Ambient Groundwater Quality of the McMullen Valley Basin:
A 2008-2009 Baseline Study – June 2011
A baseline groundwater quality study of the McMullen
Valley basin was conducted in 2008-2009 by the Arizona
Department of Environmental Quality (ADEQ) Ambient
Groundwater Monitoring Program. ADEQ conducted this
monitoring pursuant to Arizona Revised Statutes §49-225
that calls for ongoing monitoring of waters of the state
including its aquifers. This fact sheet is a synopsis of the
ADEQ Open File Report 11-02.1
The McMullen Valley groundwater basin encompasses
approximately 591 square miles in west-central Arizona.2
The western portion of the basin is located in La Paz
County, the southeastern portion is in Maricopa County,
and a small northeastern portion is in Yavapai County.
Salome, Wenden and Aguila are small communities located
within McMullen Valley where agriculture is the major
industry. Approximately 14,600 acres were farmed in 2007.3
There are two irrigation districts: the Aguila Irrigation
District and the McMullen Valley Water Conservation
District. All wells and ditches are privately owned in both
districts as neither has a consolidated distribution system.
Groundwater is the primary source for agricultural, municipal,
stock and domestic water supply within the basin;
it’s estimated that 15.1 million acre-feet of ground-water
is contained in the basin.3
The McMullen Valley basin is one of the few
groundwater basins in Arizona designated for out-of-
basin transport of groundwater. The City of
Phoenix has purchased or leased 16,000 acres of
agricultural land to obtain the water rights for
potential future transport of groundwater to the
Phoenix Active Management Area for municipal
uses.3 Until this groundwater transfer occurs, the
city is managing these agricultural properties by
leasing them to farm operators.4
McMullen Valley is located within the Basin and
Range physiographic province and is a kidney-shaped
basin, oriented northeast-to-southwest and
about 15 miles wide and 48 miles long. At the
southwest end of McMullen Valley is Harrisburg
Valley, oriented perpendicular to the axis of
McMullen Valley. The basin is drained by Centennial
Wash (Figure 1), an ephemeral tributary of the Gila River
that heads about 20 miles east of Aguila and discharges from
the basin through “the Narrows” into the Harquahala basin.2
The main source of groundwater in the basin is the
Regional aquifer.4 Found throughout McMullen Valley, strati-graphic
data suggest it’s composed of coalescing heteroge-neous
deposits of poorly sorted, coarse gravel and sand.
Although thought to be hydrologically connected, the sedi-ment’s
heterogeneous nature results in highly variable
hydraulic properties throughout the aquifer. Intergranular
cementation also impacts the aquifer’s hydraulic properties.
In general, cementation increases in the basin from west to
east, around the basin’s margins, and in proximity to
For the purposes of this study, the Regional aquifer is
subdivided into five aquifers based on partial structural con-trols
and groundwater quality differences. Heavy pumping
for agriculture in two areas, in the vicinity of Aguila (Figure 2)
and around Wenden/Salome, have produced a groundwater
divide near the La Paz-Maricopa County line creating
Eastern and Western Regional aquifers.2 In terms of spatial
extent and groundwater storage these are the largest
aquifers in the basin.2
Figure 1 - McMullen Valley occasionally has prolific surface water flows
such as when the ephemeral Centennial Wash, with flows peaking at
9,938 cubic feet per second, flooded the nearby community of Wenden
during heavy precipitation in mid-January, 2010.
WATER QUALITY SAMPLING RESULTS
Groundwater sample results were compared with the
Safe Drinking Water Act (SDWA) water quality standards.
Public water systems must meet these enforceable, health-based,
water quality standards, called Primary Maximum
Contaminant Levels (MCLs), when supplying water to their
customers. Primary MCLs are based on a daily lifetime (70
years) consumption of two liters of water.7
Of the 124 sites sampled, 54 sites (44 percent) had con-centrations
of at least one constituent that exceeded a
Primary MCL (Map 1). Constituents exceeding Primary
MCLs include arsenic (24 sites) (Map 2), fluoride (27 sites),
nitrate (25 sites), and selenium (2 sites). Primary MCLs for
radionuclides were exceeded at 9 of the 50 sites (18 per-cent)
including gross alpha (9 sites) and uranium (4 sites).
