Unconsolidated and semiconsolidated sand and gravel aquifers

Science Center Objects

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. Porosity is intergranular, and hydraulic conductivity is moderate to high.

Unconsolidated sand and gravel aquifers

Unconsolidated sand and gravel aquifers can be grouped into four categories:

  1. basin-fill aquifers, which also are called "valley-fill aquifers"
  2. blanket sand and gravel aquifers
  3. glacial-deposit aquifers
  4. stream-valley aquifers which are of generally small extent and not mapped.

All four types have intergranular porosity, and all contain water primarily under unconfined or water-table conditions. The hydraulic conductivity of the aquifers is variable, depending on the sorting of aquifer materials and the amount of silt and clay present, but generally it is high. Aquifer thickness ranges from a few meters or tens of meters in the blanket sands along the eastern Atlantic coast of the United States to several hundred meters in the basin-fill aquifers of the southwestern United States. The unconsolidated sand and gravel aquifers are susceptible to contamination because of their generally high hydraulic conductivity. Groundwater in these aquifers flows along relatively short flow paths typical of local flow systems; however, all of the basin-fill aquifers have intermediate flow systems, and the thick basin fill of California's Central Valley aquifer system has a regional flow system. Likewise, the thick blanket sands of the High Plains aquifer and the Mississippi River Valley alluvial aquifer of the central United States have regional flow systems.

Photograph showing unconsolidated deposits in North Dakota.

Permeable lenses of sand and gravel within till show a wide variation in bed thickness, grain size, and sorting of grains. Photograph showing unconsolidated deposits in North Dakota. 

Basin-fill or valley-fill aquifers were deposited in depressions formed by faulting or erosion or both. Fine-grained deposits of silt and clay form local confining units in these aquifers, and thick sequences of the unconsolidated deposits become more compact and less permeable with depth. Most basins are bounded by low-permeability rocks, but some in the western United States are hydraulically connected to adjacent carbonate-rock aquifers. Some basin-fill aquifers, such as those in the Central Valley aquifer system of California and in parts of Arizona, have supplied large amounts of water for irrigation and other uses.

Widespread, blanket-like deposits of sand and gravel form aquifers in lowland areas of Alaska, atop lava plateaus in Washington, along the Atlantic and eastern Gulf coasts, along part of the lower reaches of the Mississippi River, and in the High Plains. These aquifers mostly consist of alluvial deposits. They commonly contain water under unconfined conditions, and most groundwater flow in them travels short to intermediate distances from recharge to discharge areas. The High Plains aquifer is the most intensively pumped aquifer in North America.

Glacial-deposit aquifers form numerous local, and some regional, highly productive aquifers in the area north of the line of glaciation. These aquifers consist of outwash, terrace, or ice-contact deposits, and they mostly occupy bedrock valleys or areas of interlobate ice marginal deposition. In places, the valley deposits are buried beneath low-permeability till. Groundwater flow in the glacial-deposit aquifers is primarily local, from recharge areas near stream valley walls to discharge in the streams.

 

Diagram showing distribution of sediments that compose semiconsolidated sand aquifers

The sediments that compose semiconsolidated sand aquifers (yellow) are in the general shape of a wedge that thickens seaward. 

Semiconsolidated aquifers

Semiconsolidated aquifers consist of semiconsolidated sand interbedded with silt, clay, and minor carbonate rocks. Porosity is intergranular, and the hydraulic conductivity of the aquifers is moderate to high. The aquifers underlie the Coastal Plains of the eastern and southern United States, and they are of fluvial, deltaic, and shallow marine origin. The aquifers are in a thick wedge of sediments that dips and thickens coastward; in places, the sands of the aquifers are more than 650 meters thick. The varied depositional environments of these sediments have caused complex interbedding of fine- and coarse-grained materials. Accordingly, some aquifers are local whereas others extend over hundreds of square kilometers. The numerous local aquifers can be grouped into several regional aquifer systems that contain groundwater-flow systems of local, intermediate, and regional scale. Water in topographically high recharge areas is unconfined, but, it becomes confined as it moves coastward. Discharge is by upward leakage to shallower aquifers or to saltwater bodies in coastal areas. Because flow is sluggish near the ends of regional flow paths, the aquifers commonly contain unflushed saline water in their deeply buried, downdip parts. Where shallow aquifers have been heavily pumped near the coasts, saltwater intrusion has locally contaminated the groundwater. During 1985, more than 30 million cubic meters per day was withdrawn from these aquifers.

Map of principal unconsolidated and semiconsolidated sand and gravel aquifers

Map of principal unconsolidated and semiconsolidated sand and gravel aquifers.

 

Unconsolidated sand and gravel aquifers include:

  • Basin and Range basin-fill aquifers (southwestern U.S.)
  • Rio Grande aquifer system
  • California Coastal Basin aquifers
  • Central Valley aquifer system (California)
  • Pacific Northwest basin-fill aquifers
  • Puget Sound aquifer system
  • Willamette Lowland basin-fill aquifers
  • Columbia Plateau basin-fill aquifers
  • Snake River Plain basin-fill aquifers
  • Northern Rocky Mountains Intermontane Basins aquifer system
  • High Plains aquifer
  • Sand and gravel aquifers of alluvial and glacial origin
  • Pecos River Basin alluvial aquifer (Texas and New Mexico)
  • Mississippi River Valley alluvial aquifer
  • Seymour aquifer (Texas)
  • Surficial aquifer system (eastern U.S.)
  • Unconsolidated-deposit aquifers (Alaska)
  • South Coast Aquifer (Puerto Rico)

Semiconsolidated sand and gravel aquifers include:

  • Coastal lowlands aquifer system (Gulf Coast)
  • Texas coastal uplands aquifer system
  • Mississippi embayment aquifer system
  • Southeastern Coastal Plain aquifer system
  • Northern Atlantic Coastal Plain aquifer system