01The verdict: is San Carlos water safe?
San Carlos's drinking water has significant quality concerns based on EPA testing data. With a grade of D (45.2/100), the system has issues across multiple categories. A water filter is recommended for all residents. The city's 7 water systems serve approximately 11,217 residents using groundwater (wells).
296 unresolved violations on record
EPA records show open compliance items for this system. Most are administrative, but unresolved items are worth watching.
See what was cited ↓Lead is well under the limit
90th-percentile lead is 5.0 ppb — beneath the 15 ppb federal action level and under the 5 ppb watch threshold.
See the trend ↓lithium is approaching its limit
Measured at 72.8000 µg/L — 120% of the 0.004 µg/L limit. Within the rules, but with less headroom than you'd want.
See the ledger ↓San Carlos's drinking water received a grade of D (45.2 out of 100), indicating poor water quality. The city's 7 water systems serve approximately 11,217 residents using groundwater.
Lead levels were measured at 5.0 ppb (90th percentile), well within EPA limits. UCMR 5 testing detected 2 PFAS compounds in the water supply. The system has 2043 violations on record, including 140 health-based violations. 296 remain unresolved.
San Carlos ranks #229 out of 271 cities in Arizona for water quality, placing it among the lowest-rated in the state. San Carlos relies on groundwater, which is generally less vulnerable to surface contamination but can be affected by naturally occurring minerals like arsenic and nitrate, as well as agricultural and industrial runoff. PFAS compounds were detected in testing, though levels remain within current EPA limits. Residents seeking extra precaution may consider an activated carbon or reverse osmosis filter. The system has seen 216 violations in the past five years, suggesting a pattern of compliance challenges that residents should monitor closely.
02What's in San Carlos's water
Every contaminant flagged in San Carlos, on one shared scale: distance to its own legal limit. The red line is the limit — anything past it is out of compliance, and the closer a bar gets, the less headroom the system has.
Full contaminants report
| Contaminant | Detected Level | EPA Limit | Unit | Category | Status |
|---|---|---|---|---|---|
| Lead (90th percentile) LeadHeavy Metal A toxic heavy metal that can leach into drinking water from older pipes, solder, and fixtures. No amount of lead in water is considered safe. Health EffectsBrain and nervous system damage in children, kidney damage, high blood pressure, and reproductive problems in adults. EPA Limit15 ppb action level Common SourcesCorrosion of lead pipes, lead solder, brass faucets, and household plumbing. | 5.0 | 15 | ppb | Inorganic | Safe |
| 11Cl-PF3OUdS | ND | HI | µg/L | PFAS | Not Detected |
| 4:2 FTS | ND | HI | µg/L | PFAS | Not Detected |
| 6:2 FTS 6:2 FTSPFAS A fluorotelomer sulfonate commonly found at sites contaminated with aqueous film-forming foam (AFFF) used in firefighting. Health EffectsPotential liver toxicity and endocrine disruption. Less studied but identified as a contaminant of concern. EPA LimitNo individual MCL (not yet regulated) Common SourcesFirefighting foam (AFFF), airports, military bases, and industrial facilities. | ND | HI | µg/L | PFAS | Not Detected |
| 8:2 FTS | ND | HI | µg/L | PFAS | Not Detected |
| 9Cl-PF3ONS | ND | HI | µg/L | PFAS | Not Detected |
| ADONA | ND | HI | µg/L | PFAS | Not Detected |
| HFPO-DA HFPO-DA (GenX)PFAS A replacement for PFOA in manufacturing, marketed as safer but still a persistent "forever chemical." Also known as GenX. Health EffectsLiver and kidney effects, reproductive toxicity, immune system effects, and potential cancer risk. EPA Limit10 ppt MCL Common SourcesFluoropolymer manufacturing (used as PFOA replacement), industrial wastewater discharge. | ND | 0.01 | µg/L | PFAS | Not Detected |
| lithium LithiumInorganic A naturally occurring alkali metal found in groundwater. Monitored under UCMR 5 to assess occurrence in drinking water. Health EffectsKidney effects at high doses. Low-level exposure effects under study; some research suggests neurological effects. EPA LimitNo MCL (monitoring only under UCMR 5) Common SourcesNatural mineral deposits, geothermal water, and industrial discharge. | 72.800 | HI | µg/L | PFAS | Detected |
| NEtFOSAA | ND | HI | µg/L | PFAS | Not Detected |
| NFDHA | ND | HI | µg/L | PFAS | Not Detected |
| NMeFOSAA | ND | HI | µg/L | PFAS | Not Detected |
| PFBA PFBAPFAS One of the shortest-chain PFAS compounds. Very mobile in water and difficult to remove with standard filtration. Health EffectsThyroid effects, potential developmental toxicity. Shorter half-life in body than long-chain PFAS. EPA LimitNo individual MCL (not yet regulated) Common SourcesDegradation of longer-chain PFAS, industrial discharge, and firefighting foam. | ND | HI | µg/L | PFAS | Not Detected |
