USGS Groundwater Information
Aquifer by rock type
Aquifer Basics Links
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.
Sandstone retains only a small part of the intergranular pore space that was present before the rock was consolidated; compaction and cementation have greatly reduced the primary pore space. Secondary openings, such as joints and fractures, along with bedding planes, contain and transmit most of the groundwater in sandstone. Accordingly, the hydraulic conductivity of sandstone aquifers is low to moderate, but because they extend over large areas, these aquifers provide large amounts of water.
This map of sandstone aquifers in the United States shows the shallowest principal aquifer. In some places, other, sometimes more productive, aquifers underlie those mapped. Only small areas of some aquifers may be shown on the map because they are covered in many places by aquifers closer to the surface. In other places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped. Local aquifers are not shown because of the scale of the map. Some aquifers in sedimentary rocks are overlain by confining units, and the aquifers extend into the subsurface beyond the areas shown on the map.
The sandstone aquifers are level or gently dip. Because they are commonly interbedded with siltstone or shale, most of the water in these aquifers is under confined conditions. Groundwater-flow systems in mostly level, relatively thin sandstone aquifers are local to intermediate. Regional, intermediate, and local flow is present in the sandstone aquifers in the western United States, except for those in Oklahoma, where flow is mostly local. Many sandstone aquifers contain highly mineralized water at depths of only a few hundred meters.
In Wisconsin and adjacent states, three Cambrian and Ordovician age sandstone aquifers are combined into an aquifer system that is as much as 650 meters thick. Paleozoic through Cenozoic age sandstones that extend northeastward from Wyoming form the Northern Great Plains aquifer system, which has permeable parts of more than 2,000 meters thick in some places in a deep structural basin. Not all of these thick aquifers, however, contain freshwater.
See also: Sandstone and carbonate-rock aquifers