Effects of sorption kinetics on the fate and transport of pharmaceuticals in estuaries.

Many current fate and transport models based on the assumption of instantaneous sorption equilibrium of contaminants in the water column may not be valid for certain pharmaceuticals with long times to reach sorption equilibrium. In this study, a sorption kinetics model was developed and incorporated into a water quality model for the Patuxent River Estuary to evaluate the effect of sorption kinetics. Model results indicate that the assumption of instantaneous sorption equilibrium results in significant under-prediction of water column concentrations for some pharmaceuticals. The relative difference between predicted concentrations for the instantaneous versus kinetic approach is as large as 150% at upstream locations in the Patuxent Estuary. At downstream locations, where sorption processes have had sufficient time to reach equilibrium, the relative difference decreases to roughly 25%. This indicates that sorption kinetics affect a model's ability to capture accumulation of pharmaceuticals into riverbeds and the transport of pharmaceuticals in estuaries. These results offer strong evidence that chemicals are not removed from the water column as rapidly as has been assumed on the basis of equilibrium-based analyses. The findings are applicable not only for pharmaceutical compounds, but also for diverse contaminants that reach sorption equilibrium slowly.