R2 AM Contaminated Harbor and River Sediment
Thursday, 17 November 2005: 8:00 AM - 11:40 AM in Ballroom 2

(THI-1117-852650) Soluble contaminant release during bed sediment removal and resuspension-Chemodynamic model predictions.

thibodeaux, l¹, birdwell, j¹, reible, d², ¹ louisiana state university² university of texas-austin

ABSTRACT- Particles are placed in the water column by both natural and engineered processes. The numerical mass quantities typically suspended per mass extracted by dredges and the loadings observed following storm runoff flow events impacting streams will be reviewed. While in suspenstion a fraction of the particle-sorbed contaminant mass is solubilized. Numerous kinetic experiments involving a wide range of organic chemical types suggest that a bi-phasic rate process consisting of a fast and slow release fraction has been consistently observed by many investigators. These data sets including summaries of rate constants, soluble fractions, etc., will be presented and reviewed briefly prior to presenting the chemodynamic model developed for quantifying the release processes of dredges operating in flowing streams. A kinetics-based rather that the usual thermodynamics, chemical equilibrium-based model will be developed for forecasting water column concentrations need for estimating biota up-take quantities and other water quality parameters. The processes modeled using a mass balance approach include: particle generation, particle settling, fast and slow solubilization from particles, solubilization from the bed sediment bottom layer and evaporation to air for assumed plug-flow stream hydraulics. For steady-state dredging rate a key model output is concentration in water vs distance downstream. Typical results for mechanical and hydraulic dredges operating at a well characterized site containing PCBs(Aroclor 1242)will be presented. Elevated suspended solids levels of 6.0 to 20 mg/litre decreased exponentially but low levels remained up to 4 days. The total, sorbed plus souble, PCB concentration decreased similarly from initial values of 1.3 to 2.6 ug/litre. Interestingly, the soluble concentration was highest well downstream the dredge site. Levels of 1.5 to 1.9 ug/litre above the equilibrium ones were predicted to occur 5 to 15 hours flow-time or at 0.6 to 1.7km distance downstream. Although site specific these results should be of interest to those designing dredging operations for minimizing biota and related water quality impacts. Steady-state concentration levels were predicted to occur at 4 to 5 days flow-time or 11 to 14km downstream.

Key words: dredge design for remediation, chemical fate and transport, PCB behavior in aquatic streams, contaminant solubilization in water