Water Quality of Stormwater Runoff into Lake Wānaka, New Zealand

The potential contaminant load from stormwater runoff into Lake Wānaka is an emerging problem for the management of urban waters in the Queenstown Lakes District of New Zealand. This research combined a field-research approach, using a nested hydrographic sampling scheme and required periodic sampling throughout single rain events to capture the changing concentrations of contaminants during storm flows. Baseflow sampling occurred at least 72 hours after a storm event under fair and dry conditions, compared to stormflow sampling which proceeded when rainfall depths exceeded 2.5 mm in a 24-hour period and were sampled within the first 30 minutes of storm discharge to obtain maximum concentrations of stormwater contaminants. Over the past 60 years there has been an exponential growth in impervious land cover in the Wanaka township, from 44.2 ha in 1956, to 384.4 ha by 2018. Imperviousness was significantly correlated to increased turbidity (p = 0.024), suspended sediment (p = 0.024), and total phosphorus (p = 0.042). Contaminant concentrations varied considerably between stormwater sub-catchments in the Wānaka township, however, were consistent with national and international studies when comparing stormflow observations. As expected under baseflow conditions, contaminant concentrations were lower than those observed during stormflow, supporting that most of the contaminants associated with urban land uses were only mobilised during event flows. It was concluded that the first flush phenomenon in Wānaka was very sporadic in nature and that heavy metals and total nitrogen were quickly flushed through stormwater and peaked within the first hour of a rain event. Suspended sediment and turbidity on the other hand, usually peaked within the first hour, but also responded to increased rainfall intensity throughout an event. Both suspended sediment and turbidity were weakly related (0.2 > r r < 0.34, p < 0.05). Median contaminant concentrations were also consistently higher for stormwater drain sites (i.e., for suspended sediment, total nitrogen, total phosphorus, dissolved reactive phosphorus, and total recoverable aluminium and zinc) when compared to natural watercourses and/or tributaries, with the exclusion of the nitrate compound. At present, all nutrient, sediment, and E. coli storm water indicators are areas of concern to the nearshore discharge zones of Lake Wānaka and require specific interventions to be implemented by local and regional territories. National stormwater guidelines, for instance, need to be derived for urban land uses and also need to reference sensitive environments such as the pristine lakes located in the Otago Region. Community awareness, education, and engagement of local residents is also essential in alleviating the impacts of urban developments and stormwater discharges