Laboratory Experiments on Mud Flocculation Dynamics in the Fluvial and Estuarine Environments
Due to the flocculation process, suspended mud aggregates carried by rivers and streams can undergo changes in their size, shape, and settling velocity in response to environmental drivers such as turbulence, sediment concentration, organic matter (OM), and salinity. Some have assumed that salt is necessary for floc formation, and that mud, therefore, reaches the estuary unflocculated. Yet mud flocs exist in freshwater systems long before the estuarine zone, likely due to the presence of OM and ions in the water that facilitate binding and aggregation of mud particles. This research aimed to examine the flocculation state of mud over the fluvial as well as fluvial to marine transition (FtMT) zones of the Mississippi River basin and how salinity, or the ion concentration of water, and organic matter independently and together affect flocculation. Suspended mud was found to be mostly flocculated in the headwaters of the Mississippi River in southwest Virginia, USA. However, increasing the ion concentration of water samples to levels measured following winter storms changed the size distribution of suspended particles, led to more of the mud existing in large flocs, and resulted in an overall increase in average size by about 40%, thereby increasing the settling rate of the mud relative to the suspensions without salt. These results suggested that potential negative effects of road salts on mud deposition should be investigated further. Additional experiments were used to examine the flocculation of a natural mud sample with and without OM showed that the rate of floc growth and equilibrium size both increase with salinity regardless of the presence or absence of OM. However, the response of both to salinity was stronger when OM was present. In deionized water, natural sediment with OM was seen to produce large flocs. However, the size distribution of the suspension tended to be bimodal. With the addition of salt, increasing amounts of unflocculated material became bound within flocs, producing a more unimodal size distribution. Here, the enhancing effects of salt were noticeable at even 0.5 ppt, and increases in salinity past 3 to 5 ppt only marginally increased the floc growth rate and final size. A salinity-dependent model to account for changes in floc growth rate and equilibrium size was presented. Laboratory experiments on the sediment suspended in the lower reaches of the Mississippi River were used to provide further insight on the mud flocs behavior in the FtMT. Turbulence shear rate, a proxy for the river hydrodynamics, was found to be the most influential factor in mud floc size. While artificial increase in salinity by adding of salts did not lead to considerable increase in floc size, addition of water collected from the Gulf of Mexico enhanced the flocculation. These effects were speculated to originate from the biomatter composition of the Gulf water, particularly where the nutrient-rich Mississippi River water reaches the marine water.