Minor aquifers, confining units, and areas identified as "not a principal aquifer"

Science Center Objects

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.

Diagram of a local bedrock aquifer

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.

Diagram of minor aquifers

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 However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and in many places they are the only reliable source of water supply.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.

Photograph of igenous rocks

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.

Map showing other rocks aquifers

Map of minor aquifers, confining units, and areas identified as "not a principal aquifer"

Sand and gravel aquifers of alluvial and glacial origin

 

Minor aquifers