Secondary Hydrogeologic Regions of the United States

Map showing the Secondary Hydrogeologic Regions (SHRs) of the conterminous United States.  SHRs are areas outside of Principal Aquifers and within which the rocks or deposits are of comparable age, lithology, geologic or physiographic setting, and relationship to the presence or absence of underlying Principal Aquifers or overlying glacial deposits. Source: Figure 3, Belitz and others, 2018 (article); Fig 1, Belitz and others, 2018 (data release).