U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3034

April 2006 Printed on recycled paper

The middle San Pedro watershed 

is a northwest-southeast trending 

basin in Cochise and Pima Counties 

in southeastern Arizona (fig. 1). The 

watershed includes parts of two groundwater basins designated by the Arizona 

Department of Water Resources: the 

southern part of the Lower San Pedro 

Basin and the northern part of the 

Upper San Pedro Basin. Benson is the 

largest city in the watershed and had 

a population of about 4,900 in 2005. 

The population of the watershed has 

grown moderately by a factor of about 

1.5 during the past 20 years (Arizona 

Department of Water Resources, 2005; 

U.S. Census Bureau, 2006), but it is 

expected to rapidly increase during the 

next 20 years as residential developments 

are constructed for retirement 

communities or for satellite communities 

of nearby Tucson, Arizona. If the 

approved and planned developments are 

constructed as scheduled, the population 

of Benson is expected to increase to more 

than 50,000 by 2025 (“Arizona Daily 

Star,” 2005).

Ground water is the primary source 

of water and is essential to support 

a growing population, agriculture, 

industry, and the riparian habitat of the 

San Pedro River. The city of Benson, 

Cochise County, the public, and other 

interested parties are concerned about 

the future availability and sustainability 

of the water supply. Increased groundwater pumping could have several 

undesirable consequences, such as (1) 

loss of available ground-water storage, 

(2) increased cost of pumping, (3) need 

for deepening existing wells, (4) need 

for new wells, (5) decreased quality of Figure 1. Location of middle San Pedro watershed, southeastern Arizona.

Prepared in cooperation with the

ARIZONA DEPARTMENT OF WATER RESOURCES

Hydrogeologic Investigation of the Middle San Pedro

Watershed, Southeastern Arizona: 

A Project of the Rural Watershed Initiative

SANTA CATALINA MOUNTAINS

MOUNTAIN

R

S

INCON

MOUNTAINS

WHETSTONE

GALIURO

MOUNTAINS

Redington

Tombstone

Benson

St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

(09471800)

(09471550)

 Tombstone gaging

station

 Redington Bridge

gaging station (09472050)

(09472000)

 Redington gaging

station

Narrows gaging

station

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

THE

NARROWS

DRAGOON MOUNTAINS

San Pedro

Riparian National

Conservation Area

San

R

vi er

P

de

ro

BENSON SUBAREA

SIERRA VISTA SUBAREA

BENSON

SUBAREA

NARROWS-REDINGTON

SUBAREA

Tucson

ARIZONA

Middle San Pedro

watershed

P

R

G

S

River

ila

an de ro

UNITED STATES

MEXICO

iver

GRAHAM CO

COCHISE CO

COCHISE CO

PIMA CO
drinking water, (6) land subsidence, 

and (7) damage to the riparian habitat 

of the San Pedro River. To be effective 

and acceptable to all interested parties, 

planning and management of the groundwater supply needs to be guided by an 

objective and comprehensive scientific 

understanding of the ground-water 

system.

In 2005, the U.S. Geological Survey 

(USGS) began an investigation of the 

hydrogeology of the middle San Pedro 

watershed in cooperation with the 

Arizona Department of Water Resources 

(ADWR). This project is part of the 

Rural Watershed Initiative (RWI), which 

is a program established by the State of 

Arizona and managed by the ADWR. 

Other RWI projects that started in 2005 

include an investigation of several basins 

in Mohave County in northwestern 

Arizona and an investigation of 

the Willcox and Douglas Basins in 

southeastern Arizona. RWI projects that 

started in 1999 include studies of the 

Coconino Plateau in northern Arizona, 

the Mogollon Highlands in central 

Arizona, and the upper and middle Verde 

watersheds in central Arizona (Arizona 

Water Science Center, 2003).

The primary objective of this project 

is to improve the understanding of the 

hydrogeology of the middle San Pedro 

watershed. Specific objectives are to:

1. Assess the current state of 

knowledge of the ground-water 

system.

2. Describe and quantify the 

hydrogeologic framework.

3. Describe and quantify the groundwater flow system, including 

recharge, discharge, water in storage, 

and historical changes in the system.

4. Construct a predictive numerical 

model of the ground-water system 

that can be used to help guide 

decisions by planners and water 

managers about future development.

Physical Setting and Surface Water

The area of the middle San Pedro 

watershed is about 1,370 mi2, the length 

is about 50 mi, and the average width 

is about 25 mi. Land-surface altitudes 

range from about 2,800 to 5,000 ft for 

the alluvial valley and basin upland 

and from about 5,000 to 8,700 ft for 

the surrounding bedrock mountains. 

