Nature-based solutions for nutrient and emerging contaminant reduction in a Mediterranean Ramsar wetland.

Constructed wetlands are widely recognized as nature-based solutions for improving water quality in systems receiving treated wastewater. However, their effectiveness depends primarily on hydrological connectivity, contaminant properties, and hydraulic residence time. In Mediterranean wetlands, where strong hydrological variability and seasonal water scarcity influence water quality dynamics, understanding attenuation processes under contrasting water-circulation scenarios is relevant for protecting sensitive downstream ecosystems. This study assessed the capacity of a semi-natural pond system i.e., a series of interconnected shallow ponds comprising both natural and managed water bodies that receive treated wastewater while retaining natural wetland characteristics, located upstream of the Fuente de Piedra Ramsar wetland main basin (southern Spain) to reduce nutrients and emerging contaminants under contrasting water-circulation scenarios. Across 26 sampling events, substantial reductions were observed when treated wastewater flowed through the semi-natural pond system before reaching the main wetland basin, including decreases of 75% in total nitrogen. Marked attenuation was detected for emerging contaminants with shorter environmental half-lives, such as trimethoprim, naproxen, caffeine and selected triazine metabolites showed removals exceeding 70-90%, whereas more persistent substances, including carbamazepine and gemfibrozil, exhibited limited reduction. In contrast, negligible changes occurred during a temporary bypass configuration put in place for restoration works, in which treated wastewater avoided most of the semi-natural ponds before reaching the main wetland basin. Dry-season conditions enhanced contaminant attenuation, largely reflecting lower flows and extended hydraulic residence times. Cumulative ecotoxicological risk, estimated using risk quotients (RQsample) based on measured environmental concentrations and the lowest predicted no-effect concentrations for freshwater organisms, declined downstream under the regular flow pathway, particularly for pesticides, but moderate risk persisted under the bypass scenario. Overall, the semi-natural pond system demonstrates a strong but hydrologically dependent capacity to mitigate nutrient and emerging contaminant loads entering a Mediterranean Ramsar wetland. Nevertheless, elevated phosphorus concentrations and the persistence of certain emerging contaminants highlight the limitations of natural attenuation alone and support the integration of complementary management and treatment strategies to ensure long-term ecosystem protection.