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
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.
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.
Leaching From Saline Soils or Road Salt
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.
Injection, Leakage, or Infiltration of Brine From Oil and Gas Wells
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.
Below are other science projects associated with the National Brackish Groundwater Assessment.
Brackish Groundwater Assessment
About the USGS National Brackish Groundwater Assessment
National Brackish Groundwater Assessment: Sources of Dissolved Solids in Brackish Groundwater
National Brackish Groundwater Assessment: How is Brackish Groundwater Being Used?
National Brackish Groundwater Assessment: Pilot Saline Studies
National Brackish Groundwater Assessment: Previous Work
- Overview
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
Sources/Usage: Some content may have restrictions. Visit Media to see details.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.
Sources/Usage: Public Domain.Map of United States in Late Cretaceous Time (66 to 100 million years ago). Source: USGS Bulletin 1291.
Sources/Usage: Public Domain.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.
Sources/Usage: Public Domain.Movement of saline groundwater from adjacent aquifer. Source: USGS Circular 1262. Leaching From Saline Soils or Road Salt
Sources/Usage: Public Domain.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.
Sources/Usage: Public Domain.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
Sources/Usage: Public Domain.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.
- Science
Below are other science projects associated with the National Brackish Groundwater Assessment.
Brackish Groundwater Assessment
All water naturally contains dissolved solids that, if present in sufficient concentration, can make a water resource "brackish", or distastefully salty. The amount of fresh or potable groundwater in storage has declined for many areas in the U.S., leading to concerns about the future availability of water. Using brackish groundwater could supplement or, in some places, replace the use of...About the USGS National Brackish Groundwater Assessment
The objectives of the National Brackish Groundwater Assessment were to identify and characterize aquifers that contain brackish groundwater in the United States. For purposes of this study, brackish groundwater is defined as having dissolved solids at concentrations between 1,000 and 10,000 milligrams per liter.National Brackish Groundwater Assessment: Sources of Dissolved Solids in Brackish Groundwater
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.National Brackish Groundwater Assessment: How is Brackish Groundwater Being Used?
Industry and public drinking-water suppliers are increasingly turning to brackish groundwater to supplement or replace the use of freshwater. Brackish groundwater is either directly used or treated.National Brackish Groundwater Assessment: Pilot Saline Studies
Three pilot studies were conducted (2010-2012) to assess saline groundwater resources at regional scales. The goals of the pilot studies were to determine the availability of data for assessing the distribution and character of saline groundwater and test and develop methodologies for assessing the resource.National Brackish Groundwater Assessment: Previous Work
Interest in the distribution and classification of brackish groundwater for use as a source of water supply has been longstanding. These studies provided valuable background for the National Brackish Groundwater Assessment.