The effect of divalent metals and laminar shear on the formation of large freshwater aggregates

Aquatic sediment from Hamilton Harbor were suspended under controlled Couette shear to measure the changes in particle size distribution when the bulk concentration of divalent cations Cd²⁺, Cu²⁺, Ni²⁺ and Zn²⁺ was increased 500 ppb above ambient values. The size distribution of particles followed a bimodal distribution, at diameters of 20 and 200 m, and was modeled with a curvilinear collision model, using a logarithmic size scale to compensate for the decreasing density of larger aggregates. Although collision frequencies decreased with particle size, there was a limit (160 m) above which shear no longer affected collision. Addition of divalent metals caused formation of non-porous large aggregates greater than 300 m, at shears lower than 3 dynes cm^–2. The sharp increase in aggregate volume that resulted from metal addition indicated that a partitioning threshold exists in the harbor, coinciding with an imaginary line along the shore, where wind driven agitation causes a bottom shear of 3 dynes cm^–2. This threshold can be visualized as the area near shore where bottom sediments consist of sands with nominal size greater than 250 m. Calculations, using Stoke's settling, predict settling of large aggregates near thermocline depth, coincident with the appearance of fine clays on the sediment surface.