Groundwater sample results were also compared with
SDWA water quality guidelines. Public water systems are
encouraged to meet these unenforceable, aesthetics-based
water quality guidelines, called Secondary MCLs, when sup-plying
water to their customers. Water exceeding
Secondary MCLs may be unpleasant to drink and/or create
unwanted cosmetic or laundry effects but is not considered
a health concern.7
Of the 124 sites samples, 87 sites (70 percent) had con-centrations
of at least one constituent that exceeded a
Secondary MCL water quality guideline (Map 1).
Constituents above Secondary MCLs include chloride (13
sites), fluoride (69 sites), manganese (2 sites), pH (19 sites),
sulfate (8 sites), and TDS (31 sites).
Figure 2 - Well #23 pumps groundwater for use on the near-by
irrigated fields of cantaloupe near the town of Aguila. Like
many wells in the Eastern Regional aquifer, samples from the
well exceeded aesthetics-based standards for fluoride.
Figure 3 - A 180-feet-deep domestic well located near farm-land
north of the town of Salome. Like all samples from wells
drawing water from the Perched aquifer, it exceeded health-based
water quality standards for at least one constituent.
Low hills east of Aguila that minimize groundwater
movement divides the Eastern Regional aquifer from the
Forepaugh aquifer.5 A subsurface extension of the
Harquahala Mountains that limits groundwater movement
separates the Western Regional aquifer from the Southern
Regional aquifer located in Harrisburg Valley.2 Another sub-surface
geologic feature separates the Harcuvar aquifer
from the Southern and Western Regional aquifers lying to
Groundwater movement between the Western Regional
aquifer and the overlaying Perched aquifer (Figure 3) is
restricted by the Lake-bed Unit, a layer of fine-grained
sediments.4 These lake-bed deposits, however, are absent
in a small area one mile northeast of Salome where the
merging of the Regional and Perched aquifers are termed
the Mixed aquifer. 4
METHODS OF INVESTIGATION
To characterize regional groundwater quality in the
McMullen Valley basin, samples were collected from 124
sites consisting of irrigation, domestic, municipal and stock
wells located throughout the basin. Inorganic constituents
and oxygen and deuterium isotopes were collected at all
sites. At selected sites, radon (79 sites), radiochemistry (50
sites) and pesticide (2 sites) samples were also collected.
Twelve (12) additional sites were sampled for field parameters
Sampling protocol followed the ADEQ Quality Assurance
Project Plan. The effects of sampling equipment and proce-dures
were not found to be significant based on seven quality
assurance/quality control tests.6
Map 1 - Sample sites in the McMullen Valley basin are color-coded according to their water quality standard status.
Map 2 - Sample sites in the McMullen Valley basin are color-coded according to their arsenic concentrations.
Radon is a naturally occurring, intermediate breakdown
product from the radioactive decay of uranium-238 to
lead-206. Of the 79 sites sampled for radon, 3 exceeded the
proposed 4,000 picocuries per liter (pCi/L) standard that
would apply if Arizona establishes an enhanced multimedia
program to address the health risks from radon in indoor
air. Sixty-eight (68) sites exceeded the proposed 300 pCi/L
standard for states that would apply if Arizona doesn’t
develop a multimedia program.7
There were no positive detections of any of the 20
organochlorine compounds analyzed in the 2 pesticides
samples collected from shallow wells near irrigated
GROUNDWATER CHEMICAL COMPOSITION
Groundwater in the McMullen Valley basin was predom-inantly
of sodium-chloride or sodium-mixed chemistry.
Levels of pH-field were slightly alkaline (above 7 su); 63 sites
had pH-field levels over 8 su and 6 sites had pH-field levels
over 9 su. TDS concentrations were considered fresh
(below 1,000 mg/L) at 108 sites, slightly saline (1,000 to
3,000 mg/L) at 14 sites and moderately saline (3,000 to
10,000 mg/L) at 2 sites. Hardness concentrations were soft
(below 75 mg/L) at 57 sites, moderately hard (75 – 150 mg/L)
at 38 sites, hard (150 – 300 mg/L) at 19 sites, and very hard
(above 300 mg/L) at 10 sites.
Nitrate (as nitrogen) concentrations at most sites may
have been influenced by human activities. Nitrate concen-trations
were divided into natural background (0 sites at
<0.2 mg/L), may or may not indicate human influence (53
sites at 0.2 – 3.0 mg/L), may result from human activities (56
sites at 3.0 – 10 mg/L), and probably result from human
activities (17 sites >10mg/L).8
Most trace elements such as antimony, beryllium, cadmium,
copper, iron, lead, manganese, mercury, nickel, selenium,
silver, thallium and zinc were rarely–if ever—detected.
