River-bed sediments are active zones for pesticide deposition and subsequent movement by di †usion, mass transport and sorption to solids. The aim of this work was to investigate the importance of di †usion as a means of pesticide movement. In laboratory experiments, simazine and lindane were introduced to well-mixed aqueous solutions overlying two di †erent river sediments. Sediment cores were sectioned horizontally and analyzed for pesticide content by supercritical Ñuid extraction. Experiments were used to determine sorption isotherms of the compounds to suspended sediments at 10¡C. Vertical proÐles of the pesticides in the sediments showed that the compounds reached a maximum depth of 89 mm over a period of 37 days.
A mathematical model was developed to describe pesticide transport by di †usion within the sediment porewaters and sediment sorbed phases, taking into account sorption of the compounds to sediment particles. E †ective di †usion coef-Ðcients ((0.5È1.6) ] 10~10 m2 s~1) were obtained for simazine and lindane in the characterized sediments. These were used to calculate values for di †usion in the dissolved phase (0.38 ] 10~10 and 6É16 ] 10~10 m2 s~1 for simazine and lindane respectively) and di †usion in the sorbed phase (0.39 ] 10~10 m2 s~1 for simazine and negligible for lindane). Sorption onto the sediment signiÐcantly inÑuenced the rate of penetration of the compounds into the sediment ; thus although lindane had a larger e †ective di †usion coefficient than simazine, its larger sorption affinity and negligible di †usion in the sorbed phase led to less penetration into the sediment.