Land in arid and semiarid regions of the

world is irrigated to sustain agriculture,

urban yards and lawns, and other vegetation.

To prevent the accumulation of salts in the

root zone, the quantity of water applied

must be sufficient to flush the salts beyond

the root zone as well as meet the plant

requirements. Many factors determine the

extent and severity of salt accumulation

including chemical composition of the water

supply; nature and composition of the soil

and subsoil; topography of the land; quantity

of water used and the methods of applying

it; kinds of crops grown; climate of the

region, especially the quantity and

distribution of rainfall; and nature of groundwater and surface-water drainage systems

(Hem, 1985). Irrigated agriculture can result

in rising water tables, waterlogged soils,

progressive mineralization of water and

soils, briny wastewater-disposal problems

and concerns, and contamination of ground

water by fertilizers and pesticides applied

to the land and by chemicals in treated

sewage effluent when it is used for irrigation

(Bouwer, 1990; Bouwer and others, 1998).

 In south-central Arizona, the conversion

of desert and rangeland to irrigated

agricultural and urban land has been possible

because of the impoundment of rivers, the

pumping of ground water, and the

importation of water. The salts that remain

in the soil when these waters are used for

irrigation are of concern because they can

adversely affect crop production; quality

of the underlying ground water; and

domestic, municipal, and industrial water

uses. In order to understand the causes and

effects of salt accumulation in water and

soils and how to manage or mitigate those

effects, we need to understand where the

salts come from and where they go.

What are salts?

 The terms “salt content” or “salinity” of

water actually refer to the quantity of

mineral constituents that are dissolved in

the water. The dissolved minerals or salts

in water typically are reported as the

 The accumulation of salts in soils and

ground water in arid and semiarid regions

as a result of agricultural and irrigation

practices is as much a concern to modern

civilizations as it was to ancient civilizations.

For example, the flood plain of the Tigris

and Euphrates Rivers, known as the “Fertile

Crescent” in ancient Mesopotamia (presentday Iraq), was first irrigated more than 6,000

years ago. The resulting agricultural surplus

provided the foundation upon which the

civilization was built; however, canals built

in 4000 B.C. did not sufficiently drain excess

water from the agricultural areas, and salts

accumulated in water and soils. Progressive

waterlogging and salinization were

evident from the historical succession

of crops—a 50/50 split of wheat and

barley was grown in about 3500 B.C.;

by 2500 B.C., the more salt-tolerant

barley represented 80 percent of the

crop, and finally by 1700 B.C., wheat

could not be grown because of the salts

that had accumulated in the ground

water and soil. Centuries of irrigating

poorly drained soil with highly

mineralized water in an arid climate

left a thick crust of salt on the land

surface and soil hardened by salt deposits.

By 1950, 60 percent of the tillable land in

the area was affected by salt accumulation

(Earthscan, 1984). A more recent example

is the western desert of Egypt on the fringe

of the Nile River delta. Irrigation began in

1956, and in 5 years, salinization of ground

water and waterlogging of crop lands were

causing deterioration in crop production

(Hassan and others, 1979). As Scofield

(1938) noted, “The application of irrigation

water in abundance to soils in arid regions

may have consequences unsuspected by

those engaged in developing projects and

farmers settling on the land.”

USGS Fact Sheet 170-98

June 1999

A lesson from an ancient civilization

SYRIA

IRAN

Caspian

Sea

Persian

Gulf

Area of ancient

Mesopotamia

IraqTigris River

Euphrates River

U.S. Department of the Interior

U.S. Geological Survey

National Water-Quality Assessment Program

Introduction

Where do the salts go?

