Particle pathways of Niagara river water in Lake Ontario affecting bottom sediment contamination

Abstract

The results of integrated multidisciplinary studies of the transport and pathways of toxic contaminants entering Lake Ontario through the Niagara River are summarized. The factors controlling the transport and distribution of contaminants are: (i) Niagara River mixing characteristics; (ii) prevailing local wind conditions and large scale lake-wide circulation features, and (iii) suspended sediment load and the partitioning of the contaminants onto the suspended solids. Physical limnological studies included a variety of direct measurements to delineate the nearfield and farfield mixing characteristics of the river plume in terms of the prevailing winds and large scale lake circulation. In the ‘nearfield’ region of the plume, around the river mouth, contaminant transport was affected by the initial momentum and buoyancy of the river water mass and a sharp thermal front that is generally present through the stratified season. After the initial momentum is dissipated, the weakly buoyant river plume responds to the prevailing winds and lake-wide circulation. Adsorbed contaminants rapidly settle once the initial momentum of the river plume is dissipated but the dissolved contaminants are transported into the farfield regions in the lake and after becoming entrained in the strong south shore eastward flowing boundary current are carried toward the St. Lawrence River outflow. Only about ten percent of this eastward flowing transport exits via the St. Lawrence River, the remainder is recirculated back into the main lake. A contaminant transport model of the Niagara River Plume incorporating a hydrodynamic model, observed plume mixing characteristics and chemical partitioning was used to simulate the compartmental distribution (adsorbed vs dissolved) of selected toxic chemicals distribution observed in the nearfield region of the plume.