Groundwater Quality in Arizona:
A 15-Year Overview of the ADEQ Ambient Monitoring Program
(1995-2009)
by Douglas Towne and Jason Jones

ADEQ Water Quality Division • Surface Water Section, Monitoring Unit
1110 W. Washington St. • Phoenix, Arizona 85007-2935
Open File Report 11-04

ii

Groundwater Quality in Arizona: A 15-Year Overview
of the ADEQ Ambient Monitoring Program (1995-2009)
By Douglas C. Towne and Jason Jones
Maps by Jason Jones

Arizona Department of Environmental Quality
Open File Report 2011-04
ADEQ Water Quality Division
Surface Water Section
Monitoring Unit
1110 West Washington St.
Phoenix, Arizona 85007-2935

Thanks:
Photo Credits:

Douglas Towne

Report Cover:

Pumps powered by solar energy, electricity and wind are used to produce
water from wells throughout Arizona.

.

iii

iii

Table of Contents
Abstract..................................................... 1

By site.................................................. 17

Introduction............................................. 2

By basin............................................... 17

Background.............................................. 2

Conclusions.............................................. 22

Objective................................................... 3

Discussion................................................ 22

Investigation Methods............................ 6

Recommendations................................... 23

Sampling Protocol................................... 6

References................................................. 24

Sampling Results...................................... 8
By constituent..................................... 10

Maps:
Map 1.
Map 2.
Map 3.
Map 4.
Map 5.
Map 6.
Map 7.
Map 8.

ADEQ Ambient Groundwater Monitoring Program activity by basin, 1995-2009...................................
Arsenic concentrations at ADEQ sampled sites, 1995-2009.........................................................................
Fluoride concentrations at ADEQ sampled sites, 1995-2009........................................................................
Nitrate concentrations at ADEQ sampled sites, 1995-2009...........................................................................
Gross alpha concentrations at ADEQ sampled sites, 1995-2009..................................................................
Total dissolved solids (TDS) concentrations at ADEQ sampled sites, 1995-2009......................................
Multiple exceedances of the “Big Four” constituents at ADEQ sampled sites, 1995-2009........................
Primary/Secondary MCL exceedances at ADEQ sampled sites, 1995-2009...............................................

Tables:
Table 1. Summary of ADEQ Ambient Groundwater Monitoring Program activities............................................
Table 2. U.S. EPA Safe Drinking Water Act Primary and Secondary Maximum Contaminant
Level Water Quality Standards........................................................................................................................
Table 3. Summary of types of samples collected, ADEQ Ambient Groundwater
Monitoring Program, 1995-2009.....................................................................................................................
Table 4. Summary of water quality status of sampling results by groundwater basin, ADEQ Ambient
Groundwater Monitoring Program, 1995-2009............................................................................................
Table 5. Summary of arsenic, fluoride, nitrate and gross alpha sampling results by groundwater basin,
ADEQ Ambient Groundwater Monitoring Program, 199-2009.................................................................
Figures:
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.

iv

Irrigation well near Casa Grande.................................................................................................................
Brown Spring and Little Ash Creek.............................................................................................................
Domestic well near Date Creek....................................................................................................................
Primary MCL exceedances by constituent.................................................................................................
Secondary MCL exceedances by constituent.............................................................................................
The “Big Four” of Primary MCL exceedances...........................................................................................

5
12
13
14
15
16
18
19

4
7
9
20
21
2
2
3
10
11
23

Groundwater Quality in Arizona:
A 15-Year Overview of the State Ambient Monitoring Program (1995-2009)

Abstract - In pursuing its mandated mission to characterize groundwater quality in the state, the
Arizona Department of Environmental Quality (ADEQ) has collected samples from 1,477 sites over
a 15-year period between 1995 and 2009. The sample sites consisted mainly of domestic, stock, irrigation and municipal wells and, to a lesser extent, springs used predominantly for watering stock and
wildlife. Sampling activity was conducted within 35 of the state’s 51 officially designated groundwater basins and covered much of Arizona with the exception of Native American tribal lands.
All groundwater samples were analyzed for most inorganic constituents listed in the U.S. Environmental Protection Agency Safe Drinking Water (SDW) Act. Approximately a third of the wells also
had samples collected for SDW radionuclide constituents and lesser numbers of samples were collected for Volatile Organic Compounds and pesticide analyses.
Of the 1,477 sites sampled, 31 percent exceeded at least one health-based water quality standard,
which provides a rough estimate of the percentage of wells state-wide not meeting SDW standards.
Over 97 percent of exceedances were caused by elevated concentrations of four constituents: arsenic
(41 percent), fluoride (22 percent), nitrate (18 percent) and gross alpha (16 percent). The data provide comprehensive and reliable information on the occurrence and concentrations of groundwater
contaminants. This is critical knowledge for the estimated 100,000 private domestic wells in the state
whose owners represent about 5 percent of Arizona’s population.
Unlike public water systems, private domestic wells are not subject to SDW regulations. Thus,
collecting and analyzing water samples from private wells is not required and only occasionally
conducted. One factor in well owner’s reluctance to have their domestic water tested is expense; a
comprehensive inorganic suite costs over $650. Testing for only the four constituents that constitute
97 percent of the water quality exceedances in this study is an economical ($85) alternative for private
well owners in Arizona. Although ADEQ recommends sampling for all the SDW constituents, testing
for arsenic, fluoride, nitrate and gross alpha would be an important initial step in evaluating the suitability of water for domestic use.