| PFBS PFBSPFAS A short-chain PFAS used as a replacement for PFOS. While it clears the body faster than long-chain PFAS, it still persists in the environment. Health EffectsThyroid effects, reproductive and developmental toxicity, kidney effects. EPA LimitPart of Hazard Index (1.0) Common SourcesIndustrial discharge, firefighting foam, and as a replacement chemical in manufacturing. | 0.054 | HI | µg/L | PFAS | Detected |
| PFDA | ND | HI | µg/L | PFAS | Not Detected |
| PFDoA | ND | HI | µg/L | PFAS | Not Detected |
| PFEESA | ND | HI | µg/L | PFAS | Not Detected |
| PFHpA PFHpAPFAS A medium-chain PFAS compound found in various environmental samples. Less studied than PFOA/PFOS but still considered a contaminant of concern. Health EffectsLiver effects, potential developmental toxicity, and endocrine disruption. EPA LimitPart of Hazard Index (1.0) Common SourcesDegradation of longer-chain PFAS, industrial discharge, and contaminated water sources. | ND | HI | µg/L | PFAS | Not Detected |
| PFHpS | ND | HI | µg/L | PFAS | Not Detected |
| PFHxA PFHxAPFAS A short-chain PFAS replacement chemical widely used after manufacturers phased out longer-chain PFAS. Very commonly detected in water. Health EffectsLiver and kidney effects, potential thyroid disruption. Considered less toxic than long-chain PFAS but still persistent. EPA LimitNo individual MCL (not yet regulated) Common SourcesIndustrial processes, firefighting foam (AFFF), food packaging, and textile treatment. | ND | HI | µg/L | PFAS | Not Detected |
| PFHxS PFHxSPFAS A medium-chain PFAS found in firefighting foam and consumer products. It has a long half-life in the human body, similar to long-chain PFAS. Health EffectsImmune system effects, thyroid disruption, and potential reproductive and developmental harm. EPA LimitPart of Hazard Index (1.0) Common SourcesFirefighting foam (AFFF), waterproof textiles, food packaging, and industrial discharge. | ND | HI | µg/L | PFAS | Not Detected |
| PFMBA | ND | HI | µg/L | PFAS | Not Detected |
| PFMPA | ND | HI | µg/L | PFAS | Not Detected |
| PFNA PFNAPFAS A long-chain PFAS compound used in manufacturing fluoropolymers. It bioaccumulates in the body and is very persistent in the environment. Health EffectsDevelopmental effects, liver toxicity, immune suppression, and potential cancer risk. EPA LimitPart of Hazard Index (1.0) Common SourcesFluoropolymer manufacturing, industrial emissions, and contaminated water sources. | ND | HI | µg/L | PFAS | Not Detected |
| PFOA PFOAPFAS A long-chain PFAS ("forever chemical") once widely used in nonstick coatings and firefighting foam. It persists in the body and environment for years. Health EffectsLinked to kidney and testicular cancer, thyroid disease, elevated cholesterol, and reproductive issues. EPA Limit4.0 ppt MCL Common SourcesIndustrial discharge, firefighting foam (AFFF), nonstick cookware manufacturing, and contaminated groundwater. | ND | 0.004 | µg/L | PFAS | Not Detected |
| PFOS PFOSPFAS A long-chain PFAS compound historically used in stain-resistant coatings and firefighting foam. One of the most studied and persistent PFAS chemicals. Health EffectsLiver damage, immune system suppression, thyroid disruption, increased cholesterol, and potential cancer risk. EPA Limit4.0 ppt MCL Common SourcesFirefighting foam (AFFF), industrial sites, stain-resistant fabric treatments, and contaminated groundwater. | ND | 0.004 | µg/L | PFAS | Not Detected |
| PFPeA PFPeAPFAS A short-chain PFAS compound commonly detected in drinking water. One of the most frequently found PFAS in UCMR 5 monitoring. Health EffectsLess studied than PFOA/PFOS. Potential liver and thyroid effects. Research is ongoing. EPA LimitNo individual MCL (not yet regulated) Common SourcesIndustrial discharge, firefighting foam degradation, and consumer products. | ND | HI | µg/L | PFAS | Not Detected |
| PFPeS | ND | HI | µg/L | PFAS | Not Detected |
| PFTA | ND | HI | µg/L | PFAS | Not Detected |
| PFTrDA | ND | HI | µg/L | PFAS | Not Detected |
| PFUnA | ND | HI | µg/L | PFAS | Not Detected |
Cities ranked by 90th-percentile lead PF PFAS in Arizona
Cities ranked by compounds detected Cu Copper in Arizona
Cities ranked by 90th-percentile copper
03San Carlos's ten-year track record
Each box is one year. A green check means no EPA violations; a numbered badge counts that year's violations — red if any were health-based, amber if they were monitoring or paperwork issues. The system has 2043 violations on record, including 140 health-based violations. 296 remain unresolved.
