Herbicide application method effects on napropamide complexation with dissolved organic matter

Napropamide [2-(alpha-naphthoxy)-N,N-diethyl propionamide] has been shown t o form soluble complexes with dissolved organic matter (DOM). Batch equilib rium and soil column leaching studies were performed to evaluate the effect of a drying event following herbicide application on napropamide-DOM compl ex formation. Napropamide was applied directly to the soil and allowed to d ry and compared with studies where the herbicide was not allowed to dry. In batch studies the presence of napropamide-DOM complexes was inferred from reduced sorption coefficients measured on extracts of treated soils as comp ared with those from aqueous solutions. In column studies, drying was shown to induce herbicide movement with the wetting front even when preferential flow pathways were eliminated. The formation of napropamide-DOM complexes was verified by dialysis tubing techniques. Increased napropamide movement through soil was the result of stable napropamide-DOM complexes that lacked an adsorption affinity. Although less than 6% of the total herbicide appli ed moved by facilitated transport, this amount of rapidly mobile pesticide could significantly increase the potential for ground water contamination. Results suggest that preventing the applied herbicide solution from drying before irrigation could reduce or eliminate the threat of facilitated trans port of napropamide by DOM. We suggest that a short irrigation period after herbicide application could allow for herbicide movement beyond the soil s urface to deter drying while preventing deep movement of pesticide with the water front.