Water quality disruptions following an extreme flood: Microbial and geochemical responses to the 2024 DANA in a Mediterranean wetland.

In late October 2024, an extreme Isolated Depression at High Levels (DANA) caused widespread flooding and the failure of wastewater treatment and sewerage systems across several municipalities in the Valencia region (Spain). A six-month monitoring program was conducted in Albufera Natural Park and the Poyo and Picassent gullies. Microbiological fecal indicators (Escherichia coli, somatic coliphages, and viral markers) showed an immediate response to the flooding, indicating substantial inputs of untreated wastewater. Although most indicators declined over time, several remained above pre-event levels, suggesting incomplete recovery and persistent contamination hotspots. Geochemical analyses revealed elevated major and trace elements, marked increases in carbon and nitrogen fractions, and significant post-event enrichment of key anions in sediment samples. Changes in water nutrients and anions were less pronounced but remained detectable, reflecting runoff and wastewater overflows. Multiple contamination indices were used to assess water and sediment quality, highlighting the strong influence of reference values on interpretation. Integrative indices based on pollution load and ecological risk (PLI and PERI) indicated moderate to low current geochemical contamination. Overall, the 2024 DANA caused persistent microbiological and geochemical disturbances, underscoring the vulnerability of Mediterranean wetlands to extreme hydrometeorological events and the need for more resilient wastewater and flood-management infrastructure.