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 500 m-long, 25 m-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, ~ 1 cm rainfall event from a 16 ha agricultural field. Water samples (1 L) were collected every 30 min within the first 4 h, then every 4 h until 48 h, and again on days 5, 7, 14, 21, and 28 post-amendment at distances of 0 m, 10 m, 40 m, 300 m and 500 m from the amendment point within the wetland for suspended solids, nutrient, and pesticide analyses. Peak sediment, nutrient, and pesticide concentrations occurred within 3 h of amendment at 0 m, 10 m, 40 m, and 300 m 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 48 h. By day 28, all amendments were near or below pre-amendment concentrations. Water samples at 500 m showed no changes in sediment or nutrient concentrations; pesticide concentrations peaked within 48 h but at ≤ 11% of upstream peak concentrations and had dissipated by day 28. Managed riverine wetlands ≥ 1 ha and with hydraulic residence times of days to weeks can efficiently trap agricultural runoff during moderate (1 cm) 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 3 h of dosing. ► Agrichemicals were rapidly attenuated by 42–98% within 48 h. ► By day 28, all agrichemicals were near or below pre-dosing concentrations. ► Managed wetlands can efficiently trap agricultural runoff after rainfall events.