Model simulations of upwelling of Persistent Bioaccumulative Toxic (PBT) organic pollutants from contaminated bed sediments for base flow events

In river systems, streambed sediments have been recognized as an important factor in the transport of contaminants since they have a strong capacity to adsorb contaminants such as pesticides, nutrients, toxic hydrocarbons and heavy metals. A thorough understanding of the contaminant transport processes occurring between the streambed and stream water is very important in predicting the fate of pollutants in rivers. The focus of this study was the release of Persistent Bioaccumulative Toxic (PBT) organic pollutants from contaminated streambed sediments into clean waters, i.e., upwelling, under low-flow stream conditions and the corresponding biological effects. A multi-phase reactive transport model which accounts for the physical exchange process occurring between the streams and streambeds and physicochemical processes like sorption/desorption was developed. This model simulates the release of PBT organic pollutants from contaminated streambed sediment and the biouptake of PBT organic pollutants by benthic organisms in the stream and streambed sediments under low flow rates. The advective pumping theory was used to model the physical exchange process. PBT organic pollutant's sorption/desorption was modeled using a two-site nonlinear equilibrium/kinetic model and a two-stage desorption model. A bioconcentration factor (BCF) equation was used to model biouptake of PBT organic pollutants by benthic organisms. Due to the small amount of organism used, it is assumed that the biouptake in the system will not affect significantly the contaminant concentration distributions during transport. Simulation results indicate an increase in the stream water contaminant concentration due to the release of contaminant from streambed and a decrease of the stream water contaminant concentration due to the adsorption of contaminant to the streambed sediments. Stream water contaminant concentration increases rapidly due to a high desorption of contaminants from the streambed and low or no re-adsorption. Meanwhile, a high re-adsorption results in reduced release of contaminant from the streambed into the stream even when a high desorption is used. Therefore, desorption and re-adsorption processes are both controlling the release of contaminant from the streambed. This study contributes to the understanding and predicting the transport and biouptake of PBT organic pollutants present in streambed sediments of natural streams.