1 of the last ten years was clean. The numbered years had EPA violations — red when any were health-based, amber when they were paperwork or missed-testing issues.
▼ Down 0 ppb since 2004. Latest reading: 5 ppb (2025) — 33% of the action level. Sampled at higher-risk homes (90th percentile).
04Where San Carlos's water comes from
San Carlos's drinking water is drawn from underground aquifers through wells.
Groundwater
Underground aquifers reached by wells — naturally filtered, but can carry minerals like arsenic and nitrate.
Local treatment
Groundwater generally needs less treatment — typically disinfection and, where needed, mineral removal.
7 systems
7 water systems, operated under native american ownership.
11,217 people
Residents served across San Carlos's service area.
Groundwater is naturally filtered through rock and soil layers, generally requiring less treatment than surface water. However, it can contain naturally occurring contaminants like arsenic, radon, and minerals. Agricultural activity, septic systems, and industrial operations near well fields can introduce nitrates, pesticides, and volatile organic compounds. The system is operated by native american ownership and serves approximately 11,217 people through 7 water systems.
| System name | PWSID | Population | Source |
|---|---|---|---|
| SCUA Upper Seven Mile | 090400114 | 8,478 | Groundwater |
| SCUA Lower Peridot | 090400112 | 702 | Groundwater |
| SCUA Beverly Hills | 090400699 | 612 | Groundwater |
| SCUA Cutter | 090400239 | 574 | Purchased Groundwater |
| SCUA Lower Peridot Bush | 090400111 | 342 | Groundwater |
| SCUA Upper Peridot | 090400242 | 275 | Groundwater |
| SCUA Skill Center | 090400273 | 234 | Groundwater |
Water bodies near San Carlos
These water bodies shape the regional watershed and can indirectly affect groundwater quality.
05Pressure on San Carlos's watershed
Outside forces that shape what reaches the treatment plant. None of these is a verdict on the tap by itself — but each adds pressure worth knowing about.
Flood & storm history
9 federal disaster declarations on record for Gila County. Flooding and severe storms can overwhelm treatment plants, cause sewage overflows, and push runoff into supplies.
06How San Carlos compares
Scores for nearby Arizona cities on one line, with the average marked.
dashed line = Arizona average (41/100)
San Carlos scores 45.2/100 — on par with the Arizona average of 41/100. It outscores 8 of 10 nearby cities. Full Arizona rankings →
37.5 / 100FTucson, AZ
38.1 / 100FMesa, AZ
40.6 / 100FChandler, AZ
40.5 / 100FGilbert, AZ
34.8 / 100FScottsdale, AZ
43 / 100
07What to do at home
Steps matched to what testing actually found in San Carlos's water — not generic advice.
Filter drinking & cooking water
Activated carbon or reverse osmosis filter. 2 PFAS compounds detected
Compare tested filters →Flush taps & check your service line
Lead leaches from pipes as water sits. Run cold water 30 seconds before drinking (especially mornings), and ask the utility whether your address still has a lead service line.
Lead risks explained →Watch for San Carlos alerts
This area has a history of flood declarations — after severe weather, watch for boil-water advisories from the utility, and check the live alerts on this page.
All water alerts →Frequently asked questions
Is San Carlos, AZ tap water safe to drink?
San Carlos's water quality earned a grade of D (45.2/100). Significant issues have been found. A water filter is strongly recommended. The city ranks #229 out of 271 cities tested in Arizona.
What contaminants are in San Carlos's water?
Lead was measured at 5.0 ppb (90th percentile). 2 PFAS compounds were detected. 2043 violations are on record.
How is San Carlos's water quality grade calculated?
The grade is based on four factors: violation history (40%), lead and copper levels (25%), PFAS contamination (25%), and regulatory compliance (10%). The score is also adjusted based on how complete the available data is. See our methodology page for full details.
Do I need a water filter in San Carlos?
PFAS compounds have been detected. A filter with activated carbon can help reduce exposure.
Where does San Carlos's water come from?
San Carlos's water is sourced from Groundwater. The city has 7 water systems serving approximately 11,217 residents.
What health violations has San Carlos's water system had?
San Carlos has 140 health-based violations on record. The most recent violation was recorded in December 2025. Health-based violations mean the water exceeded EPA maximum contaminant levels (MCLs) for a regulated substance. 296 violations remain unresolved.
Is San Carlos's groundwater at risk of contamination?
San Carlos uses groundwater, which can be affected by naturally occurring contaminants like arsenic, radon, and nitrate, as well as agricultural runoff and industrial activity. The system has 2043 violations on record that may relate to groundwater quality. Groundwater systems are generally less susceptible to surface contamination but should be monitored for emerging contaminants like PFAS.
How does San Carlos's water compare to other cities?
San Carlos ranks #229 out of 271 cities in Arizona (better than 15% of state cities) and #13885 out of 15382 cities nationally (10th percentile). The grade of D reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.