62 • Ohinemuri River at Waikino (downstream of the OGNZL site and Queens Head, downstream of confluence with the Waitekauri River) • Ohinemuri River at Karangahake Gorge (downstream of OGNZL site and but upstream of the Waitawheta River). Water quality data from the past five years from three sites is summarised in Table 21 below and forms the primary data for discussion on the water quality of the Ohinemuri River. WRC measures a range of factors (such as water clarity, temperature, nutrients) to enable an overall assessment and to classify the water quality condition as excellent, satisfactory or unsatisfactory for: • Ecology – supporting plants and animals that live in water. • Swimming – people can fully immerse themselves in the water. 13.4 Summary of Water Quality of the Ohinemuri River catchment Water quality data from NIWA and Waikato Regional Council is reported on the Council website15. Summary data from the last of the selected water quality parameters are provided in Table 21. LAWA16 reports the water quality of the Ohinemuri River at SH25 and Karangahake Gorge (respectively upstream and downstream of the existing treated water discharge) as within Attribute band A NPSFM attribute state for ammoniacal nitrogen, DRP and nitrate nitrogen and trends were indeterminate or improving (especially total phosphorus). Note that at the Queens Head and Karangahake sites (downstream of the OGNZL site), the LAWA site reports ammoniacal nitrogen (annual median) as meeting NOF Attribute band B, with a degrading trend at the lowest catchment site. DRP was in NOF DRP Attribute band A at all sites within the Ohinemuri River. LAWA report E. coli as NOF Attribute band D (for human contact in lakes and rivers) at all four sites within the Ohinemuri River17. For the most part water quality variables met the WRC ‘Excellent’ category for the physicochemical components (pH, dissolved oxygen) of water quality within the Ohinemuri River. Not all attributes monitored by WRC are the equivalent of those used in the NPS categories listed but the median value for total phosphorus suggests at least satisfactory quality (WRC water quality categories). Water quality within the Ohinemuri River has elevated levels of Nitrogen (satisfactory – unsatisfactory) but lower levels of phosphorus (~satisfactory-excellent). Biological indicators of water quality (cf; periphyton, MCI and QMCI) show a poor to moderate water quality condition of the Ohinemuri River at Waihi, and indicative of moderate nutrient enrichment (LAWA18, Golder 2017), mostly resulting from land use activity within the catchment. For the Ohinemuri River monitoring site at SH25 upstream of Waihi township, LAWA show MCI (five-year median of 90.3) as within Attribute band C of the NOF requiring action plans, 15 https://www.waikatoregion.govt.nz/environment/natural-resources/water/rivers/water-quality-monitoring-map-and-sites/ 16 www.lawa.org.nz/explore-data/waikato-region/river-quality/waihou-river 17 Attribute D = 20-30% of the time, the estimated risk is >=50 in 1000 (>5% risk). The predicted average infection risk is >3% (The predicted average infection risk is the overall average infection to swimmers based on a random exposure on a random day, ignoring any possibility of not swimming during high flows or when a surveillance advisory is in place (assuming that the E. coli concentration follows a lognormal distribution). Actual risk will generally be less if a person does not swim during high flows.) 18 https://www.lawa.org.nz/explore-data/waikato-region/river-quality/waihou-river/ohinemuri-river-at-sh25-br-mci_swq/
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