www.valenza-engineering.com 381_R_04_Rev 0 OGNZL_WUG_Phase1_Conceptual_Mitigation 9 The conceptual hydrogeological model (CHM) produced by GWS (ref 11) was developed to assist in the assessment of effects on groundwater (summarised in Section 5.1.1). Groundwater inflow analyses completed by GWS indicate that between 1.1 and 5GL per year of groundwater will be removed from the Waihou groundwater catchment during the development of the WUG access tunnel and 4.6GL per year from the Otahu groundwater catchment (Waihi Basin deep aquifer). The Waihou groundwater catchment, which is sub-divided into the shallow and deep aquifer systems, is part of the Waihi Basin AMA. The Willows Farm access tunnel from the portal to the vent shaft lies within the Mataura Stream surface water sub-catchment which drains south to the Ohinemuri River. The upper reaches of this stream are steep, and flows are unlikely to be sustained by baseflow to any degree. A multi-level VWP installation in the vent shaft area suggests a vertical downward hydraulic gradient. In the flatter, lower reaches depths to groundwater are small and there is evidence from borehole hydraulic tests of a moderately permeable, perched aquifer in the pyroclastic materials forming the bedrock., with relatively shallow hydraulic gradients (0.02) towards the stream. Groundwater inflow analyses completed by GWS indicate drainage into the Willows Farm tunnel of 0.2GL per year. The twin tunnel section of the access tunnel from the junction of the two tunnels to the mine lies within the Otahu groundwater catchment beneath the Waiharakeke, Thompson and Wharekirauponga Streams. The upper reaches of these streams are steep with little baseflow contribution. Stream baseflow is expected to be mostly sourced from the shallow regolith soils, with low flows fed by bedrock discharge from rhyolite outcroppings. These areas are within the conservation estate. The acceptable effects envelope in these areas is particularly stringent, with the activity needing to be managed such that there is no measurable change in the natural flows or water quality of natural state surface water systems within conservation land and no measurable dewatering of the soil Regolith which could have an adverse effect on vegetation. Groundwater removed from the Otahu groundwater catchment is transferred via the access tunnel for treatment and discharge or use within the Waihou groundwater basin. 3.2.1. HYDROGEOLOGICAL UNDERSTANDING Groundwater catchments and resultant flow systems generally align with the surface water catchment and topographic divides that follow the ridgeline within the Coromandel Ranges to the north of Waihi. These general surface water and groundwater flow directions will be influenced locally by the structural controls that follow a similar NNE trend. To the south of the divide, in the first third of the tunnel alignment, relatively steep hydraulic gradients are anticipated reflecting the topography and the vertical recharge to the deeper groundwater. Towards Waihi, the groundwater system is present as a shallow unconfined aquifer in the surficial deposits of alluvium and younger volcanic materials with a low hydraulic gradient towards the south. Groundwater flow is controlled by paleochannels perched above the eroded basement andesites. In the area around Waihi township, hydraulic gradients steepen considerably and swing westwards towards the Martha open pit under the influence of mine drainage. The underlying andesites through which the immediate tunnel alignment passes from the portal form non-aquifers (they do not store or transmit any significant quantity of groundwater) with pore pressures also affected by the mining and dewatering of the Martha and Favona vein systems. The influence of these pore pressure reductions on the first section of tunnelling is likely to decrease rapidly advancing northwards. To the north of the divide, groundwater is present within valley-fill colluvial, perched aquifers that are topographically controlled, confined between bedrock outcrop and of limited lateral extent. Hydraulic gradients to the north and northeast mimic the topography. The depth of the tunnel alignment for the remaining two-thirds length from the divide to the WUG and the low hydraulic conductivity of the andesites
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