Trend analysis and mass‐discharge estimation of atrazine in southwestern Ontario Great Lakes tributaries: 1981–1989

Aqueous concentration time series of the herbicide atrazine and its phytotoxic metabolite desethylatrazine obtained from 1981 to September 1990 in the Thames, Grand, and Saugeen rivers draining agricultural southwestern Ontario were analyzed to ascertain temporal trend, seasonal variation, and mass‐discharge patterns Substantial year‐to‐year changes were evident. Data suggest that the greatest applications of the decade might have occurred in 1984; however, in 1985 significant decreases in levels were observed at all three sites, apparently due to an abrupt shift away from corn cultivation. Mass‐discharge estimates generally parallel the concentration trends but also reflect hydrometeorologic trends. In 1984 for the Thames basin, extremely high concentrations together with high flows combined to export about 13 tonnes (t) atrazine plus metabolite, the largest single annual loss to surface‐water systems observed over the decade. Driven by the annual spring‐summer application cycle, aqueous concentrations surge in late May to reach annual peaks in late June and early July Despite the declining levels of recent years, Thames River data suggest that the probability of exceeding the Canadian water quality guideline of 2 μg/L for protection of aquatic life, primarily algae and vascular plants, may yet approach 65%. The greatest risks to susceptible species are likely associated with large summer storm runoff in areas of high soil to surface water transfer potential. Herbicide usage in general continues in large quantities, and continued surveillance appears warranted. Further research is required to ascertain the ultimate fate of these compounds in end receiving waters such as the Great Lakes