Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans associated with settling particles in Lake Ontario.

Sediment traps were deployed at seven sites in the western and central basins of Lake Ontario for calculation of concentrations and down fluxes for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) to assess ongoing loadings to Lake Ontario from the Niagara River watershed. Traps were deployed at multiple depths (beginning at 20 m) during two seasonal time periods at stations impacted by the outflow of the Niagara River, and stations reflecting deeper water offshore conditions. Settling particles were collected seasonally to assess the influence of physical characteristics of the water column, i.e., isothermal conditions vs. stratified conditions, on concentrations and fluxes of PCDD/Fs. At all stations and for all depth intervals, PCDD/F concentrations were higher in the winter sampling period (range of 3120-10,600 pg g), compared to the spring - summer - fall time period (range of 320-6900 pg g). These results indicated bottom sediments in central and western Lake Ontario were more highly-contaminated, compared to contemporary particulate material entering the lake via the Niagara River or resulting from shoreline erosion. However, assessment of PCDD/F congener profiles and ratios also indicated source areas within the Niagara River watershed continued to episodically contribute loadings to Lake Ontario. The results also indicated changes in discharges of PCDD/Fs from sources in the Niagara River result in changes in congener profiles in settling particles, which can be detected by continued monitoring.