1

Introduction

Background

In pursuing its mandated mission to characterize
groundwater quality in the state (ADEQ, 2009)
the Arizona Department of Environmental Quality
(ADEQ) sampled 1,477 wells (including dozens of
springs) throughout Arizona with the exception of
Native American tribal lands. Sampling occurred
over a 15-year period between 1995 and 2009.
Groundwater samples were tested for most inorganic
constituents listed in the U.S. Environmental Protection Agency (USEPA) Safe Drinking Water (SDW)
Act (see Table 2). Approximately a third of the wells
also were tested for SDW radionuclide constituents
and lesser numbers were tested for Volatile Organic
Compounds (VOCs) and pesticide analyses.

Groundwater constitutes about 3.1 million acre-feet
or 43 percent of Arizona’s annual water use (ADWR,
2010). The vast majority of groundwater in Arizona
is used to irrigate crops (Figure 1) and for public
water supplies (82%).

The percentage of sampled wells that are safe to be
used for domestic purposes was determined from
the analytical results and provides an estimate of
the overall percentage of wells in Arizona that meet
SDW standards. Based on specific constituent exceedance patterns, economical suggestions for water
quality testing are provided for private well owners
who choose not to sample for the full suite of SDW
constituents as recommended by ADEQ.

Figure 1 - A 1,350-foot deep well produces water to irrigate crops near Casa Grande.
2

To a lesser extent, groundwater is also used for mining, industrial, domestic, stock, and other purposes
throughout the state. Groundwater discharge creates
the base flow for many streams, lakes, and wetlands
thereby directly impacting surface water quality
(Figure 2).
Groundwater quality is of major importance, especially when utilized for municipal and domestic
water supply purposes. All aquifers in the state
are protected for drinking water designated use by
Arizona’s 1986 Environmental Quality Act and
Aquifer Boundary and Protected Use Classification

Figure 2 - Brown Spring contributes flow to Little Ash
Creek and its riparian area located in the Agua Fria basin.

(R18-11, Article 5). Arizona’s Aquifer Water Quality
Standards (R18-11, Article 4) are protective of the
drinking water use and are equivalent to the SDWA
standards except for arsenic.
Despite this safeguard, groundwater contamination
can be a serious problem. Potential point sources in
Arizona include industrial waste, underground storage tanks, landfills, mines, and wastewater treatment
plants. These activities are specifically regulated and
monitored through programs operated by ADEQ.
However, there are other major groundwater pollution sources that are not comprehensively addressed
by ADEQ programs. These nonpoint sources include
agricultural activities and septic wastewater disposal
systems. The largest influence on groundwater quality
is natural sources which, obviously, are unregulated.
The extent and impact of both nonpoint and natural
sources on groundwater quality throughout Arizona is
not well known in many areas of the state.
To fill this data gap, ADEQ pursues its mandated
mission to characterize groundwater in the state. The
agency also investigates the influences of agricultural practices, septic systems, and natural sources on
water quality. The majority of the sampled wells (98
percent) were collected as part of baseline investigations of groundwater quality in 30 of the state’s
51 groundwater basins officially designated by the
Arizona Department of Water Resources (Table 1)
(Map 1).

Figure 3 - A 520-foot domestic well located by Date Creek
in the Bill Williams basin is being purged in preparation
for sampling.

These studies were designed to examine broad,
regional groundwater quality conditions existing
within the basins. Limited sampling consisting of
31 samples was conducted in five additional basins
because of special sampling requests or basin studies
were not completed.

• To highlight groundwater quality concerns on a
state-wide scale.
• To provide recommendations for private well owners interested in a limited, cost-effective strategy
for determining if groundwater they use meets
health-based, water quality standards.

Below is the link for the studies.
http://www.azdeq.gov/environ/water/assessment/
ambient.html

The quality of water delivered through public supplies is strictly regulated; this resource is routinely
monitored to verify it meets state and federal standards set to protect public health. However, there are
more than 100,000 private domestic wells (Figure 3)
whose owners represent about 5 percent of Arizona’s
population that are not subject to Safe Drinking Water regulations required of public water systems, and
thus, are not required to conduct water quality tests
(ADWR, 2010).

Objective
This study will examine sample results collected
over a 15-year period beginning in 1995 by ADEQ.
The purpose of this comprehensive analysis is
twofold:

3

Table 1. Summary of ADEQ Ambient Groundwater Monitoring Program Activities
Basin
Yuma
Douglas
Duncan Valley
Upper San Pedro
Virgin River
Prescott AMA
Upper Santa Cruz
Sacramento Valley
Willcox
Lower San Pedro
Hualapai Valley
Meadview
Avra Valley
San Rafael
Detrital Valley
San Simon
Cienega Creek
Salt River
Tonto Creek
San Bernardino
Lake Mohave
Aravaipa Canyon
Big Sandy
Bill Williams
Upper Hassayampa
Little Colorado River
Gila Valley
Dripping Springs
Agua Fria
Phoenix AMA
Pinal AMA
McMullen Valley
Butler Valley
Harquahala
Ranegras Plain

Year
Sampled
1995
1995-96
1995-2002
1996-97
1997
1997-98
1998
1999
1999
2000
2000
2000-03
2001
2002
2002
2002
2002
2002
2002
2002
2003
2003
2003-04
2003-09
2003-09
20032004
2004-05
2004-06
20042005-06
2008-09
200820082009-

Year Report
Published
1997
1999
1999
1999
2001
2000
2001
2001
2002
2005
2005
2003
2003
2004
2011
2005
2006
2011
2010
2011
2008
2008
2011
-

Comments

Part of Upper Gila Watershed Report
Joint Study w/ U.S. Geological Survey
Joint Study w/ U.S. Geological Survey

Special ADEQ sampling

Special ADEQ sampling
Basin sampling not yet completed
Basin sampling not yet completed
Basin sampling not yet completed

