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Herbicide levels in rivers draining two prairie agricultural watersheds (1984).

A monitoring survey was conducted during 1984 on the Ochre and Turtle Rivers, which flow into Dauphin Lake in western Manitoba, Canada, to determine levels of the herbicides MCPA, diclofop-methyl, dicamba, bromoxynil, 2,4-D, triallate and trifluralin which were widely used in each watershed. Triallate concentrations exceeded 4 ng/L in 50% and 10% of the 21 samples taken from each of the Turtle and Ochre River, respectively, during the period March to October 1984. Trifluralin concentrations exceeded 3 ng/L in 14% and 10% of the samples from the respective rivers. Maximum concentrations did not exceed 25 ng/L and were unrelated to changes in river flow. Bromoxynil and diclofop were detected in the Turtle River, at concentrations of 113 and 476 ng/L, respectively, following a major high water event in late June, but were undetectable (less than 2 and 12 ng/L, respectively) at other sampling times. Dicamba and 2,4-D were detectable throughout most of the sampling period in both rivers at low levels (less than 100 ng/L). High levels of dicamba and 2,4-D (5476 and 2568 ng/L respectively) were found in water from the Turtle River prior to the high water event event possibly caused by spraying of ditches or rights-of-way near the river. Discharges (grams/year) of all seven herbicides monitored in the study represented less than 0.1% of the amounts estimated to be used in each watershed. Unit loadings (mg/ha/year) of 2,4-D and dicamba were similar to those reported for Southern Ontario streams in the mid-1970's. The results indicate that herbicide contamination of Manitoba streams draining agricultural areas is generally low except when major runoff occurs during the application period in May and June.