U:\2021\BM210482_IBo_Waihi_North_Project\Documents\WNP_Freshwater_Report_May_2022\FINAL_Freshwater_Report_FOR DELIVERY_June 2022\FINAL DELIVERED 17 JUNE 2022\WAI-985-000-REP-LC-0007_Rev0_Freshwater_20220621_FINAL.docx 20.2 Predicted Ohinemuri River Water Quality Compliance 20.2.1 Water Quality GHD (2022A) provides modelled maximum daily mean and modelled maximum concentrations of the consented trace elements within the Ohinemuri River, and we repeat these in Table 34 below. Most elements model a mean and maximum concentration at least one order of magnitude below the consented limits ensuring there is significant buffer available to account for any underestimation of discharge concentrations. Based on these predictions GHD (2022A) concludes that water quality of the Ohinemuri River receiving environment will be compliant with the current consented water quality conditions. 20.2.2 Mass Loads GHD (2022A) also reports on the mass loads for individual elements of water quality and concludes that all water quality elements are within the mass load limits of the existing consent. We understand that if discharge concentrations approach the consented concentrations for the operational regime, the RO plant can always be utilised to further improve the treatment process and is a contingency measure available to OGNZL. We note that, based on the calculated contaminant mass load at OH6, GHD (2022A) has calculated the proportion of load attributable the WTP discharge and natural background. Where the WTP discharge and natural background load do not account for the full contaminant load observed at OH6, the additional load is assumed to be from a combination of uncaptured seepage / runoff from current site operations and other potential non-mining sources. GHD (2022A) calculated these loads where flow data and water quality align at the various monitoring locations throughout the 2019 period with data points covering a range of different flow conditions. For most trace elements (i.e., Sb, Ni, Zn and Se) the calculated difference is negligible (< 0.2 kg/day). However, GHD (2022A) did note that for manganese and sulphate there is a distinct pattern of additional calculated loadings at most flow levels suggesting that another source apart from the known sources is possible.
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