Research into the immediate and long-term impacts of a major forest wildfire on peri-urban and urban stream water quality has recently concluded in Ōtautahi/Christchurch, New Zealand. The wildfire occurred in February 2017, in the Port Hills on the edge of the city, where the popular recreational area is largely covered in pine forest, growing on surficial loess and other fine sediment deposits. The ephemeral streams in the hills drain into the spring-fed urban streams and rivers that are a feature of the city.
Immediately following the fires, stream monitoring commenced in waterways draining the burnt areas, using an unburnt catchment as a control. Monitoring of stream discharge, and of suspended sediment (TSS), nutrient, major ion, organic carbon and trace element (Fe, Mn, Cu, Pb, Zn, Co, Ni, Cr, As, Cd, Sb, Ge, U and V) concentrations continued for over 3 years. An immediate but short term increase in PO4 concentration was observed, followed by another more prolonged release from the soils after a year. NO3-N concentrations were highly elevated immediately following and for 6 months after the fire. NHx-N concentrations were elevated but highly variable for the duration of sampling. The concentrations of dissolved Mn, As, Mo, Co and W were higher than in the control catchment, and suspended particulate material from the fire-affected catchments was enriched in P, As and Mn. No significant change in organic carbon, or contamination by carbon-based combustion products, was observed. Suspended sediment concentrations in streams were high (up to 1900 mg/L measured in the 6 months after the fire) but were clearly also influenced by heavy rainfall events.
We conclude that the effects of such peri-urban wildfires on downstream urban stream water quality can be significant, enriching nutrient concentrations and adding to the suspended sediment and trace element burden such streams typically already carry.