The potential effects and management of salt accumulation in south-central Arizona

Phoenix

Flagstaff

Tucson

Riv

Gila

er

Salt

114° 112° 110°

36°

34°

32°

River

River Gila

Yuma

100 KILOMETERS

0

0

100 MILES

Colorado

River

Phoenix

Tucson

Gila

Salt River

River

PIMA

Salt River Valley

19

60

60

89

89

10

10

Tempe

Scottsdale

Tucson

Phoenix

PINAL

0

0

25 MILES

25 KILOMETERS

Central

Arizona

Project

Canal

Central

Arizona

Project

Canal

Arizona

Mesa

MARICOPA
“dissolved-solids concentration” in

milligrams of dissolved salts in one liter of

water (mg/L). Water with a dissolved-solids

concentration of less than 500 mg/L—about

a quarter of a teaspoon of salts per gallon

of water—generally is suitable for most

uses (Swenson and Baldwin, 1965). Water

may have a mineralized or salty taste when

the dissolved-solids concentration exceeds

500 mg/L, which is the Federal secondary

maximum contaminant level for drinking

water (U.S. Environmental Protection

Agency, 1994). At concentrations greater

than 2,000 mg/L, water generally is

unsuitable for many uses including longterm irrigation (Swenson and Baldwin,

1965). The salts that constitute a major part

of the dissolved-solids concentration in

waters of south-central Arizona are calcium,

magnesium, sodium, sulfate, chloride, and

bicarbonate. Concentrations of nitrate,

fluoride, and trace metals such as arsenic

or selenium are particularly significant

because they affect the suitability of water

for certain purposes (Hem, 1985).

Where do the salts come from?

 Natural processes add salts to surface

water and ground water. The concentration

of salts in water is determined by many

factors including reactions with minerals

in the soil and rock formations across which

and through which the water moves. For

example, the average dissolved-solids

concentration of the Salt River as it enters

the Salt River Valley east of Phoenix (Salt

River below Stewart Mountain Dam) is

about 480 mg/L (Baldys and others, 1995).

The average concentration of imported

lower Colorado River water used in the

area (Central Arizona Project Canal at 7th

Street in Phoenix) is about 580 mg/L (David

Anning, hydrologist, U.S. Geological

Survey, oral commun., 1998). Ground water

in south-central Arizona generally has a

dissolved-solids concentration of less than

500 mg/L; however, higher concentrations

are present in many areas.

 Human activities also can add salts to

natural waters. For example, irrigation water

may leach mineral constituents from the

soil and deeper geologic formations and

carry them to the ground water, which in

turn, can discharge to surface water where

the water table intersects the land surface.

Mining activities can release dissolved

mineral constituents to local streams and

ground water. Storm runoff from urban

areas and municipal wastewater also can

contribute salts and chemicals.

 The Salt and Colorado Rivers bring not

only water into central Arizona, but also

salts—about 1.1 million tons for the

estimated 1.4 million acre-feet (Central

Arizona Project, 1998) of Colorado River

water imported in 1997 through the Central

Arizona Project (CAP) canal, and about

520,000 tons in the roughly 0.8 million

acre-feet (Tadayon and others, 1998) of Salt

River water that flowed into the greater

Phoenix area in 1997. This is a total

imported salt load of about 1.6 million tons

per year, the equivalent to a half-ton pickuptruck load of salts entering the area about

every 10 seconds or about 900 pounds of

salts per person per year for each of the 3.6

million people in south-central Arizona in

1997 (estimate for Maricopa, Pinal, and

Pima Counties from Valerie Rice, University

of Arizona, Economic and Business

Research Program, oral commun., 1998).

An equal quantity of salts would have to

leave the area to maintain a salt balance.

Yet there is no substantial removal of salts

from the area because almost all of the

water from the Salt River and the CAP

canal is used in south-central Arizona (David

Anning, oral commun., 1998). So where

do the salts go?

Where do the salts go?

 The answer is simple—The salts go where

the water goes, and they accumulate in soils

where evapotranspiration (combined

evaporation from soils and transpiration by

plants) exceeds combined precipitation and

irrigation. For south-central Arizona, salts

accumulate in irrigated agricultural and

urban areas (parks, golf courses, and

residential yards). The water that is returned

to the atmosphere by evapotranspiration

essentially is distilled water, leaving the

The total imported salt load in

south-central Arizona equals

about 900 pounds of salts per

person per year!

