Artificial wetland for mitigation of non-point source agricultural pollution in a French drained context: lessons from a 10-years monitoring.

Managing water flow at the outlet of subsurface drainage networks is an important issue for water authorities aiming to preserve freshwater quality. As buried drains are directly connected to arterial ditches, dynamic and specific hydrological functioning of drained plots contribute to export part of chemical compounds derived from the farm inputs application, such as the exceed pesticides or fertilizers. Provided the necessary changes of agricultural practice at the farm scale, intercepting water at the outlet of subsurface drainage network in artificial wetland is considered as an efficient mitigation measure and constitutes a nature-based solution that addresses multiple ecological objectives.

From the pilot experimental subcatchment of Rampillon in France (355ha of intensive farming in the agricultural region of Brie, rural Parisian basin), we propose a synthesis of the results obtained from the 10 years of performance assessment of this artificial wetland in buffering nitrates and pesticides from drained fields. We also provide a feedback from the 15-years collaboration with the local and national stakeholders strongly involved from the early beginning of this long-term project. After an initial stage of dialog and co-construction with the stakeholders (5 years), the experimental artificial wetland (0.5ha or 0.15% of upscale watershed) was constructed in 2010. The full monitoring of the wetland for water quality and quantity started in 2012 and was complemented by biodiversity surveys in 2017 and 2021.

We will present yearly and seasonal variations of removal efficiency of nitrate and pesticides and discuss the limits and interests of artificial wetland in this context, including the high potential for removal with high seasonal variability due to thermal sensitivity and hydrological effects. Concerning heavy metals, a former study showed that agricultural fields do not significantly contribute to their transfer and that the artificial wetland retained the major part of them. In a hydraulic study using conservative tracers, we highlight the role of vegetation patches on the hydraulic performance of the wetland and on the pollutant mitigation. All those results allowed us to develop a modeling approach based on Tank In Serie (for nitrate and pesticides) to define a standard design aiming 50% annual removal efficiency.

After ten years, involved farmers are now the best ambassadors to disseminate the role of nature-based solutions in helping mitigate the unintentional pollutions from agricultural activities, not considering artificial wetland as a right to pollute but a complementary tool to Best Management Practices.

Finally, part of our works focuses on the role of artificial wetland in biodiversity maintenance in agricultural landscape by monitoring the ecosystem dynamics and the seasonal exposure of different vertebrate and invertebrate communities using ecotoxicological approach. In fine, this will allow us to better understand the intricacy of the ecological functions ensured by these artificial ecosystems and to assess their potential in supporting multiple ecosystem services such as water quality preservation, biodiversity protection, landscape connectivity and recreational activities to local populations.