PERSISTENCE AND LEACHING OF ATRAZINE IN CORN CULTURE IN THE EXPERIMENTAL SITE OF LACOTE-SAINT-ANDRE (ISERE, FRANCE)

The fate of atrazine in corn culture was studied under field condition s during the years 1991-1992. After a preemergence treatment (1000 g/h a atrazine used under a flowable formulation) occurring at the end of April, samples of soil-water and soil were collected periodically at d ifferent depths in the one-meter-thick upper layer of the soil, suppor ting the corn culture at the Experimental Farm of La Cote Saint Andre (Isere, France). Atrazine and its metabolites deethylatrazine and deis opropylatrazine, 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 mu g/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 metab olites) leached annually was tentatively estimated as the product of t he weekly concentration of the soil water at 80 cm depth by the total volume of water drained for the corresponding week. The total amount o f 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 rai nfall in the fluvio-glacial type of soil studied here, there most prob ably was an important leaching of free atrazine on the occasion of abu ndant rains during the first month after treatment. The amount dissolv ed in soil-water seemed to be but remotely associated with the adsorbe d content in soil since partition equilibrium conditions, as shown, fo r example, in experiments establishing Koc values, were probably only rarely obtained between soil and soil-water under field conditions. At razine, initially exclusively located on the soil surface, appears pro gressively at all levels of the one-meter layer studied. One year afte r the spraying of 1 kg/ha atrazine, only 15% of the initial deposit wa s still present (in the form of atrazine, deethyatrazine, or deisoprop ylatrazine) 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 pla yed by plant absorption and formation of non-extractable residues rema ined low. Just before a new crop season, the average amount of atrazin e and metabolites in the one-meter layer was measured to be close to 4 00 g/ha (150, 125, and 125 for atrazine, deethylatrazine, and deisopro pylatrazine, respectively).