Aquifer maps often include large-to-small areas that are designated "minor aquifer," "not a principal aquifer," or "confining unit.” These are usually areas are underlain by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because they are of local extent, poorly permeable, or both.
In the "Ground Water Atlas of the United States" and Principal Aquifers map, there are areas identified as "other" include large-to-small areas that are designated "minor aquifer," "not a principal aquifer," or "confining unit." These areas are underlain by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because they are of local extent, poorly permeable, or both. Permeability is the relative ease with which water will move through a rock unit; aquifers are more permeable than confining units. Rocks and deposits with minimal permeability, which are not considered to be aquifers, consist of intrusive igneous rocks, metamorphic rocks, shale, siltstone, evaporite deposits, silt, and clay.
Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rock. These igneous and metamorphic rocks are permeable only where they are fractured, and they generally yield only small amounts of water to wells. However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and in many places they are the only reliable source of water supply. Because the crystalline rocks have minimal permeability, they are not mapped as principal aquifers, but they are mapped as other rocks.
Surficial stream valley aquifers or buried principal aquifers are also sometimes in some places categorized as "other". Local stream-valley alluvial aquifers south of the line of continental glaciation that yield small-to-large amounts of water are in the valleys of many major streams that cross principal aquifers, but the stream-valley aquifers are not mapped consistently between states. Important unconsolidated sand and gravel aquifers are discussed by Ground Water Atlas chapter or are grouped as sand and gravel aquifers of alluvial and glacial origin. Many of the principal aquifers are overlain by confining units, and they extend into the subsurface beyond the uppermost extent areas shown on the map.
Sand and gravel aquifers of alluvial and glacial origin
- Surficial aquifers (Arizona, Colorado, New Mexico, Utah; Chapter C)
- Alluvial aquifers along major streams (Oklahoma, Texas; Chapter E)
- Surficial aquifer system (Arkansas, Louisiana, Mississippi; Chapter F)
Minor aquifers
- Western Interior Plains aquifer system (Kansas, Missouri, Nebraska; Chapter D)
- Great Plains aquifer system (Kansas, Missouri, Nebraska; Chapter D)
- Minor aquifers in Texas (Oklahoma, Texas; Chapter E)
- Minor aquifers (Arkansas, Louisiana, Mississippi; Chapter F)
- Intermediate aquifer system (Alabama, Florida, Georgia, South Carolina; Chapter G)
- Alaska - Bedrock aquifers (Chapter N)
- Puerto Rico Alluvial Valley aquifers (Chapter N)
- Jacobsville Aquifer (Iowa, Michigan, Minnesota, Wisconsin; Chapter J)
- Crystalline aquifers (Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont; Chapter M)
Below are other web pages related to principal aquifers.
Principal Aquifers of the United States
Unconsolidated and semiconsolidated sand and gravel aquifers
Sandstone aquifers
Sandstone and carbonate-rock aquifers
Carbonate-rock aquifers
Igneous and metamorphic-rock aquifers
- Overview
Aquifer maps often include large-to-small areas that are designated "minor aquifer," "not a principal aquifer," or "confining unit.” These are usually areas are underlain by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because they are of local extent, poorly permeable, or both.
In the "Ground Water Atlas of the United States" and Principal Aquifers map, there are areas identified as "other" include large-to-small areas that are designated "minor aquifer," "not a principal aquifer," or "confining unit." These areas are underlain by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because they are of local extent, poorly permeable, or both. Permeability is the relative ease with which water will move through a rock unit; aquifers are more permeable than confining units. Rocks and deposits with minimal permeability, which are not considered to be aquifers, consist of intrusive igneous rocks, metamorphic rocks, shale, siltstone, evaporite deposits, silt, and clay.
Sources/Usage: Public Domain.Local aquifers in bedrock in western Montana and Wyoming have sufficient permeability so that water moves laterally through them to recharge unconsolidated-deposit aquifers in an adjacent stream valley. Ultimately, the water is discharged from the unconsolidated-deposit aquifers to lakes or streams.
