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Metolachlor Transport in Surface Runoff

Dep. of Geography, Trent Univ., Peterborough, ON, Canada K9J 7B8.

^* Corresponding author.

ABSTRACT

In 1987 metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was applied to a 5.5-ha corn field in southern Ontario, Canada, in two separate areas—as a preemergence surface spray with cross-contour plowing and as a pre-plant incorporated treatment with contour plowing. The application rate for both treatments was 2.64 kg a.i. ha^–1. Metolachlor concentrations in soils and in the dissolved and adsorbed phases in runoff from natural rainstorms were monitored throughout the growing season. Persistence in soils decreased exponentially after application, although there was evidence of temporary accumulation of metolachlor in footslope areas. Dissolved concentrations in runoff decreased with time, while temporal trends in adsorbed concentrations reflected changes in metolachlor persistence in soils. Average ratios of adsorbed to dissolved herbicide concentrations ranged from 7 to 57, and were dependent upon hydrological and pedologic conditions within the treatments. Sediment carried between 9 to 58% of the total metolachlor yield from runoff plots during individual storms, and 20 to 46% of the total yield over the monitoring period. Herbicide incorporation and contour plowing led to significant reductions in dissolved and adsorbed concentrations, and in total metolachlor loss in runoff, relative to application as a preemergence spray with cross-contour plowing. Incorporation was associated with an increase in the relative importance of sediment in metolachlor transport. Variations in herbicide losses and the relative importance of transport vectors within a given treatment were linked to local hydrological processes.