These studies can be accessed on the ADEQ website:
(http://www.azdeq.gov/environ/water/assessment/ambient.html)

4

4

Map 1. ADEQ Ambient Groundwater Monitoring
Program Activity by Basin, 1995-2009

5

Private well owners often have not had analytical
tests conducted on the quality of water produced by
their wells and may be unaware of the presence of
contaminants that could adversely affect their health.
The large numbers of untested private domestic
wells make comprehensive and reliable information on the occurrence and levels of contaminants
in groundwater essential to protect public health in
Arizona.
Previous assessments of groundwater quality in
Arizona have indicated distressing results. Marrero-Ortiz and others (2009) reported 95 percent of
wells sampled for microbial, physical, and chemical
constituents exceeded at least one U.S. Environmental Protection Agency (EPA) Primary and/or Secondary drinking water standard or guideline based on 49
wells sampled in seven counties in Arizona.
In a national survey of about 2,100 domestic wells,
23 percent of sampled wells contained one or more
contaminants at a concentration greater than a
human-health benchmark. These contaminants were
most often inorganic chemicals with all but nitrate
derived primarily from natural sources. Almost
half (48 percent) of the sampled wells contained at
least one constituent at a concentration outside the
range of aesthetic values recommended by U.S. EPA
Secondary drinking water guidelines (DeSimone and
others, 2009).

Investigation Methods
Several factors were considered in selecting wells
and springs for sampling. Important considerations
included physical characteristics, land uses, hydrologic complexity (such as the presence of multiple
sub-basins, aquifers and/or perennial streams) and
the number and distribution of wells and springs.
Three strategies were used to characterize basin
groundwater quality:
• stratified random sampling using computer generated, equal area polygons
• stratified random sampling using township blocks
and/or physiographic areas
• random sampling.
6

Targeted sampling was often subsequently used near
sites having constituent concentrations with healthbased water quality standards in order to determine
the spatial extent of impacted groundwater quality.

Sampling Protocol
Production wells used for domestic, stock, irrigation,
and public water supply were commonly sampled
for the studies. Monitoring wells originally installed
to delineate the extent of fuel leaks from underground storage tanks were occasionally sampled to
assist in characterizing shallow aquifers. Springs for
stock and/or domestic use were sometimes sampled
especially in remote areas lacking wells.
Sampling protocol followed the ADEQ Quality Assurance Project Plan (ADEQ, 1991) with only minor
deviations. In all instances, the collected sample
consisted of freshly pumped groundwater as determined by well casing capacity and field parameters
such as temperature, pH, and specific conductivity.
In some instances, less than three bore volumes
were evacuated before sampling because of factors
inherent in field work. These issues range from concerns of the well owner to uncertainty of how long
some wells, such as windmills, would continue to
pump water. However, in all cases field parameters
indicated water freshly pumped from the aquifer was
sampled.
At each site, an inorganic sample was collected for
physical parameters, general mineral characteristics,
major ions, nutrients and trace elements for analysis by the Arizona Department of Health Services
(ADHS) laboratory in Phoenix, Arizona. The inorganic suite incorporated the vast majority of constituents regulated by the U.S. Environmental Protection Agency (USEPA) Safe Drinking Water (SDW)
Act including those having health-based, water quality standards called Primary Maximum Contaminant
Levels (MCLs) and those having aesthetics-based,
water quality guidelines called Secondary MCLs.
These water quality standards are provided in Table
2 (USEPA, 2010).

Table 2. US EPA Safe Drinking Water Act Primary* and Secondary Maximum
Contaminant Level Water Quality Standards
Primary Constituent
Nitrite (NO2-N)
Nitrate (NO3-N)
Antimony (Sb)
Arsenic (As)*
Barium (Ba)
Beryllium (Be)
Cadmium (Cd)
Chromium (Cr)
Copper (Cu)
Fluoride (F)
Lead (Pb)
Mercury (Hg)
Nickel (Ni)
Selenium (Se)
Thallium (Tl)
Gross Alpha
Ra-226+Ra-228
Radon **
Radon **
Uranium
Secondary Constituent
pH - field
TDS
Chloride (Cl)
Sulfate (SO4)
Fluoride (F)
Iron (Fe)
Manganese (Mn)
Silver (Ag)
Zinc (Zn)

Nutrients

Primary MCL
1.0
10.0

Trace Elements

Radiochemistry Constituents

Physical Parameters

15
5
300
4,000
30
Secondary MCL

General Mineral Characteristics
Major Ions
Trace Elements

0.006
0.01 / 0.05
2.0
0.004
0.005
0.1
1.3
4.0
0.015
0.002
0.1
0.05
0.002

<6.5 ; >8.5
500
250
250
2.0
0.3
0.05
0.1
5.0

All units are mg/L except gross alpha, radium-226+228 and radon (pCi/L), uranium (ug/L) and pH (su).
Health-based drinking water quality standards are based on a lifetime consumption of two liters of water
per day over a 70-year life span.
* All established Primary MCLs are also Arizona Aquifer Water Quality Standards with the exception of
arsenic at 0.05 mg/L.
** Proposed EPA Safe Drinking Water Act standards for radon in drinking water.

