Mobility of 2,4‐D and 2,4,5‐T n‐Butyl Esters in Soils Following Massive Application to Field Mini‐Lysimeters

Concentrations of 2,4‐dichlorophenoxyacetic acid (2,4‐D) and 2,4,5‐trichlorophenoxyacetic acid (2,4,5‐T) in soils and leachates from field mini‐lysimeters were monitored for up to 28 months to characterize the patterns of herbicide mobility in soils under climatic conditions of the Palouse region of the Pacific Northwest. The lysimeters contained soils from Washington, Wyoming, or Mississippi, and were treated with 1,120 or 5,600 kg/ha of a 1:1 mixture of 2,4‐D and 2,4,5‐T n ‐butyl esters in 1976, 1977, or 1978. The winter of 1976–1977 was mild and relatively dry, and little leachate could be collected. However, subsequent winters were wet and the soil columns were saturated by the following springs. Less than 1 ppm of 2,4‐D was found in leachates from Washington and Wyoming soils receiving the herbicide application in 1976 or 1977, though over 30 ppm was found in leachates from Washington soil treated in 1978. Over 10 ppm of 2,4‐D could be found in leachates from the acidic Mississippi soil receiving the high rate of herbicide application in 1977 or 1978. Concentrations of 2,4,5‐T in the same leachates were higher for the Washington and Wyoming soils, but lower (than corresponding concentrations of 2,4‐D) for the Mississippi soil. In all cases, 96–99% of the undegraded herbicides in the lysimeters remained in the 0‐ to 10‐cm soil layer after two to three winters, and only 0.14–1.65% of the 2,4‐D and 0.33–3.7% of the 2,4,5‐T remaining in the lysimeters were present in the 10‐ to 30‐cm depths. Mobility characterization by soil thin‐layer chromatography showed that the ester form of both herbicides was relatively immobile, the acid form was less mobile in the Mississippi soil than in the Washington and Wyoming soils, and 2,4‐D was slightly more mobile than 2,4,5‐T under corresponding conditions.