GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 101 5.3 Conceptual groundwater model (TSF3 CGM) 5.3.1 Overview Groundwater beneath the TSF3 site is interpreted to be present as a subdued expression of topography, with recharge occurring in the hills to the north/northeast and groundwater flow toward the Ruahorehore Stream to the south. Two groundwater systems at the TSF3 site (shallow and deeper) are referred to in this assessment: – The shallow system refers primarily to alluvial/colluvial/ash layers at the site and any perched water in the hills. This system is influenced by farm drainage and the Ruahorehore Stream. – The “deeper system refers to groundwater within rhyolite units (lava, tuff) that flow beneath the proposed TSF3 site. Trace elements in groundwater are naturally detectable due to the natural catchment conditions (e.g. aluminium, iron, zinc; refer to tabulated water quality data in Appendix E). The conceptual understanding of TSF3 is in general agreement with that of TSF1A and TSF2 provided by GWS (2019), which is shown in Figure 5.8 below and modified to represent TSF3. 5.3.2 Shallow groundwater Shallow groundwater is present within shallow volcanic ash/alluvium/colluvium including within the paleo-gully. Shallow groundwater is recharged from run-off from the hills and from direct rainwater infiltration. Shallow groundwater discharges near the rhyolite foothills, which produces the springs encountered in gullies (refer to the locations of the Ruahorehore Tributary head waters in Figure 5.2). Shallow groundwater levels within the lower permeability alluvium in the lower valley (near wells AP01 and AP04; Figure 5.9) are at ground level during high rainfall (winter) conditions, creating boggy ground conditions for a few months each year. Recharge rates to the shallow system are expected to be limited, with recharge unlikely to occur in the lower lying areas, indicated by upward vertical hydraulic gradients between well pairs (Appendix C), limited vadose zone and poor drainage. Rainfall is inferred to predominantly generate run-off to the farm drains and the Ruahorehore Stream across the majority of the TSF3 footprint. The low permeability of the surficial ash and alluvial units provides limited transmissivity for directing flow to streams. The Ruahorehore Stream is relatively neutral in flow, changing from losing to gaining and back again along its length (Figure 5.9). It is considered to be in connection with shallow groundwater in its immediate vicinity, but receives only a small proportion of groundwater flow as interflow (Figure 5.8). This is consistent across the catchment (580 ha), which typically has only a small number of low-flow surface water features, but has a high water table. This suggests a limited groundwater catchment for the Ruahorehore Stream at the proposed TSF3 site.
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