GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 110 J.2.1 Drain capture – mass flux calculation The mass flux of parameters to the drains was calculated by multiplying the predicted leachate concentrations by the above predicted drain capture. The leachate parameter concentrations were sourced from the water balance model, provided in the GHD (2022a) water management report. These values are based on the 95th percentile for TSF1A seepage data. The results of the calculation are provided in Table 6.4 of the main report. J.3 Water quality assessment J.3.1 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. This has been determined to be the Mataura Stream, which is part of the Ohinemuri River catchment (Section 6.2.3 of the main report). The analysis conservatively considered straight mixing of the leachate into the stream (i.e. dilution by groundwater not considered) using the following mass balance analytical solution: Where: C = Concentration of parameter in g/m3 V = Flow volume in m3/day CV = Mass flux of parameter in g/day 1 = Existing stream quality 2 = WRS leachate discharge 3 = Stream water quality after mixing of 1 and 2 This analysis considers the following inputs: – Existing Mataura Stream water quality (C1): The background stream quality was adopted from the median concentrations from samples collected since 2006. This data is presented in Table 6.5 of the main report. – Leachate parameter concentrations (C2): Sourced from the water balance model, provided in the GHD (2022a) water management report (as per the drain capture mass flux analysis above; Section J.2.1). For this analysis, both the predicted median and 95th percentile concentrations were assessed. – Mataura Stream flow (V1): A range of (rounded) flows was considered, based on the lowest predicted flow (1,000 m3/day) and median stream flow (10,000 m3/day). – Predicted leachate discharge to ground (V2): From groundwater modelling results above (10 m3/day). J.3.2 Results The range of results are presented in Table J.5. Due to the relatively low discharge volume to ground, most predicted parameter concentrations report no differences between the analysed scenarios within the levels of accuracy available. The key parameters that do vary across the scenarios are iron, manganese and sulphate. Selected results are reported in Table 6.5 of the main report. The mass flux of parameters entering the Ohinemuri River catchment via groundwater discharge were calculated by multiplying the predicted leachate concentrations with the volume of leachate discharged to ground. The results are provided in Table 6.6 of the main report. Table J.5 Predicted stream concentrations (g/m3) Parameter Median stream flow (10,000 m3/day) Low flow (1,000 m3/day) Maximum Median predicted leachate concs. 95th%ile predicted leachate concs. Median predicted leachate concs. 95th%ile predicted leachate concs. Al 0.04 0.04 0.04 0.04 0.04 As 0.001 0.001 0.001 0.001 0.001 Cu 0.0005 0.0005 0.0005 0.0005 0.0005 Fe 0.04 0.06 0.04 0.23 0.23 Hg 0.00008 0.00008 0.00008 0.00008 0.00008
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