The flow of water through cohesive clay sediments

The cohesive sediments existing in water bodies such as estuaries and lakes are highly porous with porosities that often exceed 0.9. These sediments are significant repositories for contaminants such as organics and heavy metals as well as nutrients. Research on solute transport across the sediment-water interface often treats the transport as a diffusion process. However, because of the large porosity of sediments, advective water flow through sediments could be significant. This study experimentally determined the permeability of uncompacted cohesive clay sediments under hydraulic gradients typical of field conditions and found that the permeabilities were high enough that the advective transport of solutes could be one or two orders of magnitude larger than diffusive transport, and that the permeabilities were predominately determined by the development of preferential flow network in the sediment.; Pressure driven constant flow experiments were designed and conducted to measure the permeability of freely settled clay aggregate sediments under very low flows. Two kinds of clay--kaolinite and montmorillonite, two kinds of salt water solution--NaCl and CaCl{dollar}sb2,{dollar} and two salt concentrations--0.01M and 1.00M were used in the experiments. The permeability of clay sediments was found to range from 0.01 to 100 darcy, and the median of experimental values was 1 darcy for a range in an approach velocities of 0.1 to 7 cm/hr. The largest clay sediment permeability reported in the literature is only 0.01 darcy. The permeability of clay aggregate sediment does not depend on clay mineralogy or water chemistry, unlike compacted clay systems.; A compressible porous media computer model was developed to study the pressure drop evolution during constant water flow through sediments. The model provides the theoretical basis for the observed multilayer sediment headloss evolution curves and the determination of the two phase compressions where the permeability of clay sediment could be directly calculated from Phase II headloss data.; An idealized steady state preferential flow tube model was developed to estimate the size and number of tubes from measured headloss and flow rate data. The critical shear strength of clay sediment controlled the number of the tubes. The presence of cracks affects the pressure field in porous media and the further development of preferential flow network. These effects were analyzed by a model of two dimensional porous media with cracks. The pressures from different locations at a horizontal cross section could be quite different, which echoes the experimentally observed phenomena. Cracks develop and branch at their tips where the pressure gradient is the highest. They develop with an accelerated pace due to the increasing pressure gradient at the tip.; The advective water exchange across sediment-water interface under progressive surface water waves was estimated using measured clay sediment permeabilities. Advective flow is significant in transporting solutes compared to diffusion. The ratio of the advective to diffusive transport mechanisms for the top 30 cm of a cohesive sediment under a progressive surface water wave was found to be on the order of 10 to 100, indicating the importance of advective transport.