GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 102 I.3.3 Water levels and flow rates The total groundwater flows measured in SEEP/W downgradient of TSF3 are provided in Table I.21, along with the proportion of TSF3 seepage in groundwater. It is noted that the total groundwater flow is not necessarily equivalent to TSF3 total seepage plus the base model groundwater flow. This is due to minor changes in the hydraulic gradient across the model as the TSF is constructed where the groundwater levels re-equilibrate to accommodate the added recharge to the groundwater system. The proportion of TSF3 seepage as groundwater has been calculated from the above discharge values from the down gradient flow rate. Groundwater flows used for groundwater quality analysis below are presented in Table I.22. Comparison of the upgradient flow in this table (715 m3/day) to the down gradient flow in Table I.21 provides recharge of 4 m3/day of the TSF3 site prior to construction, which is consistent with the conceptual model. Table I.21 Summary of modelling outputs for proportion of groundwater flow Analysis TSF3 total seepage* (m3/day) Total Groundwater Flow** (m3/day) Proportion of flow as TSF3 seepage Base model 0 719 0% Scenario 3 0 723 0% Scenario 4 45 743 6% Scenario 5 95 766 13% Comparison Scenario 3 – no HDPE liner 539 838 64% *Combined tailings and embankment infiltration to the receiving environment, from Table I.18. ** Result extracted from SEEP/W at the same location downgradient of TSF3 for all scenarios. Table I.22 Summary of groundwater flow volumes for water quality assessment Aspect Groundwater flow Base model upgradient flow for mixing analysis (all flow for full aquifer mixing) 715 m3/day Flow extracted from beneath tailings area based on flow lines (limited flow for direct/local groundwater mixing) 118 m3/day I.4 Water quality assessment I.4.1 Methodology A mixing assessment has been undertaken to predict the impact that the leachate (Section J.2) may have on the water quality of the receiving environment. The analytical model allows for only mixing (dilution). Other processes that influence water quality such as adsorption to aquifer materials are not adequately represented in the model. As such, the predicted results are considered to be highly conservative, providing confidence that actual effects are likely to be less than predicted in this assessment. The following mass balance analytical solution was used: Where: C = Concentration of parameter in g/m3 1 = Existing water conditions V = Flow volume in m3/day 2 = TSF3 discharge CV = Mass flux of parameter in g/day 3 = Water quality after mixing of 1 and 2 The following scenarios analysed are provided in Table I.23. No analysis is provided for the starter embankment on the basis that no discharge is predicted to discharge to the receiving environment (Table I.18). For added conservatism, the analysis considered straight mixing of the TSF3 leachate into surface water, i.e., dilution by groundwater not considered as part of the analysis for the Ohinemuri River or Ruahorehore Stream.
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