Effect of subsurface drainage on runoff losses of atrazine and metolachlor in southern Louisiana

The effect of subsurface drainage on runoff losses of atrazine and metolachlor was investigated in plots with or without subsurface drainage. The drains (10 cm diameter corrugated and perforated polyethylene tubes) were 1 m deep and were spaced at 10, 20 or 30 m intervals. The atrazine and metolachlor in runoff were analysed using gas chromatography and electron capture and Hall Electrolytic conductivity detection. Drain spacing had no effect on herbicide concentrations in the runoff. Both herbicides underwent a rapid reduction in concentration for the first 34 days after application. Concentrations of the herbicides were significantly higher (65 and 60 per cent for atrazine and metolachlor respectively) from plots without subsurface drains for the first runoff event. After this the presence of absence of subsurface drains had no significant effect on herbicide concentrations in the runoff. There was over twice as much runoff from plots without subsurface drains. The reduction in runoff losses from subsurface-drained plots was not offset by an increase in leaching losses from these plots.