Implications for constructed wetlands to mitigate nitrate and pesticide pollution in agricultural drained watersheds

- Losses of nitrates and pesticides increase after their application.
- CWs in drained watersheds remove 20-90% nitrates and 40-90% pesticides.
- The on-stream FWS CWs are more suitable for nitrate removal.
- The off-stream CWs of regulated flow interception are better for pesticide removal.
- 50% of nitrates and pesticides can be reduced by devoting 1% of the contributing area to a CW.

In the context of subsurface drainage, the mitigation of agricultural pollutants means intercepting water flows using green infrastructures such as constructed wetlands (CWs). First, based on a scientific review, this paper analyses how efficiently CWs can remove nitrate and pesticides from the runoff in drained agricultural watersheds. Average efficiency ranges from 20 to 90% and from 40 to 90% for pesticides and nitrate respectively. The main processes involved are based on microbiological activities, for which hydraulic residence time is a key factor. In order to successful implementation of such a wetland system, hydrological diagnosis of water flow and pollutant transfer at different watershed scales should be provided. Typically, the transport and transformation of pollutants shows clear seasonality depending on the application of nitrate (throughout the whole year) and pesticides (only after application periods).We suggest two interception strategies based on field experiments. The “on-stream” strategy means the establishment of free water surface (FWS) CWs directly on streams/ditches and the interception of all drainage flows: a solution suitable for nitrate removal. The “off-stream” strategy requires the establishment of CWs outside of streams/ditches, whereas interception targets only the most polluted water flow, for instance during the period after pesticide application, requiring farmer's involvement.Suggestions are also made for FWS CW design (a geotechnical survey, topography constraints, etc.) respecting ecological engineering concepts. A following size range is proposed: 76 m3 per drained hectare, equivalent to 1% of the upstream area, given a maximum water depth of 0.8 m. Nevertheless, CWs must be considered as a complementary tool dedicated to transfer reduction, and as part of broader actions aimed at reducing pollutant loading at the plot scale.