Influence of hydrodynamics on the water pathway and spatial distribution of pesticide and metabolite concentrations in constructed wetlands.

ABSTRACT

Constructed wetlands (CWs) are likely to reduce pesticide levels reaching surface water. However, the distribution of the water flow path between the main channel and isolated areas may influence global pesticide mitigation. Little information is known about the influence of water pathways on pesticide mitigation. Thus, we performed tracer experiments at low and high flow rates (0.5 L/s and 4-7 L/s) in a pond CW and ditch CW to determine the localization of various hydraulic zones and to understand their implication on pesticide mitigation. The hydraulic performance reflecting the fraction of water transported from inlet to outlet passing through the whole of CW, was greater for the pond CW than for the ditch CW regardless of the flow rate, and greater at mean flow rates (MF) than at low flow rates (LF) due to a lower proportion of isolated areas at a MF (11%-68%) than at LF (38%-89%). Dispersion governed the water transport inside the isolated areas and the water convection inside the main channel. Consequently, dissolved pesticide concentrations are heterogeneously distributed in the CWs, i.e., in the main channel and isolated area, for both flow rates. However, one month after a no-flow period, this heterogeneity disappears, and dissolved pesticide concentrations become similar in the water of the whole CW due to dispersion. Furthermore, sedimentation and storage in sediments were greater in the isolated area than in the main channel, which is possibly due to a lower speed flow rate and a higher hydraulic residence time (HRT) in the isolated area than in the main channel. Thus, isolated areas act as effective's zones to mitigate pesticides from dissolved and particulate phases inside the CW during a complete drainage season (i.e., succession of high/low/no-flow periods).