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Upper San Pedro
Geography
The Upper San Pedro basin is in southeastern Arizona and contains
approximately 1,875 square miles. The basin consists of the northwest-trending
San Pedro River Valley and the surrounding mountains.
Elevations along the valley floor range from 4,200 feet above mean sea level
at the International Boundary with Mexico to 3,300 feet above mean sea level at
"the Narrows", which forms the basin's northern boundary.
The mountains bordering the basin range from 5,000 to nearly 10,000 feet in
elevation. The Huachuca, Mustang, Whetstone, and Rincon Mountains form the
basin's western boundary and the Mule, Dragoon, Little Dragoon, and Winchester
Mountains form the eastern boundary.
Water
The San Pedro River is the basin's major surface-water drainage. The San
Pedro River enters the basin at the International Boundary near Palominas, AZ
and flows northwest for about 62 miles before leaving the basin north of Benson
at "the Narrows". The San Pedro River is mostly ephemeral and only
flows in response to local rainfall. The river does have a perennial stretch of
about 18 miles between Hereford and a point just south of Fairbank. The
perennial reach, near Charleston, is created by bedrock that forces groundwater
to the surface.
Groundwater is found in two major units in the Upper San Pedro basin:
- Streambed alluvium that forms the San Pedro River's channel and
floodplain, and
- The alluvial basin-fill sediments that fill the valley.
The streambed alluvium is more water-permeable than the basin-fill. But the
alluvium is limited in size, and for that reason it is an important local
aquifer only in the central valley along the San Pedro River's floodplain.
The alluvial basin-fill sediments form the basin's principal aquifer.
Consolidated bedrock found in the surrounding mountains yields only small
amounts of water from localized aquifers.
The hydrologic characteristics of the regional aquifer vary widely. Well
yields of 100 to 2,800 gallons per minute have been reported.
Past readings indicated groundwater in the basin-fill was found in both
unconfined (water table) and confined (artesian) conditions. In 1978, depth to
water in unconfined areas of the basin-fill ranged from 50 to 570 feet below
land surface.
Water levels are generally stable in the basin except in the Fort
Huachuca-Sierra Vista area where groundwater pumpage has created a large cone of
depression . The cone first was documented in 1973. Water-level declines within
the cone averaged 1.4 feet per year from 1968 to 1986. Further hydrologic
studies have been performed recently to determine the impact of long-term
groundwater withdrawals in the Fort Huachuca-Sierra Vista area on flows in the
San Pedro River.
Confined or artesian conditions in the regional aquifer occur near the center
of the basin where fine-grained beds restrict vertical groundwater movement.
There are two areas where confined conditions are found:
- In the southern part of the basin between Palominas and Hereford and
- To the north near Benson and St. David.
Deep wells drilled in or near the floodplain of the San Pedro River in these
areas encounter artesian conditions. The artesian area between Palominas and
Hereford is about 10 miles long and one mile wide; the artesian aquifer is
encountered in wells drilled deeper than 200 feet.
Artesian conditions also are found from about six miles south of St. David to
Benson in wells drilled deeper than 200 feet. North of Benson, an artesian
aquifer is encountered at 500 to 1,000 feet below land surface.
Historical Waterflow
The streambed alluvium that occupies the San Pedro River's stream channel and
floodplain is very permeable; in the past, well-yields ranged from 200 to 1,800
gallons per minute. Groundwater in the alluvium was unconfined, and water levels
were usually less than 50 feet below land surface. The streambed alluvium is
recharged primarily by surface-water flows in the riverbed. As a result, water
levels in the alluvium fluctuate seasonally in response to surface-water flows
in the riverbed: rising slightly in the spring and early summer, then declining
in the fall and winter.
The consolidated bedrock of the mountains surrounding the basin yields water
only where sufficiently faulted and fractured. The small, localized aquifers
created by these fault zones provide only enough water for stock and low-use
domestic wells. Most springs in the bedrock tend to be small and measured
discharges varied greatly. The largest springs in the basin are located in the
Huachuca Mountains and had discharges as high as 1,800 gallons per minute.
However, many of the springs also had periods of very little or no flow.
