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. 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. Permeable lenses of sand and gravel within till show a wide variation in bed thickness, grain size, and sorting of grains.
Photograph shows unconsolidated deposits in North Dakota.
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. 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. Limit of continental glaciation. North of this line, glacial sand and gravel aquifers overlie bedrock aquifers in many places. 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.