Critical Review of Zinc Trends in Auckland Streams, Estuaries and harbours

Zinc in stormwater has been identified by Auckland Council as a priority contaminant of concern for Auckland harbours, estuaries and streams. This is particularly evident in the source control measures specified in the Proposed Auckland Unitary Plan. Such concerns are primarily based on observations of temporal trends in contaminant concentrations for marine sediment, where it is claimed that some sites have seen increasing zinc levels. The present paper critically assesses that analysis and, as a result thereof, identifies serious concerns with the reliability of those conclusions. Furthermore, the present paper examines the temporal trends for zinc in freshwater streams using data obtained from Auckland Council. It is concluded that where statistically significant trends occur for zinc levels in streams, estuaries and harbours, they are mostly observed to be reducing. Freshwater streams in particular are consistently observed to have declining levels of zinc over the last two decades of monitoring. This finding is consistent with the comprehensive stormwater contaminant modelling by NIWA of the Upper Waitemata Harbour and Southeastern Manukau Harbour, which predicted a decreasing trend in zinc loads between 2001 and 2015-20 as existing galvanised roofs are replaced by zinc aluminium coated steel, followed by a slow increase as vehicles become the dominant source of zinc. This paper concludes that instituting zinc source control for roofing and cladding is not justified as it would have an insignificant impact on contaminant levels in stormwater.