Non-principal aquifer

Crystalline-rock aquifers

Links to photo and diagram of crystalline-rock aquifers.

Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rocks. Spaces between the individual mineral crystals of crystalline rocks are microscopically small, few, and generally unconnected; therefore, porosity is insignificant. 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 However, because these rocks extend over large areas, large volumes of ground water are withdrawn from them, and, in many places, they are the only reliable source of water supply. Accordingly, the crystalline rocks of northern Minnesota and northeastern Wisconsin, northeastern New York and the New England States, and the Piedment and Blue Ridge Physiographic Provinces, which extend from eastern Alabama to southeastern New York are mapped as aquifers in the Altas chapters that describe those areas. Because the crystalline rocks have minimal permeability, they are not mapped as principal aquifers.

Although crystalline rocks are geologically complex, movement of water through the rocks is totally dependent on the presence of secondary openings; rock type has little or no effect on ground-water flow. This map of crystalline-rock aquifers in the United States shows crystalline rocks that are not overlain by principal aquifers or confining units. In some places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped.

Publication source

Crystalline-rock aquifers

• Jacobsville and Crystalline-rock aquifers (Iowa, Mich., Minn., and Wis.; chapter J)
• Crystalline-rock aquifers (Conn., Maine, Mass., N.H., N.Y., R.I., Va.; chapter M)

See also: Other rocks

• Introduction and National Summary (chapter A)