110 We also assess additional parameters of the treated water discharge and the condition of the Ohinemuri River. In particular, given the concerns raised by WRC regarding elevated temperatures within the Ohinemuri River (WRC 2019,and section 3 above), and our understanding of potential elevated sulphate levels within the treated water discharge, we provide further commentary on the effects of elevated temperature and sulphate within the Ohinemuri River in this section. 19.2 Ecological Values We have outlined the past and present water quality and ecological values of the Ohinemuri River earlier in this report. Most notably, the information available shows that the ecosystem attributes and the ecological values of the Ohinemuri River have not varied throughout the duration of the current treated water discharge to which the USEPA (1986) receiving water criteria have applied and has not been adversely affected by the OGNZL treated water discharge. Despite the MCI and QMCI biological indices recorded from the annual biological monitoring programme returning generally poor-fair scores for water and habitat quality (both before and after the establishment of the WTP discharge), the ecological metrics for the Ohinemuri River both upstream and downstream of the discharge show fluctuations in ecological values from poor to high values. At-Risk fish have been recorded in the Ohinemuri River, and the river is also an important rainbow trout fishery, with spawning grounds in the river tributaries, and is classified as a significant trout fishery. 19.3 Water Management GHD (2022A) outlined the water management approach for the WNP and describe the water treatment process at the WTP. Mine water, cyanide water and potentially impacted runoff water are treated at the WTP before discharge to the Ohinemuri River at two locations (E1 and E2) via multi-port diffusers (see chapter 20). Treated water is stored in Polishing Ponds prior to discharge where it is tested for compliance with the relevant discharge criteria. The volumes and water quality of the various mine water and cyanide water sources are heavily dependent on a number of environmental and operational variables. The primary controlling variables are rainfall, ore and rock geochemistry, processing rate, dewatering rate, river flow rate, and tailings facility age. There are currently five consented operating regimes29. Each operating regime has a range of discharge quality limits that enable compliance with in-stream water quality criteria. Trace element removal rates are described in GHD (2022A). The WTP discharge regime is shown in Table 33. 29 Resource consent (RC AUTH971318.01.13).
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