The 2012 Great Lakes Water Quality Agreement states that “the Waters of the Great Lakes should be free from the harmful impact of contaminated groundwater.”
The Groundwater indicator status is assessed as Good based on chloride and nitrate data, however, the trend is Undetermined due to insufficient long-term data. The concentrations of nitrate in groundwater are primarily from agricultural practices. Chloride is mainly from the urban use of road deicing salt. Elevated concentrations of both of these constituents in water can have detrimental impacts to ground- and surface water quality, aquatic ecosystems, and human health. Groundwater plays an important role as a reservoir of water that, if contaminated, has the potential to become a source of contamination to the Great Lakes.
The assessment is based on data obtained from over 6,550 shallow wells throughout the Great Lakes basin. These data were used to assess the status of tributary watersheds which are incorporated into each lake assessment and an overall Great Lakes assessment.
Groundwater quality is assessed as Good in all individual Great Lake basins except Lake Ontario where it is assessed as Fair. Monitoring results indicate that chloride concentrations are generally highest in developed areas, while nitrate concentrations are generally highest in areas with intense agricultural land use. Sites with groundwater contamination do exist within the Great Lakes basin and these locations are being actively investigated and remediated by environmental agencies. Currently trends are Undetermined due to a lack of ongoing and consistent monitoring. A better understanding of the concentrations and fluxes of various contaminants (including, but not limited to nitrate and chloride) in groundwater discharging to nearshore areas, and their impacts on aquatic life, is also important.