U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3054

July 2017

GROUNDWATER RESOURCES FOR THE FUTURE

Brackish Groundwater and its Potential to Augment Freshwater Supplies

By Jennifer S. Stanton and Kevin F. Dennehy

Secure, reliable, and sustainable water resources are fundamental to the Nation’s food production, energy independence, 

and ecological and human health and well-being. Indications are that at any given time, water resources are under stress in 

selected parts of the country. The large-scale development of groundwater resources has caused declines in the amount of 

groundwater in storage and declines in discharges to surface water bodies (Reilly and others, 2008). Water supply in some 

regions, particularly in arid and semiarid regions, is not adequate to meet demand, and severe drought intensifies the stresses 

affecting water resources (National Drought Mitigation Center, the U.S. Department of Agriculture, and the National Oceanic 

and Atmospheric Association, 2015). If these drought conditions continue, water shortages could adversely affect the human 

condition and threaten environmental flows necessary to maintain ecosystem health.

In support of the national census of water resources, the U.S. Geological Survey (USGS) completed the national brackish groundwater assessment to provide updated information about brackish groundwater as a potential resource to augment or 

replace freshwater supplies (Stanton and others, 2017). Study objectives were to consolidate available data into a comprehensive database of brackish groundwater resources in the United States and to produce a summary report highlighting the distribution, physical and chemical characteristics, and use of brackish groundwater resources. This assessment was authorized by 

section 9507 of the Omnibus Public Land Management Act of 2009 (42 U.S.C. 10367), passed by Congress in March 2009. 

Before this assessment, the last national brackish groundwater compilation was completed in the mid-1960s (Feth, 1965). 

Since that time, substantially more hydrologic and geochemical data have been collected and now can be used to improve the 

understanding of the Nation’s brackish groundwater resources. 

What is Brackish Groundwater?

All water naturally contains dissolved solids that, if present in sufficient 

concentration, can make a surface-water 

or groundwater resource “brackish”. 

In general, brackish groundwater is 

groundwater that has a dissolved-solids 

content greater than freshwater but not as 

much as seawater; however, a variety of 

classification schemes have been used to 

quantitatively describe waters that have 

different dissolved-solids concentrations. 

Dissolved solids (also referred to as total 

dissolved solids) is a measure of the 

concentration of all organic and inorganic 

dissolved substances (like minerals, metals, and salts) present in a water sample. 

The U.S. Environmental Protection 

Agency secondary maximum contaminant level, a nonmandatory standard that 

only applies to public water systems, 

advises a level of 500 milligrams per 

liter (mg/L) for dissolved solids (U.S. 

Environmental Protection Agency, 2015), 

although numerous water supplies exceed 

this level. Water with dissolved-solids 

levels exceeding 1,000 mg/L is generally considered undesirable for human 

consumption. Water that is high in 

dissolved solids may taste bitter, salty, 

or metallic; smell unpleasant; or may 

be toxic. Water with higher dissolvedsolids concentrations can be used for a 

variety of purposes other than drinking 

water. For the national brackish groundwater assessment, brackish groundwater 

was quantitatively defined as having a 

dissolved-solids concentration between 

1,000 and 10,000 mg/L. Saline groundwater commonly refers to any waters 

having dissolved-solids concentrations 

of at least 1,000 mg/L and includes the 

brackish concentration range and higher 

salinity water. For comparison, seawater 

has a dissolved-solids concentration of 

35,000 mg/L (table 1).

Table 1. Water categories based on 

dissolved-solids concentration.

Categories of 

water1

Dissolved-solids 

concentration, 

in milligrams per liter

Fresh Less than 1,000

Brackish 1,000 to 10,000

Highly saline Greater than 10,000

1

Seawater contains about 35,000 milligrams per liter.

Where is Brackish Groundwater?

The national brackish groundwater assessment focused on defining the 

occurrence of brackish groundwater at 

depths less than 3,000 feet (ft) below land 

surface because few data were readily 

available below that depth. Groundwaterchemistry data for about 380,000 

groundwater samples were compiled from 

33 sources (Qi and Harris, 2017) for this 

assessment. Those data were summarized 

with a coarse, three-dimensional grid that 

indicated occurrences of brackish groundwater at some depth within 3,000 ft below 

land surface in every State except New 

Hampshire and Rhode Island (fig. 1). 

Most of the known brackish groundwater 

is within in the Western Midcontinent 

region. Other significant occurrences of 

brackish groundwater generally are in 

the Coastal Plains, Eastern Midcontinent, 

and Southwestern Basins regions (fig. 1). 

