Atrazine and Metolachlor in Subsurface Drain Water in Louisiana

Atrazine and metolachlor are applied preemergent to corn in plots that are either drained (10-cm plastic tubes 1-m deep) or undrained. Herbicide concentrations in the soil profile down to 30 cm and in the drain water are measured for 243 days. The pesticides show a steady disappearance from the soil with pseudo first-order half-lives (no difference between drained and undrained plots) in the top 2.5 cm of soil of 35-36 days for atrazine and 20-23 days for metolachlor. Total loss of atrazine through soil leaching amounts to 0.038% of the application rate; 97% of this loss occurs within 78 days after application during a period of 530 mm of rain. All (0.13% of that applied) of the metolachlor in the drain water is observed within 59 days after application. The results of the study illustrate the strong correlation between pesticide soil persistence, water solubility, rainfall timing, and pesticide leaching through soil.