GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 147 The TSF3 discharges to groundwater are inferred to ultimately discharge to the Ohinemuri River via a long flow path. Whilst the conservative assessment suggests there may be some minor increases in major ion concentrations (such as sulphate) in the river, such changes would be significantly delayed and if allowing for attenuation reactions would likely to be unmeasurable by the time discharge to the river occurs. It is considered unlikely that parameter concentrations in the river would exceed the RWQC as a direct result of the TSF3 groundwater discharges. While the Ruahorehore stream is not predicted to be a receptor of discharges from the tailings or TSF3 embankment, run-off from the pond buttress may flow to the stream. Given the small area of contact and degree of dilution expected during periods of stormwater run-off, the stream quality is expected to remain well within the RWQC. 7.4 Willows Rock Stack WRS will be a temporary storage area, with rock ultimately being used to backfill underground workings. No liner is proposed for the WRS, given the constraints on constructing an effective liner in the steep sided gullies. Instead, leachate collection drainage will be installed at the base of the gullies, where erosion has exposed the underlying andesite. The assessment of effects resulting from construction of WRS are discussed in in Section 6.5, with the below summarising the predicted effects: – A loss of tributary flow from the WRS gully is expected, due to capture of seepage through the rock by the WRS drainage system. The daily rates of capture are expected to vary as a function of rainfall and are expected to range from 1 – 2 % of the total Mataura Stream flow. Based on the available gauging, this is expected to be in the range of 260 – 310 m3/day. Such reductions in stream flow are considered unlikely to be discernible from the background variability in flow to the Mataura Stream and wetland (GHD, 2022e). – The WRS discharge to groundwater is expected to comprise <0.1% of the median stream flow (after the WRS gully contribution loss is considered). Whilst there may be some measurable increases in major ion concentrations (such as sulphate) in the stream, no parameter concentrations are expected to exceed the RWQC. – There are no water users reported within 1.5 km of the site. The nearest downstream surface water users is 5 km downstream of the site. No water users are expected to be measurably affected by the WRS seepage.
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