Figure 1. Principal aquifers and locations of 15 basins previously studied by the NAWQA Program in the Southwest Principal Aquifers study area.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3015

March 2009

The National Water-Quality Assessment (NAWQA) Program 

of the U.S. Geological Survey is conducting a regional analysis of 

water quality in the principal aquifers in the southwestern United 

States. The Southwest Principal Aquifers (SWPA) study is building a better understanding of the susceptibility and vulnerability of 

basin-fill aquifers in the region to ground-water contamination by 

synthesizing the baseline knowledge of ground-water quality conditions in 15 basins previously studied by the NAWQA Program 

(fig. 1). The improved understanding of aquifer susceptibility and 

vulnerability to contamination (these terms are defined on page 3) 

is assisting in the development of tools that water managers can 

use to assess and protect the quality of ground-water resources. 

The NAWQA Program is performing similar regional investigations for other principal aquifers across the country (Lapham and 

others, 2005). 

Southwest Principal Aquifers Regional Ground-Water Quality 

Assessment

WYOMIN

G

Basin and Range basin-fill aquifers

California Coastal Basin aquifers

Central Valley aquifer system

Rio Grande aquifer system

EXPLANATION

35°

Pacific Ocean

U.S. Geological Survey digital data, 1:500,000 and 750,000 scale, 1992–1997 and 2006

National Elevation Data 1:24,000 1999

Albers Equal Area Conic projection, NAD 83

40°

120° OREGON

115°

NEVADA

IDAHO

110°

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas

Valley

Upper Santa Cruz

Basin

West Salt River Valley

Sierra Vista

Subbasin

San Luis Valley

Salt Lake

Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal

Basin

Middle Rio Grande

Basin

SWPA study area

boundary

105°

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

0 5 10 15 200 Miles

0 5 10 15 200 Kilometers MEXICO

Basin-fill aquifers underlie about 49 percent of the 409,000 

mi2 study area and are the primary ground-water supply for cities 

and agricultural communities. In several areas, these aquifers 

provide base flow to streams that support important aquatic and 

riparian habitats. When aggregated across the study area, the 

basin-fill aquifers comprise four of the principal aquifers of the 

United States: the Basin and Range basin-fill aquifers in California, Nevada, Utah, and Arizona; the Rio Grande aquifer system in 

New Mexico and Colorado; and the Coastal Basin aquifers and the

Central Valley aquifer system in California (fig. 1; U.S. Geological 

Survey, 2003a). 

About 46.6 million people live in the SWPA study area 

(LandScan Global Population Database, 2005), mostly in urban 

metropolitan areas, but also in rural agricultural communities that 

tend about 14.4 million acres of cropland (U.S. Geological Survey, 

2003b). Other rural areas contain small communities with mining, retirement, and 

National Water-Quality Assessment Program
Tributary 

recharge 

(2%)

Mountain-front

recharge 

(2%)

River

infiltration 

(58%)

Riparian

evapotranspiration 

(15%)

Subsurface

recharge 

(5%)

Interior

drains 

(23%)

Riverside

drains

(36%) Crop-irrigation

seepage 

(6%) Canal

seepage 

(16%)

Inflow

of water from 

aquifer storage 

(10%)

Septic-field

seepage 

(0.7%)

Ground-water

withdrawal 

(26%)

Eastern Mountains

Piedmont

Inner valley

Crystalline 

rock

Basin fill Sedimentary rock

Tributary 

recharge 

(7%) Mountain-front

recharge 

(9%)

River

infiltration 

(60%)

Riparian

evapotranspiration 

(100%)

Rio

Grande

Subsurface

recharge 

(24%)

Shallow

alluvial 

aquifer

EXPLANATION

Direction of recharge

Direction of discharge

Direction of groundwater movement

(b) Total ground-water flux 575,000 acre-feet per year

(a) Total ground-water flux 130,000 

acre-feet per year

2

(or) tourism/recreational-based economies. Due to the generally 

limited availability of surface-water supplies in the arid to semiarid climate, cultural and economic activities in the region are 

particularly dependent on good-quality ground-water supplies. In 

the year 2000, about 33.7 million acre-feet of surface water was 

diverted from streams, and about 23.0 million acre-feet of ground 

water was withdrawn from aquifers in the SWPA study area (U.S. 

