Herbicide use in the management of roadside vegetation, western Oregon, 1999-2000; effects on the water quality of nearby streams

The Oregon Department of Transportation (ODOT) uses the herbicides Krovar (active ingredients diuron and bromacil), Oust (active ingredient sulfometuron-methyl) and Roundup (active ingredient glyphosate) to control roadside vegetation. The purpose of this study was to assess whether the use of these herbicides could contribute to the load of herbicides carried by Oregon streams.

In spring of 1999, three test plots were constructed on a road shoulder near a crossing of Bull Creek, a small stream near Colton, Oregon, in the Willamette Valley. Simulated rainfall of 0.3 in/hr was applied to the experimental plots 1 day, 1 week, and 2 weeks after herbicide application. The simulated rainfall experiments yielded an upper limit concentration of 1 mg/L (milligram per liter, or parts per million) glyphosate and diuron, and of a few hundred micrograms per liter (µg/L, or parts per billion) of sulfometuron-methyl in runoff from the roadside when rainfall was applied 24 hr after spraying. Concentrations in the ditch itself would be less under natural conditions because of dilution by drainage water from the entire contributing drainage area.

The road shoulder was resprayed at the end of September, and data were collected from late October 1999 through early January 2000, during natural rainfall. Diuron concentrations in the direct runoff from the road shoulder ranged from 1 to 10 µg/L throughout the 3-month sampling period; during the period, concentrations in the roadside ditch decreased from about 10 µg/L in October to about 0.1 µg/L in January, indicating progressive dilution of the roadside herbicide runoff during the fall/winter rainy season. No diuron was detected in Bull Creek downstream from the drainage ditch. A mass balance calculation confirmed that the load to Bull Creek from the drainage ditch was too low to result in detectable concentrations in the stream during October to January.

Sulfometuron-methyl concentrations in runoff from the road shoulder ranged from 0.1 to 1 µg/L throughout the 3-month sampling period, and in the drainage ditch decreased from about 1 µg/L in October and November to about 0.2 µg/L in January. It was never detected in Bull Creek. Bromacil concentrations were similar to those of diuron. Glyphosate was never detected in fall samples from the road shoulder, the drainage ditch, or the stream.