States along the Atlantic coast have the 

largest number of observations; however, the groundwater in these States is 

generally freshwater with little brackish 

groundwater occurrence except along 

the coastline. In general, dissolved-solids 

Printed on recycled paper
130° 120° 110° 100° 90° 80° 70°

40°

30°

20°

0 250 500 MILES

0 250 500 KILOMETERS

EXPLANATION

PACIFIC

OCEAN

ATLANTIC

OCEAN

HAWAII

ALASKA

<50

50 to <500

500 to <1,500

1,500 to 3,000

Brackish or highly saline groundwater

not observed

Minimum depth to brackish or highly saline 

groundwater, in feet below land surface

U.S. TERRITORIES

Base map modified from Esri and U.S. Geological Survey digital data, 1:2,000,000 and other scales, variously dated. 

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Albers Equal-Area Conic projection, standard parallels 29°30’ N. and 45°30’ N., central meridian 96°00’ W., latitude of origin 23°00’ N.

North American Datum of 1983

Western Mountain

Ranges

Northwestern

Volcanics

Southwestern

Basins Western

Midcontinent

Eastern Mountains

and Uplands

Eastern

Midcontinent

Coastal

Plains

Western Mountain

Ranges

Northwestern

Volcanics

Southwestern

Basins Western

Midcontinent

Eastern Mountains

and Uplands

Eastern

Midcontinent

Coastal

Plains

Figure 1. Observed minimum depth to brackish or highly saline groundwater between 0 and 3,000 feet below land surface.

concentrations are higher and brackish groundwater is more 

frequently detected with increasing depths below the land 

surface. Across the United States, for example, about 70 percent 

of sampled wells between 1,500 and 3,000 ft below land surface 

produced brackish or highly saline groundwater; whereas, less 

than 20 percent of sampled wells between 0 and 50 ft below 

land surface had brackish or highly saline groundwater. Similarly, the median dissolved-solids concentration observed across 

the United States is 334 mg/L for all wells between 0 and 50 ft 

below land surface and 3,692 mg/L for all wells between 1,500 

and 3,000 ft below land surface.

What Chemical Factors Affect the Usability of 

Brackish Groundwater?

In addition to limitations caused by high dissolved-solids 

concentrations, other chemical constituents can be present 

that affect the usefulness of brackish groundwater. The chemical composition of brackish groundwater is spatially variable 

because it depends in part on local geologic, hydrologic, and 

climatic conditions. Chemical variations in brackish groundwater are important because they can have implications for the 

feasibility, treatability, and associated cost of using brackish 

groundwater (McMahon and others, 2015). Specific chemical 

constituents in brackish groundwater can exceed standards for 

a particular use, and identifying the constituents is important 

to determine if brackish groundwater must undergo specialized 

treatment beyond what is required to reduce overall salt content. 

For example, arsenic can exceed the U.S. Environmental Protection Agency primary drinking-water standard of 10 micrograms 

per liter for drinking water (fig. 2). Other chemical characteristics also can limit brackish water use; for example, mineral scaling can impede conveyance, storage, and treatment of brackish 

groundwater.

Mineral scaling refers to the deposition or precipitation 

of minerals on a surface or membrane that stores, transmits, or 

filters water and can impede flow. For example, scaling can precipitate minerals along the walls of pipes, reducing the interior 

diameter of the supply lines. Corrosion, which is less common 

with brackish groundwater, can cause deterioration of metal 

surfaces. Water treatment is commonly necessary to remove constituents that are related to scaling or that exceed health benchmarks. Reverse osmosis is the most common form of desalination treatment in the United States. Reverse osmosis systems 

include a thin, semipermeable barrier that transmits water under 

pressure, while excluding solutes. Without proper treatment of 

source water to reduce scaling potential, reverse osmosis systems 

can fail; therefore, information about the minerals that are present and their concentrations will enable a cost/benefit analysis of 

the resource and a given treatment technology.

Detailed information about brackish groundwater chemical characteristics and scaling potentials was derived from 

geochemical modeling (Parkhurst and Appelo, 1999) and other 

approaches. For example, the Langelier saturation index is an 

indicator of the degree of saturation of water with respect to 

calcite and is commonly used by the water-supply industry to 
determine potential for corrosion or scaling (Langelier, 1936). A 

negative Langelier saturation index value indicates the water is 

not likely to precipitate calcite and, thus, could lead to corrosion; 

whereas, a positive index value will indicate the water is likely 

to deposit calcite in the distribution or treatment system. Only 

4 percent of the evaluated groundwater samples (n=14,380) 

with a dissolved-solids concentration of 1,000 mg/L or more 

exceeded a Langelier saturation index value of 1 (increased scaling potential), and only 2 percent of samples had an index value 

less than -1 (increased corrosion potential) (fig. 3). Some minerals identified as potential scaling components include calcite, 

barite, and chalcedony, depending on the source of the water.

