Persistence and leaching of atrazine in corn culture in the experimental site of La Côte Saint André (Isère, France)

Abstract

The fate of atrazine in corn culture was studied under field conditions during the years 1991–1992. After a preemergence treatment (1000 g/ha atrazine used under a flowable formulation) occurring at the end of April, samples of soil-water and soil were collected periodically at different depths in the one-meter-thick upper layer of the soil, supporting the corn culture at the Experimental Farm of La Côte Saint André (Isère, France). Atrazine and its metabolites deethylatrazine and deisopropylatrazine, measured by GC analysis, were present throughout the year in soil samples and soil-water samples at every sampling date and at each depth. Atrazine concentration varied from 0.2 to 14.7 μg/L in soil-water samples. The total content of atrazine, deethylatrazine, and deisopropylatrazine dissolved in the soil water of the one-meter-thick upper soil layer varied throughout the year from 33 to 94 g/ha. Under our conditions of sampling, the total amount (atrazine and metabolites) leached annually was tentatively estimated as the product of the weekly concentration of the soil water at 80 cm depth by the total volume of water drained for the corresponding week. The total amount of atrazine plus metabolites obtained through this estimation procedure was close to 50 g/ha, representing approximately 5% of the treatment applied (0.6% when atrazine alone was taken into account). Due to the importance of the rapid macropore transport of water after a heavy rainfall in the fluvio-glacial type of soil studied here, there most probably was an important leaching of free atrazine on the occasion of abundant rains during the first month after treatment. The amount dissolved in soil-water seemed to be but remotely associated with the adsorbed content in soil since partition equilibrium conditions, as shown, for example, in experiments establishing Koc values, were probably only rarely obtained between soil and soil-water under field conditions. Atrazine, initially exclusively located on the soil surface, appears progressively at all levels of the one-meter layer studied. One year after the spraying of 1 kg/ha atrazine, only 15% of the initial deposit was still present (in the form of atrazine, deethyatrazine, or deisopropylatrazine) in the one-meter-thick upper layer. The major part of the atrazine deposit (close to 80–85%) was dissipated essentially through metabolization, early leaching, and volatilization, while the role played by plant absorption and formation of non-extractable residues remained low. Just before a new crop season, the average amount of atrazine and metabolites in the one-meter layer was measured to be close to 400 g/ha (150, 125, and 125 for atrazine, deethylatrazine, and deisopropylatrazine, respectively).