U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3016

August 2018

and its Potential as a Resource in the Southwestern United States

Brackish Groundwater

120° 110°

PACIFIC

OCEAN

0 150 300 MILES

0 150 300 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, copyright 2014, and is used herein under 

license.

Albers Equal-Area Conic projection.

North American Datum of 1983. 

40°

30°

EXPLANATION

Central Valley aquifer system. Brackish groundwater

occurs in upper parts of unconfined system; saline 

water occurs in marine sediments at depth.

Rio Grande aquifer system. Shallow brackish groundwater occurs in closed basins, and areas with upward-flowing deep basin groundwater that is brackish

as a result of long flow paths. 

Basin and Range basin-fill aquifers. Shallow 

brackish groundwater occurs in closed basins 

near playas, above confining units, and areas with

upward flowing deep basin groundwater that is 

brackish as a result of long flow paths. Water at 

depths greater than a few thousand feet yield 

saline water that is confined and has poor

hydraulic connection with shallower systems.

Basin and Range carbonate-rock aquifers. Brackish 

groundwater occurs in areas with geothermal 

activity and with groundwater discharge.

California Coastal Basin aquifers. Shallow 

brackish groundwater results from concentration 

by evapotranspiration, application of saline waters

(oil-field brines and irrigation waters) at land 

surface, and seawater intrusion. Deeper brackish

groundwater occurs in unconsolidated to semiconsolidated marinesediments containing saline 

water.

Principal aquifers and description of brackish zones 

Brackish groundwater assessment boundary for 

the Southwestern United States

ARIZONA

UTAH COLORADO

NEW

MEXICO

TEXAS

NEVADA

CALIFORNIA

IDAHO

MEXICO

Figure 1. Map of the five principal aquifers 

in the southwestern United States and 

description of the settings where the brackish zones of each aquifer occur.

Secure, reliable, and sustainable water resources are

fundamental to food production, energy independence, and 

the health of humans and ecosystems. But the large-scale 

development of fresh groundwater resources has stressed 

aquifers in some areas, causing declines in the amount 

of groundwater in storage and decreases in discharge to 

surface-water bodies like rivers and springs (Reilly and 

others, 2008). In some parts of the southwestern United 

States, the water supply is not adequate to meet demand 

without substantial effects on groundwater storage or 

surface discharge, and severe drought intensifies the stresses 

affecting water resources. 

In support of the national census of water resources, 

the U.S. Geological Survey (USGS) completed the 

national brackish groundwater assessment to provide 

information about brackish groundwater as a potential 

resource to augment or replace freshwater supplies 

(Stanton and others, 2017). The objectives of the 

brackish groundwater assessment were to consolidate 

available data into a comprehensive database of 

brackish groundwater resources in the United States 

and to produce a summary report about the distribution, 

physical and chemical characteristics, and use of 

brackish groundwater. This fact sheet summarizes 

the occurrence of brackish groundwater and factors 

affecting its usability in the southwestern United States 

(specifically the Southwest Basins region) reported for 

the national study. The map below (fig. 1) summarizes 

the brackish zones for the five largest principal aquifers 

within the southwestern United States, along with 

groundwater resources in the remaining part of the 

region (Reilly and others, 2008).
Brackish Groundwater Occurrence 

Previous regional studies have found that groundwater quality 

for most of the aquifers in the southwestern United States varies 

horizontally and vertically and is related to regional geology, 

mineralogy, structure, drainage patterns, and development 

(Thiros and others, 2014). The Basin and Range basin-fill aquifers, California Coastal Basin aquifers, Central Valley aquifer 

system, and Rio Grande aquifer system consist of unconsolidated 

to semi-consolidated stream and lake sediments that in places are 

interspersed with volcanic rocks. The Basin and Range carbonate-rock aquifers are of marine origin and consist of interbedded 

limestone, dolomite, sandstone, shale, and volcanic rocks. Within 

these principal aquifers, brackish groundwater occurs in different 

settings and at different depths. Brackish groundwater conditions 

also occur outside of these principal aquifers.

