Exchange of DDE and DDE associated fine suspended sediments between streams and streambeds

Stream-subsurface exchange has important implications for contaminant transport in streams because it can cause the retention of contaminants in the streambed and also has the potential to enhance the release of contaminants from the streambed. Hydrophobic organic contaminants (HOCs), such as dichloro-diphenyl-dichloroethane (DDE) have the potential to strongly adsorb onto sediment particles and their release from contaminated sediment can be influenced by factors such as changes in water chemistry, and by the presence of high molecular weight organic substances known as dissolved organic carbon (DOC). Thus, to accurately determine the fate and transport of HOCs and the ecological risks involved, it is essential to understand their fluxes between sediments and overlaying water under varying stream conditions.;In this study, laboratory batch and flume experiments were conducted to demonstrate the DDE transport behavior under varying stream conditions including pH, ionic strength, and in the presence of DOC. DDE transport from stream water to streambed and DDE transport from streambed into stream water were examined under a defined physical (the presence of streambed topography) condition. Fundamental process-based multiphase stream-subsurface exchange models were applied to interpret the flume experimental results.;Results showed that changes in water chemistry influenced the release of p,p'- DDE from contaminated sediment and associations between p,p'-DDE and DOC. Decreasing pH decreased p,p'-DDE -DOC associations and enhanced the release of p,p' -DDE from contaminated sediment in the batch experiments; however the effect of pH was undistinguishable in the flume experiments. Increases in ionic strength increased p,p'-DDE -DOC associations and led to a decrease in p,p'-DDE release from the contaminated sediment, which was observed in both the batch and flume experiments. The observed p,p'-DDE exchange was successfully predicted by the process-based models, thus indicating that these models were able to properly represent the physical and chemical processes controlling the transport of p,p'-DDE in stream systems. This study contributes to the mechanistic understanding of HOC transport and the factors governing HOC transport in river systems, and to the development of reliable, predictive models for the assessment of impacted streams.