Assessment of water pollution and environmental impacts in the Pineios river basin before and after extreme flood events in Thessaly, Greece.

Extreme flood events increasingly threaten water quality in agriculturally intensive regions, particularly under accelerating climate change. In September 2023, the Pineios river basin in Thessaly, Greece, the country's most productive agricultural region, experienced catastrophic flooding caused by Storm Daniel and subsequently Storm Elias. This study investigates the occurrence, distribution, and temporal variation of phthalate esters, alternative plasticizers, per- and polyfluoroalkyl substances (PFAS), and fecal indicator bacteria in surface waters before and after these extreme weather events. A total of 24 sampling locations were monitored across the river basin. Water samples were analyzed using validated targeted screening methods based on gas or liquid chromatography tandem mass spectrometry (GC- or LC-MS/MS), while microbial contamination was assessed using International Organization for Standardization (ISO) compliant culture-based techniques. Results revealed widespread contamination by both legacy phthalates, particularly di(2-ethylhexyl) phthalate (DEHP, median concentrations up to 0.36 µg/L), di-n-butyl phthalate (DnBP, 1.9 µg/L) and dimethyl phthalate (DMP, 3.5 µg/L), and emerging alternative plasticizers, such as di(2-ethylhexyl) terephthalate (DEHT, 0.15 µg/L), which was the most abundant of the emerging compounds. Statistically significant post-flood increases (p < 0.05) were observed at several estuarine sites, with total median plasticizer concentrations reaching up to 5.4 µg/L. Among 63 monitored PFAS, 13 compounds were consistently detected across all samples. These included perfluorooctanoic acid (PFOA, median up to 0.47 ng/L), perfluorooctanesulfonic acid (PFOS, 0.2 ng/L), and 6:2 fluorotelomersulfonic acid (6:2 FTS, 1.7 ng/L). Concentrations for PFAS regulated groups, namely PFAS-4 and PFAS-24, frequently approached or exceeded proposed EU water quality thresholds. Fecal indicator bacteria, including Escherichia coli and Enterococcus spp., frequently exceeded the EU bathing water standards, particularly near wastewater discharge points and areas affected by agricultural flooding. Peaks in contamination were linked to carcass decomposition, damage to wastewater treatment infrastructure, and surface runoff following the flood events. These findings highlight the vulnerability of Mediterranean river basins to complex pollutant mixtures following extreme weather phenomena. The study provides novel regional data and emphasizes the urgent need for integrated environmental monitoring aligned with One Health principles to inform future regulation and management of emerging contaminants in flood-prone areas.