GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 82 . diversion drains along the south-eastern boundary of the NRS (where groundwater is anticipated to be intercepted). 4. NRS assessment: Development of a steady-state cross-section model (developed within GeoStudio 2021 SEEP/W finite element numerical modelling software) to predict the rate of leachate capture and seepage through the Zone A liner. The assessment modelled the following scenarios: a. NRS Operation: Rock stack to proposed maximum operational capacity. b. NRS Closure: Final proposed elevation of rock stack including capping layer. 5. Water quality assessment: Development of a contaminant mass mixing model to predict impacts to groundwater and surface water quality from leachate seepage to the receiving environment. Results from AECOM (2021c) geochemical equilibrium modelling to generate a groundwater quality equilibrated for mineral saturation for the NRS leachate were used alongside predicted groundwater and leachate flow from the above assessments and existing surface water quality data to provide water quality predictions for the surface water receiving environment. Refer to Appendix H for detailed description of the assessment methodology. 4.5 NRS assessment results 4.5.1 Construction dewatering and groundwater flow Groundwater flow at the base of the Rhyolite Dome, upgradient of the proposed NRS site, has been estimated using Darcy’s Law (Darcy, 1856) to be approximately 720 m3/day (see Appendix H for detailed calculations). Taking into consideration additional groundwater recharge under existing conditions as groundwater flows across the alluvial flats (450 m3/day) total groundwater discharge to the Ohinemuri River is estimated to be in the order of 1,170 m3/day. Groundwater is anticipated to be intercepted during construction of the uphill diversion drains at the south-eastern boundary of the NRS. Construction of the drains will involve excavation of rhyolite rock in this area, with dewatering likely to be required where the design invert levels of the uphill diversion drain are below the existing groundwater table. Initial groundwater inflow rates and distance of influence of groundwater drawdown predicted during excavation and dewatering for the installation of the uphill diversion drains presented in Table 4.3. The following is noted: – The inflow rates determined assume that the full length of excavation required will be undertaken at the same time, however it is likely that the excavation will be progressively advanced. These results are therefore considered to be conservative. – Groundwater inflow rates are predicted to reduce over time as groundwater levels and gradients towards the uphill diversion drains reduce as the system equilibrates to the change in conditions. Table 4.3 NRS groundwater inflow rates and distance of influence of dewatering for uphill diversion drain construction Uphill Diversion Drain Dewatering Assessment Result range Initial groundwater inflow rate (m3/day) 10 – 70 Distance of influence (m) 1 – 10 - Considering assessment uncertainty, results are rounded. 4.5.2 Rock stack leachate The predicted rate of leachate capture within leachate collection drains, and rate of leachate seepage through the Zone A liner is presented in Table 4.4, predicted leachate and groundwater capture within sub-soil drains is presented in Table 4.5 and the predicted leachate flow within groundwater that is anticipated to migrate to the Ohinemuri River is presented in Table 4.6. The results represent steady state conditions during average rainfall conditions, and present predicted seepage results for 30% and 50% rainfall recharge in recognition of uncertainty
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