Redox conditions by principal aquifer rock type

Groundwater is predominantly oxic in the volcanic-rock and unconsolidated sand and gravel aquifers, which are found mostly in the western United States. Anoxic conditions are more common in the glacial aquifer system and in aquifers of several other rock types that are found mostly in the North and East. These differences in redox conditions can affect the persistence of some contaminants, including nitrate and some pesticides and volatile organic compounds.  This graphic is Figure 3-9 in USGS Circular 1360, Water Quality in Principal Aquifers of the United States, 1991–2010.