Sediment quality Atlantic Ocean watershed: chapter 3

Lake Saint-Louis

Located downstream from Lake Saint-François, Lake Saint-Louis has also seen a decline in mercury levels in surface sediments. Scientists have observed an average decline of 70% in mercury concentrations between 1985 and 2003. Levels are now below the most stringent quality criteria, except in one area in the southern part of the lake.

In 2003, in addition to mercury, sediments in Lake Saint-Louis contained polycyclic aromatic hydrocarbons (PAHs) and PCBs at concentrations exceeding the probable effects level (PEL). The southern part, i.e., the mouth of the Saint-Louis River, is the most heavily contaminated sector of the lake. The source of the contamination is likely a tributary, the Saint-Louis River, as well as industrial effluent discharged to the lake.

While most chemical contaminants appear to be declining over time, there are some that appear to be rising. They include arsenic, which has been detected in sediments in the northern part of the lake at twice the concentration present in 1985. The presence of arsenic may be due to the geology of the area.

As in Lake Saint-François, PBDEs and TBT were first detected in Lake Saint-Louis sediments in 2003. The highest PBDE concentrations are observed in the northern part of the lake and in the stretch of the Ottawa River, west of Île Perrot. The highest TBT concentrations are measured in sediments from the Dorval marina and from the Saint-Louis River marina in Beauharnois.