GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 42 The 2D groundwater model cross sections were used to predict groundwater levels, flow paths and fluxes as follows: Groundwater flow scaling for current conditions and GOP excavation scenario For the purposes of estimating groundwater discharges across the pit boundary, which is extracted from the groundwater models on a per unit metre basis in the four directions modelled (north, east, south and west), these fluxes have been scaled up to represent the full water balance of the pit. These fluxes were scaled up using the adopted pit perimeter sections presented in Figure G.2. The justification for each adopted section is presented in Table G.2. While providing appropriate analysis of change in water levels, to provide balanced flow estimates the circular the area represented by each model required further scaling (flow scaling) to accommodate the circular nature of Gladstone Hill and the proposed GOP. This change addresses the inconsistencies in simulating radial discharge using a cross sectional model approach. To determine the degree of scaling required the following was undertaken: – Modified long-term cross section models were developed including NAF rockfill above the tailings to match the existing landform. A closure cap infiltration rate of 1x10-8 m/s (AECOM, 2020) was applied to these models. – The total modelled discharge from the pit to shallow groundwater was determined as the sum of the discharge to each perimeter area (each determined by factoring the cross section model discharge by the pit perimeter lengths detailed in Table G.2). – Total recharge to the pit area was estimated by applying the adopted recharge rate (1x10-8 m/s) and factoring it by the total pit area. – The scaling factor was determined by dividing the total recharge to the pit area by the total modelled discharge. The scaling factor was applied to all estimates of flow from the model area, including the simulation of current conditions. Groundwater flow scaling for TSF discharges For the purposes of estimating total groundwater discharge through the base of the pit and towards the TSF drainage system for the operational TSF and TSF closure scenarios, the flow results from each cross-section were scaled up using the average dimensions of the TSF: – An average of the following: North-south model discharge flux was multiplied by 520 m (dimension of TSF in east-west direction) East-west model discharge flux was multiplied by 262 m (dimension of TSF in north-south direction) Due to the influence of greater deep groundwater pressures on groundwater flows and TSF discharges for the TSF closure scenario, which considers complete cessation of TSF drainage after rewatering of the deep groundwater system, groundwater flows were assessed differently from the other TSF scenarios. The methodology for the long-term TSF scenario comprised comparison of results from the two cross section models. The modelled lowest point of groundwater discharge from the pit, that could accommodate all groundwater discharge from the TSF, was adopted as the preferential location for water discharge from the pit. The cross section representing this discharge was scaled as per the TSF dimensions provided above. Scaling of groundwater flow to surface water bodies during all GOP and TSF scenarios Total groundwater discharge to each surface water receiving environment from each model section was multiplied by the length of the water body anticipated to be receiving groundwater discharges from the full modelled area during all model scenarios. The following lengths were adopted: – Gladstone wetland / Ohinemuri tributary (TB4) – 280 m – Ohinemuri tributary (TB5) – 230 m – Ohinemuri River east of Gladstone (OH3) – 247 m – Ohinemuri River west of Gladstone (OH6) – 342 m
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