Facilitated Transport of Napropamide by Dissolved Organic Matter Through Soil Columns

Contamination of groundwater by pesticide percolation is of great concern. Field observations have revealed that some pesticides move deeper into the soil profile than would be expected from predictions made by solute transport models. The discrepancies have been attributed to preferential flow of water carrying pesticides via macropores in field soils. The same phenomenon may also be explained by transport facilitated by a carrier such as dissolved organic matter (DOM). A laboratory study was conducted to evaluate the transport of napropamide [2-(a-naphthoxy-N,N-diethylpropionamide] through soil via DOM. Soils were sieved, packed in columns, and treated at the surface with [sup.14]C-labeled napropamide. Water was applied to the columns by flooding and leachate was collected. It was found that [sup.14]C-labeled napropamide was present in the first 0.22 cm of leachate. The [sup.14]C and DOM concentrations were highest in the initial leachate and decreased with increasing leachate. Napropamide concentration fell below detection at some depth in all columns and recovery in the soil averaged 95% of the applied napropamide. Gas chromatographic analyses verified that [sup.14]C activity in the leachate was associated with napropamide. A dialysis equilibrium technique determined that 17 to 56% of the napropamide in the leachate was retained inside a 500-Da dialysis membrane. The rapid leaching of a small fraction of napropamide was not a result of preferential flow in our experiments but is due to DOM-facilitated transport. Thus, under field conditions rapid pesticide leaching could be the combined effects of preferential flow and facilitated transport.