The U.S. Geological Survey (USGS) previously identified and mapped 62 Principal Aquifers (PAs) in the U.S., with 57 located in the conterminous states. Areas outside of PAs, which account for about 40% of the conterminous U.S., were collectively identified as “other rocks.” This paper, for the first time, subdivides this large area into internally‐consistent features, defined here as Secondary Hydrogeologic Regions (SHRs). SHRs are areas of other rock within which the rocks or deposits are of comparable age, lithology, geologic or physiographic setting, and relationship to the presence or absence of underling PAs or overlying glacial deposits. A total of 69 SHRs were identified. The number and size of SHRs identified in this paper are comparable to the number and size of PAs previously identified by the USGS. From a two‐dimensional perspective, SHRs are complementary to PAs, mapped only where the PAs were not identified on the USGS PA map and not mapped where the PAs were identified. SHRs generally consist of low permeability rocks or deposits, but can include locally productive aquifers. The two maps, taken together, provide a comprehensive, national‐scale hydrogeologic framework for assessing and understanding groundwater systems.
Regional‐scale classification of groundwater systems provides a basis for understanding and management of groundwater resources (Barthel ; Dennehy et al. ). In the United States (U.S.), two classification systems have been widely used: an approach developed by Heath (, , ), and subsequent work by the U.S. Geological Survey (USGS) presented in “The Ground Water Atlas of the United States” (Miller ). The Ground Water Atlas included a map of Principal Aquifers, which was subsequently updated by the USGS (). In the five paragraphs that follow, the work of Heath and the USGS are reviewed. Those studies provide the foundation for the work presented in this paper. The review of previous work is followed by two paragraphs describing the purpose and potential value of the current work.
Heath ( and ) provided a classification system for identifying groundwater regions, which were defined as areas in which the composition, arrangement, and structure of rock units are similar. Heath, building upon the work of Meinzer () and Thomas (), identified 15 groundwater regions in the U.S., Puerto Rico, and the Virgin Islands. Heath () subsequently expanded his classification system to North America, identifying 28 hydrogeologic regions. Heath's classification system has been widely used. For example, it provided the framework for describing the hydrogeology of North America (Back et al. ) and provided a foundation for the initial development of the widely cited DRASTIC method for assessing vulnerability to pollution (Aller et al. ). Heath's groundwater regions are large and focus on the general characteristics of groundwater systems that provide water supply. Heath did not explicitly identify specific aquifers or aquifer systems within those regions.
The USGS (Miller ) subsequently published “The Ground Water Atlas of the United States” which described the “location, extent, and geologic and hydrologic characteristics of the most productive aquifers in the United States.” The productive aquifers were identified as principal aquifers (PAs) and were characterized as belonging to one of six lithologic categories (Figure 1A). Areas outside of PAs were simply identified as “not a principal aquifer” by Miller () and as “other” on the USGS PA map (USGS ). Areas mapped as other were described by the USGS () as “rocks that are minimally permeable but may contain locally productive aquifers.” For the purposes of discussion, areas outside of PAs are defined here as “other rocks.”
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