U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Fate of Alachlor, Atrazine, and Bromide in a Corn Plot near Topeka, Kansas

Abstract

In April 1991, a bromide tracer and the herbicides atrazine and alachlor were applied to a 0.75-hectare (ha) field plot of Eudora silt loam in the Kansas River Valley near Topeka, Kansas. The chemicals were immediately disked into the top 7.5 cm of soil, and the plot was planted with corn. Composite soil cores were collected from 12 contiguous cells in the plot, each 15 meters (m) by 15 m, at 15-centimeter (cm) depth intervals to 90 cm. Cores were collected immediately after application, then after 30, 60, and 120 days. Mean atrazine losses from the soil profile, relative to the immediate postapplication core results, were 75 percent at day 30, 92 percent at day 60, and 96 percent at day 120. Mean alachlor losses were 93 percent at day 30, 99 percent at day 60, and nearly 100 percent by day 120. The ratio of desethylatrazine metabolite to atrazine concentrations steadily increased in the top 15-cm soil layer during the 120-day period. An analysis of the soil-water budget and bromide distribution in the soil profile indicates that downward movement of the chemicals was restricted by a lack of significant downward soil-water flux during the growing season, when evapotranspiration was greatest. Loss of the herbicides from the soil profile is attributed mainly to degradation and transformation, and loss of bromide is attributed mainly to uptake by the crop. Low but persistent herbicide concentrations 120 days after application indicate that herbicide half lives are not constant and that herbicide residues could be tightly bound within solid-phase organic carbon and unavailable for biotic degradation.