120° 110° 100° 90° 80° 70°

40°

30°

EXPLANATION

Arsenic concentration in brackish groundwater,

 in micrograms per liter (n = 6,427)

>10

≤10

0 250 500 MILES

0 250 500 KILOMETERS

Base map modified from Esri and U.S. Geological Survey digital data, 1:2,000,000 and other scales, variously dated. 

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Albers Equal-Area Conic projection, standard parallels 29°30’ N. and 45°30’ N., central meridian 96°00’ W., latitude of origin 23°00’ N.

North American Datum of 1983

PACIFIC

OCEAN

ATLANTIC

OCEAN

Figure 2. Concentrations of arsenic in brackish groundwater in the conterminous United States 

(modified from McMahon and others, 2015).

40°

30°

130° 120° 110° 100° 90° 80° 70°

PACIFIC

OCEAN

ATLANTIC

OCEAN

HAWAII

ALASKA

0 250 500 MILES

0 250 500 KILOMETERS

EXPLANATION

Langelier saturation

index

≤–1

>–1 to 0

>0 to 1

>1

Base map modified from Esri and U.S. Geological Survey digital data, 1:2,000,000 and other scales, variously dated. 

Base map image is the intellectual property of Esri and is used herein under license. Copyright © 2014 Esri and its licensors. All rights reserved.

Albers Equal-Area Conic projection, standard parallels 29°30’ N. and 45°30’ N., central meridian 96°00’ W., latitude of origin 23°00’ N.

North American Datum of 1983

Figure 3. Distribution of Langelier saturation index values in brackish groundwater.

What Physical Factors Affect the Usability of 

Brackish Groundwater?

When evaluating brackish groundwater as a potential 

resource, understanding the ability of brackish groundwaterbearing aquifers to store and transmit usable amounts of water 

is important. This ability is controlled by the physical properties 

of the aquifer sediments, such as hydraulic conductivity, storage 

coefficient, specific yield, and porosity. Information about these 

properties is limited for most brackish zones; therefore, wellyield data (available for about 16 percent of wells) were used to 

determine some measure of the ability of wells producing brackish groundwater to provide usable amounts of water.

Although compaction and 

cementation of deeper sediments 

could limit development of brackish groundwater resources in some 

areas, reported well yields indicate 

that shallow and deep wells could 

produce brackish groundwater at 

sufficient rates for many uses. In 

the regions with the most brackish 

groundwater, many of the sampled 

wells that produce brackish groundwater have reported well yields that 

are at least 10 gallons per minute. 

In the Southwestern Basins region 

(fig. 1), a large part of the sampled 

wells producing brackish groundwater can provide yields of at least 100 

or 1,000 gallons per minute (fig. 4). 

These results probably represent 

minimum values because data are 

from reported pumping rates and do 

not represent maximum potential 

well yields. However, high yields 

do not guarantee sustained production or production without adverse 

consequences.

Information about additional 

physical features of the hydrologic 

system, such as areas of recharge and 

discharge and the connection between 

brackish groundwater and fresh 

groundwater or surface water, also is 

needed to evaluate the sustainability 

of developing brackish groundwater. 

An understanding of these characteristics provides the ability to assess the 

potential for extraction of brackish 

groundwater to cause (1) enhanced 

movement of more mineralized water 

into freshwater zones, and vice versa; 

(2) substantial groundwater-level 

declines and land subsidence; and 

(3) cascading effects on streamflow 

and other surface water bodies as the 

hydrologic system attempts to adjust 

to these new withdrawals.
Percentage of brackish groundwater samples

Brackish groundwater region

Coastal 

Plains

Eastern 

Midcontinent

Southwestern 

Basins

Western 

Midcontinent

100

80

60

40

20

0

Reported well yield of sampled well, 

in gallons per minute

10 or more 100 or more 1,000 or more

EXPLANATION

Figure 4. Percentage of brackish groundwater samples that are 

from wells that meet or exceed reported well yields of 10, 100, and 

1,000 gallons per minute.

Can Brackish Groundwater be Used as an 

Alternative to Freshwater Resources?

