Pesticides in the Red River and its Tributaries in Southern Manitoba: 1993—95

Abstract Pesticide concentrations and loadings at three locations along the Red River 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 the initial year of the study (1993) between June and October. Pesticide recoveries ranged from 14 to 115%, and those with < 80% recovery (bromoxynil, 2,4-D,diclofop, MPCA and triclopyr) were recovery corrected. Maximun concentrations were related to regional use periods, despite greatest runoff occurring during spring melt and early summer rains. Pesticides were not present at elevated levels during this period. The pesticides detected in the highest frequency in the Red River tributaries generally were those used most extensively (e.g., MCPA). MCPA concentaraions for all sites ranged from < 0.01 to990 ng/L. MCPA had the largest loadings of all herbicides in each tributary, although loads were low (9-3100 g) compared to usage. Atrazine and alachlor 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 River were similar to loadings of the highly used and less persistant phenoxyacids, although it was used only to a minor extent in Manitoba. Alachor, althiugh not in 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 usage of chlorpyrifos, 2,4 D and MCLA contributed to loadings along the Red River. Pesticide concentrations generally were well below Canadian water quality guidelines in the Red River drainage basin; however, chlorpyrifos concentrations occassionally exceeded guideline levels for the production of freshwater aquatic life.