Central Arizona Project Canal, west of Phoenix,

Arizona. (Photograph by Darryl Webb, Mesa

Tribune, published with permission.)

Salt River, east of Phoenix, Arizona.

(Photograph by Gail Cordy.)
Central Arizona Project

(CAP) water

~300 feet

Ground-water

table

Evaporation/

transpiration

by plants Irrigation

Salt River

water

Fertilizer and pesticide Root zone

CAP water residues

Deep percolation

500

5 580 480

4.5 2,400

Salt River water

Deep percolation

2,900

4.5

4.5

Rate of downward movement

of water (deep percolation)

in unsaturated zone, in feet

per year

Rate of ground-water rise, 4.5

in feet per year

Saturated zone

Unsaturated zone

Quantity of irrigation applied,

evapotranspiration, or deep

percolation per year for a unit

area, in feet 5

Diagrammatic illustration showing quantity of irrigation, evapotranspiration, or deep percolation; rate of ground-water movement; and

dissolved-solids concentration for waters in the Salt River Valley, Phoenix, Arizona.

4

salts in surficial soils and in the root zones

of plants. To avoid salinity damage and

possibly killing plants or crops, the salts

brought in by irrigation water must be

leached from the root zone by applying

more irrigation water than can be evaporated. The leaching results in sustainable

crop production and plant growth.

Generally, the leach water or deeppercolation water continues to move

downward through the soil and basin-fill

sediments of the unsaturated zone until it

reaches the ground water (saturated zone).

 As an example (see illustration above),

consider cotton grown in south-central

Arizona with an efficient irrigation system

that applies 5 feet of water per year, of

which 4 feet are evaporated or transpired

(Erie and others, 1982), and about 1 foot

leaches salts from the root zone. The

leaching process produces 1 foot per year

of deep-percolation water that moves down

to the ground-water table; however, this 1

foot of water per year contains almost all

of the dissolved salts from 5 feet of irrigation

water. As a result, salt concentrations in the

deep-percolation water will be as much as

5 times higher (Bouwer, 1990) than those

of the original irrigation water—about

2,400 mg/L if water from the Salt River is

used and 2,900 mg/L if water from the CAP

canal is used.

What are the potential effects of

salt accumulation in ground water?

 During much of this century, more ground

water has been withdrawn in south-central

Arizona than has been replenished by

natural and artificial means (Arizona

Department of Water Resources, 1994). As

a result, ground-water levels generally have

moved downward more quickly than the

deep-percolation water from agricultural

fields and urban areas, and the quality of

deep ground water has not been degraded

by the slower moving salty water (Bouwer,

1997). Since the mid-1980’s, the trend has

been to rely less on ground water and use

more CAP water, especially for agriculture

(Cordy and others, 1998). This trend could

result in ground-water levels declining more

slowly or even beginning to rise. Water

levels also would rise if salty deeppercolation water reaches the ground-water

table. 

 In the southeastern part of the Salt River

Valley where irrigation has continued but

ground-water pumping has stopped, groundwater levels have risen an average of about

4.5 feet per year in the last 15 years, and

the concentration of salts in shallow ground

water has increased (Karol Wolf, hydrogeologist, Salt River Project, oral commun.,

1998). By applying this rate of water-level

rise to other areas in south-central Arizona,

ground water at a depth of 300 feet today

could rise to the land surface in about 70

years if pumping was discontinued and all

of the deep-percolation water continued to

reach the water table. Rates of rise in

ground-water levels depend on the water

storage capacity (interconnected pore space

between soil grains) and water content of

the soils above the water table, the quantity

of irrigation water applied, the quantity of

evapotranspiration, and the natural recharge

from precipitation and surface runoff. As

previously noted, the deep-percolation water

could contain about 2,400 mg/L of salts if

water from the Salt River is used for

irrigation and 2,900 mg/L if water from the

CAP canal is used. In addition to the salts,

ground water may contain elevated

concentrations of contaminants from

fertilizer and pesticide applications,

especially where ground-water levels are

near the land surface.