Water quality is determined, in part, by dissolved-solids 

concentrations, which vary across a basin. Concentrations are 

typically lower in groundwater recharge areas, such as along 

mountain fronts, and higher in groundwater discharge areas, 

which often occur in the middle of the basins. In many parts of 

the southwest, brackish groundwater occurs primarily in two 

zones: a shallow zone affected by the concentration of minerals from evaporative processes, particularly at the terminus of 

closed basins; and a deep zone affected by long flow paths, the 

presence of soluble salts (such as gypsum, anhydrite, and halite), 

inflow from adjacent geologic units, or saline water contained 

2

within marine sediments. Other processes that are associated 

with elevated dissolved-solids concentrations are stream leakage, 

thermal springs, oil-field brines, and seawater intrusion along the 

California coast.

The national brackish groundwater assessment reported 

dissolved-solids concentration data for groundwater samples 

from about 34,000 wells sampled by Federal, state, and local 

agencies in the southwestern United States (see table; Qi and 

Harris, 2017). These data were then added to a three-dimensional 

model that provided a framework to analyze the spatial distribution of brackish groundwater conditions. The model grid consists 

of 4 layers of cells with 6.2 mile (mi) by 6.2 mi horizontal 

extents, and each layer extends through different depth intervals 

below the land surface (bls): 0–50 feet (ft), 50–500 ft, 500–1,500 

ft, and 1,500–3,000 ft. Although the model grid in its entirety 

represents about 216,000 mi3 of the subsurface, only about 14 

percent of that volume (30,300 mi3) is represented by sampled 

wells, and the remaining 86 percent has unknown brackish 

conditions. 

Brackish groundwater occurs in many parts of the southwestern United States (fig. 2). The model grid used for the brackish 

groundwater assessment maps both the extent of brackish conditions above 3,000 feet bls—about 9,300 mi3

—and the percent 

volume indicated by well data—about 31 percent. Most of the 

observed brackish groundwater occurred between 50 and 1,500 ft 

bls (fig. 3).

Water containing moderate to high concentrations of dissolved solids such as minerals, metals, 

and salts is considered brackish. Important chemical constituents contributing to dissolvedsolids concentrations are typically cations (positively charged molecules) such as calcium, 

magnesium, sodium, and potassium; anions (negatively charged molecules) such as sulfate, 

chloride, and bicarbonate; as well as charge-neutral silica. The U.S. Environmental Protection 

Agency advises, but does not require, that public drinking-water systems provide water with 

less than 500 milligrams per liter (mg/L) in total dissolved solids concentrations. Numerous water 

supplies, however, exceed this concentration. Water exceeding 1,000 mg/L in dissolved solids 

is generally considered undesirable for human consumption and may smell unpleasant or taste 

bitter, salty, or metallic. Water with higher dissolved-solids concentrations can be used for a 

variety of purposes other than drinking water. For comparison, the dissolved solids concentration of seawater is about 35,000 mg/L. For this assessment, brackish groundwater was defined 

as having a dissolved-solids concentration between 1,000 and 10,000 mg/L, and highly saline 

groundwater was defined as having dissolved-solids concentrations greater than 10,000 mg/L.

What is Brackish Groundwater?
Areas underlain by the principal aquifers of the southwestern 

United States could benefit from the development of brackish 

water as a resource to complement fresh water supplies. 

Occurrence near the surface makes the brackish groundwater 

potentially less expensive and more feasible to extract through 

wells. Brackish groundwater resources can be found within the 

50 feet of the land surface in extensive areas of the Central Valley 

aquifer system in California and in the Rio Grande aquifer system 

of south-central New Mexico (figs. 1 and 2). Brackish groundwater 

can be found within 50 to 500 ft bls in extensive areas in the 

California Coastal aquifers, the Basin and Range basin-fill aquifers 

of southwestern Arizona and western Utah, and the Rio Grande 

aquifer system of central New Mexico and west Texas (figs. 1 

and 2). Between 20 and 33 percent of the wells completed in each 

southwest principal aquifer produced brackish groundwater, and 

1 to 2 percent produced highly saline groundwater (see table). 

