U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2008-3076

2008

T

Dissolved Solids in Basin-Fill Aquifers and Streams in 

the Southwestern United States — Executive Summary

he U.S. Geological Survey (USGS) 

recently completed a regional 

study in the Southwestern United 

States to characterize dissolvedsolids conditions in major water 

supplies, including important rivers 

and aquifers. High concentrations 

of dissolved solids can degrade 

a water supply’s suitability for 

important uses, such as drinking 

water or crop irrigation. In an effort 

to ensure the continued availability 

of clean surface and groundwater, 

USGS scientists identified areas 

where there have been both 

increasing and decreasing trends in 

dissolved-solids concentrations. 

A U.S. Geological Survey scientist collects a sample from the Colorado River for analysis of dissolved solids 

and other water-quality constituents (USGS photograph by Nancy Bauch).

Major water supplies support significant 

economic and cultural activities in the Southwestern United States. These water supplies 

include the Colorado River, the Rio Grande, 

their tributaries, several smaller rivers, and basin-fill aquifers, such as the Rio Grande aquifer system, Basin and Range basin-fill aquifers, and California Coastal Basin aquifers. 

All water naturally contains dissolved solids 

as a result of the weathering and dissolution of 

minerals in soils and geologic formations. Major ions, such as bicarbonate, calcium, chloride, magnesium, potassium, silica, sodium, 

and sulfate, constitute most of the dissolved 

solids in water and are an indicator of salinity. 

High concentrations of dissolved-solids in water, generally those greater than 500 mg/L, can 

degrade a water supply’s suitability for certain 

uses. For domestic, municipal, and industrial 

users, high concentrations can result in objectionable taste to drinking water; greater watertreatment costs, increased use of detergents 

and soaps, and the encrustment or corrosion 

of metallic surfaces and associated reduction 

in appliance or equipment lifespan. For irrigation users, high concentrations can result in 

decreased crop production or crop death, and 

thus, decreases in economic returns. 

The U.S. Geological Survey (USGS) National Water-Quality Assessment Program 

recently completed a regional study in the 

Southwest to characterize dissolved-solids 

conditions in these important water supplies. 

The study focused on addressing questions 

regarding (1) spatial variation, (2) effects of 

natural and human factors, (3) sources, transport, accumulation, and (4) long-term trends 

of dissolved solids in these water supplies. 

This summary highlights some of the major 

findings of the report, “Dissolved-Solids 

in Basin-Fill Aquifers and Streams in the 

Southwestern United States,” USGS Scientific 

Investigations Report 2006-5315, available at 

http://pubs.usgs.gov/sir/2006/5315/.

National Water-Quality

 Assessment Program
How Variable are Dissolved-Solids Concentrations Across the Southwest?

Dissolved-solids concentrations of ground 

water in the basin-fill aquifers of the Southwest ranged from less than 500 mg/L near 

basin margins where ground water is recharged from nearby mountains to more than 

10,000 mg/L in topographically low areas of 

some basins or in areas adjacent to specific 

streams or rivers in the Basin and Range and 

Rio Grande aquifer systems. The U.S. Environmental Protection Agency non-enforceable secondary drinking water regulation of 500 mg/L 

for dissolved solids was established to govern 

esthetic qualities of water, such as taste. The 

area of the basin-fill aquifer systems with dissolved-solids concentrations less than or equal 

to 500 mg/L was about 57% for the Rio Grande 

aquifer system, 63% for the Basin and Range 

basin-fill aquifers, and 44% for the California 

Coastal Basin aquifers. At least 70% of the area 

of each of these three basin-fill aquifer systems 

had dissolved-solids concentrations less than 

or equal to 1,000 mg/L. Dissolved-solids conAs shown on this map, dissolved-solids concentrations in basin-fill aquifers and in streams vary 

across the Southwestern United States and can degrade the suitability of a water supply for drinking 

water or irrigation where over 500 mg/L. 