9

7

Federal Primary MCLs are synonymous with state
Aquifer Water Standards: Drinking Water Protected
Use (AWQ) with three exceptions. There is no copper
or turbidity standard and arsenic has 0.05 mg/L state
standard compared with the 0.01 mg/L Primary MCL.
Other types of samples were sometimes collected at
these sites depending on budgetary considerations
and whether there was a probable chance of detecting elevated concentrations of naturally occurring
radionuclide constituents and/or detecting anthropomorphic compounds. See Table 3 for a summary of
analyses by parameter for the basins sampled.
Radionuclide samples were collected and analyzed
according to USEPA SDW protocols by the Arizona
Regulatory Radiation Agency (ARRA) laboratory
in Phoenix, Arizona at 553 sites (38 percent) mostly considered to have high potential for elevated
radiation activity because of nearby geology and/or
mining land use.
Volatile Organic Compounds (VOCs) samples were
collected at 287 sites (19 percent) mostly in urban
areas. Samples for currently registered pesticides
were collected at 72 sites (5 percent) and for banned
pesticides at 43 sites (3 percent) in areas of irrigated
farmland. Both VOC and pesticide analysis was conducted by the ADHS laboratory in Phoenix, Arizona.
The effects of sampling equipment and procedures
were evaluated using quality control samples including equipment blanks, duplicate samples, split samples and, occasionally, spiked samples. Data were
also validated using seven measurements including
cation/anion balances.
In two studies conducted in conjunction with the
U.S. Geological Survey, the field protocols and laboratories of each agency were evaluated using split
samples (Coes and others, 2000). Based on these
indices, the impacts of sampling procedures and lab
analysis were found not to be significant except in
very specific circumstances.

8

Sampling Results
Primary MCLs / AWQ Standards - Water quality
data from 1,477 wells and springs were compared to
inorganic USEPA SDW requirements and/or AWQ
standards. SDW Primary MCLs are water quality
standards that public water systems must meet when
providing water to their customers. There are 16
inorganic constituents that have Primary MCLs including antimony, arsenic, asbestos, barium, beryllium, cadmium, chromium, copper, cyanide, fluoride,
lead, mercury, nitrate, nitrite, selenium, thallium, and
turbidity (USEPA, 2010).
The ADEQ Ambient Groundwater Monitoring program routinely tests for turbidity but elevated levels
were not considered Primary MCL exceedances as
this standard applies only to public systems using
surface water or groundwater under the direct influence of surface water. Turbidity has no health effects
itself, but can interfere with disinfection and provide
a medium for microbial growth (USEPA, 2010).
Asbestos and cyanide were not routinely tested for
because of their rare occurrence and the specialized
sampling and testing procedures required for these
constituents.
Of the 1,477 sites sampled, 391 sites (26 percent)
exceeded at least one inorganic water quality standard. Three constituents commonly exceeded water
quality standards: arsenic, fluoride, and nitrate. Maps
are provided for each of these constituents that show
in a very general way, the spatial variability of concentrations across Arizona.

Table 3. Summary of Types of Samples Collected, ADEQ Ambient Groundwater
Monitoring Program 1995 – 2009
Basin
Yuma
Douglas
Duncan Valley
Upper San Pedro
Virgin River
Prescott AMA
Upper Santa Cruz
Sacramento Valley
Willcox
Lower San Pedro
Hualapai Valley
Meadview
Avra Valley
San Rafael Valley
Detrital Valley
San Simon
Cienega Creek
Salt River
Tonto Creek
San Bernardino
Lake Mohave
Aravaipa Canyon
Big Sandy
Bill Williams
Upper Hassayampa
Ltl Colorado River
Gila Valley
Dripping Springs
Agua Fria
Phoenix AMA
Pinal AMA
McMullen Valley
Butler Valley
Harquahala
Ranegras Plain
Total

Sites
Sampled
55
52
55
73
38
58
65
48
58
63
26
8
42
20
28
77
20
41
23
14
43
15
56
101
34
7
65
12
46
11
86
124
2
4
7
1477

Inorganic
Samples
55
52
55
73
38
58
65
48
58
63
26
8
42
20
28
77
20
41
23
14
43
15
56
101
34
7
65
12
46
11
86
124
2
4
7
1477

Radionuclide
Samples
7
7
20
10
10
40
44
19
16
2
22
5
11
23
6
35
18
15
29
55
14
3
20
7
33
25
53
1
3
553

VOC
Samples
13
10
2
36
48
54
25
21
1
19
2
10
22
8
14
287

Pesticide
Samples
57
7
12
3
2
4
4
2
4
4
14
2
115

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9

Sampling Results by Constituent
Arsenic - Arsenic concentrations at 276 sites (or
19 percent) exceeded the 0.01 milligram per Liter
(mg/L) Primary MCL that became effective January
26, 2006). In contrast, only 30 sites (or 2 percent)
had arsenic concentrations that exceeded the former
Primary MCL and current state AQW standard of
0.05 mg/L. Although elevated arsenic concentrations
are found throughout Arizona, the highest concentrations are generally located in southeastern and
western Arizona (Map 2).
Fluoride – Fluoride concentrations at 141 sites (10
percent) exceeded the 4.0 mg/L federal Primary
MCL/state AQW standard (Map 3). Also shown on
the map are fluoride concentrations (367 sites or 24
percent) that exceed the aesthetics-based, Secondary
Maximum Contaminant Levels. Although fluoride
concentrations elevated over the Primary Maximum
Contaminant Level are found throughout Arizona,
the highest concentrations are generally located in
southeastern and western.

or 18 percent) (Map 5), uranium (42 sites or 8 percent) and radium-226 and 228 (12 sites or 2 percent).
All uranium and radium-226 and 228 exceedances
occurred at sites where gross alpha concentrations
also exceeded health-based, water quality standards.
Thus, gross alpha was considered representative of
radionuclide results; uranium and radium-226 and
228 were not used in further analyses. Gross alpha
exceedances occur throughout the state, but most
commonly occur in northwestern Arizona especially
in the Cerbat and Hualapai mountains.
Of the 1,477 sites sampled, 459 sites (31 percent)
exceeded at least one inorganic and/or radionuclide
water quality standard. Four constituents commonly
exceeded water quality standards: arsenic, fluoride,
nitrate, and gross alpha (Figure 4).

