Impacts of the 2024 flash flood on water quality, pathogenic bacteria and organic contaminant risks in the Albufera Natural Park (Valencia, Spain)

Climate change is increasing flash flood frequency and intensity around the globe, which can contribute to the mobilization of urban and agricultural contaminants into aquatic ecosystems. In October 2024, a massive flash flood affected the Valencia region (Spain), which discharged contaminated water into the Albufera Natural Park (ANP), a protected Mediterranean wetland. In this study we provide one of the most comprehensive assessments to characterize the contamination footprint produced by a flash flood event in downstream aquatic ecosystems. We evaluated physical-chemical parameters, pathogenic bacteria, and 241 organic contaminants, including pesticides (PEST), pharmaceuticals and personal care products (PPCPs), perfluoroalkyl substances (PFAS), organophosphorus flame retardants (OPFRs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs), in 15 sampling sites along the ANP. Our study reveals an increase of pathogenic bacteria and chemical exposure concentrations in the north-west areas of the ANP, correlating with elevated ammonia and reduced oxygen levels. Comparison with pre-flood data revealed significant concentration increases for PPCPs, PFAS, PAHs, and OPFRs, attributed to wastewater treatment failures and urban/industrial chemical mobilization. Several pollutants were found to significantly exceed established ecological protection thresholds (risk quotients > 10), with the most significant risks identified for ibuprofen, PFOS, and fluoranthene in water samples, along with 13 sediment-associated contaminants (including PFOS and several PESTs and PPCPs). This study provides a basis for guiding future monitoring and interventions designed to assess and prevent contamination risks from flash floods in freshwater ecosystems.