Uranium and gross alpha in Maricopa County drinking water: what the data shows

Maricopa County tap water is routinely tested for radioactive contaminants, including uranium, radium-226, radium-228, and gross alpha activity. Every public water system in Arizona with more than 25 service connections must submit radionuclide monitoring results to the U.S. Environmental Protection Agency and publish them in an annual Consumer Confidence Report (CCR). The data is public, searchable at epa.gov/ccr, and consistently shows that Maricopa County utilities keep radionuclide concentrations well below EPA Maximum Contaminant Levels (MCLs). However, naturally occurring radioactive materials in the desert geology beneath Phoenix, Scottsdale, Mesa, and Gilbert mean that low-level radionuclide readings are routine, not exceptional. Understanding what those readings mean, and what treatment effectively reduces them, gives homeowners an accurate picture of their water.

Where do radionuclides in Arizona water come from?

Arizona sits on some of the oldest exposed rock in North America. Precambrian granite formations, which underlie much of Maricopa County at depth, naturally contain uranium-238, thorium-232, and their radioactive decay products. When groundwater moves through fractured granite or uranium-bearing sedimentary layers, it picks up dissolved uranium and radium. Gross alpha activity, the broad measure of all alpha-emitting radionuclides in a water sample, reflects the sum of these dissolved radioactive materials.

The U.S. Geological Survey has documented uranium in Arizona groundwater through its National Water Quality Assessment (NAWQA) program and National Water Information System (NWIS) at waterdata.usgs.gov. USGS sampling of Arizona alluvial and basin-fill aquifers shows uranium concentrations ranging from less than 1 microgram per liter in some zones to above 30 micrograms per liter in areas where groundwater contacts uranium-bearing rock for extended periods. The eastern and southern portions of Maricopa County, which draw on aquifers in contact with older geological formations, tend to show higher uranium activity than CAP-fed surface water zones in central Phoenix.

What are the EPA limits and what do Maricopa County utilities actually measure?

Federal drinking water regulations under the Safe Drinking Water Act establish four radionuclide MCLs that apply to all community water systems in Arizona:

• Gross alpha particle activity: 15 picocuries per liter (pCi/L). This is the summary measure that triggers further radionuclide investigation when exceeded.
• Combined radium-226 and radium-228: 5 pCi/L. Radium is a decay product of uranium that can concentrate in groundwater-reliant systems.
• Uranium: 30 micrograms per liter (mcg/L or ppb). The MCL was set in 2001 and applies to all community water systems. USEPA rule at 40 CFR Part 141, Subpart B.
• Beta particle and photon activity: 4 millirems per year, relevant only for systems drawing on water near nuclear facilities, not applicable to Maricopa County utilities.

Maricopa County's major utilities consistently report radionuclide levels below these MCLs. The City of Phoenix 2024 Consumer Confidence Report (phoenix.gov/waterservices) shows gross alpha activity between 0 and 2 pCi/L, radium-226 and radium-228 at or near non-detect, and uranium at 0 to 3 mcg/L, all well within federal limits. Scottsdale's 2024 Water Quality Report (scottsdaleaz.gov/water) shows similar surface-water-dominated results with gross alpha below 2 pCi/L. Mesa Utilities, which blends more groundwater into its supply, reports gross alpha in the 2 to 5 pCi/L range in its eastern distribution zones. Gilbert Water, which relies more heavily on groundwater from the San Tan area, documents uranium at 2 to 8 mcg/L in its 2024 CCR, still below the 30 mcg/L MCL. Chandler and Tempe, serving primarily surface-water-fed zones, show uranium and gross alpha results comparable to Phoenix. All Maricopa County results cited here are available in each utility's Consumer Confidence Report at epa.gov/ccr.

Why gross alpha matters even at low levels

Alpha particles cannot penetrate skin from outside the body, but alpha emitters that are ingested are a different matter. When uranium or radium is consumed in drinking water over years or decades, the alpha radiation is absorbed by surrounding tissue. The EPA set the uranium MCL of 30 mcg/L based on risk of kidney toxicity and a small increase in cancer risk over a lifetime of consumption at that concentration (EPA fact sheet at epa.gov/sites/default/files/2015-09/documents/2002_radionuclides_final_rule_factsheet.pdf). The MCL is designed to protect against lifetime chronic exposure, not single-event consumption.

