Multi-event monitoring of pollutant dynamics and treatment performance in a free water surface constructed wetland under combined agricultural and urban stormwater pressures

Free water surface constructed wetlands (FWSCWs) provide multiple ecosystem services, including water quality improvement, yet their response to stormwater pulses under combined agricultural and urban pressures remains insufficiently characterised. The Tancat de la Pipa Constructed Wetland, located within the Albufera de València Natural Park (Spain), was monitored during 13 rainfall events in 2023–2025 to quantify pollutant concentrations in the storm-driven inflows reaching the Albufera Lagoon and assess treatment performance for conventional pollutants, trace metals and emerging contaminants. Rainfall events triggered pronounced but contrasting concentration increases at both inflows: NH₄ + -N dominated the urban-influenced inflow response suggesting CSO contributions, while NO₃ − -N, TN and PO₄ 3− -P responded most rapidly at the agricultural inflow, consistent with distinct catchment sources and drainage dynamics. Despite these pollutant surges, the wetland achieved EMC-based total removal efficiencies of 78% for TP, 67–74% for TN, 81% for NH₄ + -N, 84–90% for NO₃ − -N and 86–90% for TSS, with outlet concentrations statistically indistinguishable from long-term dry-weather values. Of the 22 trace metals monitored, 16 were consistently attenuated; high ecological risk (RQ > 10) was recorded for Al, Hg and Pb at both inlets, substantially reduced though not fully eliminated at the system outlets. Eight of the 21 screened emerging contaminants were detected, with variable removal efficiencies ranging from 4% to >82% across compound groups; mixture toxicity assessment revealed persistent moderate ecological risk at the final outlets. The results demonstrate that a large-scale FWSCW can effectively buffer the broad spectrum of pollutants mobilised by stormwater in a sensitive Mediterranean landscape.