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The U.S. Geological Survey (USGS) previously identified 62 Principal Aquifers (PAs) in the U.S., with 57 located in the conterminous states. The USGS characterized areas outside of PAs as “other rocks;” other rocks account for about 40% of the area of the conterminous states. This paper subdivides the large area identified as other rocks into Secondary Hydrogeologic Regions (SHRs). SHRs are defined as areas of other rock within which the rocks are of comparable geologic age, lithology, and relationship to the presence or absence of underling PAs or overlying glacial deposits. A total of 69 SHRs were identified. SHRs were identified in two phases. In the first phase, Other Rock Regions (ORRs) were defined as regions...
    map background search result map search result map Data Release for Secondary Hydrogeologic Regions of the Conterminous United States (ver. 2.0, June 2022) Data Release for Secondary Hydrogeologic Regions of the Conterminous United States (ver. 2.0, June 2022)