As salty ground water approaches

land surface, plants begin to show

signs of salinity damage and die

from salty water in the root zone and

waterlogging, basements may flood,

water levels may rise into landfills,

and underground pipes can be

damaged.

 If water levels are allowed to remain at or

near land surface, salt marshes and salt flats

could form. How can the salty ground water

be managed to prevent these problems?

How can the salty ground water be

managed?

 Because salinization of ground water and

soils is a common problem in arid and

semiarid parts of the world where land is

irrigated, many solutions have been

proposed and tested. The salt load can be

reduced through improved irrigation

practices or modifications in cropping

practices that reduce deep-percolation losses

(Ayars and other, 1997). Improved irrigation

practices might include reducing preplanting

irrigation, using different irrigation

technologies such as drip systems that

deliver water directly to each plant, and

Deep

percolation 1

Dissolved-solids concentration,

in milligrams per liter 480
using shallow ground-water management

techniques such as tile drains to collect the

salty water where ground-water levels are

high. Cropping modifications could include

allowing some land to lie fallow, growing

crops using dryland techniques, and retiring

land from agricultural use.

 If ground water in the upper parts of the

aquifers is contaminated by deeppercolation water, it could be too salty for

drinking or irrigation of salt-sensitive crops;

however, there are several options for

managing the salty water. One option is to

pump ground water from the upper parts

of the aquifers to stabilize ground-water

levels at acceptable depths (Bouwer, 1997).

This salty water could then be disposed of

in evaporation lakes after minimizing the

volume of water and maximizing the salt

content by sequential irrigation of

increasingly salt-tolerant plants (Shannon

and others, 1997). In this process, the deeppercolation water from salt-sensitive crops

like vegetables is captured and used to

irrigate a more salt-tolerant crop, such as

cotton, from which the deep-percolation

water could be used on very salt-tolerant

plants ending with halophytes (extremely

salt-tolerant plants). The salty water at the

end of the process could be managed in

evaporation lakes; however, these lakes can

become environmental hazards by creating

areas of high salt concentrations that can

be detrimental to animals and plants.

 Another option is to desalt the pumped

ground water using reverse osmosis or other

membrane-filtration processes. Desalting

produces a reject brine that can be stored

indefinitely in lined evaporation ponds.

Salty deep-percolation water and (or) reject

brines could be injected into deep wells far

below the potable ground water; however,

Federal regulations must be met in the

selection of disposal wells, and the

migration of these waters into potable water

supplies cannot always be predicted or

controlled. A third option is to convey the

leach water and (or) brines by a “brine line”

to the lower end of the Colorado River for

commercial desalinization (reverse osmosis)

and (or) for expanding wetlands at the end

of the Colorado River (Bouwer, 1997).

Other options are possible, but the

accumulation of salts in ground water and

waterlogging of soils in south-central

Arizona could cause significant problems

if practices that allow salt accumulation

continue.

—Gail Cordy (USGS) and Herman Bouwer

(USDA)

References Cited

Arizona Department of Water Resources,

1994, Arizona water resources

assessment, v. I, Inventory and Analysis:

Phoenix, Arizona Department of Water

Resources report, August 1994, p. 51.

Ayars, J.E., Soppe, R.W., Cone, D., and

Wichelns, D., 1997, Managing salt load in

irrigation district drainage water, in

Findikakis, A.N., and Stauffer, Fritz, eds., 

Groundwater: An endangered resource:

International Association for

Hydraulic Research, 27th Congress,

San Francisco, California, August 10-15,

1997, Proceedings, p. 277-282.

Baldys, Stanley, III, Ham, L.K., and Fossum,

K.D., 1995, Summary statistics and trend

analysis of water-quality data at sites in the

Gila River Basin, New Mexico and Arizona:

U.S. Geological Survey Water-Resources

Investigations Report 95-4083, 86 p.

