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

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Abstract
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 (<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 (<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.