Estimating the Potential for Facilitated Transport of Napropamide by Dissolved Organic Matter

Dissolved organic matter (DOM) has been found to significantly affect the soil sorption/desorption of napropamide [2-(a-naphthoxy-N, N-diethylpropionamide] and to enhance its transport through soil columns. A method to qualitatively predict if DOM will enhance the transport of napropamide based on easily determined sorptive characteristics is investigated. Batch sorption experiments were specifically designed to simulate field application of napropamide. The formation of a napropamide–DOM complex was enhanced by pretreating soils with pesticide and allowing the carrier solvent to evaporate prior to preparing a water extract of the soil (K1). A second adsorption coefficient was measured by equilibrating K1 with untreated soil (K2). Column transport studies using the same soils as the batch equilibrium studies were performed. After allowing napropamide to dry, columns were leached. Initial leachate from all soils contained napropamide indicating that the collected napropamide had been transported through the column without adsorption. However, the amount leached was dependant on soil type and in general soils with higher organic matter content had higher concentrations of napropamide in the initial 30 mL of leachate when compared with lower organic matter (OM) soils. The ratio of the K1 and K2 solutions from the batch equilibrium studies were found to be positively correlated with the mass of napropamide in the initial leachate. Therefore the ratio of K1 to K2 provided a better qualitative predictor of the potential for facilitated transport of napropmamide than soil OM content. MOST AGRICULTURALLY APPLIED pesticides are slightly soluble organic compounds that potentially degrade the environment. They can be transported by runoff from treated fields and appear in surface water bodies or they may be carried by water flowing through the soil profile and degrade ground waters. The focus of this paper will be on the potential for ground water degradation by surface applied napropamide, and the development of a qualitative predictor for the facilitated transport of this pesticide by DOM. The transport of pesticides through soil by flowing water is retarded by adsorption to the soil. The retardation factor (R), defined as the ratio of the rate of pesticide transport to the rate of water flow, is related to the adsorption coefficient by the equation