National Water Census: Brackish Groundwater Assessment

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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 freshwater sources and enhance our Nation's water security.

The amount of fresh or potable groundwater in storage has declined for many areas in the United States and has led to concerns about the future availability of water for drinking-water, agricultural, industrial, and environmental needs. Use of brackish groundwater could supplement or, in some places, replace the use of freshwater sources and enhance our Nation's water security. However, a better understanding of the location and character of brackish groundwater is needed to expand development of the resource and provide a scientific basis for making policy decisions. To address this need, the Department of Interior's WaterSMART initiative, through the USGS Water Availability and Use Science Program, conducted a national assessment of brackish aquifers.

 

Brackish Groundwater discharge through a spring in Death Valley, CA

Groundwater in basin-fill aquifers of the Southwest U.S. often increases in dissolved-solids content as it travels along its flow path as a result of geochemical interactions with the aquifer matrix and through evaporative processes. At the end of its flowpath, groundwater may be brackish or saline and discharges to the surface through springs, such this one in Death Valley shown in the photograph above. (Credit: David Anning, USGS)

What is "Brackish"?

All water naturally contains dissolved solids that, if present in sufficient concentration, can make a surface-water or groundwater resource "brackish", typically defined as distastefully salty. Although quantitative definitions of this term vary, it is generally understood that brackish groundwater is water that has a greater dissolved-solids content than occurs in freshwater, but not as much as seawater (35,000 milligrams per liter*). It is considered by many investigators to have dissolved-solids concentration between 1,000 and 10,000 milligrams per liter (mg/L). The term "saline" commonly refers to any water having dissolved-solids concentration greater than 1,000 mg/L and includes the brackish concentration range.

*Milligrams per liter (mg/L) is generally equivalent to parts per million (ppm) when dissolved-solids concentrations are less than about 7,000 mg/L. For larger concentrations, a density correction should be used when converting from mg/L to ppm.

 

Why Study Brackish Groundwater?

There is growing recognition that brackish groundwater is a resource that could be further developed in many parts of the United States.

Alternative Water Sources Are Needed

In many parts of the country, groundwater withdrawals exceed recharge rates and have caused groundwater-level declines, reductions to the volume of groundwater in storage, lower streamflow and lake levels, or land subsidence. It is expected that the demand for groundwater will continue to increase because of population growth, especially in the arid West. Further, surface-water resources are fully appropriated in many parts of the country, creating additional groundwater demand. Development of brackish groundwater as an alternative water source can help address concerns about the future availability of water and contribute to the water security of the Nation.

Percentage Change in Population by State and Decade: 1980 - 1990 to 20

Percentage change in population by state and decade: 1980-1990, 1990-2000, and 2000-2010. As the population continues to grow in many areas of the U.S., so does the demand for water. (Credit: U.S. Census Bureau. For more information, see Population Distribution and Change 2000-2010.)

Available Precipitation - Average from 1934 to 2002 In Inches Per Year

Available precipitation (difference between precipitation and potential evapotranspiration), an estimate of how much water is available for recharge to the groundwater system or as runoff to streams. Areas with low amounts of average precipitation may not be able to meet the water needs of continued population growth. (Source: USGS Circular 1323)

 

Potential Untapped Future Water Resource

Brackish groundwater is potentially abundant. Early studies indicated that mineralized groundwater underlies most of the country. Further, advances in desalination technologies are making treatment and use of brackish groundwater for potable water supply more feasible.

 

USGS WAUSP Water Census: Map of National Brackish Groundwater Assessment

Map of predicted depth to brackish groundwater from Brackish Groundwater In The United States (USGS Professional Paper 1833).