Only arsenic (Map 2), barium, boron, chromium and
fluoride were detected at more than 50 percent of the sites.
Many statistically significant groundwater quality patterns
were found between aquifers in the McMullen Valley basin.
Generally, concentrations of many constituents, including
TDS, magnesium, sodium, chloride, sulfate, and nitrate
(Figure 4), were significantly higher in the Perched and
Mixed aquifers than in the other five aquifers. In addition,
hardness, calcium, potassium, barium and gross beta were
significantly higher in the Mixed aquifer than the other
aquifers; similarly, turbidity and boron were significant higher
in the Perched aquifer than the other aquifers (Kruskal-
Wallis with Tukey test, p <_ 0.05).
There were a few exceptions to the Perched and Mixed
aquifers having the highest concentration of constituents in
the McMullen Valley basin. Fluoride concentrations were
significantly higher in the Forepaugh aquifer than the other
aquifers except for the Perched aquifer (Figure 5).
Figure 4 - Samples collected from wells in the Perched and
Mixed aquifers have significantly higher nitrate concentrations
than well samples collected from the other McMullen Valley
aquifers (Kruskal-Wallis with Tukey test, p <_ 0.01). The
Perched and Mixed aquifers are likely impacted by nitrogen-laden
recharge from irrigated fields and, to a lesser degree,
Figure 5 - Samples collected from wells in the Forepaugh aquifer
have significantly higher fluoride concentrations than samples col-lected
from all other aquifers except the Perched aquifer. (Kruskal-
Wallis with Tukey test, p <_ 0.05). The median fluoride concentration
for both the Forepaugh and Perched aquifers exceeded the 4.0 mg/L
health based water quality standard.
The basin’s most important groundwater quality aspect
is the absence of the Lake-bed Unit northeast of Salome.4
Nearby wells commonly exceed water quality standards
and guidelines; nitrate concentrations were elevated up to
seven times the 10 mg/L health-based water quality stan-dard.
This appears to be the result of percolating irrigation
water, and to a lesser degree wastewater from septic sys-tems,
recharging the Perched aquifer with high concentra-tions
of salts and nitrate.4 With a higher static water level
than the Regional aquifer, groundwater drains downward
from the Perched aquifer into the Western Regional aquifer.
4 This impacted area is referred to in this report as the
Mixed aquifer. Besides TDS and nitrate water quality
exceedances, gross alpha and uranium exceedances also
occurred in the Mixed aquifer. The latter two constituents
are likely naturally occurring and are related to nearby
granite geology or alluvial areas of eroded granite though
the elevated levels may be exacerbated by anthropomorphic
sources such as the high alkalinity recharge liberating
naturally occurring uranium that is absorbed into aquifer
Figure 6 - Samples collected from wells in the Harcuvar
aquifer have significantly higher oxygen-18 values than samples
collected in other McMullen Valley aquifers (Kruskal-Wallis
with Tukey test, p <_ 0.05). This pattern indicates groundwater
in the Harcuvar aquifer is likely of more recent origin.
Although the plume of degraded water in the Mixed
aquifer appears too large to be significantly reduced by
pumping, wells in the area should continue to be used for
irrigation purposes to minimize the spread of the plume.4
Groundwater from wells tapping the Mixed aquifer would
require extensive treatment to be used as a municipal or
domestic source. The proposed City of Phoenix well field
locations should avoid this area.
Another important finding was that all nine sample sites
in the Forepaugh aquifer exceeded health-based water quality
standards, most commonly for fluoride and to a lesser
degree, arsenic (Figure 7). Fluoride concentrations were as
high as 15 mg/L, almost four times the health based water
quality standard. Concentrations of fluoride above 5 mg/L
are controlled by calcium through precipitation or dissolu-tion
of the mineral fluorite.11 In a chemically closed hydro-logic
system such as the McMullen Valley basin, calcium is
removed from solution by precipitation of calcium carbon-ate
and the formation of smectite clays. High concentrations
of dissolved fluoride may occur in groundwater depleted in
calcium if a source of fluoride ions is available for dissolu-tion.
11 Arsenic concentrations were as high as 0.022 mg/L,
over twice the 0.01 mg/L standard and may have been influ-enced
by exchange on clays or with hydroxyl ions. Other
factors such as aquifer residence time, an oxidizing environ-ment,
and lithology likely effect arsenic concentrations.11
Although the Eastern and Western Regional aquifers gen-erally
produced water acceptable for domestic or municipal
uses, both aquifers had areas of water quality concern.
