GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 53 . GOP groundwater inflow Groundwater inflows to the open pit from the shallow groundwater system in the GOP excavation scenario have been modelled for four sections around the pit and with the results presented in Table 3.8. The following is noted: – Greater inflow rates from the shallow groundwater system than those presented in Table 3.8 are likely to be observed during the initial excavation period, as higher groundwater levels will generate strong groundwater gradients towards the pit. – Inflows from the north, south and west are negligible as groundwater modelling indicates that the water table won’t intercept the pit wall where the shallow materials are exposed during this phase of works (Figure 3.12). Table 3.8 Estimated inflows to GOP from shallow groundwater system during GOP excavation Pit section Shallow groundwater inflow (m3/day) North Negligible East 9 South Negligible West Negligible Total 9 Notes: - Considering model uncertainty, results are rounded. Historical dewatering activities in the vicinity of Gladstone, including for the Favona-Moonlight system, is considered to provide a reasonable approximation for the likely rate of groundwater inflow to the proposed GOP from the deep groundwater system. The average daily discharge recorded during dewatering of the Favona underground is presented in Figure 3.15, alongside mine (dewatering) depth and estimated groundwater inflow (GWS, 2018a). During initial mine dewatering groundwater discharge includes water sourced from storage within the connected vein systems as well as groundwater inflow to the vein systems from the surrounding rockmass. After the groundwater from storage has been depleted, the groundwater inflows reach an equilibrium, considered by GWS (2018a) to have occurred in 2014 (Figure 3.15). This results in maximum groundwater discharge rates occurring between the commencement of mining and shortly after the maximum mine depth is reached. Prior to dewatering of the Favona system, there was no hydraulic connection to the Martha ore bodies to the west due to structural barriers to groundwater flow (Figure 2.3). Groundwater discharge in Figure 3.15 was therefore sourced from the connected vein system and surrounding rockmass within the ‘Waihi East’ area. In contrast, inflows to the proposed GOP are only anticipated to be sourced from a much smaller, currently disconnected section of the Gladstone vein system to the southwest of the pit. The qualitative assessment of groundwater inflow to the proposed GOP during excavation has conservatively assumed similar groundwater inflow rates as those recorded during mining of Favona to 1,005 mRL (the proposed GOP depth), however reduced by 50% considering the Gladstone vein system immediately northeast of the proposed GOP is recorded as dewatered (Section 3.3.3) with inflows only considered to occur from the southwest. The results of the qualitative assessment are presented in Table 3.9.
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