Flood Frequency and Duration Drive the Aquatic-Terrestrial Pesticide Transfer to Riparian Root-Zone Soil: A Mesocosm Study

The aquatic-terrestrial transition areas, riparian zones, recognized as biodiversity hotspots, are increasingly subjected to various stressors, including chemical contaminants such as pesticides. These compounds can be transported into the riparian zone, from surrounding agricultural fields, e.g., via surface runoff, but also from the aquatic environments, e.g. during flooding events. However, little is known about how flood frequency and duration affect this aquatic-terrestrial transfer. To investigate the flood-mediated pesticide transport, we conducted an experiment in 16 flow-through unit mesocosms, each unit containing a stream supplied with water from an agriculturally influenced river, and an adjacent riparian area. We simulated flooding events with varying duration of three, seven and fourteen days, repeated four times between May through September. We sampled river water every week and riparian root-zone soil 24 h after each flooding event and analysed organic pesticides using high-performance liquid chromatography coupled with tandem mass spectrometry via electrospray ionization. We found that six pesticides were detected exclusively in riparian root-zone soil following four repeated flooding events. Our findings indicate that both longer flood durations and repeated flooding events tend to increase the total concentration of pesticides in the riparian root-zone soil. These results demonstrate that flooding promotes the movement of pesticides from streams into adjacent riparian areas. As flood frequency and intensity are expected to increase due to climate change, the significance of this transport pathway is likely to increase, with potential consequences for riparian biodiversity and habitat quality.