Factors Affecting Yields of Selected Herbicides in the Susquehanna and Potomac Rivers and Tributaries, June 1994

During June 1994, stream water-quality samples were collected from 43 sites on the main stems and tributaries of the two river basins, following spring planting and the application of most pre-emergent pesticides. Samples were analyzed for concentrations of 46 herbicides and insecticides.

Instantaneous loads of five commonly found row-crop herbicides--alachlor, metolachlor, atrazine, cyanazine, and simazine--were determined. Instantaneous yields (loads normalized using basin drainage area) of agricultural basins were grouped into categories according to physiographic province or section and generalized lithology of underlying bedrock. For all five herbicides, the category of agricultural basins in the Piedmont Province had the highest median yields and largest range of yields of all the categories.

Three variables--the amount of corn grown in the basin, instantaneous streamflow yield, and herbicide application rate--were used to explain variations in herbicide yield among agricultural basins. For the five herbicides, the factor that most explained the variations was the amount of corn grown in the basin. A statistically significant relation (p <= 0.10) was found among yields of atrazine and simazine, and the amount of corn grown in the basin, streamflow yield, and the combination of the amount of corn grown in the basin and streamflow yield. At the same statistical significance level, cyanazine and metolachlor yields were related to both the amount of corn grown in the basin and to the combination of the amount of corn grown in the basin and streamflow yield. Alachlor yields were related to the amount of corn grown in the basin.

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