National Aquifer Code Reference List

National aquifers, or principal aquifers of the United States, are defined as regionally extensive aquifers or aquifer systems that have the potential to be used as a source of potable (drinkable) water. The first publication of the core of the national aquifer list in a comprehensive manner was the Ground Water Atlas of the United States (USGS Hydrologic Atlas 730, published in 2000) published as part of the National Atlas in 1998, revised 2003. A two-dimensional map representation of the principal aquifers was derived from this data and published as part of the National Atlas in 1998 (revised 2003). 

Aquifers shown on the 2003 Principal Aquifers map, with some additions, were used as the first National Aquifer Reference List in the USGS National Water Information System (NWIS) in the Aggregate Water Use Data System (AWUDS, database released in August 2001). In 2005, the “national aquifer” information was added to NWIS, allowing users to search by national aquifer code and name from the reference list below. Aquifers are identified by a geohydrologic unit code (a three-digit number related to the age of the formation, followed by a 4 or 5 character abbreviation for the geologic unit or aquifer name).

National Aquifer Reference List 

Entries in italics are no longer valid.
To learn more about a specific aquifer, visit the Principal Aquifers website for an overview, or access more extensive information from the Ground Water Atlas of the United States.

Code components: 

• N or S, where N = aquifer, S = aquifer system

• 3 digit number = rock type (4 digits used for "Other" to fill out the code) 

• 100-- Sand and gravel aquifers 
  200--Semiconsolidated sand aquifers (not used) 
  300--Sandstone aquifers 
  400--Carbonate-rock aquifers 
  500--Sandstone and carbonate-rock aquifers 
  600--Igneous and metamorphic-rock aquifers 
  9999--Areas that are not a national aquifer 

• 6 character identifier derived from the aquifer name using the code derivation procedure also used for GWSI geohydrologic unit names. (Note that the National Aquifer Code uses a different numeric component than the geohydrologic unit names.)

History of the National Aquifer Code Reference List

• First publication of the core of the national aquifer list in a comprehensive manner was the Ground Water Atlas of the United States (HA 730 A-H), published in 13 chapters over the period 1990-2000, James A. Miller, editor. 
• A single combined map (Principal Aquifers of the 48 Conterminous United States, Hawaii, Puerto Rico, and the U.S. Virgin Islands) was published as part of the National Atlas in 1998, revised 2003. 
• Aquifers shown on the 2003 Principal Aquifers map, with some additions, were used as the first National Aquifer reference list in NWIS, released in AWUDS v. 1.0 for the national compilation of water-use data for the year 2000 (database released in August 2001). Water-use data collected using this list of aquifers will be published in Circular 1279, Estimated Withdrawals from Principal Aquifers in the United States, 2000. The additions were: 
  • Alluvial aquifers--consist of stream-valley aquifers and other aquifers of unconsolidated alluvial materials, not included in another named national aquifer, south of the limit of Quarternary continental glaciation and east of the Rocky Mountains. Note that alluvial aquifers north of the limit of continental glaciation are included in Sand and gravel aquifers (glaciated regions).
  • Intermediate aquifer (Florida) 
  • New England crystalline-rock aquifers 
  • (in error) Northern Great Plains aquifer system 
• Some revisions to the lists of valid States, this code, and reference list were added to the NWIS Site File and to NWISWeb with the release of NWIS 4.5 in 2005; see the WMA Technical Memo "NWIS 4.5 -- Assigning National Aquifer Codes in the Ground Water Site Inventory (GWSI) for the NWIS 4.5 Release." 
• Revisions to the reference list since it was first used by NWIS: 
  • Northern Great Plains aquifer system was removed: this umbrella term includes four national aquifers but is not considered a national aquifer itself. The component aquifers are Lower Tertiary, Upper Cretaceous, Lower Cretaceous, and Paleozoic. The appropriate component aquifer should be used instead of Northern Great Plains. (February 2005) 
  • Valley and Ridge carbonate-rock aquifers were removed: these now should be included in Valley and Ridge aquifers. (March 2005) 
  • Surficial aquifer system: removed the following States from the valid list: Delaware, Maryland, New Jersey, Virginia. The surficial aquifer in the Coastal Plain of these States should be included in the Northern Atlantic Coastal Plain aquifer system. Surficial deposits of glacial origin (New Jersey) should be included in Sand and gravel aquifers (glaciated regions). Surficial deposits which are not of glacial origin may be coded as Alluvial aquifers. You may need to request the addition of Alluvial aquifers to the list of aquifers valid in your State. (March 2005) 
  • The name of N600NECRSN was changed from "New England crystalline-rock aquifers" to "New York and New England crystalline-rock aquifers" to better reflect the geographic extent (January 2015).