The transport and deposition of current use pesticides and PCBs to surface waters in the Red River drainage basin

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Rawn, Dorothea Florence Kenny
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Pesticide concentrations were determined in surface water, precipitation and air over a four-year period in the South Tobacco Creek Watershed, a small (70 km$\sp2$) agricultural watershed in southern Manitoba, Canada. The objectives of this project were to determine pesticide temporal trends in rivers and streams in southern Manitoba and to relate concentrations to pesticide use patterns. Samples were collected on a weekly basis between 1993 and 1995. During 1996, sample collection frequency was increased to a two-day schedule. PCBs also were measured in atmospheric and water compartments in the watershed. As part of a larger spectrum examination of pesticide levels in Manitoba surface waters, the Red River and its tributaries were sampled. Sample collection in 1993 was performed once every three weeks and sampling frequency between 1994 and 1995 was biweekly. Temporal trends of pesticides in the South Tobacco Creek Watershed generally reflected regional uses of each chemical, with maximum concentrations occurring during and extending beyond regional application periods. Creek water concentrations of pesticides were consistently at maximum levels when both precipitation and air concentrations were elevated. During the period of high pesticide concentrations in the precipitation and air, water flow was low in South Tobacco Creek and no overland flow events were observed, indicating that atmospheric compartments were significant contributors to pesticide loadings. Results in the Red River and its tributaries were similar to observations in the South Tobacco Creek Watershed, with maximum pesticide concentrations occurring during regional application times. The widely used phenoxyacid herbicides were detected in river water at highest concentrations, however, minor use products were observed. A notable exception observed between the small watershed and the Red River was the detection of alachlor in the Red River, although it is not used in Canada. It is, however, applied in the Red River drainage basin within the US which indicates that pesticide usage in the neighbouring states can impact Canadian water quality. No temporal trends of PCBs were observed in South Tobacco Creek water, however, seasonal differences in air concentrations were observed. Air concentrations were correlated with temperature, indicating PCB sources were volatilization from soil and plant surfaces. PCB concentrations in the water column were higher than anticipated, based on levels observed in large lakes. These elevated concentrations may reflect PCB inputs to the watershed via surface runoff, in addition to direct atmospheric deposition of these chemicals to the water surface.