DESORPTION DURING RESUSPENSION EVENTS - KINETIC V EQUILIBRIUM-MODEL

The spills and release of hydrophobic organic chemicals (HOCs) into wa terbodies have resulted in the contamination of bottom sediments. When these sediments are resuspended by runoff events or by dredging, part iculate-phase contaminants desorb to the water phase. Equilibrium betw een the particulate phase and the dissolved phase is usually assumed f or most modelling applications regardless of time scale. For well mixe d systems in which chemical transport is not limited by mass transfer between the bulk water phase and sediment aggregates, an intraparticle diffusion model can be applied to estimate the time required for vari ous HOCs to reach an equilibrium state between dissolved and particula te-phase concentrations. Calculations from this model show that total equilibrium time scales cover a wide range, from less than a day to a few hundred days. This study compares predicted suspension times for s ediments of various sizes with expected equilibration times for desorp tion. This comparison indicates that the equilibrium assumption is not valid for a wide range of parameter values typical of natural systems . In particular, compounds with high partition coefficients, greater t han about 10(4) mL g(-1), will have minimum equilibration times of at least one to ten days. This is likely to be greater than the expected resuspension time.