Title: Low concentrations of glyphosate in water and sediment after direct over-water application to control an invasive aquatic plant

Abstract

When an invasive wetland grass degrades a Ramsar wetland and Important Bird Area, decisive management action is called for. To limit the extent and spread of European Phragmites australis, the Ontario government began the first, large-scale application of glyphosate (Roundup Custom^Ⓡ) over standing water to control an invasive species in Canadian history. Between 2016 and 2018, over 1000 ha of marsh were treated. To assess the concentration, movement and longevity of this herbicide in treated marshes, we measured the concentration of glyphosate, its primary breakdown product aminomethylphosphonic acid (AMPA), and the alcohol ethoxylate-based adjuvant Aquasurf^Ⓡ in water and sediments in areas of the highest exposure and up to 150 m into adjacent bays. The maximum observed concentration of glyphosate in water was 0.320 mg/L, occurring within 24 hr of application. The maximum glyphosate concentration in sediment was 0.250 mg/kg, occurring within about 30 days of application. AMPA was detectable in water and sediment, indicating microbial breakdown of glyphosate in the marsh, but at low concentrations (max_water = 0.025 mg/L, max_sed = 0.012 mg/kg). The maximum distance from the point of application that glyphosate was detected in the water was 100 m, while AMPA was detectable only at the edge of where glyphosate was applied (0 m). Concentrations in water returned to pre-treatment levels (<DL) within 20-30 days of application. In sediment, glyphosate residue persisted above detection limits (>0.005 mg/kg) for over one year but less than two years. Concentrations of alcohol ethoxylates were variable in space and time, following a pattern that could not be attributed to Aquasurf^Ⓡ use. The direct, over-water application of Roundup Custom^Ⓡ with Aquasurf^Ⓡ to control invasive P. australis did not reach concentrations deemed to pose toxicological concern to aquatic biota by the Canadian Council of Ministers of the Environment.

Keywords: Aquatic invasive species; Chemical control; Coastal wetland; Environmental monitoring; European common reed; Herbicide; Phragmites australis.