Geological Survey, 2008). Irrigation and public-supply withdrawals from basin-fill aquifers in the study area for 2000 were about 

18.0 million acre-feet and 4.1 million acre-feet, respectively, and 

together account for about one quarter of the total withdrawals 

from all aquifers in the United States (Maupin and Barber, 2005). 

Basin-Fill Aquifers

Basin-fill aquifers primarily consist of sand and gravel deposits that partly fill structurally formed depressions and are bounded 

by mountains (fig. 2). In some areas, fine-grained deposits of silt 

and clay are interbedded with the porous sand and gravel deposits 

and form confining units that retard vertical movement of ground

water. Most basins contain thick sequences of basin-fill deposits, 

and the sediment becomes increasingly more compacted and 

less permeable with depth. Many basins are drained by a stream 

that flows through a gap in the consolidated rock, although some

basins are closed and ground water and surface water are removed 

only by evapotranspiration. High-energy mountain streams form 

alluvial fans with coarse-grained sediment deposited along the 

mountain fronts. The unsaturated zones below alluvial fan surfaces 

usually are several hundred feet thick and underlain by an unconfined aquifer. Steep alluvial fans transition to a relatively flat valley 

floor where lacustrine and fluvial depositional environments often 

have created layers of fine-grained sediment interbedded with 

more permeable layers of sand and gravel. This usually results in 

confined conditions and upward vertical gradients in discharge 

areas in the central part of the basin. 

The primary sources of natural recharge to the deeper parts of 

the basin-fill aquifers are precipitation on the surrounding mountains and infiltration from streams. Runoff from the surrounding 

mountains seeps into the coarser-grained stream-channel and 

alluvial-fan deposits near the basin margins. Precipitation also can 

infiltrate the consolidated mountain rock where it is fractured or 

porous and can move into the basin-fill deposits. Low precipitation 

rates, combined with high evaporation rates, result in a relatively 

small contribution of ground-water recharge from precipitation 

occurring on the basin floor. Much of that recharge is focused as 

infiltration through ephemeral stream channels. Some recharge 

occurs as subsurface inflow of ground water from adjacent 

basins. 

Before human development of water resources began in the 

alluvial basins, ground-water recharge and discharge were in 

equilibrium. Ground-water discharge typically was by evapotranspiration of shallow ground water in wetlands or playas (in 

closed basins) and by discharge to streams flowing through the 

basin. In some basins, discharge occurs as subsurface outflow to 

adjacent basins; however, in other basins, faulting and constrictions in the bedrock that surrounds the aquifer restrict groundwater outflow. 

With water development, some basin-fill aquifers have 

changed considerably as a result of an increase in the amount 

of and number of mechanisms for recharge and discharge. Total 

ground-water flux rates (recharge and discharge) increased by a 

factor of more than four in the Middle Rio Grande Basin, New 

Mexico, and by a factor of about seven in the West Salt River 

Valley, Arizona (figs. 2 and 3, respectively). Artificial recharge 

sources, several of which are shown in fig. 2 for the Middle Rio 

Grande Basin, include seepage of irrigation water applied to 

crops and lawns; seepage from canals, distribution pipes, sewer 

pipes and septic systems; infiltration of storm-water runoff 

from retention basins, recharge basins, and dry wells; seepage 

of treated wastewater through streambeds or irrigated fields; 

and infiltration in recharge ponds or well-injection of surface 

water or imported water. Recharge from these artificial sources 

introduces water to parts of the ground-water system that previously received little or no recharge from the land surface. The 

increase in recharge and redistribution of water to areas that 

previously did not receive recharge has resulted in increased 

saturated thicknesses, increased flow velocities, and changes in

flow directions.

