GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 54 G.1.3 Surface water catchment assessment methodology Surface water catchment calculations were undertaken to understand the relative potential loss of contributions (including interflow) to flow in the Ohinemuri River and local tributaries. Figure G.7 presents the existing surface water catchment areas within which the proposed Gladstone Pit is located. As the summit of Gladstone Hill forms a high point in the central eastern area of the proposed pit, stormwater runoff and interflow are separated between the four water bodies defined for the purposes of this assessment. This allows assessment for two separate areas of the Ohinemuri River (OH3 and OH6), as well as the Gladstone wetland and intermittent Ohinemuri tributaries TB4 and TB5. Changes in catchment area due to excavation of the proposed GOP and contouring of the tailings capping layer have been used to predict the reduction in stormwater runoff and interflow to each surface water catchment. The surface water catchments for four key areas (OH3, OH6, TB5 and TB4) were estimated based on topographic contours, and design of the tailings capping layer to direct all runoff and interflow towards the Gladstone wetland and TB4 catchment south of the pit. G.1.4 Water quality assessment methodology Water quality assessment is not required for the GOP excavation or operational and TSF closure scenarios, as there are no predicted discharges to the environment. After the TSF drainage system is no longer operation (long-term TSF scenario), discharge of groundwater which has interacted with the backfilled rock and tailings porewater is predicted to occur to the shallow groundwater adjacent to the pit. An assessment was undertaken to predict receiving water quality within the Ohinemuri River (OH6), which utilised the following information: – AECOM results (2021b) from geochemical equilibrium modelling to generate a groundwater quality equilibrated for mineral saturation for the TSF discharge. – Existing groundwater quality adopted from the concentrations measured in a groundwater sample from piezometer GLD04s. – Existing Ohinemuri River water quality adopted from the median concentrations measured in samples collected from OH6 (2017 – 2020). – Predicted groundwater discharge to the Ohinemuri River (derived from groundwater modelling results). – Ohinemuri River: Flow rates from the FRENDRUPS flow gauge station. The median flow rate (63,200 m3/day) from available data recorded between 1 October 2017 and 30 April 2020 was applied.
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