Nitrate – Nitrate concentrations at 115 sites (8
percent) exceeded the 10 mg/L federal Primary
MCL/state AQW standard (Map 4). Elevated nitrate
concentrations most commonly occur at groundwater sites located near major expanses of irrigated
farmland in central and western Arizona.
Other Inorganic Constituents - Antimony (6
sites), barium (1 site), beryllium (4 sites), cadmium
(2 sites), chromium (1 site), copper (0 sites), lead
(2 sites), mercury (0 sites), nitrite (0 sites), selenium (2 sites), and thallium (0 sites) were rarely, if
ever, detected at concentrations above water quality
standards. These 11 constituents combined to exceed
water quality standards at about 1 percent of sites.
Gross Alpha - There are three radionuclide constituents that have Primary MCLs: gross alpha,
radium-226 and 228, and uranium. Radionuclide
samples were collected at 553 of the 1,477 sites that
were typically in or near bedrock, particularly in
areas in proximity to mines and/or granite rock.
Of the 553 sites sampled for radionuclides, water
quality exceedances included gross alpha (101 sites
10

Figure 4 - Primary MCL Exceedances by Constituent

sites or 18 percent), chloride (184 sites or 13 percent), manganese (136 sites or 9 percent), iron (104
sites or 6 percent), and pH (83 sites > 8.5 su and 4
sites < 6.5 or 6 percent outside range).
Aluminum (0 sites), copper (0 sites), silver (0 sites),
and zinc (1 site) were rarely, if ever, detected at
concentrations above water quality guidelines. These
4 constituents combined to exceed water quality
standards at less than 1 percent of sites.
TDS – TDS concentrations at 555 sites (37 percent)
exceeded the 500 mg/L aesthetics-based, water quality standard (Map 6). TDS concentrations elevated
over the water quality standard are typically naturally occurring but may be influenced by human activities including agricultural activities and septic tank
effluent. Although TDS concentrations elevated over
the aesthetics-based, water quality standard occur
throughout the state, exceedances most commonly
occur in agricultural areas of the southern Arizona
including near the cities of Casa Grande, Safford,
and Yuma.

Figure 5 - Secondary MCL Exceedances by Constituent

Secondary MCLs - There are 15 inorganic constituents that have Secondary MCLs including aluminum, chloride, color, copper, corrosivity, fluoride,
foaming agents, iron, manganese, odor, pH, silver,
sulfate, total dissolved solids (TDS) and zinc.
The ADEQ ambient groundwater monitoring program does not routinely test for color, corrosivity,
foaming agents or odor which are somewhat subjective and can be evaluated by domestic well owners
Of the 1,477 sites where samples were collected, 761
or (52 percent) exceeded at least one water quality
guideline. The constituents that mostly commonly
exceeded the water quality standards (Figure 5)
include TDS (555 sites or 37 percent) (Map 6), fluoride (367 sites or 24 percent) (Map 3), sulfate (263

From the limited collection of VOC and pesticide
samples, few anthropomorphic organic compounds
were detected and these did not exceed Primary
MCLs. VOC detections could usually be traced to
disinfection by-products or by PVC glue used by the
well owner to create a sample port near the wellhead
a day or two before sampling by ADEQ.
VOC contamination of groundwater is typically
found in the vicinity of industrial or defense facilities. These sites are being remediated through various federal EPA programs or the state Water Quality
Assurance Revolving Fund (WQARF) program.
These sites are found throughout the state but many
are located in the Phoenix and Tucson metropolitan
areas.
A complete listing of sites is on the ADEQ website
at http://www.azdeq.gov/environ/waste/sps/siteinfo.
html

11

Map 2. Arsenic Concentrations (mg/L) at ADEQ
Sample Sites, 1995-2009. The federal health-based,
Primary Maximum Contaminant Level (MCL) for arsenic was 0.05 mg/L but was lowered to 0.01 mg/L in
2006. The state AWQ standard remains at 0.05 mg/L.
12

Map 3. Fluoride Concentrations (mg/L) at ADEQ
Sample Sites, 1995-2009. The health-based, federal
Primary MCL / state AQW standard for fluoride is
4.0 mg/L. Fluoride is unique in that it has also has a
Secondary MCL of 2.0 mg/L.
13

Map 4. Nitrate (as Nitrogen) Concentrations (mg/L)
at ADEQ Sample Sites, 1995-2009. The health-based,
federal Primary MCL / state AQW standard for
nitrate (as nitrogen) is 10 mg/L.

14

Map 5. Gross Alpha Concentrations (piC/L) at ADEQ
Sample Sites, 1995-2009. The health-based, federal
Primary MCL / state AQW standard for gross alpha is
15 pCi/L.

15

Map 6. Total Dissolved Solids (TDS) Concentrations
(mg/L) at ADEQ Sample Sites, 19952009. The aesthetics-based, Secondary Maximum
Contaminant level for TDS is 500 mg/L.

16

Sampling Results by Site
Of the 1,477 sites sampled (Map 7):
• 624 sample sites (42 percent) met all Primary and
Secondary MCLs
• 1018 sample sites (69 percent) met all Primary
MCLs
Generally, sites sampled in the southeastern quadrant
of Arizona tended to have fewer water quality exceedances except for agricultural areas located near
the communities of Casa Grande, Safford and San
Simon. Primary and Secondary MCL exceedances
tended to occur more frequently in the northwest
quadrant of the state and also in the extreme southwestern corner of Arizona. Some of these sites are
correlated with agricultural areas such as near the
communities of Salome and Yuma; other exceedances involve gross alpha concentrations in the Cerbat
and Hualapai Mountains.