Sources/Usage: Public Domain.Fractured zones at the contact between sandstone and shale beds and in fractured novaculite-chert beds are good locations from which wells can obtain water from the Ouachita Mountains aquifer in Arkansas. Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rock. These igneous and metamorphic rocks are permeable only where they are fractured, and they generally yield only small amounts of water to wells. However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and in many places they are the only reliable source of water supply. Because the crystalline rocks have minimal permeability, they are not mapped as principal aquifers, but they are mapped as other rocks.
Sources/Usage: Public Domain.Granitic igneous rocks form a large part of the bedrock surface from Connecticut north and in New York's Adirondack Mountains. Surficial stream valley aquifers or buried principal aquifers are also sometimes in some places categorized as "other". Local stream-valley alluvial aquifers south of the line of continental glaciation that yield small-to-large amounts of water are in the valleys of many major streams that cross principal aquifers, but the stream-valley aquifers are not mapped consistently between states. Important unconsolidated sand and gravel aquifers are discussed by Ground Water Atlas chapter or are grouped as sand and gravel aquifers of alluvial and glacial origin. Many of the principal aquifers are overlain by confining units, and they extend into the subsurface beyond the uppermost extent areas shown on the map.
Sources/Usage: Public Domain.Map of minor aquifers, confining units, and areas identified as "not a principal aquifer" Sand and gravel aquifers of alluvial and glacial origin
- Surficial aquifers (Arizona, Colorado, New Mexico, Utah; Chapter C)
- Alluvial aquifers along major streams (Oklahoma, Texas; Chapter E)
- Surficial aquifer system (Arkansas, Louisiana, Mississippi; Chapter F)
Minor aquifers
- Western Interior Plains aquifer system (Kansas, Missouri, Nebraska; Chapter D)
- Great Plains aquifer system (Kansas, Missouri, Nebraska; Chapter D)
- Minor aquifers in Texas (Oklahoma, Texas; Chapter E)
- Minor aquifers (Arkansas, Louisiana, Mississippi; Chapter F)
- Intermediate aquifer system (Alabama, Florida, Georgia, South Carolina; Chapter G)
- Alaska - Bedrock aquifers (Chapter N)
- Puerto Rico Alluvial Valley aquifers (Chapter N)
- Jacobsville Aquifer (Iowa, Michigan, Minnesota, Wisconsin; Chapter J)
- Crystalline aquifers (Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont; Chapter M)
- Science
Below are other web pages related to principal aquifers.
Principal Aquifers of the United States
This website compiles USGS resources and data related to principal aquifers including Aquifer Basics, principal aquifers maps and GIS data, and the National Aquifer Code Reference List.Unconsolidated and semiconsolidated sand and gravel aquifers
Unconsolidated sand and gravel aquifers are characterized by intergranular porosity and all contain water primarily under unconfined, or water-table, conditions. They are grouped into four categories: basin-fill, blanket sand and gravel, glacial-deposit, and stream-valley aquifers. Semiconsolidated aquifers consist of semiconsolidated sand interbedded with silt, clay, and minor carbonate rocks...Sandstone aquifers
Sandstone aquifers are more widespread than those in all other kinds of consolidated rocks. Groundwater movement in sandstone aquifers primarily is along bedding planes, but joints and fractures provide avenues for the vertical movement of water. Sandstone aquifers can be highly productive and provide large volumes of water.Sandstone and carbonate-rock aquifers
In scattered places in the United States, carbonate rocks are interbedded with almost equal amounts of water-yielding sandstone. In most places where these two rock types are interbedded, the carbonate rocks yield much more water than the sandstone.Carbonate-rock aquifers
Aquifers in carbonate rocks are most extensive in the eastern U.S. Most of the carbonate-rock aquifers consist of limestone, but dolomite and marble locally yield water. The water-yielding properties of carbonate rocks vary widely; some yield almost no water and are considered to be confining units, whereas others are among the most productive aquifers known.Igneous and metamorphic-rock aquifers
Igneous and metamorphic-rock aquifers can be grouped into two categories: crystalline-rock and volcanic-rock. Spaces in crystalline rocks are microscopically small, few, and generally unconnected. However, because these aquifers extend over large areas, large volumes of water can be withdrawn. Volcanic-rock aquifers have a wide range of chemical, mineralogic, structural, and hydraulic properties... - Publications