For many basins, withdrawal from pumping wells has 

become the primary source of ground-water discharge and is 

greater than ground-water recharge. For the West Salt River 

Valley, modern discharge is about 15 percent greater than modern recharge. In some basins, the imbalance between recharge 

and discharge has led to large decreases in ground-water storage 

and (or) decreases in ground-water discharge to streams and 

evapotranspiration. Water-level declines and changes in flow 

directions and magnitudes occur where ground-water withdrawals are large. The increased rates of recharge and discharge 

Figure 2. Generalized block diagram for the Middle Rio Grande Basin, 

New Mexico, showing components of the ground-water system under 

(a) predevelopment and (b) modern conditions. Component data from 

McAda and Barroll (2002).
associated with water development have increased flow through 

many basin-fill aquifers, especially from the land surface to the 

shallower parts of the aquifer. However, ground-water withdrawals from the deep wells typically used for public supply also have 

resulted in enhanced movement of ground water from shallower to 

deeper parts of basin-fill aquifers. Water development, therefore, 

typically results in aquifers being more susceptible to water-quality 

degradation by human activities occurring at the land surface and 

more vulnerable to contaminants where sources are present. 

Susceptibility and Vulnerability to Contamination

The ease with which water enters and moves through an aquifer is described as its intrinsic susceptibility (Focazio and others, 

2002). Aquifer susceptibility is dependent on the aquifer properties and other characteristics such as recharge rate, the presence or 

absence of an overlying confining unit, ground-water travel time, 

thickness and characteristics of the unsaturated zone, and pumping. The vulnerability of ground water to contamination is the 

probability for contaminants to reach a specified part of an aquifer 

after being introduced, usually at the land surface. Vulnerability 

is dependent on the properties of the ground-water system (susceptibility), the proximity of contaminant sources, and the contaminant’s chemical characteristics. Long ground-water residence 

times and slow rates of contaminant degradation in basin-fill aquifers of the SWPA study area can make the process of contaminating ground water virtually irreversible and treatment prohibitively 

expensive or otherwise impractical. Therefore, it is imperative to 

understand the primary natural and human factors associated with 

the susceptibility and vulnerability of these aquifers to contamination, which allows water managers to plan for their optimal protection and utilization.

The SWPA study has completed two regional water-quality 

investigations for basin-fill aquifers. Paul and others (2007) 

investigated water quality of the shallow, upper parts of basin-fill 

aquifers in the SWPA study area and found them to be vulnerable to nitrate contamination (concentrations above 10 milligrams 

per liter) in areas where fertilizer is used, land is irrigated, and 

Figure 3. Estimated annual recharge and discharge water-budget 

components for predevelopment and modern conditions in the West 

Salt River Valley, Arizona.

oxidizing conditions are present in the ground water. Similarly, 

Paul and others (2007) found that the occurrence of selected pesticides is affected by oxidation/reduction conditions, soil permeability, ground-water temperature, and depth to the well’s screened 

interval. Occurrence of selected volatile organic compounds was 

found to be affected by oxidation/reduction conditions, pH, and 

industrial land use. Anning and others (2007) investigated salinity 

in the basin-fill aquifers of the SWPA study area and found that 

dissolved-solids concentrations typically increase along flowpaths 

as a result of geochemical reactions with the aquifer matrix, dissolution of disseminated salts and massive evaporite deposits, and 

evapotranspiration of shallow ground water by natural vegetation 

or by agricultural crops. Mixing with inflows of ground water, 

stream seepage, or irrigation seepage with higher concentration 

also causes increases in dissolved-solids concentrations along 

flowpaths.

Regional Analysis 

Similarities in the hydrogeology, land- and water-use practices, and water-quality issues between the basin-fill aquifers 

of the SWPA study area allow for regional analysis. Regional 

analysis begins by determining the primary influential factors that 

commonly affect water quality, and the associated susceptibility 

and vulnerability of basin-fill aquifers to contamination, on the 

basis of data and information from a subset of information-rich 

basin-fill aquifers in the study area. Variations in the presence and 

magnitude of these influential factors across the basins allow for 

determination of the effects of each factor and for development 

of conceptual and mathematical susceptibility and vulnerability 

models of these effects. The models formed for these areas then 

can be extended to areas lacking ground-water quality data and 

interpretive studies. As part of the regional analysis activities, 

digital data sets that represent potential factors affecting water 

quality were produced by the SWPA study (fig. 4; McKinney and 

Anning, 2009). Potential factors such as population, land use, and 

water use (fig. 4) are general measures of the degree and type of 

human activities occurring on the land surface, and may reflect 

the magnitude and extent of contaminant sources and transport 

mechanisms. 