Rio Grande

San Juan

Green River

San Pedro River

Gila River

Little Colorado River Colorado River

Verde River

Salt

River

Sevier

River

Virgin River

Quinn

River

Humboldt River

Owens River 

Reese River

SantaClara

Mo aj ve River River

SantaAna

River

San Diego

River

Santa Cruz

River

Salton

Sea

Great Salt Lake

Carson River WalkerRiver Truckee

River

Bear River

White River

Yampa Ri ver

BASIN AND RANGE

COLORADO 

PLATEAU

WYOMING BASIN

SOUTHERN 

ROCKY 

MOUNTAINS

PACIFIC

 BORDER

CASCADE-SIERRA MOUNTAINS

LOWER 

CALIFORNIAN

MIDDLE ROCKY 

MOUNTAINS

Reno

Tucson

El Paso

Phoenix

Las Vegas

Albuquerque

Los Angeles

Salt Lake 

City

California

Nevada

Wyoming

Utah

Arizona

Colorado

New 

Mexico

San Diego

110°

30°

35°

35°

40°

40°

120°

115°

Base from U.S. Geological Survey

digital data, 100m, 2003 

Albers Equal-Area Conic projection

Standard parallels 29° 30’, 45° 30’,

central meridian 96°

0 50 100 Miles

0 50 100 Kilometers

Rio 

Grande 

aquifer 

system

Basin and Range 

California Coastal basin-fill aquifers

Basin aquifers

Colorado Plateaus aquifers

Basin and Range 

basin-fill aquifers

Colorado River

EXPLANATION

Median daily 

concentration

in streams, 

milligrams per liter

Less than 100

100 - 249

250 - 499

500 - 999

Greater than

1,000

Insufficient data

0-500

501-1,000

1,000-3,000

3,001-10,000

>10,000

Not a basin-fill aquifer

Concentration

in basin-fill aquifers, 

milligrams per liter

Physiographic Province boundary--

Annotation, BASIN AND RANGE, 

is physiographic province name

Principal Aquifer boundary--Annotation, 

Rio Grande aquifer system, is principal aquifer name
centrations greater than 3,000 mg/L were found 

in topographically low areas with brackish or 

saline lakes, playas, and terminal basins, such 

as the Great Salt Lake and Desert in Utah; the 

Mojave Desert with its many playas, Death 

Valley, and Salton Sea area in California; the 

Black Rock Desert and Carson and Humboldt 

Sinks in Nevada; and the Tularosa Basin in 

New Mexico. Dissolved-solids concentrations 

greater than 3,000 mg/L were also found in 

ground water in the Basin and Range and Rio 

Grande aquifer systems near or along drainages 

of the Virgin, Gila, and lower Salt Rivers in the 

Colorado River Basin, and the Jemez River and 

Rio Puerco in the Rio Grande Basin.

Dissolved solids in streams were described 

on the basis of median daily concentration, median annual load, and median annual yield data 

for 420 surface-water-quality monitoring sites. 

The time period for which dissolved-solids 

data was available for individual sites varied, 

but it was at least 10 or more years between 

1974 and 2003. Median dissolved-solids concentrations vary markedly among the sites in 

the Southwest, ranging between 22 and 13,800 

mg/L, and also vary between different sites 

on the same stream. Median daily dissolvedsolids concentrations less than 100 mg/L are 

predominately found at sites in the headwaters 

of the Rio Grande, Colorado, Green, San Juan, 

Truckee, and Carson Rivers. These areas are 

underlain by igneous and metamorphic rocks 

that are relatively resistant to the solvent action 

of water. Median daily dissolved-solids concentrations greater than or equal to 500 mg/L are 

predominately found in streams in contact with 

less resistant, more soluble sedimentary rocks, 

such as those found on the Colorado Plateau 

physiographic province. Median daily concentrations generally increased in a downstream 

direction for sites on the Rio Grande, Colorado, 

Yampa, White, Green, San Juan, Gila, Bear, 

and Sevier Rivers. 

What Factors Affect Dissolved Solids?

Several natural and human factors affect 

dissolved-solids concentrations in streams 

from their headwaters to their mouths and in 

ground water along flow paths from recharge 

areas to discharge areas. Factors that affect 

concentrations in upland mountain streams 

include (1) the amount of low-concentration 

runoff to the stream available to mix with and 

dilute higher-concentration base flow; (2) 

presence of sedimentary rocks in the drainage basin that are less resistant to the solvent 

action of water, especially evaporite deposits; 

(3) streamflow storage and mixing processes 

in reservoirs; (4) evapotranspiration, which 

concentrates the dissolved solids; and (5) 

transbasin diversions that result in the removal 

of low-concentration water that would otherwise serve to help dilute high-concentration 

water sources in the originating basin.

Streams flow out of upland mountain areas 

and into lowland areas that have flatter terrain 

and contain large basin-fill aquifers. Groundwater recharge of the basin-fill aquifers largely occurs along the basin margin by streamflow infiltration, or by subsurface flow from 

adjacent bedrock highland aquifers. Ground 

water in recharge areas typically has low 

dissolved-solids concentrations in comparison 

to ground water in other parts of the aquifer. 

