DIFFUSIVE TRANSPORT OF ORGANIC COLLOIDS FROM SEDIMENT BEDS

Organic colloids resulting from sediment diagenesis in porewaters are transported across the sediment-water interface via Brownian diffusion . The sediment selected for this study was from the University Lake, B aton Rouge, La. The effects of electrolytes on the sediment-to-water f lux of colloids varied depending on the type and concentration of the electrolyte. An ionic strength of 0.5 M reduced the flux of colloids; the effect being greater for KCl than for NaCl at the same ionic stren gth. The effects of different electrolytes (viz., NaCl, KCl, and CaCl2 ) on the porewater colloid concentrations were studied in batch experi ments. Below the ionic strength of 0.5 M, the electrolytes were effect ive in increasing the binding of the colloids to sediment surfaces, an d hence decreased their pore-water concentrations. Above the ionic str ength of 0.5 M, the amount of colloids released to the porewater was g reatest in the presence of CaCl2. The diffusive flux of colloids from sediment to water was modeled using an effective diffusivity as the si ngle adjustable parameter. The effective diffusivity (D-eff) was obtai ned using two complementary techniques. The first method utilized the sediment-to-water Bur of colloids, which was fit to a model to extract D-eff. For the second method, postmortem cores of the sediment were o btained after 82 d, the porewater colloids were extracted and analyzed . The porewater colloid profile as a function of sediment depth was us ed to obtain D-eff. The effects of sediment diagenesis on the porewate r colloids and the sediment total organic carbon (TOC) in a natural en vironment are illustrated using data from New Bedford Harbor (NBH), Ma ss, For this specific case only 0.43% of the organic matter delivered to the sediment was found to reappear in the water column as ''dissolv ed'' species.