Bouwer, Herman, 1990, Agricultural

chemicals and groundwater quality: Journal

of Soil and Water Conservation, v. 45,

no. 2, p. 184-189.

———1997, Arizona’s long-term water

outlook: From NIMTO to AMTO:

Arizona Hydrological Society Newsletter,

v. 14, April and May issues.

Bouwer, Herman, Fox, Peter, and Westerhoff,

Paul, 1998, Irrigating with treated effluent:

How does this practice affect underlying

ground water?: Water Environment and

Technology, v. 10, no. 9, p. 115-118.

Central Arizona Project, 1998, Achieving the

dream: Central Arizona Project 1977 Annual

Report, Phoenix, Arizona, 44 p.

Cordy, G.E., Rees, J.A., Edmonds, R.J., Gebler,

J.B., Wirt, L., Gellenbeck, D.J., and Anning,

D.W., 1998, Water-quality assessment of the

Central Arizona Basins, Arizona and northern

Mexico—Environmental setting and

overview of water quality: U.S. Geological

Survey Water-Resources Investigations 

Report 98-4097, 72 p.

Earthscan, 1984, Cropland or wasteland: The

problem and promises of irrigation: Earthscan

Press Briefing Document No. 38, 97 p.

Erie, L.J., French, O.F., Bucks, D.A., and Harris,

K., 1982, Consumptive use of water by major

crops in the southwestern United States: U.S.

Department of Agriculture, Agricultural

Research Service, Conservation Research

Report Number 29, 40 p.

Hassan, M.N., Marei, S., Motagalli, M., El Abd,

H., and Arnaout, S., 1979, Salinity hazard

and water logging in the area west of

Noubaria canal, in Bishay, Adli, and

McGinnies, W.G., eds., Advances in desert

and arid land technology and

development: International Conference on

Applications of Science and Technology for

Desert Development, Cairo, Egypt,

September 9-15, 1978, Proceedings,

p. 85-95.

Hem, J.D., 1985, Study and interpretation of the

chemical characteristics of natural water:

U.S. Geological Survey Water-Supply Paper

2254, p. 213-217.

Shannon, M., Cervinka, V., and Daniel, D.A.,

1997, Drainage water reuse, in

Madramootoo, C.A., Johnston, W.R., and

Willardson, L.S., eds., Management of

Agricultural Drainage Water Quality:

Food and Agricultural Organization of the

United Nations, Water Report No. 13,

Rome, p. 29-40.

Scofield, C.S., 1938, Soil, water supply, and soil

solution in irrigation agriculture, in Soils &

Men, Yearbook of Agriculture 1938: U.S.

Department of Agriculture, p. 705-716.

Swenson, H.A., and Baldwin, H.L., 1965, A

primer on water quality: U.S. Geological

Survey, Washington, D.C., 27 p.

Tadayon, Saeid, Duet, N.R., Fisk, G.G.,

McCormack, H.F., Pope, G.L., and Rigas,

P.D., 1998, Water resources data, Arizona,

water year 1997: U.S. Geological Survey

Water-Data Report AZ-97-1, p. 220.

U.S. Environmental Protection Agency, 1994,

National revised secondary drinking water

regulations–Secondary maximum

contaminant levels: Washington, D.C.,

U.S. Environmental Protection Agency,

U.S. Code of Federal Regulations, Title 40,

section 143.3, July 1, 1994, p. 836.

For further information contact:

Gail Cordy

U.S. Geological Survey

520 N. Park Avenue, Suite 221

Tucson, Arizona 85719-5035

(520) 670-6671 

E-mail:gcordy@usgs.gov

or visit our home page

http://az.water.usgs.gov/cazb

Dr. Herman Bouwer

U.S. Water Conservation Laboratory

4331 E. Broadway Road

Phoenix, Arizona 85040-8832

(602) 379-4356

E-mail:hbouwer@uswl.ars.ag.gov

or visit our home page

http://www.uswcl.ars.ag.gov

Printed on recycled paper