The middle watershed is divided into 

two subareas: (1) the Benson subarea is 

from the boundary with the Sierra Vista 

subarea on the south to the Narrows on 

the north, and (2) the Narrows-Redington 

subarea is from the Narrows to a 

streamflow-gaging station near Redington 

on the north (fig. 1). The Narrows is at a 

constriction in the basin where bedrock 

mountains extend into the watershed 

from the east and west.

The climate of the study area is arid 

to semiarid with hot summers and mild 

winters. Average annual precipitation 

ranges from about 11 to 16 in. in the 

alluvial valley and basin upland, and 

from about 16 to 25 in. in the mountains 

(Western Regional Climate Center, 

2005; Spatial Climate Analysis Service, 

2003). Vegetation is diverse; the riparian 

forest near the San Pedro River includes 

cottonwood, willow, salt cedar, and 

mesquite trees (fig. 2), the basin upland 

includes grasslands, desert scrub, and 

mesquite trees, and the mountains 

typically have oak woodlands in the lower 

altitudes and ponderosa pine in the highest 

altitudes (Kepner and Edmonds, 2002).

The San Pedro River drains the study 

area and flows from south to north from 

its origins in Mexico to the Gila River 

north of the study area. The river is 

mostly ephemeral in the study area with 

Figure 2. Riparian vegetation of the San Pedro River in the middle San Pedro watershed, 

southeastern Arizona.

a few short reaches of perennial flow 

in areas where the ground-water table 

is near land surface or where ground 

water flows upward. Average annual 

streamflow for concurrent records 

(1970–85) at gaging stations in the study 

area was 43,000 acre-ft at Tombstone at 

the southern boundary and 37,000 acre-ft 

at Redington near the northern boundary. 

Annual streamflow near Redington 

decreased by about 70 percent from the 

mid-1940s (about 35,000 acre-ft) to the 

late 1990s (about 10,000 acre-ft; fig. 3). 

Possible causes of this decreasing trend 

in flow are decreases in precipitation, 

changes in upland land-cover and riparian 

vegetation, and increases in ground-water 

pumping (Thomas and Pool, in press).

Hydrogeologic Framework

The hydrogeologic framework of 

a ground-water system describes the 

boundaries and internal properties of the 

aquifers and confining beds in the system. 

An aquifer is geologic material that can 

transmit usable quantities of water to 

wells. The geologic material of aquifers 

typically is coarse-grained sediments 

that have larger pore spaces between the 

solid particles and higher permeability 

than fine-grained sediments. Most usable 

water in storage in a ground-water system 

comes from the aquifers. Confining beds 
Figure 4. Generalized geologic section of the middle San Pedro watershed, southeastern Arizona.

Figure 3. Annual streamflow of San Pedro River near Redington, Arizona (09472000), 1944–97.

typically are fine-grained sediments that 

restrict the vertical movement of water 

either into or out of adjacent aquifers. 

Confining beds strongly influence the 

movement of ground water and the 

effects of ground-water pumping (Heath, 

1989).

Ground water in the middle San Pedro 

watershed occurs primarily in the basin 

fill, which is unconsolidated deposits of 

clay, silt, sand, and gravel. The lateral 

boundaries of the basin fill are the 

bedrock mountains bordering the east 

and west sides of the watershed, and the 

lower boundary is the underlying bedrock 

(fig. 4). Bedrock is mostly composed of 

granite, sandstone, and limestone. The 

aquifers in the ground-water system are 

mostly composed of sands and gravels, 

and the confining beds are composed of 

clays and silts.

The hydrogeologic framework is 

moderately well defined in the Benson 

subarea and poorly defined in the 

Narrows-Redington subarea (Arizona 

Department of Water Resources, 1994; 

Arizona Department of Water Resources, 

2005). In the Benson subarea, there is 

a regional aquifer in the basin fill and a 

small local aquifer in the alluvial material 

of the flood plain of the San Pedro 

River. The regional basin-fill aquifer 

is mostly unconfined on the margins 

of the watershed and mostly confined 

(artesian) in the center of the watershed. 

The thickness of the basin fill varies 

widely by location and ranges from about 

150 ft to more than 2,000 ft (Arizona 

Department of Water Resources, 2005). 

The flood-plain aquifer generally is 

less than 50 ft thick and ranges in width 

from tens of feet to almost 2 miles. The 

Silt and clay

VERTICAL SCALE GREATLY EXAGGERATED

FAULT—Arrows

 indicate direction

 of movement

Flood-plain

San Pedro River

alluvium

Bedrock of the 

mountains

Mostly sand

and gravel

Basin fill

Semiconsolidated rocks

ANNUAL STREAMFLOW, IN ACRE-FEET

Linear least-squares regression line

decrease of about 500 acre-ft per year

1940194519501955196019651970197519801985199019952000

100,000

200,000

10,000

1,000

100

St. David Formation is an important 

feature of the ground-water system in the 

Benson subarea. It consists of as much as 

900 ft of clays, silts, and some freshwater 

limestone. The formation contains clay 

layers that are as much as 300 ft thick 

near the center of the Benson subarea. 