Of the wells that could not be assigned to or were not completed 

within one of the principal aquifers evaluated, 20 percent of them 

yielded brackish groundwater. The observed volume of brackish 

groundwater occurring between 0 and 3,000 ft bls within grid 

cells from each of the five principal aquifers evaluated in the 

Southwestern Basin region ranged from 31 to 38 percent of the 

total volume of the region with wells (see table).

Brackish Groundwater as a Potential Resource

Brackish groundwater potentially represents a substantial 

water resource for the southwestern United States that 

could be used to augment or replace freshwater supplies 

in some areas. In the southwest, about 22.5 million acrefeet of groundwater are withdrawn annually from the 

five principal aquifers for irrigation, public supply, and 

industrial purposes (Maupin and Barber, 2005). An acrefoot of water is about 326,000 gallons, or almost enough 

to fill a football field one foot deep. Only about 0.6 million 

acre-feet (or about 3 percent) of the annual withdrawals are 

brackish or highly saline (Stanton and others, 2017), but, for 

example, if the amount of brackish groundwater that could 

be withdrawn and used from the subsurface is 1 percent of 

the total volume represented by wells observed to contain 

brackish groundwater in the southwestern United States, then 

there is about 93 mi3, or 310 million acre-feet, of brackish 

groundwater resources potentially available for use. There is 

likely even more brackish groundwater potentially available 

than this estimate, because the unobserved area (86 percent 

of the region) may also yield usable brackish water supplies. 

A predictive model from the national study indicates a high 

probability of brackish groundwater occurrence across much 

of the southwest region (Stanton and others, 2017). 

3

Las

Vegas

Tucson

Phoenix

San

Francisco

El Paso

Los

Angeles

Albuquerque

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Sacramento

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100 200 MILES

100 200 300 KILOMETERS

EXPLANATION

Minimum depth to brackish 

groundwater, in feet 

0–50

50–500

500–1,500

1,500–3,000

Brackish conditions not observed

Not evaluated

ARIZONA

UTAH COLORADO

NEW

MEXICO

TEXAS

NEVADA

CALIFORNIA

IDAHO

MEXICO

Base map from Esri, copyright 2017, used herein under license.

Transverse Mercator projection, NAD 1983, UTM zone 12N 

Figure 2. Map showing 

the occurrence of brackish groundwater in the 

southwestern United 

States. Water-quality data 

from wells were recorded 

to a depth of 3,000 feet 

below the land surface, and 

for wells having brackish 

water, the map shows the 

minimum depth at which it 

was observed.
4

Principal aquifer

Basin and 

Range basinfill aquifers

Basin and 

Range 

carbonaterock aquifers

California 

Coastal 

Basin 

aquifers

Central 

Valley 

aquifer 

system

Rio Grande 

aquifer 

system

Other1

Statistics for wells and samples

Number of sampled wells 13,874 335 3,403 6,276 2,813 7,516

Median sample dissolved-solids 

concentration, milligrams per 

liter

490 630 590 400 660 440

Sampled wells with brackish 

groundwater, in percent

24 25 26 20 33 20

Sampled wells with highly saline 

groundwater, in percent

2 2 1 2 1 1

Estimated grid-cell volume represented by samples 2

Fresh, brackish, and highly saline 

groundwater, in cubic miles 11,130 590 1,960 4,220 1,870 10,530

Brackish groundwater, in cubic 

miles (percent of represented 

volume)

3,510 (32) 200 (33) 600 (31) 1,550 (37) 720 (38) 2,680 (25)

Highly saline groundwater, in cubic 

miles (percent of represented 

volume)

