National Brackish Groundwater Assessment: Sources of Dissolved Solids in Brackish Groundwater

Science Center Objects

A variety of conditions and mechanisms can cause groundwater to become brackish. An understanding of the sources of dissolved solids that contribute to the formation of brackish groundwater can help determine where brackish aquifers are likely to exist and can provide clues about other characteristics, such as the chemical composition, of brackish aquifers.

Dissolution of Minerals in the Saturated and Unsaturated Zones

A photo of gypsum.

Gypsum forms this unusual pattern of veins in old elevated stream terrace deposits along the River Road in southern Big Bend.

Salt deposits, such as halite or gypsum, are found in many sedimentary basins in the United States and are highly soluble. Other deposits are less soluble but can contribute dissolved solids when in contact with water over longer periods of time.








Connate Seawater and (or) Dissolution of Marine Sediments

"Connate" water is water that was trapped in the sediments as they were deposited. Large parts of the United States have been covered with seawater in previous geologic time periods, and geologic systems that have not been flushed by freshwater can still contain connate seawater. Sediments deposited in a marine environment, such as carbonates, can dissolve in a groundwater system.

Map of the U.S. cover by shallow sea except areas of the Rocky Mountains, Great Lakes, Appalachian, and North East.

Map of United States in Late Cretaceous Time (66 to 100 million years ago). Source: USGS Bulletin 1291.

Map of the U.S. where most areas of the Nation are covered by sea and swamps.

Map of United States in Middle Pennsylvanian Time (about 300 million years ago). Source: USGS Bulletin 1291.










Movement of Saline or Brackish Groundwater From Adjacent Aquifers

Saline or brackish groundwater from adjacent aquifers can mix with fresh water to create brackish conditions. Well boreholes that are open to both types of water can facilitate mixing in the subsurface. In addition, pumping wells may cause water to flow from a saline aquifer to a fresh aquifer.

Cross-section of western Florida coast showing directional groundwater flow and salt-water intrusion patterns.

Movement of saline groundwater from adjacent aquifer. Source: USGS Circular 1262.








Leaching From Saline Soils or Road Salt

Cotton growing in salt-encrusted soils in California

Cotton growing in salt-encrusted soils in California. (Credit: Gary Bañuelos, U.S. Department of Agriculture, Agricultural Research Service)

Salts at the land surface can leach to shallow groundwater. Evapotranspiration can concentrate dissolved salts in the soil where it subsequently can infiltrate to shallow groundwater, especially where precipitation rates are low. In addition, millions of tons of salt are applied to roads each winter. Runoff containing dissolved road salt can eventually reach shallow groundwater. In many cases, these sources of dissolved solids are associated with localized brackish groundwater and will not be studied as part of the National Brackish Groundwater Assessment.














Seawater Intrusion

Seawater can mix with fresh water where aquifers along the coast are connected with the ocean. Pumping wells near the coast can further increase flow of seawater into fresh aquifers.

This figure shows a schematic illustration of some of the modes of saltwater intrusion in a multilayer, regional aquifer system caused by ground-water pumping at wells. Saltwater moves into the unconfined aquifer from the Atlantic Ocean and into the shallow part of the top confined aquifer from the major bay. The two freshwater-saltwater interfaces at the seaward boundary of each of the confined aquifers also move landward as saltwater is drawn inland from offshore areas.

Cross-section showing groundwater and surface water reservoirs and flow patterns.

Generalized groundwater flow patterns in a multilayer, regional aquifer system. Source: USGS Circular 1262.











Injection, Leakage, or Infiltration of Brine From Oil and Gas Wells

View of the large brine pit, the active tank battery, and the partly remediated salt scar adjacent to Skiatook Lake

Large brine pit, active tank battery, and the partly remediated salt scar adjacent to Skiatook Lake at the OSPER "B" site. (Credit: Ken Jewell, U.S. Environmental Protection Agency.)

Brine is water that contains very large amounts of dissolved salts (total dissolved-solids concentration greater than 35,000 milligrams per liter), and is a byproduct of pumping oil and gas. Most brine is injected back into the subsurface, and the rest is disposed at the land surface. Naturally fresh groundwater can mix with brine and produce brackish groundwater.