Transport of hydrophobic organic compounds by colloids through porous media 3. Diffusion from sediment porewater to overlying water in laboratory microcosms

Abstract

The diffusion of colloidal organic carbon from the porewater of a local sediment University Lake, Baton Rouge, LA) was studied in laboratory microcosms. The effects of porewater ionic strength on the flux of dissolved organic carbon (DOC) was studied using two different electrolytes, namely NaCl and CaCl 2. The sediment porewater DOC concentration frofiles 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 in porewaters. The D e values increased slightly in the presence of electrolytes in the porewaters (0.030 cm 2 d −1 for 1 M CaCl 2 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 larger 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 different regions and during different seasons. The effect of ionic strength on the depletion of a hydrophobic organic contaminant (namely pyrene) from the sediment by diffusion was found to be minimal under the conditions of these experiments.