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Water Chemistry

EPA monitors the water quality of the Great Lakes throughout the water column in accordance with the Great Lakes Water Quality Agreement. Surveys of water quality are conducted within each of the Great Lakes every spring and summer. EPA began monitoring the waters of Lake Michigan, Lake Huron and Lake Erie in 1983. Lake Ontario was added in 1985, followed by Lake Superior in 1992.

The Lake Guardian, EPA’s research vessel, collects samples throughout the water column from eight to twenty sites located in each lake. The sites are located in the offshore waters, avoiding areas that are highly influenced by rapid changes in water quality to better measure the long-term condition of the lakes.


  • A rosette sampler collects water samples through the water column from the surface to the lake bottom
  • Several samples are collected at various depths
  • Samples are analyzed on-board the vessel for pH, conductivity, alkalinity, hardness, turbidity, and dissolved reactive phosphorus
  • To understand the health of the Great Lakes, samples are preserved and later analyzed at a laboratory for water quality parameters for phosphorus, nitrogen, chloride, silica and metal concentrations
  • Data from analysis are available within 1-2 years from collection
  • New and updated data analysis is performed on an annual basis for long-term trends in changes to water quality parameters