In contrast, basins with the lowest percentage of Primary MCL exceedances such as Aravaipa Canyon,
Dripping Springs Wash and San Bernardino Valley
(all 0 percent), Avra Valley, San Rafael and Upper
San Pedro (all 10 percent), Salt River (12 percent),
Douglas (14 percent) and Cienega Creek (15 percent) were generally characterized as having limited
expanses of irrigated farmland. Generally, lower
numbers of radionuclide samples were also collected
in these basins. Most of these basins are located in
the southeastern Arizona and many of the Primary
MCL exceedances occurred at artesian wells producing very old water from deep, confined aquifers.

Of the 459 samples sites (31 percent) that had
Primary MCL exceedances, 325 (71 percent) only
exceeded water quality standards for one constituent
(Map 8). Of the remainder, 118 (26 percent) exceeded water quality standards for two constituents, 16 (3
percent) exceeded water quality standards for three
constituents, and no sites exceeded water quality for
four constituents.

Sampling Results by Basin
Water quality standards exceedances occur statewide, but there is much variability among individual
groundwater basins (Tables 4 and 5).
The basins with the highest percentage of Primary
MCL exceedances such as the Pinal Active Management Area (AMA) (70 percent), Gila Valley (46
percent), Willcox (46 percent) and McMullen Valley
(45 percent) tend to have both large expanses of
irrigated cropland that impact nitrate concentrations
along with aquifer materials that sometimes produce
elevated concentrations of arsenic and fluoride. Other basins such as Sacramento Valley (48 percent) and
Big Sandy (42 percent) mostly lack irrigated cropland but do have aquifer materials that sometimes
produce elevated concentrations of arsenic, fluoride
and gross alpha.
17

Map 7. Primary and Secondary MCL Exceedances at
ADEQ Sample Sites, 1995-2009.

18

Map 8. Number of “Big Four” (Arsenic, Fluoride,
Nitrate and Gross Alpha) Primary MCL Exceedances
at ADEQ Sample Sites, 1995-2009.

19

Table 4. Summary of the Water Quality Status of Sampling Results by Groundwater
Basin, ADEQ Ambient Groundwater Monitoring Program, 1995-2010
Basin
Yuma
Douglas
Duncan Valley
Upper San Pedro
Virgin River
Prescott AMA
Upr Santa Cruz
Sacramento Vly
Willcox
Lower San Pedro
Hualapai Valley
Meadview
Avra Valley
San Rafael
Detrital Valley
San Simon
Cienega Creek
Salt River
Tonto Creek
San Bernardino
Lake Mohave
Aravaipa Canyon
Big Sandy
Bill Williams
Upr Hassayampa
Ltl Colorado Rvr
Gila Valley
Dripping Spring
Agua Fria
Phoenix AMA
Pinal AMA
McMullen Vly
Butler Valley
Harquahala
Ranegras Plain
Total

# of Sites
Sampled
55
52
55
73
38
58
65
48
58
63
26
8
42
20
28
77
20
41
23
14
43
15
56
101
34
7
65
12
46
11
86
124
2
4
7
1477

Sites Over
PMCLs
11
20 %
8
15 %
12
21 %
7
10 %
9
24 %
17
29 %
12
19 %
23
48 %
22
46 %
19
30 %
9
35 %
3
38 %
4
10 %
2
10 %
9
32 %
25
32 %
3
15 %
5
12 %
7
30 %
0
0%
15
35 %
0
0%
24
43 %
27
27 %
9
26 %
0
0%
30
46 %
0
0%
14
30 %
5
45 %
60
70 %
56
45 %
0
0%
3
75 %
6
86 %
459 31 %

Sites Over
both MCLs
11
20 %
6
6%
8
14 %
5
7%
5
13 %
5
9%
7
11 %
22
46 %
20
42 %
17
27 %
9
35 %
2
25 %
2
5%
2
10 %
5
18 %
24
31 %
0
0%
4
10 %
2
9%
0
0%
12
28 %
0
0%
19
34 %
24
24 %
8
24 %
0
0%
30
46 %
0
0%
11
24 %
4
36 %
42
49 %
52
42 %
0
0%
3
75 %
6
86 %
369 25 %

Sites Over
only SMCLs
44
80 %
10
20 %
26
46 %
19
26 %
20
53 %
4
7%
10
15 %
6
8%
3
6%
12
19 %
8
31 %
2
25 %
8
19 %
1
5%
6
21 %
24
31 %
2
10 %
10
24 %
1
4%
6
43 %
19
44 %
4
27 %
9
16 %
25
25 %
5
15 %
4
57 %
24
37 %
0
0%
20
44 %
4
36 %
18
21 %
35
28 %
0
0%
1
25 %
1
14 %
392 27 %

Sites Under
both MCLs
0
0%
34
65 %
19
33 %
47
64 %
9
24 %
37
64 %
36
55 %
19
40 %
33
57 %
32
51 %
9
35 %
3
38 %
30
71 %
17
85 %
13
46 %
28
36 %
15
75 %
25
61 %
15
65 %
8
57 %
9
21 %
11
73 %
23
41 %
49
0%
20
59 %
3
43 %
11
17 %
12 100 %
11
24 %
2
18 %
8
9%
33
27 %
2
100 %
0
0%
0
100 %
624 42 %

PMCLs – Primary Maximum Contaminant Levels
SMCLs – Secondary Maximum Contaminant Levels
Both – Primary and Secondary Maximum Contaminant Levels

