USGS Groundwater Information
Aquifer by rock type
Unconsolidated and semiconsolidated sand and gravel aquifers
The principal water-yielding aquifers of North America can be grouped into five types: unconsolidated and semiconsolidated sand and gravel aquifers, sandstone aquifers, carbonate-rock aquifers, aquifers in interbedded sandstone and carbonate rocks, and aquifers in igneous and metamorphic rocks. This map of unconsolidated and semiconsolidated sand and gravel aquifers in the United States shows the shallowest principal aquifer.
Unconsolidated sand and gravel aquifers can be grouped into four categories: basin-fill aquifers, which also are called "valley-fill aquifers"; blanket sand and gravel aquifers; glacial-deposit aquifers; and 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.
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.
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.
Unconsolidated sand and gravel aquifers include:
Semiconsolidated sand and gravel aquifers include: