Dispersion in Unconsolidated Aquatic Sediments

Abstract

Inert tracer breakthrough experiments were used to parameterize hydrodynamic dispersion in undisturbed cores of surface sediment from lacustrine, estuarine, and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46 to 83%), mean grain size (10−5to 10−2cm), and sorting (0·48–1·26). As expected, hydrodynamic dispersion depends on the average longitudinal fluid flow velocity through the sediment plug. At linear flow velocities exceeding 10−4cm s−1, mechanical dispersion exceeds diffusion in all sediment cores studied. Compared to the classical studies on dispersion in sand columns, however, Peclet numbers based on particle size measurements do not provide a reliable guide for predicting the transition from molecular diffusion-dominated to mechanical dispersion-dominated flow regimes in the sediments. It is believed that the influence of pore structure on dispersion is much larger than that of particle size and that the characteristic pore lengths in the finest, highly porous sediments are orders of magnitude larger than the mean grain size. Aggregation, microlaminations, and a heterogeneous pore size distribution may all contribute to non-ideal flow conditions in the sediments. Tailing of the breakthrough curve occurred occasionally in fine grain sediment, signifying micro and macro scale dispersion and non-ideal flow behavior. Experiments showing significant non-ideal flow through the sediment plug were not used for calculation of hydrodynamic dispersion coefficient.