Dissolved-solids concentrations in ground water typically increase along flowpaths through 

basin-fill aquifers as a result of (1) geochemical reactions with the aquifer matrix, (2) dissolution of disseminated salts and massive 

evaporite deposits, and (3) evapotranspiration 

of shallow ground water by natural vegetaThis map shows delivery rate of dissolved 

solids from internal sources, internal 

accumulation rate, and predominant 

internal source of dissolved solids within 

accounting units in the Southwestern 

United States. Dissolved solids are 

delivered to streams from natural and 

human sources. The delivery rates and 

the relative share from these sources 

vary for hydrologic accounting units in the 

Southwestern United States. Losses of 

dissolved-solids from streams, either due 

to stream diversions or due to streambed 

infiltration, accumulate in the subsurface 

and may degrade ground water. 

Accumulation rates also vary by hydrologic 

accounting unit. 

EXPLANATION

PREDOMINANT INTERNAL SOURCE OF DISSOLVED SOLIDS 

WITHIN ACCOUNTING UNIT

Natural - geologic units

Human - cultivated and pasture lands

HYDROLOGIC ACCOUNTING UNIT BOUNDARY—

Numbers, 23/25 ; left number is delivery rate of dissolved solids from internal sources 

and right number is internal accumulation rate of dissolved solids, both in 

units of tons/year per square mile of accounting unit. nc, not computed. 

42°

40°

38°

36°

34°

32°

44°

108° 106° 112° 110° 114° 116° 118° 120°

CALIFORNIA

ARIZONA

NEW

MEXICO

UTAH

NEVADA

COLORADO

OREGON

IDAHO WYOMING

TEXAS

MEXICO

UNITED STATES

PACIFIC OCEAN

34/34

23/13

27/37

34/34

61/34

23/25

116/nc

197/nc

122/nc

26/26

9/9

148/704

11/12

58/48

12/11

48/143

141/305

290/197

99/60

69/26

32/16

22/3

36/5

41/36

54/163

432/246

110/42

63/8

62/<1

25/43

40/26

65/25

23/9

28/11

11/11

55/83 50/50

48/21

110/99

146/<1

206/<1

73/22

14/14

38/5

19/7

135/11

26/18

0 100 MILES

0 100 KILOMETERS

Base from U.S. Geological Survey digital data, 100m, 2003

Albers Equal-Area Conic projection Standard parallels 

29° 30’, 45° 30’,central meridian 96°

Phoenix

Tucson

San Diego

Los Angeles

Salt Lake City

Reno

Las Vegas

Albuquerque

El Paso
tion or by agricultural crops. Dissolved-solids 

concentrations also can change as a result of 

(1) mixing two or more subsurface waters; (2) 

recharge from irrigation seepage, septic tank 

seepage, and percolation ponds or streambeds 

that infiltrate imported water or treated municipal wastewater; or (3) seawater intrusion 

(in coastal areas). Most of these processes result in increased concentrations in the ground 

water. As streams flow through lowland areas, 

dissolved-solids concentrations in streams 

also change due to evapotranspiration, mixing 

with ground water, irrigation-return flows, or 

releases from municipal wastewater-treatment 

plants. Usually the change is an increased 

concentration downstream.

In lowland areas, the enhancement or restriction of surface-water and ground-water 

outflow affects the accumulation of dissolved 

solids in water supplies. Natural drainage 

or artificial drainage by canals or pipelines 

can enhance the outflow of water containing 

dissolved solids, thereby diminishing the accumulation of salts. Restriction of outflow 

through water use, or through natural features 

like topographic barriers that prevent surface 

outflow, promotes the accumulation of salts. 

The salts generally accumulate in areas with 

high evapotranspiration, a process that increases dissolved-solids concentrations and can impair the quality of a ground-water resource.

Where Do Dissolved Solids Come From and 

Where Do They Accumulate?

Significant dissolved-solids source and accumulation areas were determined by using 

a mass-balance analysis of the contributions 

(inflows, internal deliveries, and imports) and 

losses (outflows, internal accumulation, and 

exports) of dissolved solids for river systems 

in hydrologic accounting units of the Southwest. Quantification of contribution and loss 

terms was facilitated by using a spatially referenced regression model of contaminant transport on watershed attributes. Hydrologic accounting units are large river basins that were 

defined nationwide and average about 16,000 

square miles (mi2

) each in the Southwest.

