DISSIPATION OF GLYPHOSATE AND AMINOMETHYLPHOSPHONIC ACID IN WATER ANDSEDIMENTS OF BOREAL FOREST PONDS

Three small ponds in the boreal forest of southern Manitoba were treat ed with an aerial application of 2.1 kg/ha glyphosate. Two of the pond s had been treated the preceding year with 0.9 kg/ha glyphosate, where as the third was previously untreated. Foliage samples of plants borde ring the ponds were collected immediately after treatment, and water a nd sediment samples were collected over a period of 265 d after treatm ent. Samples were analyzed for glyphosate and its first metabolite, am inomethylphosphonic acid (AMPA). We found that glyphosate dissipated r apidly from the surface waters of all ponds (dissipation half-lives of 3.5-11.2 d). AMPA residues were detected in water samples during the first 14 d after treatment, suggesting that herbicide degradation was occurring in the water column. However, not all applied herbicide was accountable in residues in the water. Glyphosate and AMPA increased in sediment samples to day 36, suggesting that sediment adsorption was a major sink for the herbicide. Glyphosate dissipation from the water c olumn was biphasic, with a rapid initial phase followed by a slower ph ase. We hypothesize this was due to herbicide sorption and partitionin g between suspended particulate matter and bottom sediments. As was fo und after the first pond treatment, glyphosate dissipation was slower in chemically alkaline ponds than in a more dilute pond, suggesting th at herbicide complexation with ions in solution may be a significant f actor affecting the rate of dissipation in standing water.