Mitigating agrichemicals from an artificial runoff event using a managed riverine wetland

We examined the mitigation efficiency of a managed riverine wetland amended with a mixture of suspended sediment, two nutrients (nitrogen and phosphorus), and three pesticides (atrazine, metolachlor, and permethrin) during a simulated agricultural runoff event. Hydrologic management of the 500m-long, 25m-wide riverine wetland was done by adding weirs at both ends. The agrichemical mixture was amended to the wetland at the upstream weir simulating a four-hour, ~1cm rainfall event from a 16ha agricultural field. Water samples (1L) were collected every 30min within the first 4h, then every 4h until 48h, and again on days 5, 7, 14, 21, and 28 post-amendment at distances of 0m, 10m, 40m, 300m and 500m from the amendment point within the wetland for suspended solids, nutrient, and pesticide analyses. Peak sediment, nutrient, and pesticide concentrations occurred within 3h of amendment at 0m, 10m, 40m, and 300m downstream and showed rapid attenuation of agrichemicals from the water column with 79–98%, 42–98%, and 63–98% decrease in concentrations of sediments, nutrients, and pesticides, respectively, within 48h. By day 28, all amendments were near or below pre-amendment concentrations. Water samples at 500m showed no changes in sediment or nutrient concentrations; pesticide concentrations peaked within 48h but at ≤11% of upstream peak concentrations and had dissipated by day 28. Managed riverine wetlands≥1ha and with hydraulic residence times of days to weeks can efficiently trap agricultural runoff during moderate (1cm) late-spring and early-summer rainfall events, mitigating impacts to receiving rivers. ► Mitigation efficiency of a managed wetland dosed with agrichemicals was assessed. ► Peak sediment, nutrient, and pesticide loads occurred within 3h of dosing. ► Agrichemicals were rapidly attenuated by 42–98% within 48h. ► By day 28, all agrichemicals were near or below pre-dosing concentrations. ► Managed wetlands can efficiently trap agricultural runoff after rainfall events.