300 (3) 35 (6) 85 (4) 145 (3) 60 (3) 130 (1)

Well yields for shallow brackish wells, less than 500 feet depth interval

Number of sampled wells 803

Included in 

deeper 

brackish 

wells

26 38 42

Not compiled Median well yield, gallons/minute 1,570 425 600 10

Interquartile range of well yield, 

gallons/minute 449 to 2,780 40 to 800 175 to 1,500 3 to 150

Well yields for deeper brackish wells, 500 to 3,000 feet depth interval

Number of sampled wells 336 18 10 46 5

Not compiled Median well yield, gallons/minute 1,970 300 900 1,510 315

Interquartile range of well yield, 

gallons/minute 1,140 to 2,650 30 to 800 800 to 2,000

1,130 to 

2,180 200 to 1,150

Summary of observed brackish and highly saline groundwater in principal aquifers of the southwestern 

United States.

[Grid cells are categorized as “fresh,” “brackish,” or “highly saline” on the basis of the maximum observed dissolved-solids concentration found in groundwater samples collected from wells within the designated grid cell being less than 1,000 milligrams per 

liter (mg/L), between 1,000 mg/L and 10,000 mg/L, or greater than 10,000 mg/L, respectively. In this categorization, “fresh” cells 

could also contain unobserved brackish or highly saline groundwater, “brackish” cells could also contain observed or unobserved 

fresh groundwater and unobserved highly saline groundwater, and “highly saline” cells could also contain observed or unobserved 

fresh and/or brackish groundwater] 

Figure 3. This pie chart plots the 

percent of total subsurface volume 

observed to contain brackish 

groundwater in each depth interval. 

Of a mapped total 9,300 cubic miles, 

most of the observed brackish 

water in the Southwestern Basins 

region occurs between 50 and 

1,000 feet below land surface. 

Depth interval 

0–50 feet

1 percent

50–500 feet 

42 percent

500–1,500 feet 

44 percent 

1,500–3,000 feet 

13 percent 

1

Includes 6 wells from the Southern Nevada volcanic-rock aquifers and 31 wells in sand and gravel of alluvial or glacial origin. The remainder of wells are either outside the 

horizontal and vertical extent of the listed principal aquifers, or could not definitively be assigned to one of the listed principal aquifers.

2

Grid-cell volumes include air, water, and rock occupied in the subsurface to 3,000 feet below land surface. Volumes are computed based on grid-cell dimensions and on well 

construction and sample information associated with each grid cell. Estimates reported here reflect grid-cell volumes for areas where well observations are available. Entire 

principal aquifer subsurface volumes could be greater than those reported here because of the lack of observations in many grid cells.
Using Brackish Groundwater

The feasibility of using brackish groundwater may be 

limited by factors such as the concentration of individual ions or the ability of subsurface materials to yield 

groundwater in sufficient volume, but these factors do 

not necessarily preclude brackish groundwater as viable 

supply. For example, water high in dissolved solids may 

initially be unusable for drinking water, but concentrations can sometimes be reduced using processes like 

reverse osmosis, or by blending brackish water with 

freshwater supplies. Brackish groundwater supplies may 

also be used for a variety of purposes other than drinking water, such as for cooling during power generation, 

irrigation, livestock watering, aquaculture, and in the 

oil and gas industry for drilling, enhanced recovery, and 

hydraulic fracturing. 

Some chemical constituents may be found in brackish groundwater at concentrations high enough to be 

of concern for certain uses. Of the many constituents 

analyzed in brackish groundwater samples during 

the national assessment, arsenic, nitrate, and uranium 

were most likely to be present in southwest aquifers in 

concentrations greater than drinking-water standards 

allow. This finding corroborates results from other 

groundwater-quality studies of the southwestern United 

States (Thiros and others, 2014). 

