Rich Jepsen

Effects of Gas Generation on the Density and Erosion of Sediments from the Grand River

Abstract Measurements were made of the erosion rates and bulk properties of sediments from eleven sites in the Grand River in Michigan. High concentrations of gas, up to 5% by volume, were present in the sediments; this significantly affected the sediment bulk densities and erosion rates. Sediments from one of these sites were reconstructed (thoroughly mixed and then allowed to consolidate) in the laboratory where the time-dependent effects of gas generation on the densities and erosion rates of these sediments were quantified. In consolidation studies, the density was a function of depth, time, and temperature; it increased because of pore waters moving up and out of the solid-water matrix and decreased because of gas generation and movement. At higher temperatures, the rate of gas generation and the gas volume were greater and the bulk density was lower than at the lower temperatures. For the same bulk properties, temperatures, and shear stresses, erosion rates for the field and reconstructed sediments were the same. For reconstructed sediments at approximately 20°C, the effects of gas were to decrease the sediment densities by up to 10%, to increase the erosion rates by as much as a factor of sixty, and to decrease the critical shear stress for erosion by as much as a factor of twenty compared to sediments with no gas present. At lower temperatures, these effects decreased significantly.

Parameters Affecting the Adsorption of PCBs to Suspended Sediments from the Detroit River

Abstract In previous work, the adsorption and partitioning of a hydrophobic organic chemical, hexachlorobenzene, to sediments from the Detroit River were investigated by means of long-term (up to 6 months) batch mixing experiments. Effects on the adsorption and partitioning due to colloids from the water and from the sediments, the flocculation of these colloids and of the sedimentary particles, particle size, and the amount of organic matter were determined. In the present experiments, these investigations have been extended to three PCB congeners: a monochlorobiphenyl, a dichlorobiphenyl, and a hexachlorobiphenyl. Long-term experiments were done in order to be certain that a steady state was always attained. From this steady state, a unique equilibrium partition coefficient was determined for each congener which was independent of sediment concentration. The parameters on which these partition coefficients and the adsorption rates depended were also determined. For hexachlorobenzene and the three PCB congeners, the time to steady state increased as the value of the equilibrium partition coefficient increased. For each of these same four chemicals over a wide range of dissolved chemical concentrations, the amount of chemical sorbed to the sediments was linearly dependent on the amount of chemical dissolved, and therefore the sorption isotherm for each chemical is linear over this range of chemical concentrations. The slow adsorption rates that were demonstrated can significantly affect the sediment-water fluxes of contaminants and the sediment toxicity as compared with the assumption of equilibrium partitioning.