GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 140 6.4 WRS analysis results The captured water will include a mix of perched groundwater flow and rock stack infiltration. The volumes of groundwater captured by the seepage rock drain is required for input into the water management assessment (Water Management Report (GHD, 2022a)), and will be sent to the WTP for treatment. The values below are average daily rates, based on 30% and 50% average annual rainfall recharge: – A total infiltration volume of 35 – 60 m3/day is predicted through the WRS. – The seepage rock drain is predicted to capture between 80% and 90% of this (30 – 55 m3/day). – The remaining 15% - 25% is predicted to discharge to ground (5 – 10 m3/day). This seepage is inferred to ultimately discharge to the Mataura Stream, via groundwater flow in the underlying andesite rock. 6.5 WRS effects summary and discussion 6.5.1 Groundwater levels, flows and recharge – Deeper aquifer: The presence of the WRS is expected to provide some additional recharge to the deeper aquifer. This has been estimated above at 5 – 10 m3/day. It is assumed this will ultimately discharge to the Mataura Stream catchment. This is a minor contribution and is expected to result in up to 2 m of mounding directly beneath the WRS (Figure 6.10). Given the steep horizontal hydraulic gradient down the slope (~0.1), this mounding is not expected to impact the overall groundwater flow regime east toward the Mataura Stream. Figure 6.10 Groundwater level mounding of deeper aquifer (andesite)* for 50% recharge scenario *Blue dashed lines represent saturated soils. The green lines represent water flow paths – Perched aquifer: Water from the perched aquifer is assumed to discharge into the WRS gully as seeps and springs. During operation, WRS infiltration water that does not discharge to the deeper aquifer is expected to continue to flow along the perched groundwater system and discharge via existing flow paths into the underlying rock drain system. This water will be captured by the WRS toe seepage collection pond and be pumped to the WTP during WRS operation. On remediation of the site, it is expected the flow to the stream will be restored. Although intended to be temporary, the key physical influence (if any) of the WRS on the gully itself is expected to be ecological, which is not addressed in this report. – Stream: The capture of the Tributary 2 water through the rock drain is expected to result in the loss of up to 2% of flow from the Mataura Stream (based on flow gauging results in Table 6.1, which excludes Tributary 2
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