These clay layers act as a confining bed 

that restricts vertical movement of ground 

water and creates confined or artesian 

conditions in the underlying aquifer 

(Goode and Maddock, 2000). Other small 

and localized confining beds are scattered 

throughout the system, and they influence 

ground-water movement in the local 

areas.

The ground-water system in the 

Narrows-Redington subarea is similar to 

the system in the Benson subarea. Both 

have a regional aquifer in the basin fill 

and a local aquifer in the flood-plain 

alluvium. Two principal differences 

between the subareas are (1) the 

Narrows-Redington basin-fill aquifer 

is smaller than the Benson basin-fill 

aquifer— it is narrower and thinner 

(likely less than 1,000 ft thick)—and (2) 

the principal confining bed (Quiburis 

Formation) in the Narrows-Redington 

basin fill is likely thinner and less 

effective in restricting vertical groundwater movement than the St. David 

Formation in the Benson basin fill 

(Lombard, 2004). The flood-plain aquifer 

in the Narrows-Redington subarea likely 

has a similar thickness as the flood-plain 

aquifer in the Benson subarea (less than 

50 ft), but it is not as wide.

Ground-Water Flow System

Ground water in the basin fill generally 

flows from the mountains on the eastern 

and western sides of the study area to the 

San Pedro River flood plain and from 

south to north following the gradient 

of the river. Most recharge is from 

infiltration of rainfall and runoff in and 

near the mountains. Most discharge is 

from evaporation from the shallow water 

table in the San Pedro River flood plain 

and transpiration by riparian vegetation 

in the flood plain. Total recharge is 

estimated to be about 18,000 acre-ft/ yr in 

the Benson subarea (Arizona Department 

of Water Resources, 2005) and 11,000 

acre-ft/yr in the Narrows-Redington 

subarea (Arizona Department of Water 

Resources, 1994). Ground-water pumpage 

in 2002 was about 9,000 acre-ft/yr in the 

Benson subarea and 9,000 acre-ft/yr in 

the Narrows-Redington subarea (Arizona 

Department of Water Resources, 1994 and 

2005). Thus, in 2002, pumpage was about 

50 percent of recharge in the Benson 

subarea and 80 percent of recharge in 
the Narrows-Redington subarea. This 

percentage is smaller than that for many 

other ground-water basins in southeastern 

Arizona (Arizona Department of Water 

Resources, 1994), but pumpage will 

increase substantially as residential 

developments are constructed and 

municipal and domestic demands increase.

Ground-Water Quality

The quality of ground water in the 

middle San Pedro watershed is generally 

suitable for domestic, municipal, 

agricultural, industrial, and stock uses. In 

the Benson subarea, the total dissolved 

solids (TDS) concentration typically 

is between about 200 and 400 mg/L 

(Arizona Department of Water Resources, 

2005). Of 15 water samples collected 

during 1996–97 in the Benson subarea, 2 

samples exceeded the U.S. Environmental 

Protection Agency (USEPA) Secondary

Maximum Contaminant Level (SMCL) 

of 500 mg/L (Coes and others, 1999).

SMCLs are aesthetic based and are not 

enforceable. High concentrations of TDS

can corrode some materials and can impart 

an unpleasant taste to drinking water 

(U.S. Environmental Protection Agency, 

2005). There is about a 9-mi2 area of

contaminated shallow ground water about 

2.5 mi southwest of St. David. Operations 

at Apache Nitrogen Products (formerly 

known as Apache Powder Company) 

resulted in high concentrations of nitrate 

and perchlorate in ground water. The 

site has been designated as a USEPA 

Superfund site, and extensive remediation 

activities are currently being used to 

control and clean up the contamination 

(Arizona Department of Environmental

Quality, 2006). The TDS concentrations 

in the Narrows-Redington subarea (about 

200 to 600 mg/L) generally are higher 

than those in the Benson subarea. Of 

21 water samples collected during 2000 

in the Narrows-Redington subarea, 5 

samples exceeded the USEPA SMCL of 

500 mg/L (Towne, 2005).

Planned Approach of Investigation

The investigation will consist of 

(1) an evaluation of existing data and 

information, (2) data collection and 

monitoring, (3) analysis and interpretation 

of the existing and new data, and (4) 

production of a Web site and reports 

describing the results of the study. 

Several complementary methods will be 

employed to achieve the study objectives. 

Existing data and information will be 

compiled and analyzed to enhance the 

knowledge of the ground-water system 

and to identify data gaps where the project 

should focus parts of the investigation. 