24

20

Table 5. Summary of Arsenic, Fluoride, Nitrate and Gross Alpha Sampling Results
by Groundwater Basin, ADEQ Ambient Groundwater Monitoring
Program 1995 – 2010
Basin
Yuma
Douglas
Duncan Valley
Upper San Pedro
Virgin River
Prescott AMA
Upr Santa Cruz
Sacramento Vly
Willcox
Lower San Pedro
Hualapai Valley
Meadview
Avra Valley
San Rafael
Detrital Valley
San Simon
Cienega Creek
Salt River
Tonto Creek
San Bernardino
Lake Mohave
Aravaipa Canyon
Big Sandy
Bill Williams
Upr Hassayampa
Ltl Colorado Rvr
Gila Valley
Dripping Spring
Agua Fria
Phoenix AMA
Pinal AMA
McMullen Vly
Butler Valley
Harquahala
Ranegras Plain
Total

# of Sites
Sampled
55 / 7
52 / 7
55 / 20
73 / 0
38 / 10
58 / 10
65 / 0
48 / 40
58 / 44
63 / 19
26 / 26
8/2
42 / 22
20 / 5
28 / 11
77 / 23
20 / 6
41 / 35
23 / 18
14 / 0
43 / 15
15 / 0
56 / 29
101 / 55
34 / 14
7/3
65 / 20
12 / 7
46 / 33
11 / 0
86 / 25
124 / 53
2/1
4/0
7/3
1477

Sites >
Arsenic MCL
9
16 %
5
10 %
14
25 %
6
8%
9
24 %
15
26 %
9
14 %
6
13 %
9
16 %
12
19 %
3
12 %
1
3%
2
5%
0
0%
3
11 %
16
21 %
1
5%
5
12 %
5
22 %
0
0%
14
33 %
0
0%
13
23 %
10
10 %
1
3%
0
0%
21
32 %
0
0%
12
26 %
4
36 %
33
38 %
29
23 %
0
0%
3
75 %
6
86 %
276
19 %

Sites >
Sites >
Fluoride MCL Nitrate MCL
0
0%
5
9%
0
6%
1
2%
6
11 %
2
4%
3
4%
0
0%
0
0%
0
0%
4
5%
1
2%
1
2%
7
11 %
4
8%
6
13 %
8
14 %
5
9%
8
13 %
1
2%
2
8%
3
12 %
0
0%
0
0%
0
0%
0
0%
0
0%
0
0%
0
0%
3
11 %
19
25 %
3
4%
0
0%
0
0%
1
2%
0
0%
0
0%
1
4%
0
0%
0
0%
1
2%
3
7%
0
0%
0
0%
11
20 %
0
0%
4
4%
3
3%
0
0%
4
12 %
0
0%
0
0%
20
31 %
4
6%
0
0%
0
0%
5
11 %
1
2%
0
0%
1
9%
7
8%
23
27 %
32
26 %
30
22 %
0
0%
0
0%
0
0%
2
50 %
6
86 %
5
71 %
141
10 %
115
8%

Sites > Gross
Alpha MCL
0
0%
2
29 %
2
10 %
0
0%
1
10 %
1
10 %
0
0%
18
45 %
7
16 %
2
11 %
3
19 %
2
100 %
3
14 %
1
20 %
3
27 %
3
13 %
2
33 %
1
3%
2
11 %
0
0%
0
0%
0
0%
8
28 %
16
29 %
5
36 %
0
0%
3
15 %
0
0%
1
3%
0
0%
5
20 %
10
19 %
0
0%
0
0%
0
0%
101
18 %

25

21

Conclusions
The results of this study indicate that generally the
majority of groundwater sites (69 percent) meet
EPA’s Primary SDW requirements and could be
used for public water supply without any more than
standard treatment. However, this percentage should
be used somewhat cautiously as several biases in
the data may alter the rate of sites meeting Primary
MCLs. Certainly, using the state AQW standard of
0.05 mg/L instead of the federal Primary MCL of
0.01 mg/L for arsenic would also increase the percentage of groundwater sites meeting that could be
used for public water supply without any more than
standard treatment.
A major factor that might increase the percentage of
sites not meeting Primary MCLs is that radionuclide
samples were not collected at 61 percent of sites including some basins without a single well or spring
sampled for radionuclide constituents. Collecting radionuclide samples at each of the 1,477 sites would
have probably increased the frequency of Primary
MCL exceedances but by how much is unknown.
Other types of samples such as VOCs and pesticides
were collected at far lower frequencies than radionuclides. However, their results, (overwhelmingly
non-detect) indicate that sampling for these contaminants at each of the 1,477 sites would probably not
have significantly impacted the frequency of Primary
MCL exceedances.
On the other hand, some factors indicate that 69
percent might be too low since when a site exceeded a Primary MCL, especially when it was a high
outlier, additional targeted samples were collected in
the area to determine the extent of the plume. Thus,
targeting some samples around sites with extreme
examples of poor quality groundwater would tend
to increase the number of sites not meeting health
based, water quality standards.
The most significant regulatory impact to the percentage of groundwater sites meeting SDW requirements
was lowering of the arsenic standard from 0.05 mg/L
to 0.01 mg/L that took effect nationwide on January
26, 2006. The change in the standard resulted in
Primary MCL exceedances for arsenic in the 1,477
22

sites increasing from 27 sites (2 percent) evaluated
under the former 0.05 mg/L standard to 276 sites (19
percent) using the current 0.01 mg/L standard. With
the change, arsenic became, by far, the most prominent Primary MCL exceedance in Arizona.

