Water residence time and pesticide removal in pilot-scale wetlands

The use of agricultural pesticides may lead to environmental contamination. Constructed wetlands (CWs) are often recommended for decreasing non-point source pollution by pesticides. The aim of this study was to evaluate the influences of water residence time (WRT) on the effectiveness of two CWs for decreasing pollution in drainage water at the pilot scale. Thus, two successive charge/discharge steps were conducted successively with high and low flows before conducting a step with no flow. For both pilot experiments, the measured effectiveness was between 22 and 100% of the applied quantities of the five pesticides. These results were correlated with the physicochemical properties of the pesticides, particularly their adsorption and desorption capacities. In addition, the pilot scale wetlands were less effective during short WRT (from 15.2 to 100%) than during long WRT (from 17.8 to 100%), despite the greater remobilisation observed during the discharge steps with low flows. After stopping flow, the DT50 values of the pesticides in the aqueous phase were less than 4 days in the two pilot wetlands, likely due to their rapid transfer to the solid phase (soil or straw). In addition, the DT50 values measured in the solid phases ranged from 3.8 days to more than one year. Thus, the WRT must be optimised in the CW by maintaining vegetation or installing weirs, to favour retention processes (sorption, sedimentation) during the drainage periods (October to May). Next, the pesticides stored in the solid phase could be subjected to degradation processes, particularly when moisture, residence times and temperature conditions are favourable (summer).