Constructed wetlands for the mitigation of pesticide and heavy metal concentrations in a protected agrolandscape: removal efficiencies and ecological risk assessment.

Constructed wetlands (CWs) are increasingly recognised as nature-based solutions, offering key ecosystem services such as the mitigation of pollutants in agricultural landscapes. This study assessed the effectiveness of two surface-flow CWs, Tancat de la Pipa and Tancat de L'Illa, located within the Albufera de València Natural Park (Spain), in reducing pesticide and heavy metal concentrations between inlet and outlet waters (directly/indirectly connected to a protected lagoon) between 2020 and 2025 (including a historic rainfall and flooding event, leading to significantly higher concentrations of certain pesticides). Water, sediment, macrophyte, and herbivorous bird excrement samples were analysed for 33 pesticides and 12 metals/metalloids across the seasons. The results showed how the CWs reduced the concentration of 57-100 % of the fungicides, 67-86 % of the herbicides, and 67-100 % of the insecticides; overall removal efficiencies being 15 %, 25 % and 50 %, respectively. In addition to the CWs' configuration (e.g., size and type of macrophytes), pesticide retention was influenced by compound physical and chemical properties, favouring the accumulation of non-mobile ones in sediment and mobile ones in vegetation and herbivorous bird excrement. For metals/metalloids, the concentration of 50 % of the elements was reduced, with seasonal variations, reaching nearly 100 % in summer and winter. Environmental risks associated with pesticide concentrations, including banned compounds still detected in the CWs, were notably reduced in the outlets, preventing the discharge of moderate to high-risk concentrations into the protected Albufera de València lagoon. While for metals/metalloids, although general risk levels were low, mercury exceeded threshold values yet the CWs effectively mitigated it.