Data Release for Secondary Hydrogeologic Regions of the Conterminous United States (ver. 2.0, June 2022)

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 underlain by geologic units of comparable age, lithology, and geologic or physiographic setting. ORRs were an intermediate product. In the second phase, ORRs were evaluated relative to the presence of PAs that may underlie the ORRs and (or) glacial deposits that may overly the ORRs. The presence or absence of stream-valley aquifers overlying an SHR was not considered, which is consistent with the identification of PAs. Identification and mapping of ORRs and SHRs was facilitated using digital databases and geographic information system tools.

The SHRs were classified using three criteria: (1) presence or absence of underlying PAs or overlying glacial deposits, (2) primary lithology, and (3) geologic province and subprovince. The number and size of SHRs identified in this paper are comparable to the number and size of PAs previously identified by the USGS. With the identification of SHRs, all areas of the conterminous U.S. belong to an internally-consistent mapped feature, thus providing a comprehensive framework for assessing groundwater at regional and national scales.

Figures 1-3 are included for reference. Files are provided in Adobe PDF (get Adobe Reader here: https://get.adobe.com/reader/):
Figure 1. Map showing the 69 Secondary Hydrogeologic Regions of the conterminous United States.
Figure 2. Maps showing Secondary Hydrogeologic Regions (SHRs) classified by (A) Type; (B) Lithologic class, and (C) Geologic province and subprovince.
Figure 3. Map showing Community Supply Wells (Price and Maupin, 2014) classified by the type of Secondary Hydrogeologic Region (SHR) that the well plots within. Type refers to the presence of a Principal Aquifer beneath (first letter) or glacial deposits above an SHR (second letter); Y= yes, present, and N = no, not present. There are about 143,000 wells shown: 9% are in SHRs classified as Type NN, 13% in Type NY, 3% in type YN, and 4 % in Type YY; 71% are in Principal Aquifers.