RELEASE RATES OF CHLORINATED HYDROCARBONS FROM CONTAMINATED SEDIMENTS

The release rates of chlorinated hydrocarbons from contaminated intertidal sediments were measured with an apparatus that consists of a microcosm with calibrated flow regime, a continuous counterflow liquid-liquid extractor, and an aerator column. Sediment-water partition coefficients were also determined. A set of design criteria was developed and used to asses the validity of the flux measurements. The release rates of halogenated hydrocarbons (octanol-water partition coefficient 10(4) to 10(7)) conform to a diffusion model with an effective diffusion coefficient of the order of 1 to 4.10(-8) m2.s-1, 40 times higher than the estimated molecular diffusion coefficient. It is difficult to understand why the effective diffusion coefficient is so high. Using a numerical transport model that assesses the effect of pore water flushing, we have ruled out the possibility that the result is due to pore water irrigation by the secondary flow that exists in the microcosm, but more work will be needed to understand the transport mechanism. The magnitude of the fluxes suggests that contaminated sediments, such as those used in this experiments, may act as weak but persistent sources of chlorinated hydrocarbons to the water column.