64 13.5 OGNZL Treated Water Discharge Monitoring 13.5.1 Background OGNZL holds a discharge permit (Discharge Permit 971318) to discharge treated water into the Ohinemuri River (at two locations). The water is treated in a Water Treatment Plant (WTP) prior to being discharged into the river through an ‘upper’ (E1) and/or ‘lower’ (E2) discharge (Figure 14). At times the mine can also discharge water from TSF2 into an unnamed tributary of the Ohinemuri River. Condition 16 of Discharge Permit 971318 specifies the collection of water samples from sites within the Ohinemuri River and Ruahorehore Stream, while Condition 17 of the same consent requires the reporting of results to WRC. The resource consent also included conditions which set out the receiving water quality standards that are to be met. These water quality limits are provided in Appendix 6 and are discussed further in Chapter 20 of the present report. Accordingly, OGNZL undertakes a range of long-term receiving water biological, sediment and water quality monitoring. Monitoring is undertaken at nine sites; five of which are located on the Ohinemuri River and four are located on tributaries to the Ohinemuri River. The purpose of the data collection is to measure compliance with the water quality standards set within the conditions of the resource consent. Of the sites on the Ohinemuri River, one is located upstream of all mine related discharges (OC2), one is located downstream of the tributary into which TSF2 discharges but upstream of discharge E1 (OH3), two are located between discharge E1 and E2 (OH5 and OH1) and one is downstream of all mine related discharges (OH6). As for the above, it is not our intention to summarise and repeat all of the data from the consent monitoring programmes; such data is well reported and will be referenced accordingly. However, we summarise the lengthy record of OGNZL compliance monitoring data to inform our discussion on the water quality and ecology of the Ohinemuri River at Waihi. 13.5.2 Heavy Metals The existing discharge consent monitoring requires that heavy metals are sampled and measured and assessed against the limits detailed in the consent conditions, which are derived from USEPA (1986) criteria. The historical range from the collective OGNZL monitoring sites for each parameter, including sediments, is provided in Table 22. Ryder (2021) concludes that in-river metals were below compliance limits (USEPA criteria) and often below detection limit. Similar conclusions were reported in earlier compliance reports, Golder (2015a, 2016) and Ryder (2018-2020). Ryder (2021) notes that selenium trigger limit requires concentrations remain below 0.02 g/m3 97 % of the time on an annual basis and are not permitted to exceed 0.035 g/m3 in any single analysis. We note that the laboratory detection limit19 for mercury is higher than the consented compliance limit. Historical range derived from data measured May 2005 to May 2021. 19 Laboratory detection limit = the minimum measured concentration of a substance that can be reported with 99% confidence that the measured concentration is distinguishable from method blank results.
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