Removal of dissolved pesticide mixtures by a stormwater wetland receiving runoff from a vineyard catchment: an inter-annual comparison

Wetlands can collect contaminated runoff from agricultural catchments and have intrinsic physical, chemical and biological retention and removal processes useful for mitigating pesticides. However, knowledge about the ability of stormwater wetlands to mitigate pesticide mixtures in runoff is currently very limited. We show here that stormwater wetlands that primarily serve for flood protection can also be effective tools for reducing concentrations and removing loads of runoff-related pesticides and some of their degradation products into downstream aquatic ecosystems. Dissolved concentrations and loads of seven fungicides, six herbicides and four degradation products in runoff from a vineyard catchment were continuously recorded at the inlet and the outlet of the stormwater wetland during two successive periods of pesticide application (April to June). Reduction of pesticide concentrations by the wetland ranged from 50% (simazine) to 100% (azoxystrobin, cymoxanil, cyprodinil, gluphosinate, terbuthylazine and tetraconazole). Removal rates of dissolved load ranged from 26% for aminomethylphosphonic acid (AMPA) to 100% (azoxystrobin, cymoxanil, cyprodinil, diuron, 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU), gluphosinate, kresoxym methyl, terbuthylazine and tetraconazole). More than 77% of the input mass of total suspended solids was retained, underscoring the capability of the wetland to trap pesticide-laden particles via sedimentation. Inter-annual change in the removal of AMPA, isoxaben, kresoxim methyl and simazine was mainly linked to the larger vegetal cover in 2010. Our results demonstrate that stormwater wetlands can remove pesticide mixtures in agricultural runoff, although removal of individual pesticides can vary over time, depending on the characteristics of runoff events and the vegetation cover.