Loss of 2,4‐D in Runoff from Plots Receiving Simulated Rainfall and from a Small Agricultural Watershed

Abstract

AbstractMovement of 2,4‐D [(2,4‐dichlorophenoxy) acetic acid] was not significant in either surface or subsurface runoff from a small agricultural watershed on a sandy Coastal Plain soil. Surface runoff levels were highest for the first runoff event after herbicide application (0.56 kg/ha) each year, and initial concentrations were related to the time lapse between herbicide application and the date of the first runoff event. Maximum concentrations were 8.1, 6.2, and 2.5 µg/liter in 1970, 1971, and 1972, respectively. The corresponding time lapse for the same years was 20, 27, and 34 days. Persistence studies showed that the 2,4‐D concentration in the surface 0.5 cm of soil decreased 95%, from 4.7 to 0.23 ppm in only 7 days, and after 34 days the soil concentration was only 0.01 ppm. Although subsurface flow was three times greater than surface runoff during the 3‐year period, 2,4‐D movement in subsurface water was negligible. Concentrations were usually zero or < 1 µg/liter. Soil sampled to a 90‐cm depth showed no 2,4‐D accumulation or build‐up in the soil profile. Simulated rains (8.25 cm in 30 min) applied to subplots on the watershed showed that there is a potential for greater 2,4‐D losses in surface runoff when it rains soon after herbicide application. When rains were applied 1, 8, and 35 days after herbicide application, the average 2,4‐D concentrations in runoff were 25.2, 5.8, and 0.7 µg/liter, respectively.