GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 117 5.5.5 Drain Capture Volumes The volumes of groundwater captured by the subsurface drains is required for input into the water management assessment (Water Management Report (GHD, 2022a), and will be sent to the WTP for treatment. The captured water will include a mix of tailings seepage, embankment rock infiltration and groundwater. The predicted model results are summarised in Table 5.9 below. – Starter embankment: All drains are assumed to be operational and capture 100% of tailings and embankment rock seepage from TSF3 (240 m3/day). In addition to this, groundwater contributions are expected in the order of 50 m3/day. – Operational to closure: As above, all drains are assumed to be operational and capture 100% TSF3 seepage (295 m3/day). In addition to this, groundwater contributions are expected in the order of 65 m3/day. – Post-closure: All drains, except the downstream toe drain, are assumed to have failed for this scenario. No tailings seepage is predicted to be captured as noted in Section 5.5.2, and 240 m3/day of embankment rock infiltration is predicted to be captured by the drains. In addition to this, groundwater contributions are expected in the order of 30 m3/day. Table 5.9 Predicted drain capture volumes and sources Stage Total TSF3 seepage to drains (m3/day)* Total drain capture (m3/day) Water/seepage source (Proportions) Tailings Embankment rock Groundwater Portion of groundwater allocation** Starter embankment 240 290 <5% 80% 15 - 20% 1.1% Operations to closure 340 360 20% 75% 5% 0.4% Post-closure 240 260 0 90% 10% 0.4% *From values provided in Table 5.7 and 5.8. ** Percentage of the available groundwater within the Waihi Basin (1,845,000 m3year) 5.6 TSF3 effects summary and discussion The predicted influence on key environmental aspects is provided in the following sections. Recommendations are made throughout the following sections, with a summary of proposed monitoring provided in Section 5.6.4. 5.6.1 Groundwater water levels, flows and recharge During the following stages, the groundwater regime is expected to be altered. The expected effects, magnitude and management of these are discussed below. Construction dewatering The key potential effects of this activity are summarised and qualified with respect to the analysis provided in Section 5.5.1: – A reduction in recharge of the deeper aquifer: The proposed water take, with an upper bound in the order of 2,500 m3/day, is predicted to be less than 1% of the aquifer management level and temporary in nature (120 days) (Table 5.6). It is acknowledged that the surface water catchment (Ohinemuri River at Waihi Terrace) is overallocated, however, the dewatered groundwater will be returned to the catchment via one of two means: Dirty water via the WTP after treatment; and/or
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