GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 73 Table H.2 NRS model set-up Boundary type Adopted boundary condition Justification Model scenario Rainfall recharge Two recharge rates applied to rock stack: Water flux – 2 x 10-8 & 3 x 10-8 m/s 30 & 50 % of average rainfall at the site (2,110 mm/year4) NRS operation Infiltration through capping layer Applied to surface of Zone F: Water flux – 1 x 10-8 m/s 5-year average cap infiltration rate5 for Zones G & H NRS operation & closure Upgradient groundwater boundary (east) Constant Head – 1,115 mRL Based on piezometric groundwater contours (Appendix C). Downgradient groundwater boundary (west) Constant Head – 1,100 mRL Leachate collection drains Water Rate – potential seepage face review Removal of leachate through collection drains Sub-soil collection drains Water Rate – potential seepage face review Removal of groundwater through sub-soil drains Table H.3 NRS model adopted hydrogeological material parameters Geological unit Hydraulic conductivity (m/s) Anisotropy (Ky/Kx) Volumetric water content and hydraulic conductivity function(1) Hydraulic conductivity justification Shallow aquifer 6 x 10-6 0.5 Silty sand Geomean of hydraulic test results in shallow aquifer piezometers Zone A liner 1 x 10-8 1.0 Clayey silt As per design specification Zone F drainage layer 1 x 10-4 1.0 Uniform sand Considered appropriate for Zone F materials Rock stack 1 x 10-5 0.5 Silty sand Considered appropriate for rock stack materials 1. SEEP 2012 materials database H.1.4 Water quality assessment Seepage of leachate through the Zone A liner to the underlying groundwater is predicted to occur. A mixing assessment has been undertaken to predict the impact this may have on the water quality of groundwater, as well as the receiving water quality of the Ohinemuri River. This assessment utilised the following information: – AECOM results (2021c) of geochemical equilibrium modelling to generate a groundwater quality equilibrated for NRS leachate through the Zone A liner and mineral saturation in the NRS closure scenario. – Existing Ohinemuri River water quality adopted from the median sample concentrations at OH3 (2017 – 2020; for parameters not analysed during this period, older sample data was used to inform median concentrations). – Existing groundwater quality adopted from the median sample concentrations collected from DH05a (2017 – 2020). – Predicted leachate discharge to groundwater and predicted migration of leachate to the Ohinemuri River (derived from groundwater modelling results). – Ohinemuri River: Flow rates from the FRENDRUPS flow gauge station. The median flow rate (63,212 m3/day) from available data recorded between 1 October 2017 and 30 April 2020 was applied. 4 NIWA, 2019. Climate data obtained from Waihi, Station Agent No. 1550. 5 AECOM, 2020. Infiltration rates provided for TSF1A embankment slope. Email communication provided by AECOM, 27 July 2020.
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