Groundwater
Groundwater movement in the basin is from the higher elevations in the
mountains toward the valley and then northwest along the riverbed. Groundwater
moves readily between the younger and older basin-fill units and between the
streambed alluvium and the younger basin-fill unit. In the confined areas, water
from the artesian aquifers may leak upwards into the water-table aquifer. In
1990, the Arizona Department of Water Resources estimated the total groundwater
in storage in the Upper San Pedro basin to be 59 million acre-feet.
Water enters the aquifers by mountain-front recharge and by streambed
infiltration. A smaller amount enters the basin as groundwater underflow from
Mexico. Mountain-front recharge consists of surface runoff that flows off the
bedrock in the mountains. It infiltrates into the permeable basin-fill sediments
on the alluvial fans surrounding the mountains, and eventually reaches the water
table. Streambed infiltration occurs when surface-water flows in the San Pedro
River channel and its tributary washes infiltrate the coarse streambed sands
down to the water table.
Groundwater Recharge/Discharge
Mountain-front recharge is the main source of recharge for the regional
aquifer and streambed infiltration is the main source of recharge for the
streambed alluvium in the San Pedro River floodplain. Recharge also enters the
regional aquifer as infiltration from the streambed alluvium. Direct
infiltration of precipitation falling on the valley floor is considered
negligible due to high evaporation rates and low rainfall totals. Groundwater
recharge estimates are 29,000 acre-feet per year from streambed infiltration and
mountain-front recharge, and 900 acre-feet per year from underflow into the
basin from Mexico. Total groundwater recharge into the basin is estimated to be
nearly 30,000 acre-feet per year.
Groundwater is discharged from the basin by
- Pumpage from wells
- Evapotranspiration from phreatophytes and crops
- Evaporation from open water in the riverbed, and
- Discharge from springs and seeps.
Pumpage is the largest source of discharge; in 1988-1989, it was estimated to
total 40,300 acre-feet. Estimated groundwater pumpage in the Upper San Pedro
basin for 1988-1989 is presented in the following table. Irrigation is a major
water user in the basin with approximately 12,700 acres of land irrigated in the
basin (Arizona Department of Water Resources, 1990). Most irrigation wells are
located in the highly-permeable streambed alluvium. Most industrial and
domestic/public supply wells are located in the regional basin-fill aquifer.
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GROUNDWATER PUMPAGE IN THE UPPER SAN PEDRO BASIN FOR
1988-1989
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Water Use |
Pumpage (af/yr) |
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Irrigation |
30,380 |
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Public Supply/Domestic |
9,460 |
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Mining/Industrial |
430 |
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Total |
40,270 |
Water Quality
The quality of groundwater in the Upper San Pedro basin generally is suitable
for most uses. The total dissolved solids concentrations of samples collected in
1978 ranged from 200 to 2,500 milligrams per liter (mg/l). The recommended
secondary maximum contaminant level for total dissolved solids in drinking water
is 500 mg/l. The dissolved solid concentration in wells is highest in the San
Pedro River floodplain from St. David north to "the Narrows".
Fluoride concentrations for well and spring samples collected in 1978 ranged
from 0.1 to 5.9 mg/l. Fluoride concentrations were highest in wells completed in
the streambed alluvium along the San Pedro River between St. David and Benson.
The maximum contaminant level for fluoride in drinking water has been set by the
Arizona Department of Environmental Quality (ADEQ) at 4.0 mg/l.
Known groundwater-quality problems existing in the Upper San Pedro basin
include nitrate contamination of groundwater near St. David and sulfate
contamination in the Bisbee-Naco area. In St. David, groundwater is contaminated
with nitrates, lead, and sulfates, potentially due to the operation of a nearby
explosive – and - chemical manufacturing firm. The area around Apache Powder
Company is now designated as a Federal Superfund site. Site investigation and
remedial action have begun under the supervision of the U.S. Environmental
Protection Agency, Arizona Department of Water Resources, and Arizona Department
of Environmental Quality. Other potential sources of contamination include
inadequate septic tank systems and agricultural practices. In 1988 in the Bisbee
- Naco area, the infiltration of leachate from a tailings pond near Warren,
northeast of Naco, Arizona, appeared to be contributing sulfate to the
groundwater.
External Links Outside
of the ADWR Network
This Icon represents a link out of the ADWR web site. We are not
responsible for content out of our control.
Upper San Pedro Partnership
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