Pesticides in the Red River and its tributaries in southern Manitoba: 1993-95

Pesticide concentrations and loadings at three locations along the Red Rive r and on seven of its tributaries in southern Manitoba were determined over a three year period. Samples (18 L) were collected on a biweekly schedule between spring and late autumn in 1994 and 1995 and triweekly throughout th e initial year of the study (1993) between June and October. Pesticide reco veries ranged from 14 to 115%, and those with <80% recovery (bromoxynil, 2, 4-D, diclofop, MCPA and triclopyr) were recovery corrected. Maximum concent rations were related to regional use periods, despite greatest runoff occur ring during spring melt and early summer rains. Pesticides were not present at elevated levels during this period. The pesticides detected in the high est frequency in the Red River tributaries generally were those used most e xtensively (e.g., MCPA). MCPA concentrations for all sites ranged from <0.0 1 to 990 ng/L. MCPA had the largest loadings of all herbicides in each trib utary, although loads were low (9-3100 g) compared to usage. Atrazine and a lachlor were frequently detected in the Red River. Atrazine is widely used in the neighbouring U.S. states, and loadings of this chemical in the Red R iver were similar to loadings of the highly used and less persistent phenox yacids, although it was used only to a minor extent in Manitoba. Alachlor, although not used in Canada, was present (<0.060-150 ng/L) in the Red River at each sampling site, which reflected U.S. usage of this product. Urban u sage of chlorpyrifos, 2,4-D and MCPA contributed to loadings along the Red River. Pesticide concentrations generally were well below Canadian water qu ality guidelines in the Red River drainage basin; however, chlorpyrifos con centrations occasionally exceeded guideline levels for the protection of fr eshwater aquatic life.