Herbicide Runoff From Experimental Watersheds

Two commonly used herbicides in corn fields of the Rhode River Watershed were atrazine (2‐chloro‐4‐ethylamino‐6‐iso‐propylamino‐1,3,5‐triazine) and alachlor (2‐chloro‐2′,6′‐diethyl‐ N ‐methoxymethyl acetanilide). Although alachlor was applied in larger quantities, atrazine was detected more frequently in runoff waters and had greater concentrations than alachlor (0–40 µ g/L vs. 0–6 µ g/L). Atrazine was more persistent and more mobile in watershed soils. Linear regression analysis of herbicide loading rates and percentage agricultural land‐use did not give a direct relationship. Runoff waters from forest watersheds where herbicides were not directly applied, were contaminated with herbicides. During the 3‐y study period (1976–1978), a maximum of 10 µ g/L of atrazine, and up to 0.5 µ g/L alachlor were discharged in winter runoff waters from the eight experimental watersheds. In addition to quantity of herbicides directly applied to land surface, residual levels in runoff waters must be influenced by other important factors such as topography, location of croplands in relationship to drainage channel, etc. A major portion of atrazine was found in dissolved aqueous form in runoff‐water samples collected during storm events. Percolation in subsurface flow and dissolution in overland flow were believed to be important transport mechanisms.