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 Selenium levels in both plants and fish have shown to be variable over time and, other than being elevated downstream of the treated water discharge compared with those analysed upstream, without any clear pattern. The selenium concentration in eels collected at OH6 (downstream of the treated water discharge, Figure 14) exceeded the trigger limit of 8.1 mg Se/kg twice over the last ten years of monitoring (March 2018 and February 2017). In addition, the selenium concentration in bullies exceeded the trigger limit twice over the last ten years (June 2018 and September 2008). Repeat sampling immediately after each trigger limit exceedance revealed no further trigger limit exceedance. Golder (2017) suggested that the lower concentration in the repeat sampling of eels may be from changes in the diet and eating patterns of the fish or movement of eels within the river and its tributaries. Over a ten-year period, OGNZL has carried out additional monitoring and investigations to determine the cause of the trigger limit exceedances, including a statistical analysis of the data (Golder 2015b) that supported a change to the monitoring programme (a change that is reflected in the results incorporated above). 13.6.7 Riparian Vegetation As discussed in section 4, it is worth noting that extensive plantings have been undertaken by OGNZL at the Oceana Gold Waihi site. In total some 467,500 plants have been planted between 1991 and 2016, for a mix of riparian, swamp, gully and hillside enhancements. Available records show that of these some 91,600 plants covering 18.8 ha of riverbank were planted along the margins of the Ohinemuri River mainstem in the vicinity of the Oceana Gold Waihi site between 1995 and 2005. Given the length of time that has passed since even the last of these plantings, the result is a mature riparian vegetation that contributes significantly to the ecological values of the Ohinemuri River. In addition to these plantings some 107,000 plants (covering 10 ha) have been planted alongside a number of tributaries to the Ohinemuri River, including over 70,000 plants (covering 5.9 ha) alongside the Ruahorehore Stream; likewise contributing to the ecological health and values of the river system as a whole. As the Ohinemuri River in the vicinity of the Oceana Gold Waihi is a larger channel (widths range from 6-20 m, Golder 2017), the riparian vegetation does not form a full canopy cover and thus shading across the river is limited. 13.6.8 Key Conclusions from the Ohinemuri River Monitoring The key conclusions from the WRC and OGNZL treated water discharge monitoring summarise the current ecological results from the monitoring of the Ohinemuri River: • There is no evidence of any adverse ecological effects resulting from the treated water discharge on the ecological values of the Ohinemuri River. • Periphyton (benthic algae) were below the threshold for filamentous algae and algal mats as set out in the NZ Periphyton Guidelines. • Algal productivity (measured as Chlorophyll a (chl-a) exceeded the NZ Periphyton Guidelines at sites upstream and downstream of the discharges in May 2017; chl-a was highest at the upstream site.
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