Boron concentrations were greatest in Cretaceous bedrock aquifers. Although concentrations were low in Precambrian bedrock aquifers, some of the Precambrian aquifers, such as the North Shore Volcanics Group, had high concentrations. Map of Boron in Minnesota's Buried Quaternary Distribution Locations where boron concentrations are elevated in buried Quaternary aquifers coincide with the occurrence of Cretaceous bedrock aquifers. This is because water flows upward from the Cretaceous aquifers into the Quaternary aquifers. Drilling shallow wells in these locations may reduce the risk of encountering high boron concentrations.
Concentrations are much lower deep in aquifers where oxygen is absent.
Infants less than six months in age may be affected by drinking water with high nitrate concentrations. These are areas where shallow ground water is sensitive to contamination from human activity at the land surface. Mean concentrations of nitrate are very low in ground water. Concentrations are lowest in buried Quaternary and Cretaceous aquifers because they are well protected, and in Precambrian aquifers because there are low inputs of nitrogen. Nitrate concentrations greater than 1 part per million are often considered to represent aquifers impacted by human activity.