136 wastewater discharge to result in concentrations downstream that may result in effects on rainbow trout and other aquatic organisms where they are present. It therefore becomes apparent that effects will occur only where organisms sensitive to sulphate are present within the ecosystem. Trout embryos and fry were amongst the most sensitive organisms to sulphate that occur within the range of sulphate concentrations recorded from the Ohinemuri River at Waihi from Zak et al. (2021). Cladocerans were also sensitive to sulphate but this group of organisms are not typically found in fast flowing rivers such as the Ohinemuri River, although they may occur within the deeper slow-flowing pools. We are not aware of trout spawning or trout fry activity in the mainstem reaches of the Ohinemuri River at Waihi, and consider that the tributary streams, including the Ruahorehore Stream are likely to be preferred locations for those functions. In their modelling of the treated water discharge for WNP, GHD (2022A) predicted that maximum daily mean concentration of sulphate at the most downstream site at Waihi (site OH6) would be 660 g/m3 and the maximum concentration would be 1,000 g/m3. These predicted maximum concentrations are higher than those already occurring within the river. We note that GHD (2022A) go on to state that there is a distinct pattern of additional calculated loadings at most flow levels for sulphate in the Ohinemuri River suggesting that another source apart from the known sources is possible.
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