Wetland hydrodynamics and mitigation of pesticides and their metabolites at pilot-scale
Résumé
Constructed wetlands (CWs) are useful for reducing pesticide transfer from drainage into surface water, though little was known about the influence of hydrodynamics on their mitigation. We thus assessed the influence of design parameters (aspect ratio, water depth, flow rate) on hydraulic performance simultaneously to pesticide mitigation. We performed our work on four pesticides with contrasted properties: boscalid (BSC), cyproconazole (CYP), isoproturon (IPU) and dimethachlor (DMT), under three controlled flow-rate conditions (including no-flow) by using two pilots with contrasted designs (pond and ditch) over 62 days. Hydraulic performance and pesticide mitigation were less effective in a pond than in the ditch whatever the flow. Moreover, pesticide mitigation was more significant at low than at high flow rates for both pilots. M high flow rate, water transport is mainly governed by convection, but at low flow rates both water transport and mitigation are governed by hydrodynamic dispersion, inducing a longer contact time between pesticides and substrate due to longer hydraulic retention. Finally, BSC and CYP are better mitigated than DMT and IPU, even if they are also more strongly released during low flow rates. In addition, the mitigation of pesticides and some of their metabolites produced inside the pilots was almost complete during stagnation.
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