The Basin and Range basin-fill aquifers contained 

the largest percentage of brackish groundwater samples 

that exceeded livestock consumption standards for 

constituents such as arsenic, boron, and fluoride. In 

other principal aquifers, however, untreated brackish 

groundwater was found to generally be within criteria for livestock. Arsenic, boron, or fluoride are also 

potential concerns where untreated brackish groundwater is used for irrigation. 

Other chemical characteristics can also limit 

brackish water use; for example, mineral scaling can 

impede conveyance, storage, and treatment of brackish 

groundwater. 

The feasibility of using brackish groundwater as a 

resource is limited in some parts of the Nation, where 

groundwater yields are low. In the southwestern United 

States, however, well yields of brackish groundwater 

appear to be sufficiently high (see table 1). About 80 

percent of the wells evaluated in the assessment had 

yields greater than 100 gallons per minute, and almost 

60 percent of the brackish wells reported yields greater 

than 1,000 gallons per minute. Although compaction 

and cementation of deep deposits could limit development of brackish groundwater resources in some areas, 

reported yields indicated that both shallow and deep 

wells produce adequate amounts of brackish water for 

many uses. 

5

Potential uses of 

brackish groundwater include (from 

top to bottom) crop 

irrigation, in drilling and hydraulic 

fracturing for 

recovery of oil and 

gas resources, 

livestock watering, 

and cooling during 

power generation. 

Photograph by Jeff Vanuga, USDA 

National Resources Conservation Service

Photograph by Jeff Vanuga, USDA National 

Resources Conservation Service

Photograph by David Anning, 

U.S. Geological Survey

Photograph courtesy of the U.S. 

Nuclear Regulatory Commission
David W. Anning, Kimberly R. Beisner, Angela P. 

Paul, Jennifer S. Stanton, and Susan A. Thiros

Edited and designed by Claire M. Landowski

For more information contact:

Arizona Water Science Center

520 N. Park Ave., Suite 221

Tucson, AZ 85719

https://az.water.usgs.gov/

1-888-ASK-USGS (1-888-275-8747)

https://www.facebook.com/USGeologicalSurvey

https://twitter.com/USGS

Interactions between groundwater and subsurface evaporite minerals, like 

the gypsum in the photo above, can increase the dissolved-solids content 

of groundwater, making it more brackish. Below, a U.S. Geological Survey 

scientist prepares to sample a municipal well near Phoenix, Arizona, as part 

of one of many ongoing groundwater-monitoring programs in the Southwest. 

Both photographs by David Anning, U.S. Geological Survey. 

ISSN 2327-6916 (print) 

ISSN 2327-6932 (online) 

https://doi.org/10.3133/fs20183010

6

Toward Resource Development 

The USGS national brackish groundwater assessment has provided updated basic information about the 

occurrence and characteristics of brackish aquifers in 

the southwestern United States, and has indicated areas 

of interest for future research in the development of 

this resource. For more information about the national 

brackish groundwater assessment, visit https://water.

usgs.gov/ogw/gwrp/brackishgw/, where data compiled 

from more than 30 national, regional, state, and local 

sources are available for download. 

The next task toward development of brackish 

groundwater as a usable resource is to acquire detailed 

water chemistry and aquifer information for specific 

areas of interest. These data are needed to evaluate the 

sustainability of brackish groundwater development, 

including the potential effects of brackish groundwater 

withdrawals on adjacent water resources.

References Cited

Maupin, M.A., and Barber, N.L., 2005, Estimated withdrawals from principal aquifers in the United States, 

2000: U.S. Geological Survey Circular 1279, 46 p., 

https://pubs.usgs.gov/circ/2005/1279/.

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., 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.

Thiros, S.A., Paul, A.P., Bexfield, L.M., and Anning, 

D.W., 2014, The quality of our Nation’s waters—

Water quality in basin-fill aquifers of the southwestern United States; Arizona, California, Colorado, 

Nevada, New Mexico, and Utah, 1993–2009: U.S. 

Geological Survey Circular 1358, 133 p., http://dx.doi.

org/10.3133/cir1358.