Sample sites often exceeded standards for fluoride and, to a
lesser degree, arsenic in the Eastern Regional aquifer, south-east
of the town of Aguila and in the Western Regional
Figure 7 - ADEQ’s Jason Jones samples a domestic well in the
Forepaugh aquifer located east of the town of Aguila. Samples
from sites in the Forepaugh aquifer commonly had health-based
exceedances of fluoride and arsenic.
Oxygen and deuterium levels were significantly higher in
the Harcuvar aquifer than the other aquifers (Figure 6). Well
depths and groundwater depths were significantly greater in
the Eastern Regional aquifer than the other aquifers
(Kruskal-Wallis with Tukey test, p <_ 0.05).
1 Towne, D.C., 2009, Ambient groundwater quality of the McMullen
Valley basin: A 2008-2009 baseline study: Arizona Department of
Environmental Quality Open File Report 11-02, 91 p.
2 Remick, W.H., 1981, Maps showing ground-water conditions in the
McMullen Valley area, Maricopa, Yavapai, and Yuma Counties,
Arizona—1981, Arizona Department of Water Resources,
Hydrologic Map Series Report Number 6, 3 sheets, scale
3 Arizona Department of Water Resources website, 2010,
www.azwater.gov/azdwr/default.aspx, accessed 03/05/10.
4 Montgomery, James M. Consulting Engineers, Inc., 1992, City of
Phoenix Project Report: McMullen Valley Water Transfer Project
Study: Phoenix: Project Number W-886457.
5 Pool, D.R., 1987 Hydrogeology of McMullen Valley, West-Central,
Arizona: U.S. Geological Survey Water-Resources Investigations
Report 87-4140, 51 p.
6 Arizona Department of Environmental Quality, 1991, Quality
Assurance Project Plan: Arizona Department of Environmental
Quality Standards Unit, 209 p.
7 U.S. Environmental Protection Agency Web site,
www.epa.gov/waterscience/criteria/humanhealth/, accessed 8/20/09.
8 Madison, R.J., and Brunett, J.O., 1984, Overview of the occurrence
of nitrate in ground water of the United States, in National Water
Summary 1984-Water Quality Issues: U.S. Geological Survey Water
Supply Paper 2275, pp. 93-105.
9 Lowry, J.D. and Lowry, S.B., 1988, Radionuclides in drinking water.
Journal of the American Water Works Association, 80 (July), pp. 50-
10 Jagucki, M.L., Jurgens, B.C., Burow, K.R. and Eberts, S.M., 2009,
Assessing the vulnerability of public-supply wells to contamination:
Central Valley aquifer system near Modesto, California: U.S.
Geological Survey Water Fact Sheet 2009-3036, 6 p.
11 Robertson, F.N., 1991, Geochemistry of ground water in alluvial
basins of Arizona and adjacent parts of Nevada, New Mexico, and
California: U.S. Geological Survey Professional Paper 1406-C, 90 p.
aquifer near Wenden. These water quality exceedances
appear to be naturally occurring from the same processes
The Eastern Regional aquifer exhibited significantly
lower concentrations of TDS, sodium, and boron than in
the Western Regional aquifer; the opposite pattern occurs
with well depth and groundwater depth (Kruskal-Wallis
with Tukey test, p <_ 0.05). These water quality differences
may result from poor quality irrigation recharge minimally
impacting the Eastern Regional aquifer because of the great
depths needed to percolate to groundwater.
Few water quality standards were exceeded in the
Southern Regional and Harcuvar aquifers; both appear to
consist of more recent recharge than the other aquifers. In
the Southern Regional aquifer there are occasional
exceedances of nitrate in wells located near Centennial
Wash; these are probably the result of wastewater disposal
from septic systems. In the extreme southern portion of the
Southern Regional aquifer near the Narrows, gross alpha
and uranium exceedances occurred along with the highest
radon concentrations (10,241 pico curies per Liter) ever
collected by the ADEQ ambient groundwater monitoring
FOR MORE INFORMATION
Douglas C. Towne
ADEQ Hydrologist, Monitoring Unit
1110 W. Washington St. #5330D, Phoenix, AZ 85007
Web site: www.azdeq.gov/environ/water/assessment/
Maps by Jean Ann Rodine
Publication Number: FS 11-03
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