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 transpo rt models. The discrepancies have been attributed to preferential now of wa ter carrying pesticides via macropores in field soils. The same phenomenon may also be explained by transport facilitated by a carrier such as dissolv ed organic matter (DOM), A laboratory study was conducted to evaluate the t ransport of napropamide [2-(a-naphthoxy-N,N-diethylpropionamide] through so il via DOM, Soils were sieved, packed in columns, and treated at the surfac e with C-14-labeled napropamide, Water was applied to the columns by floodi ng and leachate was collected. It was found that C-14-labeled napropamide w as present in the first 0.22 cm of leachate. The C-14 and DOM concentration s were highest in the initial leachate and decreased with increasing leacha te. Napropamide concentration fell below detection at some depth in all col umns and recovery in the soil averaged 95% of the applied napropamide, Gas chromatographic analyses verified that C-14 activity in the leachate was as sociated 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 wa s not a result of preferential flow in our experiments but is due to DOM-fa cilitated transport. Thus, under field conditions rapid pesticide leaching could be the combined effects of preferential now and facilitated transport .