The relative contribution from natural 

and human sources and the delivery rates 

for internal sources of dissolved solids vary 

by accounting unit. The most significant 

dissolved-solids source areas in the Southwest 

included the Colorado Headwaters, Middle 

Gila, Lower Bear, and Santa Ana hydrologic 

accounting units, where deliveries from internal sources were greater than 150 (ton/year)/

mi2

. Overall, geologic units contribute 44% of 

the total internal deliveries for all hydrologic 

accounting units in the Southwest—about 

7% is from crystalline and volcanic rocks, 

2% is from eugeosynclinal rocks, 12% is 

from Tertiary sedimentary rocks, 12% is from 

Mesozoic sedimentary rocks, and 10% is 

from Paleozoic and Precambrian sedimentary 

rocks. Cultivated lands (44%) and pasture 

lands (12%) contribute the remaining 56% of 

the total internal deliveries for all Southwest 

accounting units. 

 Some of the dissolved solids delivered 

to streams in an accounting unit accumulate 

internally as a result of streambed infiltration 

or by infiltration of diverted water through 

soils in offstream locations. The most significant dissolved-solids accumulation areas include the Salton Sea, Lower Gila-Agua Fria, 

Middle Gila, Lower Bear, and Great Salt 

Lake hydrologic accounting units, where accumulation rates were greater than 150 (ton/

year)/mi2

. The dissolved-solids accumulation 

rate for the Salton Sea hydrologic accounting 

unit, 704 (ton/year)/mi2

, was more than twice 

as high as the second highest rate, 305 (ton/

year)/mi2

 for the Lower Gila-Agua Fria hydrologic accounting unit. 

Are Dissolved-Solids Concentrations Increasing or Decreasing Over Time?

Trends for 1974–1988, 1989–2003, and 

1974–2003 were determined for concentrations 

of dissolved solids in basin-fill aquifers and for 

flow-adjusted and seasonal-adjusted concentrations in streams. For the basin-fill aquifers, 

concentrations of dissolved solids did not 

change over time for most ground-water-quality 

monitoring wells in the analysis. The percentage of wells in basin-fill aquifers with either an 

increasing or decreasing trend in concentrations 

was 23% for 1974–1988, 32% for 1989–2003, 

and 41% for 1974–2003. Of the wells that had 

a trend in concentrations, increasing trends 

were more common than decreasing trends for 

each period. The trends were not focused in any 

particular part of the Southwest, which suggests that trends occur in localized areas and not 

across large regions. For 1989–2003, wells with 

depths to water only a few feet below the land 

surface were the most likely to have trends—

about a 42% chance. The likelihood of a trend 

in concentration occurring decreases steadily 

with depth to water below the land surface and 

is about 5% where depth to water is 1,000 feet. 

In comparison to conditions for groundwater-quality monitoring wells in the basin-fill 

aquifers, the presence of trends in dissolvedsolids concentrations in streams was much 

more common and many decreases were observed. Of the three periods, 1974–1988 had 

the greatest percentage of sites with either no 

change or an increase in concentration. For 

this period, concentrations increased at 34% of 

the sites, did not change at 24% of sites, and 

decreased at 42% of sites. During 1989–2003, 

concentrations decreased at 51% of the sites, 

and there were five river basins where concentrations decreased at 75% or more of the sites. 

For several areas in the Colorado River Basin, 

concentrations decreased during 1989–2003 at 

all sites downstream from salinity-control units, 

whereas increasing and decreasing concentrations trends occurred at sites upstream from the 

units. Decreases in concentrations occurred at 

three sites above salinity-control units but were 

much less than the decreases at sites below 

those units. For 1974–2003, concentrations 

decreased at about 70% of the sites, and the median change in concentration during this period 

for all sites was a decline of about 8%. Most of 

the sites included in the trend analysis for this 

period are on the main stem of major rivers, and 

as a result, the conclusions that are drawn from 

this data set relate more specifically to conditions in the major rivers.

High concentrations of dissolved solids can 

degrade a water supply’s suitability for important uses, such as drinking water or irrigation. 

This study of dissolved-solids conditions in 

the important water supplies of the Southwest 

is only part of the USGS National WaterQuality Assessment Program’s efforts to help 

understand and ensure the future availability 

of our Nation’s freshwater supplies.

David W. Anning

Edited by James W. Hendley II

Graphic design by Judy Weathers

For further information, contact:

David W. Anning, Hydrologist

U.S. Geological Survey Arizona 

Water Science Center 

Northern Arizona Programs

 2255 N. Gemini Drive

Flagstaff, AZ 86001

email: dwanning@usgs.gov

or 

http://az.water.usgs.gov

See also Dissolved Solids

 in Basin-Fill Aquifers and Streams in the 

Southwestern United States

 (USGS Scientific Investigations Report 2006-5315; online at 

http://pubs.usgs.gov/sir/2006/5315/)

This Fact Sheet and any updates to it are available online at 

http://pubs.usgs.gov/fs/2008/3076/

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