Coupled transport and photodegradation of napropamide in soils undergoing evaporation from a shallow water table

Photolysis in soils is effectively limited to the top 0.5 mm of the soil surface. Consequently, processes that move contaminants to this sunlight-irradiated zone may have a significant impact on the degree of photolysis in soils. The effect of upward transport in translocating napropamide [2-(alpha-naphthoxy)-N,N-diethylpropionamide] into the photic zone was examined using an experimental system in which soils were supplied with water at their base. Sunlight photolysis rates were much greater than predicted if photolysis had been limited to the soil surface, with up to 70% lost in a 3 cm deep loamy sand soil after 14 days of irradiation as compared to dark controls. Rates of loss were slower in a sandy clay loam, with 29% loss over 14 days. Rates of transport were affected by sorption, with measured values of the distribution coefficient K-d Of 0.4-0.72 and 6.6-7.2 mL/g in loamy sand and sandy clay loam soils, respectively.