Water providers are turning to brackish groundwater to 

augment or replace the use of freshwater for drinking and 

for other water uses, which include cooling water for power 

generation, irrigation, aquaculture and a variety of uses in the 

oil and gas industry such as drilling, enhanced recovery, and 

hydraulic fracturing. The use of brackish groundwater, however, 

has been hampered by the lack of basic knowledge concerning 

brackish groundwater distribution, accessibility, yield, chemical 

composition, use, treatment requirements, and effects on the 

environment.

A better understanding of the distribution and physical 

and chemical characteristics of brackish groundwater will help 

support efficient and sustainable economic development of the 

resource. Identification of new sources of brackish groundwater, especially in areas with limited freshwater resources, has 

the potential to enhance the Nation’s water security. In some 

areas, the use of brackish groundwater could reduce effects on 

freshwater resources commonly used for drinking, business, and 

recreational activities. The strategic development and treatment 

of brackish groundwater to produce new supplies of differing 

quality for a variety of uses could help water-stressed regions 

stretch their limited freshwater supplies.

The national brackish groundwater assessment has provided 

updated basic information about the occurrence and characteristics of brackish aquifers and creates a foundation for focusing 

future research in locations that could be helpful for the development of the resource. The next challenge with regard to possible 

development of brackish groundwater is to acquire detailed 

information for specific brackish groundwater aquifers that is 

needed for evaluating sustainable brackish groundwater development, including the effects of brackish groundwater withdrawals 

on adjacent water resources.

For More Information

For more information about the national brackish groundwater assessment, go to https://water.usgs.gov/ogw/gwrp/brackishgw/, which includes downloadable data compiled from more 

than 30 national, regional, state, and local sources that were used 

to assess the Nation’s brackish groundwater resources. 

References Cited

Feth, J.H., 1965, Preliminary map of the conterminous United 

States showing depth to and quality of shallowest ground 

water containing more than 1,000 parts per million dissolved 

solids: U.S. Geological Survey Hydrologic Atlas 199, 31 p., 

2 plates, scale 1:3,000,000. [Also available at https://pubs.

er.usgs.gov/publication/ha199.]

Langelier, W.F., 1936, The analytical control of anti-corrosion 

water treatment: Journal of American Water Works Association, v. 28, no. 10, p. 1500–1521. [Also available at http://

www.jstor.org/stable/41226418.]

McMahon, P.B., Böhlke, J.K., Dahm, K.G., Parkhurst, D.L., 

Anning, D.W., and Stanton, J.S., 2015, Chemical considerations 

for an updated national assessment of brackish groundwater 

resources: Groundwater, v. 54, no. 4, p. 464–475, accessed February 3, 2017, at http://dx.doi.org/10.1111/gwat.12367.

National Drought Mitigation Center, the U.S. Department of 

Agriculture, and the National Oceanic and Atmospheric 

Association, 2016, U.S. drought monitor for January 6, 2015: 

National Drought Mitigation Center, digital data, accessed 

March 13, 2016, at http://droughtmonitor.unl.edu/.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User’s guide to 

PHREEQC (version 2)—A computer program for speciation, batch-reaction, one-dimensional transport, and inverse 

geochemical calculations: U.S. Geological Survey WaterResources Investigations Report 99–4259, 312 p. [Also available at https://pubs.er.usgs.gov/publication/wri994259.]

Qi, S.L., and Harris, A.C., 2017, Geochemical database for the 

brackish groundwater assessment of the United States: U.S. 

Geological Survey data release, https://dx.doi.org/10.5066/

F72F7KK1.

Reilly, T.E., Dennehy, K.F., Alley, W.M., and Cunningham, 

W.L., 2008, Ground-water availability in the United States: 

U.S. Geological Survey Circular 1323, 70 p. [Also available at 

https://pubs.usgs.gov/circ/1323/.]

Stanton, J.S., Anning. D.W., Brown, C.J., Moore, R.B., 

McGuire, V.L., Qi, S.L., Harris, A.C., Dennehy, K.F., McMahon, P.B., Degnan, J.R., and Böhlke, J.K., 2017, Brackish 

groundwater in the United States: U.S. Geological Survey 

Professional Paper 1833, 185 p., https://dx.doi.org/10.3133/

pp1833.

U.S. Environmental Protection Agency, 2015, Secondary drinking water standards—Guidance for nuisance chemicals: U.S. 

Environmental Protection Agency, accessed March 2, 2016, 

at https://www.epa.gov/dwstandardsregulations/secondarydrinking-water-standards-guidance-nuisance-chemicals.

Any use of trade, firm, or product names is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

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ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20173054