Monitoring Shows “Cap” Functioning Well


Information about environmental monitoring completed in Peninsula Harbour was presented to the public at a lunchtime meeting at Marathon Town Hall on October 19th. Peninsula Harbour is the main harbour in the Town of Marathon, Ontario. One year post capping data related to the thin-layer cap for contaminated sediment was presented.

A 23 hectare thin-layer cap was put in place in 2012 in the area of highest harbour contamination. The cost of this cap was $7 million, shared by the federal and provincial governments and industry formerly located at the site.

Representatives of Environment and Climate Change Canada (ECCC), the Ontario Ministry of Natural Resources and Forestry (MNRF), the Ontario Ministry of the Environment and Climate Change (MOECC) and Lakehead University were in attendance.

2013 data showed that the cap was not moving and that there was no movement of contaminants up through the cap. Underwater video taken as part of the monitoring process showed that aquatic vegetation was beginning to colonize the cap. Collection of more substantive monitoring data will be completed in 2017.

Peninsula Harbour was contaminated by mercury used in the former chlor-alkali plant located beside the Marathon mill. This chlor-alkali plant produced chlorine for the bleaching of paper. Harbour PCB levels were also elevated.

Rationale for construction of the cap was not to “lock off” the contamination. Sand and coarser material was used which would actually mix with the top few millimeters of contaminated sediment. Rather, placement of the 15 to 20 cm. thick cap on the harbour floor was meant to speed up and help the natural process of sedimentation in the clear, low sediment waters of Superior. The cap construction started in July and was completed in August of 2012. The medium and coarse grade sand (more like gravel) placed on the contaminated harbour floor reduced the exposure of aquatic organisms to mercury and PCBs promoting a healthier ecosystem.

Public questions and comments made during the presentation included the following:

  • The boat launch area should be checked for cap sand to ensure this sand is not migrating to the area of the boat launch
  • Fish in Peninsula Harbour should be checked for contamination. In response to this comment, RAP government agency representatives noted that such monitoring is done on an ongoing basis. The Guide to Eating Ontario Fish is available here.
  • Signage noting where the Guide to Eating Ontario Fish is available should be put in place at the boat launch
  • How much sediment from the surrounding harbour area will end up on the cap? ECCC personnel responded to this question by saying that some contaminated sediment from outside the capped area would be deposited on the cap, and that some fine sediment in the capping material released during the capping activity may have also been deposited on the cap.
  • A couple of recreational vessels are mooring in the area of the cap. Mooring on the cap should be prevented by signage and outreach.
  • Please make the recently published Lake Superior Action and Management Plan more widely available.

Peninsula Harbour was identified as a Great Lakes Area of Concern requiring a Remedial Action Plan, or cleanup plan, in 1987. Problems identified include the following:

-bacterial contamination
-aesthetic impairment
-degraded fish and benthic communities
-high levels of toxic contamination (i.e. mercury and PBCs) in fish and bottom sediments.






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