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(614) A direct soluble release process for contaminants in stable streambed sediments-field data and theory.
Thibodeaux, Louis*,1, Libbers, Paul1, Turner, Carrie2, Erickson, Michael3, 1 Louisiana State University, Baton Rouge, LA2 LTI Environmental Engineering, Ann Arbor, MI3 Blasland, Bouck and Lee, Ann Arbor, MI
ABSTRACT- Most of the mass of hydrophobic organic chemicals and heavy metals that reside in sediment beds are sequestered onto the fine solid particles. Increased water velocities, dredging activities, decommissioning of dams, watervessel traffic and other such bed disturbances, places particles into the water columnn where desorption partitioning favors mass transport to the aqueous phase and subsequent downstream movement. The resulting particles retains little of their loosely-bound contaminant mass and significantly more of their tightly-bound mass. These cleaner particles are eventually redeposited downstream. Most contaminated sediment sites of environmental significance have been stable entities for decades and in some cases for centuries. In the absence of dredging these episodic resuspension events are few and far between. During these events the entrained-particle release process is rapid and dominant. Data will be presented to show that at many contaminated sediment sites a slower, non-particle resuspension release process exist and that it may be the most significant one. Less particle agressive but otherwise rapid chemical release processes that occur within and on stable bed sediments will be described. Although these operate at slower rates than the resuspension ones they do so on a continuous time basis whereas the resuspension driven ones are episodic and of short time durations. If these slower in-bed soluble release processes are in fact significant as they appear to be then they are a dominant factor in the natural recovery rate of streams and need to be considered when the natural recovery remediation option is selected. In addition, key chemical process elements in riverine fate and transport models will need to be reformulated. To this end the presentation will include a theoretical structure and model algorithms for the soluble release process. Bioturbation enhanced diffusion appears to be a likely suspect in the suite of candidate processes. Data on the kinetic paramerers reflecting the direct-from-the-bed soluble release process observed on several streams will be presented along with other information. Theoretical model simulations and corresponding field measurements of representative release kinetic mass transfer coefficients will be presented as well.
Key words: stable bed sediments , soluble release, contaminant transport, bioturbation
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