Discussion
Some general observations may be made concerning the hydrologic conditions and spatial locations
where these constituents tend to be found at concentrations that exceed water quality standards. However, the maps contained in this report should be used
cautiously since there are severe limitations in representing a three-dimensional groundwater unit on
a two-dimensional map. As an example, two wells
adjacent to one another could be drawing groundwater from different aquifers that have a very different
water quality. Generally, fluoride, arsenic, and gross
alpha are naturally occurring although the latter may
be increased by mining activity (Lowry, 1988).
Elevated fluoride concentrations may occur throughout Arizona but tend to be most common in the
southeastern and west central portions of the state.
Fluoride concentrations above 5 mg/L are controlled
by calcium through precipitation or dissolution of
the mineral fluorite. In a chemically closed hydrologic system, calcium is removed from solution
by precipitation of the calcium carbonate 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 dissolution (Robertson, 1991). Thus, wells
having soft, sodium-dominated, older water, such as
artesian wells drawing from deep aquifers in southeastern Arizona, are likely to have elevated fluoride
concentrations.
Arsenic concentrations tend to be the most difficult
to predict with concentrations elevated over the
Primary MCL occurring throughout Arizona. Arsenic concentrations may be influenced by similar
reactions as fluoride, including exchange on clays
or with hydroxyl ions. Other factors such as a long
aquifer residence time, an oxidizing environment,
high pH levels, and an abundance of trace elements
in alluvial sediments are also likely to impact arsenic
concentrations (Robertson, 1991; Spencer, 2002).

Radionuclide concentrations such as gross alpha
usually occur in areas located in or near granite
rock or alluvial areas composed of eroded granite
(Lowry, 1988). Mining activity also increases gross
alpha concentrations because of the increased rock
surface exposure. Gross alpha exceedances occurred
where other radionuclide exceedances such radium
226+228 and uranium occurred, making this constituent an important harbinger of elevated radionuclides concentrations in general.
Elevated nitrate concentrations most commonly
occur in groundwater samples collected from shallow wells located near major expanses of irrigated
farmland. In Arizona, these areas are located within
several basins including the Pinal AMA, McMullen
Valley, Willcox, Safford, and Yuma. Nitrate concentrations can also occasionally exceed water quality
standards in domestic wells impacted by septic
systems especially in high densities, and in isolated
stock wells situated by corrals where livestock linger
for extended periods. In some Sonoran desert areas
of Arizona, naturally occurring nitrate concentrations
are elevated from nitrogen that has been accumulated in the soil by native legume plants (Thiros and
others, 2012).

(Figure 6). The “Big Four” constituents that most frequently exceed Primary MCLs in groundwater in Arizona.
Sampling for these four constituents is highly recommended
and this option would run only $85. (Test America, 2010).

However, while there are sites where elevated
concentrations of arsenic, fluoride, nitrate and gross
alpha are likely to occur (Figure 6), domestic well
owners should be aware that exceedances of these
constituents can potentially occur in water produced
by any well.

Local county extension offices can also provide
technical assistance and occasionally offer limited
water quality testing for well owners.

likely reveal most water quality standard exceedances. Arsenic, fluoride, nitrate and gross alpha are
the four constituents (nicknamed the “Big Four”)
that caused 97 percent of health based water quality
exceedances in 1,477 wells scattered across Arizona.
Domestic well owners interested in testing their
groundwater quality can find additional information
on the EPA website at http://water.epa.gov/drink/
index.cfm.

Recommendations
Although ADEQ recommends homeowners using a
private well for domestic use have the water tested
for the full suite of inorganic and radionuclide SDW
requirements to ensure safety, this is an expensive
venture. For 18 constituents (excluding asbestos, cyanide and turbidity), these combined analytical tests
would cost $664.80 at a local laboratory certified
by the Arizona Department of Health Services (Test
America, 2010). The costs for inorganic constituents
typically range from $7 - $45.
However, if the well owner is unable to afford such
extensive testing, there is an alternative that will
23

References
Arizona Department of Environmental
Quality, 1991, Quality Assurance Project Plan:
Arizona Department of Environmental Quality Standards Unit, 209 p.
Arizona Department of State,
Department of Water Quality Standards, 2012,
ADOS website, http://www.azsos.gov/public_services/Title_18/18-11.htm
Arizona Department of Water
Resources, 2010. ADWR Website: Arizona’s Water
Supplies and Water Demands, www.azwater.gov/.
Coes, A.L., D.J. Gellenbeck, D.C.
Towne, and M.C. Freark, 2000. Ground-water quality in the Upper Santa Cruz basin, Arizona, 1998,
U.S. Geological Survey Water-Resources Investigations Report 00-4117, 55 p.
DeSimone, L.A., Hamilton, P.A., and
Gilliom, R.J., 2009. The quality of our nation’s
waters—Quality of water from domestic wells in
principal aquifers of the United States, 1991-2004—
Overview of major findings: U.S. Geological Survey
Circular 1332, 48 p.
Lowry, J.D., and S. B. Lowry, 1988.
“Radionuclides in drinking waters,” American Water Works Association Journal 80 (July), p. 50-64.
Marrero-Ortiz, Roberto, Kelley R. Riley,
Martin K. Karpiscak and Charles. P. Gerba,
“Groundwater quality of individual wells and small
systems in Arizona,” American Water Works Association Journal, Sept. 2009, Vol. 101, Issue 9, pp.
89-102.
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,
pp. 90.

24

Spencer, J., 2002, Natural occurrence of
arsenic in Southwest ground water. Southwest Hydrology, May/June, pp. 14-15.
Test America Laboratory, 2010. Billing
invoice received December 2, 2010.
Thiros, S.A., Paul, A.P., Bexfield, L.M.
and Anning, D.W., 2012. Water quality in basin-fill
aquifers of the Southwestern United States, Arizona,
California, Colorado, Nevada, New Mexico, and
Utah, 1993-2009, U.S. Geological Survey Circular
1358, 95 p.
U.S. Environmental Protection Agency,
2010. USEPA Website: Human Health Criteria,
http://water.epa.gov/drink/standardsriskmanagement.cfm