The Effects of Cohesive Particles on the Resuspension of Sediment

Raquel Escatel, Dr. James Bonner*, Michael Sterling, Dr. Cheryl Page, and Temitope Ojo

           Over the past twenty years, there has been increased attention directed to how water pollutants interact with solid matter.  Today modeling of both suspended and bottom sediment have been heightened by the recognition that many toxic substances associate with solid matter. Solids can be classified as either organic or inorganic particles, both of which are laterally carried by water currents.  The time it takes for the particles to settle to the bottom of the water source depends on the magnitude and frequency of the currents that they are exposed to.  Although a portion will remain permanently at the bottom, solids can be reintroduced into the water by turbulence.  This process is known as resuspension and its occurrence is due to strong current and wind-mixing. The lab experiments performed were conducted to determine the critical shear stress of clay and silica.  These sediments are found in natural bodies of water and due to their large surface area and cohesiveness are responsible for the reintroduction of contaminants to the water surface.  For cohesive sediments from both lakes and oceans, experimental work has determined that the resuspension rate and the total amount of sediment that can be resuspended at a particular stress are functions of (a) the turbulent stress at the sediment-water interface and (b) the time after deposition (Gailanai et al.).  Lab experiments were set up a manner such that the shear stress to which the sediment was exposed could be varied from 20 revolutions per minutes to 50 revolutions per minute.  The sediment was packed to the bottom of a 40 liter reactor vessel to form a benthic layer with a height of approximately ¼ inch. De-ionized water was then added to the reactor vessel and then left undisturbed for at least one day, allowing for the sediment to completely settle.  After mixing for two hours, samples were collected at two locations of the reactor.  The particles in the samples were analyzed using the particle size distributions and grametric weight analysis. When running the experiment with a concentration of 600 ml of sediment, the Total Suspended Solids (TSS) of clay and silica at time zero were almost equal.  Throughout the experiment, an increase in mixing speed was followed by an increase in TSS.  When this mixing speed was kept constant for a total time of twenty-five minutes, the amount of resuspended clay increased for an average of ten minutes then began to decline until the mixer speed was amplified.  This decrease in TSS is attributed to the cohesive characteristic of clay responsible for the flocculation of particles, hence their increase in size and mass resulting in the settling of the sediment.  Although the size and mass of silica particles also increased throughout the experiment, the change was not as dramatic as that of clay resulting on a steady amount of TSS at a given mixing speed.

*Conrad Blucher Institute, Texas A&M University-Corpus Christi

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