01The verdict: is San Antonio water safe?
San Antonio's drinking water has significant quality concerns based on EPA testing data. With a grade of D+ (50.9/100), the system has issues across multiple categories. A water filter is recommended for all residents. The city's 3 water systems serve approximately 554 residents using groundwater (wells).
Lead is over its legal limit
Measured at 54.0 ppb against the EPA Action Level of 15 ppb. The ledger below shows how far over — and which filter removes it.
See the measurement ↓Top half in Northern Mariana Islands
San Antonio ranks #4 of 7 graded Northern Mariana Islands cities.
See the comparison ↓14 violations in the last five years
Recent compliance history is the best predictor of future problems — the year-by-year record is below.
See the record ↓San Antonio's drinking water received a grade of D+ (50.9 out of 100), indicating poor water quality. The city's 3 water systems serve approximately 554 residents using groundwater.
Lead levels were measured at 54.0 ppb (90th percentile), which exceeds the EPA action level of 15 ppb. This system has not yet been tested for PFAS under the EPA UCMR 5 program. The system has 62 violations on record, including 20 health-based violations. 22 remain unresolved.
San Antonio ranks #4 out of 7 cities in Northern Mariana Islands for water quality, placing it mid-range in the state. San Antonio 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. Lead levels exceed the EPA action level of 15 ppb, which typically indicates aging lead service lines or lead solder in the distribution system. An NSF 53-certified filter is strongly recommended for drinking and cooking water. As a small community water system, San Antonio may have fewer resources for advanced treatment technologies and infrastructure upgrades compared to larger utilities. The system has seen 14 violations in the past five years, suggesting a pattern of compliance challenges that residents should monitor closely.
02What's in San Antonio's water
Every contaminant flagged in San Antonio, 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.
Over Limit — EPA Action Level: 15 ppb
Exceeds the EPA action level. Lead enters at the service line, not the treatment plant — your address matters more than the city average. Lead service line replacement and an NSF 53-certified point-of-use filter are recommended.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. | 54.0 | 15 | ppb | Inorganic | Over Limit |
03San Antonio'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 62 violations on record, including 20 health-based violations. 22 remain unresolved.
7 of the last ten years were 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 2013. Latest reading: 0.50 ppb (2025) — 3% of the action level. Sampled at higher-risk homes (90th percentile).
04Where San Antonio's water comes from
San Antonio'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.
3 systems
3 water systems (1 wholesaler), operated under private ownership.
554 people
Residents served across San Antonio'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 private ownership and serves approximately 554 people through 3 water systems.
| System name | PWSID | Population | Source |
|---|---|---|---|
| Pacific Islands Club | MP0000012 | 554 | Groundwater |
| Aqua Water | MP0000177 | 170 | Groundwater |
| Winners Residence | MP0000005 | 124 | Groundwater |
05How San Antonio compares
Scores for nearby Northern Mariana Islands cities on one line, with the average marked.
dashed line = Northern Mariana Islands average (60/100)
San Antonio scores 50.9/100 — below the Northern Mariana Islands average of 60/100. It outscores 3 of 6 nearby cities.
40.2 / 100BLower Base, MP
73.4 / 100DSan Jose, MP
50 / 100BSinapalu, MP
78.2 / 100DChalan Lau Lau, MP
48.3 / 100CAs Matuis, MP
68.4 / 100
06What to do at home
Steps matched to what testing actually found in San Antonio's water — not generic advice.
Filter drinking & cooking water
NSF 53-certified pitcher or under-sink filter. Lead level (54.0 ppb) exceeds the EPA action level of 15 ppb
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 Antonio alerts
We refresh EPA data monthly and surface new violations, advisories, and boil-water notices for your county on this page.
All water alerts →Frequently asked questions
Is San Antonio, MP tap water safe to drink?
San Antonio's water quality earned a grade of D+ (50.9/100). Significant issues have been found. A water filter is strongly recommended. The city ranks #4 out of 7 cities tested in Northern Mariana Islands.
What contaminants are in San Antonio's water?
Lead was measured at 54.0 ppb (90th percentile). 62 violations are on record.
How is San Antonio'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 Antonio?
Yes — lead levels exceed the EPA action level of 15 ppb. We recommend an NSF 53-certified filter or reverse osmosis system. Based on current data, basic filtration should suffice for additional peace of mind.
Where does San Antonio's water come from?
San Antonio's water is sourced from Groundwater. The city has 3 water systems serving approximately 554 residents.
What health violations has San Antonio's water system had?
San Antonio has 20 health-based violations on record. The most recent violation was recorded in February 2023. Health-based violations mean the water exceeded EPA maximum contaminant levels (MCLs) for a regulated substance. 22 violations remain unresolved.
Is San Antonio's groundwater at risk of contamination?
San Antonio 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 62 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 Antonio's water compare to other cities?
San Antonio ranks #4 out of 7 cities in Northern Mariana Islands (better than 43% of state cities) and #12727 out of 15382 cities nationally (17th percentile). The grade of D+ reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.