Impact of wildfire on water quality in Caramulo Mountain ridge (Central Portugal)

Fire is a major ecosystem process affecting forests and wooded areas in the Mediterranean basin. In recent years, a clear increase in wildfires in Portugal has taken place with strong impacts on ecosystems. The destruction of vegetation cover and the change in soil properties lead to modifications in the hydrological regime, due to the contribution of each component of the hydrological cycle, and qualitative aspect, with the mobilization of substances that are forwarded to water bodies by overland flow and infiltration. This study evaluates the impact of a large wildfire on groundwater and surface water quality by various constituents of concern, particularly polycyclic aromatic hydrocarbons (PAHs) in Caramulo Mountain (Central Portugal). Water samples from burnt and unburnt areas were collected in eight sampling points, over five campaigns, during 19 months, to evaluate the influence of precipitation and the time elapsed since the wildfire on the magnitude of pollutant loads entering the reservoirs as well as the impact attenuation. Fourteen of the 16 priority PAHs were found, especially naphthalene and benzo(ghi)perylene. The concentration of PAHs followed the precipitation profile, proving the influence of overland flow and infiltration on the input of these pollutants into surface water and groundwater. The behavior of nutrients and other trace elements was also evaluated, revealing similar fluctuations for several major ions (Mg⁺, Cl⁻ and SO₄²⁻) and metallic elements (Fe, Mn and Pb). The research results have contributed to improve the understanding of fundamental physical and biogeochemical processes controlling the formation and transport of PAHs in hydrological systems, and the role of natural remediation processes.