Flagstaff, Arizona
Flagstaff Water Quality
A mountain lake, springs on the San Francisco Peaks in summer, and deep wells. The 2025 report's everyday facts: arsenic below the limit but present, chlorine dioxide pretreatment and very low sodium.
Arsenic 6.9 ppb in 2025 — under the 10 ppb limit, from natural deposits.
The short answer
Flagstaff blends Upper Lake Mary (typically 20–30% of the year), Inner Basin springs on the San Francisco Peaks (typically May–September) and deep groundwater wells, so the water at your tap is not the same mix all year.
The 2025 report lists arsenic at 6.9 ppb against a limit of 10, chlorine dioxide pretreatment for the lake water, and sodium of 3.2–6.8 ppm. It prints no hardness figure. If anything here justifies equipment, it is arsenic margin and disinfectant taste at the kitchen tap — confirmed by a test, not assumed.
Get a free test
Tell us your Flagstaff address. We confirm coverage first, then schedule a licensed technician to test at your tap — no charge, no obligation.
What filtration does — and does not
Softening, carbon and reverse osmosis solve different problems. Flagstaff's report is an arsenic and disinfectant story, not a hardness purchase order.
Where Flagstaff water comes from
A lake, mountain springs and deep wells, in a mix that shifts with the season — a different starting point from the Phoenix and Tucson systems.
Upper Lake Mary, 20–30% of the year
Raw water is carried about 5 miles to the Lake Mary Water Treatment Plant, pretreated with chlorine dioxide, run through conventional filtration and then mixed with groundwater from the Lake Mary wellfield. The report says the lake typically makes up 20–30% of annual drinking water.
Inner Basin springs, May to September
Developed springs, infiltration galleries and three shallow wells in the Inner Basin of the San Francisco Peaks feed the North Reservoir Filtration Plant, typically from May through September. The City's 2026 report to its Water Commission says about 76 million gallons came from the Inner Basin in summer 2025 after the waterline restoration project.
Deep groundwater wells
Wells in two wellfields outside town, plus several local wells inside city limits, make up the rest. Which mix reaches a given house depends on the season and on where it sits in the distribution system.
Rated low risk by ADEQ
The Arizona Department of Environmental Quality's source water assessment gives Flagstaff's sources a low-risk designation, meaning most protection measures are already in place or the hydrogeology limits the benefit of more.
What the 2025 Flagstaff report actually says
Limits are fixed. The results next to them are from the City of Flagstaff's Consumer Confidence Report for Calendar Year 2025 — system-level figures, not a reading at your tap. A later report supersedes them.
Arsenic — 6.9 ppb (limit 10)
Arsenic was detected at 6.9 ppb in 2025 against an MCL of 10 and a health goal of 0, from erosion of natural deposits. The report carries the EPA's standard advisory: the water meets the standard but does contain low levels of arsenic.
Chlorine dioxide and chlorite
Because Lake Mary water is pretreated with chlorine dioxide, the report lists chlorine dioxide up to 112 ppb (MRDL 800) and its byproduct chlorite up to 0.360 ppm (MCL 1). Chlorine residual is listed at 0.69 ppm with a range of ND to 1.44.
Trihalomethanes and haloacetic acids
As printed, the TTHM row shows 266 ppb in the highest-amount column with a range of ND–56, and the HAA5 row shows 125 ppb with a range of ND–24. Both are marked no violation. Those columns do not reconcile on the page, so we quote them as printed rather than interpret them.
Sodium — 3.2 to 6.8 ppm
Very low. Nitrate ranged from 0.087 to 2.0 ppm against a limit of 10, and fluoride was listed at 0.072 ppm. Uranium, last sampled in 2021, was up to 1.7 ppb against a limit of 30.
PFBA — 6.1 ppt in federal monitoring
Under the federal UCMR5 program the City found one PFAS compound, perfluorobutanoic acid (PFBA), at up to 6.1 ppt in 2024 (minimum reporting level 5). PFBA is not one of the PFAS with a federal limit. No other PFAS appears in the table.
Lead — 3.6 ppb at the 90th percentile
In the 2023 round, lead was 3.6 ppb at the 90th percentile with 0 of 30 sites above the 15 ppb action level; copper was 0.15 ppm. The City's service line inventory is published online.
Why Flagstaff is not Phoenix at altitude
Phoenix treats Colorado River water from the CAP canal and Salt and Verde water from SRP reservoirs, and Tucson recharges Colorado River water into its aquifer. Flagstaff has no canal water at all: its surface source is a local lake, its summer source is snowmelt springs, and the rest is deep groundwater. That is why arsenic, a groundwater mineral, and chlorine dioxide, used on the lake water, are the two findings that matter here — and why a quote built on a valley city's report is a quote for the wrong house.
Where these figures come from
Everything on this page traces to the City of Flagstaff. Independent compilations of water quality data exist and are not used here.
- City of Flagstaff — Consumer Confidence Report for Calendar Year 2025 (PDF) — sources, ADEQ source water assessment, regulated substances, lead and copper, UCMR5 and the 2025 reporting violations.
