Ground water quality sub-indicator report

The overall status of groundwater quality in the Great Lakes Basin is assessed as “Good” (Figure 1). For the assessed fraction of the basin (84% of the total area), the groundwater quality is “Good” in 58% of the area, “Fair” in 41% of the area, and “Poor” in 1% of the area, resulting in an overall assessment of “Good”. The portions of the basin that have insufficient data (16% percent of the total Basin area; e.g., the northern portion of the Lake Superior basin) are not assessed, and their indicator status is classified as “Undetermined” (see Basin-by-Basin Assessments below). The overall trend in groundwater quality in the basin is “Undetermined” primarily due to a lack of repeated sampling for most sites: most sites have only one sample result. However, increasing (upward) trends in chloride and nitrate concentrations in groundwater have been reported for various watersheds within the basin.

The overall status of groundwater quality has changed from “Fair” in the previous report (2019) to “Good” in this report, which is attributed to the improved geospatial data coverage. Across the basin, the number of sites with available sample data and the spatial distribution of sites increased substantially for this assessment (670 data points in 2019 versus 6,554 in 2022). Although not all newly added samples were collected since the last report, the data were not available for the previous assessment.