TRANSPORT OF HYDROPHOBIC ORGANIC-COMPOUNDS BY COLLOIDS THROUGH POROUS-MEDIA .3. DIFFUSION FROM SEDIMENT POREWATER TO OVERLYING WATER IN LABORATORY MICROCOSMS

The diffusion of colloidal organic carbon from the porewater of a loca l sediment (University Lake, Baton Rouge, LA) was studied in laborator y microcosms. The effects of porewater ionic strength on the flux of d issolved organic carbon (DOG) was studied using two different electrol ytes, namely NaCl and CaCl2. The sediment porewater DOC concentration profiles were used to obtain the effective diffusivities (D-e) in the sediment. The values of D-e (0.019 and 0.021 cm(2)d(-1)) obtained for the sediment sample used in this work indicated low colloid mobility i n porewaters. The D, values increased slightly in the presence of elec trolytes in the porewaters (0.030 cm(2) d(-1) for 1 M CaCl2 and 0.023 cm(2) d(-1) for 0.5 M NaCl). The porewater concentration of colloidal DOC was consistently low in the presence of electrolytes, indicating l arger adsorption to the sediment solids. The D-e values were found to range from 0.019 to 0.103 cm(2) d(-1) for sediments collected from dif ferent regions and during different seasons. The effect of ionic stren gth on the depletion of a hydrophobic organic contaminant (namely pyre ne) from the sediment by diffusion was found to be minimal under the c onditions of these experiments. (C) 1997 Elsevier Science B.V.