The hydrogeologic framework will be 

defined using geophysical methods, 

existing geologic information, and 

information from existing wells, such 

as lithologic logs, borehole geophysical 

logs, and ground-water levels. 

Characteristics of the ground-water flow 

system, including recharge, discharge, 

storage, flow directions, and changes 

over time, will be defined using existing 

information and new data collected 

during the study. Such new data will 

include measurements and descriptions 

of the characteristics of soils, upland 

and riparian vegetation, agricultural 

crops, spring flows, and ground-water 

chemistry; and measurements of changes 

over time in precipitation, ground-water 

levels, ground-water storage, groundwater pumpage, and streamflow of the 

San Pedro River and its tributaries. 

A predictive numerical model of the 

ground-water system will be constructed 

by using results of the study. The model 

will be capable of estimating changes 

to the ground-water system caused by 

increased pumpage or changes in recharge 

or discharge. A long-term hydrogeologic 

monitoring network will be established 

to determine changes in ground-water 

levels and storage over time, and effects 

of ground-water pumpage.

—Blakemore E. Thomas

References Cited

Arizona Daily Star, 2005, Growth crawls toward 

Benson: Arizona Daily Star newspaper article 

on March 27, 2005, accessed July 25, 2005, at 

URL http://www.dailystar.com/

Arizona Department of Environmental Quality, 

2006, accessed January 4, 2006, at URL http://

azdeq.gov/environ/waste/spr/download/state/

apache.pdf

Arizona Department of Water Resources, 1994, 

Arizona water resources assessment, volume 

II, hydrologic summary, August 1994: Arizona 

Department of Water Resources, 236 p.

Arizona Department of Water Resources, 2005, 

Groundwater resources of the Upper San Pedro 

Basin, Arizona, technical report to the Upper 

San Pedro Basin AMA review report: Arizona 

Department of Water Resources, 91 p.

Arizona Water Science Center, 2003, Arizona Rural 

Watershed Initiative, accessed January 5, 2006, 

at URL http://az.water.usgs.gov/rwi-ii/.

Coes, A.L., Gellenbeck, D.J., and Towne, D.C., 

1999, Ground-water quality in the Sierra Vista 

subbasin, Arizona, 1996-97: U.S. Geological 

Survey Water-Resources Investigations Report 

99–4056, 50 p.

Goode, T.C., and Maddock, Thomas III, 2000, 

Simulation of groundwater conditions in the 

Upper San Pedro Basin for the evaluation of 

alternative futures: University of Arizona, 

Tucson, Arizona, Department of Hydrology and 

Water Resources, HWR No. 00–030, 113 p.

Heath, R.C., 1989, Basic ground-water hydrology: 

U.S. Geological Survey Water-Supply Paper 

2220, 84 p.

Kepner, W.G., and Edmonds, C.M., 2002, Remote 

sensing and geographic information systems for 

decision analysis in resource administration: A 

case study of 25 years of landscape change in a 

southwestern watershed: U.S. Environmental 

Protection Agency report EPA/600/R-02/039, 23 p.

Lombard, J.P., 2004, Results of Benson Narrows 

investigation—Contract AZFO-031031: Study 

conducted for the Arizona Chapter of the Nature 

Conservancy: James P. Lombard, R.G., private 

consultant, Tucson, Arizona. 

Spatial Climate Analysis Service, 2003, PRISM 

products: Corvallis, Oregon, Oregon State 

University, accessed June 30, 2003, at URL 

http://www.ocs.orst.edu/prism/.

Thomas, B.E., and Pool, D.R., in press, Trends in 

streamflow of the San Pedro River, southeastern 

Arizona, and regional trends in precipitation 

and streamflow in southeastern Arizona and 

southwestern New Mexico: U.S. Geological 

Survey Professional Paper 1712.

Towne, 2005, Ambient groundwater quality of the 

Lower San Pedro Basin: A 2000 baseline study: 

Arizona Department of Environmental Quality 

Open-File Report 2002–01, 39 p.

U.S. Census Bureau, 2006, Annual estimates of the 

population for incorporated places in Arizona: 

U.S. Census Bureau accessed January 6, 2006, 

at URL http://www.census.gov/prod/www/abs/

decennial/index.htm

U.S. Environmental Protection Agency, 2005, List 

of Drinking Water Contaminants and MCLs, 

accessed December 15, 2005, at URL http://

www.epa.gov/safewater/mcl.html

Western Regional Climate Center, 2005, Arizona 

Climate Summaries, accessed December 16, 

2005, at URL http://www.wrcc.dri.edu/summary/

climsmaz.html/

For further information, contact:

Blakemore E. Thomas 

U.S. Geological Survey

Arizona Water Science Center

520 North Park Avenue, Suite 221

Tucson, Arizona, 85719-5035

Email: bthomas@usgs.gov or visit home 

page at http://az.water.usgs.gov