Hydrological tracers for assessing transport and dissipation processes of pesticides in a model constructed wetland system
Authors: Fernández-Pascual Elena , Bork Marcus , Hensen Birte , Lange Jens 
Published: January 2020

Abstract. Studies that have used hydrological tracers to investigate the fate and transport of pesticides in constructed wetlands have often considered such systems as a “black box”. Consequently, internal temporal and spatial mechanisms that dominate pesticide transport and dissipation (e.g., sorption, transformation and plant uptake) are still not fully understood. Here we present a novel approach that combines the use of
tracers with different sorptive and reactive properties – i.e., bromide (Br−), uranine (UR) and sulforhodamine B (SRB) – with high vertical resolution sampling and monitoring to evaluate transport and
dissipation processes of three selected pesticides (boscalid, penconazole
and metazachlor) inside a model constructed wetland system on a long-term
basis and detailed spatial scale. Moreover, the influence of vegetation and
alternating different hydrologic conditions on transport and dissipation
processes was evaluated by comparing a vegetated with a non-vegetated
section and by alternating periods of saturation and drying. Breakthrough
curves obtained at different sampling depths pointed out that the solutes
were not equally distributed within the constructed wetland. Data revealed
that a higher mass of solutes was transported to the vegetated part of the
uppermost layer, which was associated with possible lateral transport at or
near the surface and/or a shortcut effect produced by the roots. In contrast, the middle layers showed retardation, most likely due to the
presence of water-filled pores before the injections and low pore
connectivity in the vicinity of the sampling ports. The strong temporal and
spatial correlation found between Br−, UR and metazachlor indicated that transport was the dominant process for these solutes. Conversely, SRB, boscalid and penconazole most likely underwent sorption, as evidenced by their absence in the middle layers, the rapid decrease in their concentrations after the injections and the gradual increase in accumulated mass recovery at the outlet. The overall tracer mass balance allowed us to identify three dissipation pathways: sorption, transformation and plant uptake. The detection of metazachlor transformation products (TPs) confirmed the contribution of transformation to metazachlor dissipation, whereas no TPs for boscalid and penconazole were detected; however, their transformation could not be ruled out in the present study. Hot spots of sorption and transformation were found in the uppermost layer, whereas hot moments were detected at the beginning of the experiment for sorption and after promoting aerated conditions for transformation. The use of hydrological tracers coupled with high vertical resolution sampling and monitoring proved to provide valuable information about the transport vectors and dissipation processes of pesticides inside a constructed wetland. This study represents a first approximation, and further experiments need to be carried under field conditions in combination with modeling.