GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 68 This appendix presents the methodology and results for technical assessment of the proposed Northern Rock Stack (NRS). All elevations are presented in metres reduced level (mRL) with reference to Waihi Mine datum, which is 1,000 metres below sea level. H.1 Assessment methodology H.1.1 Groundwater flow assessment Groundwater flow from upgradient of the NRS footprint under existing conditions has been estimated using Darcy’s Law analytical solution1: ൌ Where: Q = Volumetric groundwater flow rate (m3/day) A = Cross sectional area of groundwater flow (m2) K = Saturated hydraulic conductivity (m/day) i = Groundwater hydraulic gradient (dimensionless) The following inputs were adopted for assessment using Darcy’s Law: – 25 m average shallow aquifer thickness based on geology encountered during drilling. – 557 m approximate length of NRS perpendicular to groundwater flow. – 6 x 10-6 m/s shallow aquifer hydraulic conductivity, based on hydraulic testing undertaken in monitoring wells. – 0.1 hydraulic gradient at base of Rhyolite Dome east of proposed NRS. – Total groundwater flow to the Ohinemuri River has been calculated by including the additional recharge expected across the alluvial flats; multiplying the recharge rate 2x10-8 m/s (30% of rainfall) by the approximate footprint area of the NRS (260,000 m2). H.1.2 Construction dewatering assessment Groundwater is anticipated to be intercepted by the uphill diversion drains at the south-eastern boundary of the NRS, approximately between chainage 400 – 630 (Figure H.1). 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 groundwater drawdown distance of influence have been estimated using the Sichardt analytical solutions2: ൌ ሺ െ ℎሻ√ Where: L = Distance of influence (m) C = Empirical calibration factor (1750 for planar flow cases) H-h = Target drawdown in excavation (m) k = Saturated hydraulic conductivity (m/sec) ൌ ሺ ଶ െ ℎଶሻ Where: Q = Volumetric groundwater inflow to excavation (m3/sec) x = Length of slot (trench) (m) – The following inputs were adopted for assessment using the Sichardt solution: – 2 m of groundwater drawdown at the location of the excavation/drain invert. – 230 m section (length of trench) along which dewatering is anticipated. – 1 x 10-7 m/s – 1 x 10-5 m/s range of rhyolite hydraulic conductivity, based on hydraulic testing undertaken in monitoring wells. 1 Darcy, H. 1856. Les fontaines publiques de la vile de Dijon. Paris:Dalmont. 2 Preene, M., Robers, T.O.L., Powrie, W. 2016. Groundwater control: design and practice, second edition. CIRIA, C750. London, 2016.
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