharekirauponga Stream amounts to 68L/s and occurs primarily from the headwater springs that flow at rates between 1.5 and 10L/s (GWS). Underground mining experience elsewhere at Waihi suggests groundwater inflows outside the vein systems can be expected to be low where the mine and tunnel formations are unaltered (GWS).
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