Sediment-Water Fluxes of Hydrophobic Organic Chemicals Due to Molecular Diffusion

The flux of hydrophobic organic chemicals (HOCs) between the bottom sediments and overlying water is primarily due to sediment erosion/deposition, molecular diffusion, and bioturbation. All of these processes are continuously and often significantly modified by the finite rate of adsorption and desorption of the HOC between the solid particles in the sediments and the surrounding pore water. Molecular diffusion can be a significant contributor to the overall flux, is always present, and modifies all of the other flux processes. In order to investigate the flux of HOCs due to molecular diffusion as modified by sorption, experiments have been conducted in two different sediments with eight HOCs which have a range of partition coefficients, K_p, from 5 x 10⁴ to 10 L/kg. Lengths of the experiments ranged from 32 to 256 days. For large K_p's, the results demonstrate that (1) significant changes in HOC concentrations are limited to a few millimeters near the sediment-water interface, (2) measured values of the HOC concentration are generally much less than their equilibrium value, and (3) the flux depends on K_p and is much greater than that for a non-sorbing chemical or for a sorbing chemical with equilibrium sorption assumed. As K_p decreases, the results reduce to those expected for pure molecular diffusion without sorption. A theoretical model which includes non-equilibrium sorption has been developed and is uniformly valid for all HOCs tested. Good agreement between the numerical calculations, asymptotic approximations, and experimental results is obtained.