During its first data-collection and analysis phase from 1991 

to 2001, the NAWQA Program sampled wells and established 

baseline water-quality conditions for basin-fill aquifers in 15 

basins across the study area (fig. 1). Ground-water quality also 

was investigated for relations to natural and human factors on the 

basis of a wide suite of constituents including major ions, nutrients, trace elements, pesticides, and volatile organic compounds. 

These studies developed detailed knowledge of local conditions 

and factors affecting ground-water quality for each basin, and the 

SWPA program is developing a regional understanding by synthesizing results from the 15 basin studies into a common set of factors and themes found to affect water-quality in basin-fill aquifers 

across the Southwestern United States. The synthesis consists of 

three major components:

1. A review that summarizes the status of, trends in, and influential factors affecting ground-water quality of basin-fill aquifers 

in the 15 individual basins previously studied by NAWQA;

2. Development of a conceptual model of the primary natural 

and human factors commonly affecting ground-water quality, 

leading to a regional understanding of the susceptibility and 

vulnerability of basin-fill aquifers to contaminants; and, 

3. Development of statistical models that relate specific groundwater quality constituent concentrations or occurrence to natural and human factors linked to the susceptibility and vulnerability of basin-fill aquifers to contamination. 

Predevelopment Modern

68,000 acre-feet 457,200 acre-feet

68,000 acre-feet 524,000 acre-feet

Discharge Recharge

Well pumpage

95%

Evapotranspiration

59%

Subsurface

outflow

41%

Stream

losses

53%

Artificial: Irrigation

losses

68%

Subsurface

inflow

38%

Evapotranspiration

3%

Mountain front and

mountain block

9%

Subsurface outflow

1%

Stream gain

1%

Stream

losses

15%

Mountain front and

mountain block

1%

Artificial:

other

10%

Subsurface inflow

6%

Predevelopment data from Freethey and Anderson (1986) and 

modern conditions from Corkhill and others (2004).

3
Resource managers and scientists will be able to use the 

results of the SWPA regional water-quality studies in assessing the 

susceptibility and vulnerability of ground water to contamination 

in basins across the study area. By identifying natural and human 

factors and processes affecting the occurrence and transport of contaminants, the assessments will allow managers and scientists to 

apply findings to broader classes of contaminants. Regional-scale 

models and other decision-support tools that integrate aquifer characteristics, land use, and water-quality monitoring data will help 

water managers to estimate water-quality conditions in unmonitored areas, to assess the susceptibility and vulnerability of ground 

water under different future basin-development scenarios, and to 

develop cost-effective ground-water monitoring programs. 

References

Anning, D.W., Bauch, N.J., Gerner, S.J., Flynn, M.E., Hamlin, S.N., 

Moore, S.J., Schaefer, D.H., Anderholm, S.K., and Spangler, L.E., 

2007, Dissolved solids in basin-fill aquifers and streams in the 

Southwestern United States: U.S. Geological Survey ScientificInvestigations Report 2006–5315, 168 p. Available at http://pubs.

usgs.gov/sir/2006/5315/

Corkhill, F., Bota, L., Jahnke, P., Mason, D., and Hipke, W., 2004, 

Revised groundwater modeling results for the East Valley Water 

Forum Regional Water Planning Study, Arizona Department of 

Water Resources, obtained October 13, 2006, at: http://www.

water.az.gov/dwr/content/Publications/files/EVWF_revised_

run1_12_2_04.pdf

Focazio, M.J., Reilly, T.E., Rupert, M.G., and Helsel, D.R., 

2002, Assessing ground-water vulnerability to contamination—Providing scientifically defensible information for 

decision makers: U.S. Geological Survey Circular 1224, 33 

p. Available at http://pubs.usgs.gov/circ/2002/circ1224/

Freethey, G.W., and Anderson, T.W., 1986, Predevelopment 

hydrologic conditions in the alluvial basins of Arizona and 

adjacent parts of California and New Mexico: U.S. Geological Survey Hydrologic Investigations Atlas 664, 3 sheets.

