Spatial and temporal trends in sediment contamination in Lake Ontario

A Lake Ontario sediment survey was conducted in 1998 to characterize spatial and temporal trends in contamination, and for comparison with data from previous surveys in order to assess any changes in environmental quality since the advent of measures to reduce contaminant sources. This survey was also designed to assist in tracing possible sources and vectors of contamination, and to identify areas where contamination exceeded Canadian sediment quality guidelines for the protection of aquatic biota. In addition, levels of a suite of eight metals were compared to pre-colonial concentrations, and surficial sediment enrichment factors were calculated. The highest levels of contaminants were observed at stations within the three major depositional basins; the spatial distributions across the individual basins were similar. Lake-wide averages for polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/PCDFs) were 100 ng/g and 111 pg/g TEQs, respectively. Concentrations of mercury and lead were observed to have decreased from 0.79 mug/g and 125 mug/g, respectively, in 1968 to 0.59,mug/g and 69 mug/g, respectively, in 1998. Exceedances of the Canadian Sediment Quality Probable Effect Level (PEL) guidelines were most numerous for arsenic (67%), PCDDs/PCDFs (58%), mercury (62%) and lead (38%). Concentrations of PCBs at all sampling stations were below the Canadian PEL of 277 ng/g.