For Maricopa County homeowners, the practical implication is that measured levels in utility reports are well below regulatory concern for the distribution system as a whole. Groundwater-heavy zones in the east valley carry higher baseline readings than surface-water-fed zones in central Phoenix and Scottsdale. Households served by small private wells in Maricopa County, which are not regulated under community water system rules, are responsible for their own testing. The ADEQ Well Water Program at azdeq.gov provides guidance for private well owners on radionuclide sampling.

Which treatment methods actually remove uranium and radium?

Not all common home filtration technologies are equally effective against dissolved radionuclides. Point-of-use options vary significantly in their removal capability.

• Reverse osmosis (RO): The most effective point-of-use technology for radionuclide reduction. An RO membrane with a pore size in the nanofiltration range (0.0001 micron) rejects dissolved ions including uranium and radium at rates of 90 to 99 percent, depending on membrane quality and operating conditions. The NSF/ANSI 58 standard covers RO systems certified for lead and other contaminants; the EPA also recognizes RO as an effective treatment for uranium at the household level in its document at epa.gov/ground-water-and-drinking-water.
• Ion exchange: Water softeners using cation exchange resin will remove radium-226 and radium-228 along with calcium and magnesium, because radium behaves chemically like calcium. Standard sodium-cycle softeners are not optimized for uranium, which exists as a uranyl carbonate complex in Arizona's alkaline water and does not exchange well on standard cation resin.
• Activated carbon filters: Ineffective for dissolved uranium and radium. Carbon targets organic compounds, not dissolved heavy metals or radionuclides.
• Sediment and whole-house filters: Do not remove dissolved radioactive ions. These address particulate matter, not dissolved contaminants.

For Phoenix metro homeowners who want to reduce radionuclide exposure at the kitchen tap, a four-stage under-sink RO system with a certified 50 or 75 GPD membrane is the practical and cost-effective path. The same unit that reduces TDS, PFAS, arsenic, and nitrate also addresses uranium and gross alpha at the point of consumption.

How to read your utility's radionuclide data

Every Consumer Confidence Report lists radionuclide test results in a standard table showing: the contaminant name, the year sampled, the highest level detected during the monitoring period, the MCL, and the MCLG (Maximum Contaminant Level Goal, which is zero for most radionuclides). The reports are free, publicly available, and accessible through the EPA's Consumer Confidence Report database at epa.gov/ccr. Enter your utility name or zip code to retrieve your utility's most recent report. For Maricopa County, the following utilities post their own CCRs directly:

• City of Phoenix: phoenix.gov/waterservices
• Scottsdale Water: scottsdaleaz.gov/water
• Mesa Utilities: mesaaz.gov/residents/utilities
• Chandler Water: chandleraz.gov/government/departments/public-works-utilities/water
• Gilbert Water: gilbertaz.gov/departments/public-works/water-resources
• Tempe: tempe.gov/government/community-services/tempe-utilities
• Peoria: peoriaaz.gov/government/departments/water
• Glendale: glendaleaz.com/live/city-services/water-services

If your water comes from a system that has not posted its CCR online, the EPA CCR portal at epa.gov/ccr allows a national search by water system name or PWSID number. All Arizona public water systems reporting to ADEQ are also searchable through the ADEQ online public water supply database at azdeq.gov.

What this means for Phoenix metro homeowners

Maricopa County utilities are in compliance with all federal radionuclide MCLs. The distribution-system data confirms that even groundwater-heavy utilities like Gilbert and parts of Mesa stay below EPA thresholds for uranium, radium, and gross alpha. This is the water arriving at your service connection.

Where radionuclide reduction at the household level makes sense: households on private wells in Maricopa County, homeowners in groundwater-heavy zones who want to reduce uranium to the lowest practical level at the kitchen tap, and anyone combining radionuclide reduction with TDS, PFAS, and arsenic reduction in a single system. In those cases, an NSF 58-certified RO unit handles all of them simultaneously. Aqua Wellness installs four-stage reverse osmosis systems in Phoenix, Scottsdale, Tempe, Mesa, Chandler, and Ahwatukee. Free water test available by calling (928) 451-1591.