LandScan Global Population Database, 2005, Oak Ridge, TN: 

Oak Ridge National Laboratory, accessed September 2006, 

at http://www.ornl.gov/landscan/

Lapham, W.W., Hamilton, P.A., and Myers, D.N., 2005, 

National Water-Quality Assessment Program—Cycle II 

Regional Assessments of Aquifers: U.S. Geological Survey 

Fact Sheet 2005-3013, 4 p. Available at http://pubs.usgs.

gov/fs/2005/3013/

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. Available at http://

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

McAda, D.P. and Barroll, Peggy, 2002, Simulation of groundwater flow in the Middle Rio Grande Basin between Cochiti 

and San Acacia, New Mexico: U.S. Geological Survey 

Water-Resources Investigations Report 02-4200, 81 p. 

Available at http://nm.water.usgs.gov/publications/abstracts/

wrir02-4200.html

McKinney, T.S., and Anning, D.W., 2009, Geospatial data to 

support analysis of water-quality conditions in basin-fill 

aquifers in the Southwestern United States: U.S. Geological 

Survey Scientific Investigations Report 2008-5239. Available at http://pubs.usgs.gov/sir/2008/5239/

Figure 4. Factors that likely affect the water quality of basin-fill aquifers 

include population, irrigated land, total water use for public supply and 

irrigation, and percentage of total water use supplied by ground water 

within the basin. Graphs above show approximate conditions for years 

2000–2005. 

For additional information about the Southwest Principal Aquifers 

Water-Quality Assessment, contact:

Lead Scientist Fact Sheet Author

Susan Thiros David Anning

sthiros@usgs.gov dwanning@usgs.gov

(801) 908-5063 (928) 556-7139

http://water.usgs.gov/nawqa/studies/praq/swpa

By D.W. Anning, S.A. Thiros, L.M. Bexfield, T.S. McKinney, 

and J.M. Green

Paul, A.P., Seiler, R.L., Rowe, T.G., and Rosen, M.R., 2007, Effects 

of agriculture and urbanization on quality of shallow ground 

water in the arid to semiarid western United States, 1993–2004: 

U.S. Geological Survey Scientific-Investigations Report 2007–

5179, 56 p. Available at http://pubs.usgs.gov/sir/2007/5179/

U.S. Geological Survey, 2003a, Principal aquifers, in National 

Atlas of the United States of America, 1 sheet, accessed May 6, 

2008, at http://nationalatlas.gov/mld/aquifrp.html

U.S. Geological Survey, 2003b, National Land Cover Database 

(NLCD 2001) multi-zone download site, accessed February 

2007, at http://www.mrlc.gov/nlcd_multizone_map.php

U.S. Geological Survey, 2008, Water use in the United States, 2000 

data for counties, accessed May 6, 2008, at http://water.usgs.gov/

watuse/. 

POPULATION

1,000

10,000

100,000

1,000,000

10,000,000

100

1,000

10,000

100,000

1,000,000

10,000,000

IRRIGATED AGRICULTURAL

LAND, IN ACRES

Less than 1,000 acres

1,000

10,000

100,000

1,000,000

10,000,000

100,000,000

TOTAL WATER USE FOR PUBLIC

SUPPLY AND IRRIGATION,

IN ACRE-FEET PER YEAR

0

20

40

60

80

100

Sierra Vista Subbasin, AZ

Upper Santa Cruz Basin, AZ

West Salt River Valley, AZ

Santa Ana Coastal Basin, CA

Santa Ana Inland Basin, CA

San Jacinto Basin, CA

Sacramento Valley, CA

San Joaquin Valley, CA

Carson Valley, NV and CA

Eagle Valley, NV

Spanish Springs Valley, NV

Truckee Meadows, NV

Las Vegas Valley, NV

Middle Rio Grande Basin, NM

San Luis Valley, CO and NM

Salt Lake Valley, UT

Central

Valley

Locations for basins are shown in figure 1

Data from McKinney and Anning (2009)

PERCENTAGE OF TOTAL WATER

USE SUPPLIED BY GROUND WATER

4