Assessing Ash‑Derived Contaminants in Freshwaters Following Wildfires in Northern Portugal

Background: Wildfires are increasing in frequency and intensity worldwide, with Portugal recording the second-largest burned area during the summer of 2025 1. Wildfire-derived ash acts as reservoir of contaminants, releasing trace elements and other potentially toxic compounds that may alter water chemistry when mobilized through surface runoff during post-fire precipitation events 2. Despite this threat, the impact of ash-derived pollutants on freshwater systems in Northern Portugal remains insufficiently characterized, particularly regarding environmental forensic and public health implications 3. Objective: This study aimed to evaluate key physicochemical parameters (nitrate, pH, and electrical conductivity) associated with ash-derived contamination in wildfire-affected areas, comparing pre- and post-precipitation conditions. Methods: Surface water samples were collected from six wildfire-affected sites in Northern Portugal (Lousada, Penafiel, and Paredes) on 17 October 2025 (pre-rainfall baseline) and 11 December 2025 (post-rainfall) to assess the effects of runoff-driven contaminant mobilization. Nitrate concentrations were determined using UV–Vis spectrophotometry (Unicam UV/Vis spectrophotometer, ATI Unicam) and expressed in mg/L. pH and electrical conductivity were measured using a Crison GLP21 pH meter and a Crison GLP31 conductimeter, respectively. Data were analyzed using Jamovi (v2.6.44) and are presented as mean ± standard error of the mean (SEM). Results: Mean nitrate concentrations across all sites increased slightly from 0.14 ± 0.04 mg/L (pre-rainfall) to 0.16 ± 0.04 mg/L (post-rainfall), although this difference was not statistically significant (p > 0.05). Similarly, pH values showed a non-significant increase from 6.15 ± 0.11 to 6.48 ± 0.26 (p > 0.05). In contrast, electrical conductivity decreased from 173.2 ± 45.7 to 153.0 ± 45.1 µS/cm, also without statistical significance (p > 0.05). Site D3 exhibited the highest pre-rainfall values (nitrate: 0.29 mg/L; pH: 6.43; conductivity: 321 µS/cm), whereas post-rainfall maxima were observed at site D1 (nitrate: 0.39 mg/L; pH: 7.07; conductivity: 365 µS/cm). Conclusions: Post-fire rainfall induced only minor, non-significant changes in water quality, though site-specific variability highlights the need for continued monitoring.