Post-fire water contamination risk assessment in Portugal

Wildfires in the Mediterranean basin, especially in Portugal, have increased in extent and frequency over the last few years. One of the many impacts of wildfires on humans and ecosystems is on the water quality of surface waters. Ashes and increased erosion rates might elevate the influx of nutrients, sediments, or other water quality-related components, possibly affecting the water supply. This study has three main objectives. (1) Identifying post-fire water contamination events in over 60 Portuguese reservoirs, through changepoint analysis of historical time series for (2) Testing the relationship between post-fire water contamination events with fire-, watershed-, reservoir-, and climatic drivers through logistic regression using generalized additive models. (3) The modelling and evaluation of post-fire water supply contamination risks in Portugal, using a deterministic approach. Results showed increases in TSS in 13.6% of all wildfires. Most changes fell into the unusually large fire seasons of 2003-2005 and 2017, while the most significant impacts could be seen in southern reservoirs after 2005. Fire size was identified as the main driver of post-fire water contamination, while reservoir and climate-related characteristics like water levels also played a significant role in TSS. Increased levels of suspended sediments were identified as a potential threat to the water supply, especially when large wildfires coincide with drought-induced low reservoir water levels. The modelling of past water contamination episodes shows a similar spatial distribution as the structural fire risk in Portugal, identifying the centre (and southern) regions as the most affected areas. This study may support numerous case and modelling studies and inform water managers about possible future threats.