- City of Flagstaff — 2026 Report to the Water Commission (CY 2025) — Inner Basin production restarted in 2025 after the waterline restoration project.
- City of Flagstaff — Water Quality — earlier reports and the source water assessment.
The report also lists two 2025 reporting violations (paperwork submitted late, back in compliance within a month); the City states there are no health concerns related to them.
Matching treatment to the finding
Flagstaff's report supports an arsenic-margin or disinfectant-taste conversation at the kitchen tap — not a softener default.
Arsenic: point-of-use, certified for it
Arsenic is below the limit, not absent. A household that wants it lower usually starts with a kitchen-tap reverse osmosis system certified for arsenic reduction, and verifies the result with a test after installation.
Taste: carbon for the disinfectant
Chlorine and chlorine dioxide are the taste and odour story. Activated carbon at the tap or as a whole-house stage reduces both well.
Hardness: test, do not assume
The 2025 report does not print a hardness figure, and a lake, mountain springs and deep wells will not share one. A tap test answers it for your house before anyone sizes a softener.
Older homes: check the service line
System lead results are low. A house with an older or unknown service line is answered by the City's online inventory and a tap test, not by the 90th-percentile figure.
How the free test works
A licensed technician tests at your kitchen tap, walks the plumbing, and tells you which of the findings above applies to your house and which does not. There is no charge and no obligation.
Seamless Home is the licensed contractor of record on the work that follows. Installation is performed by vetted installing partners engaged as our subcontractors, and the workmanship obligation stays with us. Coverage is confirmed per service area rather than promised as blanket availability.
An RO unit needs a legal drain connection — where a water softener is allowed to discharge sets out the indirect-waste rule that applies to treatment drain lines.
What a water treatment job with us covers
Installation and replacement of whole-home and point-of-use treatment, specified against what your water actually contains.
Sediment filtration
Installation of the first stage that protects everything downstream of it, sized to the service line and the household draw.
Softening
Installation of ion-exchange softening where hardness is the problem, including the drain connection the plumbing code treats as indirect waste.
Carbon filtration
Installation of carbon stages for taste, odour and disinfection by-products, specified from your utility's own water quality report.
Reverse osmosis
Point-of-use installation at the kitchen sink where the target is dissolved solids rather than whole-home flow.
Disinfection
Installation of UV or equivalent treatment where the supply is a private well or is otherwise not continuously disinfected.
System replacement
Replacing equipment that has reached the end of its service life, including the discharge and air gap details an inspector looks at.
This is installation and replacement work, specified from a published water quality report or a test of your supply — not from a sales chart. Coverage is confirmed per service area rather than promised as blanket availability.
What the work actually looks like
Treatment equipment is plumbing. Most of the job is the connection, the drain and the code detail behind it.
Flagstaff water, answered plainly
Where does Flagstaff's water come from?+
Three sources: Upper Lake Mary, treated at the Lake Mary Water Treatment Plant and typically 20–30% of the year's water; springs, infiltration galleries and shallow wells in the Inner Basin of the San Francisco Peaks, typically May through September; and deep groundwater wells in two wellfields outside town plus local wells inside city limits.
Is there arsenic in Flagstaff water?+
Yes, at a low level. The Consumer Confidence Report for calendar year 2025 lists arsenic at 6.9 ppb against the federal limit of 10 ppb, from erosion of natural deposits, with no violation. A reverse osmosis system certified for arsenic reduction is the usual point-of-use option for a household that wants it lower.
Why does Flagstaff use chlorine dioxide?+
Lake Mary water is pretreated with chlorine dioxide for raw-water oxidation and disinfection and to limit the formation of disinfection byproducts. That is why the report lists chlorine dioxide (up to 112 ppb) and chlorite (up to 0.360 ppm), both below their limits.
Is Flagstaff water hard?+
The 2025 report does not print a hardness figure, so we do not quote one. With lake water, mountain springs and deep wells in the mix, a tap test at your house is the reliable answer.
Is there PFAS in Flagstaff water?+
Federal UCMR5 monitoring found one compound, PFBA, at up to 6.1 ppt in 2024. PFBA does not have a federal drinking water limit, and the report lists no other PFAS detection. A certified lab test at your tap is the direct answer for your house.
Where do these figures come from?+
The City of Flagstaff's Consumer Confidence Report for Calendar Year 2025 (public water system AZ-04-03008) and the City's 2026 Report to the Water Commission. Third-party water-quality aggregators are not the source used here. A later report supersedes these figures.
Related resources
Home water purification and treatment
The national service page behind this one.
Learn more →Phoenix water quality
Canal and reservoir water in the valley — the contrast.
Learn more →Tucson water quality
Recharged Colorado River water in southern Arizona.
Learn more →Where a water softener can discharge
Indirect-waste rules for treatment drain lines.
Learn more →All home services
Water, battery, HVAC and solar under one licensed contractor of record.
Learn more →Get a free test at your tap
A licensed technician tests at your kitchen tap and tells you which of the findings above applies to your house — arsenic, disinfectant taste, hardness or none of them. No charge, no obligation, and coverage confirmed at your address first.
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