Skip to main content
U.S. flag

An official website of the United States government

Secondary hydrogeologic regions of the conterminous United States

July 18, 2018

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.

Publication Year 2018
Title Secondary hydrogeologic regions of the conterminous United States
DOI 10.1111/gwat.12806
Authors Kenneth Belitz, Elise Watson, Tyler D. Johnson, Jennifer B. Sharpe
Publication Type Article
Publication Subtype Journal Article
Series Title Groundwater
Index ID 70198156
Record Source USGS Publications Warehouse
USGS Organization WMA - Earth System Processes Division