In October 2017, the Mondego hydrological basin was devastated by multiple wildfires. To understand the impacts that these had on surface water quality, monthly monitoring campaigns were implemented in the affected catchments. The campaigns oversaw the water’s physical characteristics and major, minor and trace constituents. Analysis of major changes was performed, as well as comparisons with geochemical backgrounds, determined using publicly available data collected prior to the fires. The backgrounds allowed us to verify which water parameters were most influenced by the fires. Wildfires were responsible for increasing turbidity on all watercourses, and electrical conductivity increased in watercourses with flatter, granitoid/sedimentary-dominated watersheds. Cl⁻, NO₃⁻, and SO₄²⁻ surpassed background values in most monitored basins. Changes in Al, Fe, Mn, As, Ba, and Zn concentrations were observed in all watercourses after the wildfires and decreases after intense rainfall. Background values were particularly exceeded for Al in all monitoring points and As in basins with extensive agricultural areas. Burnt plant material and organic matter partially account for the increases in Cl⁻, NO₃⁻, SO₄²⁻, Al, As, Fe, and Mn content. High Al, Fe and Zn with low water conductivity suggests that these elements were associated with sediment loads after the removal of vegetation.

As water is facing increasing pressures from population and economic growth and climate change, it becomes imperative to promote the protection, restoration and management of this resource and its watersheds. Since water quality depends on multiple factors both natural and anthropic, it is not easy to establish their influences. After the October 2017 fires that affected almost 30% of the Mondego hydrographic basin in Central Portugal, 10 catchments were selected for periodic physical-chemical monitoring. These monitoring campaigns started one month after the fires and lasted for two hydrological years, measuring the electric conductivity (EC), pH, dissolved oxygen (DO), turbidity (Turb), alkalinity (Alk), major and minor ions, and trace elements. The obtained data were then statistically analysed alongside the geomorphological characteristics of each catchment coupled with features of land-use and occupation. From the results, it was possible to establish that fire-affected artificial areas, through the atmospheric deposition and surface runoff of combustion products, had the most impact on surface water quality, increasing As, K⁻, Ca²⁺, Mg²⁺, NO₃⁻, SO₄²⁻ and Sr, and consequently increasing electrical conductivity. Agricultural land-use seems to play a major influence in raising the water’s EC, Cl, K⁻ and Na²⁺. Regarding natural factors such as catchment geology, it was found that the extent of igneous exposures influences As, and the carbonate sedimentary units are a source of Ca²⁺ and HCO₃²⁻ concentrations and impose an increase in alkalinity. Rainfall seems, in the short term, to increase the water concentration in Al and NO₃⁻, while also raising turbidity due to sediments dragged by surface runoff. While, in the long-term, rainfall reduces the concentrations of elements in surface water and approximates the water’s pH to rainfall features.