01The verdict: is San Luis water safe?
San Luis's drinking water has significant quality concerns based on EPA testing data. With a grade of D (46/100), the system has issues across multiple categories. A water filter is recommended for all residents. The city's 4 water systems serve approximately 1,275 residents using groundwater (wells).
Copper is over its legal limit
Measured at 1.62 mg/L against the EPA Action Level of 1.3 mg/L. The ledger below shows how far over — and which filter removes it.
See the measurement ↓Lead is well under the limit
90th-percentile lead is 0.0 ppb — beneath the 15 ppb federal action level and under the 5 ppb watch threshold.
See the trend ↓26 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 Luis's drinking water received a grade of D (46 out of 100), indicating poor water quality. The city's 4 water systems serve approximately 1,275 residents using groundwater.
Lead levels were measured at 0.0 ppb (90th percentile), well within EPA limits. This system has not yet been tested for PFAS under the EPA UCMR 5 program. The system has 242 violations on record, including 9 health-based violations. 35 remain unresolved.
San Luis ranks #193 out of 244 cities in Colorado for water quality, placing it among the lowest-rated in the state. San Luis 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. As a small community water system, San Luis may have fewer resources for advanced treatment technologies and infrastructure upgrades compared to larger utilities. The system has seen 26 violations in the past five years, suggesting a pattern of compliance challenges that residents should monitor closely.
02What's in San Luis's water
Every contaminant flagged in San Luis, 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: 1.3 mg/L
Exceeds the EPA action level. Copper leaches from household plumbing — flush taps for 30 seconds before drinking, and consider a certified filter.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. | 0.0 | 15 | ppb | Inorganic | Safe |
| Copper (90th percentile) CopperInorganic A metal that enters drinking water mainly through corrosion of copper plumbing. Small amounts are essential for health, but excess levels are harmful. Health EffectsGastrointestinal distress (nausea, vomiting, diarrhea) at short-term high levels; liver and kidney damage from long-term exposure. EPA Limit1.3 mg/L action level Common SourcesCorrosion of copper household plumbing, erosion of natural deposits. | 1.62 | 1.3 | mg/L | Inorganic | Over Limit |
Cities ranked by 90th-percentile lead PF PFAS in Colorado
Cities ranked by compounds detected Cu Copper in Colorado
Cities ranked by 90th-percentile copper
03San Luis'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 242 violations on record, including 9 health-based violations. 35 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 4 ppb since 1993. Latest reading: 0 ppb (2025) — 0% of the action level. Sampled at higher-risk homes (90th percentile).
1.6mg/L(2007)
One sampling round on record — 125% of the 1.3mg/L EPA action level. Sampled at higher-risk homes (90th percentile).
04Where San Luis's water comes from
San Luis'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.
4 systems
4 water systems, operated under local government ownership.
1,275 people
Residents served across San Luis'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 local government ownership and serves approximately 1,275 people through 4 water systems.
| System name | PWSID | Population | Source |
|---|---|---|---|
| SAN LUIS WSD | CO0112900 | 629 | Groundwater |
| COSTILLA COUNTY WS | CO0112300 | 561 | Groundwater |
| VIEJO SAN ACACIO | CO0112810 | 45 | Groundwater |
| SAN ACACIO DOMESTIC WA | CO0112850 | 40 | Groundwater |
05Pressure on San Luis'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
3 federal disaster declarations on record for Costilla County. Flooding and severe storms can overwhelm treatment plants, cause sewage overflows, and push runoff into supplies.
Drought
Costilla County is in D4 (exceptional drought) per the U.S. Drought Monitor (week of Jul 2026). Drought can raise disinfection-byproduct levels and taste/odor issues as utilities draw from lower reservoirs.
06How San Luis compares
Scores for nearby Colorado cities on one line, with the average marked.
dashed line = Colorado average (45/100)
San Luis scores 46/100 — on par with the Colorado average of 45/100. It outscores 7 of 10 nearby cities. Full Colorado rankings →
39.6 / 100FAurora, CO
38.1 / 100FColorado Springs, CO
33.5 / 100DFort Collins, CO
46.8 / 100FLakewood, CO
36.9 / 100FThornton, CO
38 / 100
Compare all cities in the county — grades, violations, and contaminant data side by side.→
07What to do at home
Steps matched to what testing actually found in San Luis's water — not generic advice.
Filter drinking & cooking water
NSF-certified water filter. Copper exceeds the EPA action level of 1.3 mg/L
Compare tested filters →Read your utility's annual report
Every utility must publish a yearly Consumer Confidence Report with its own testing results. It pairs well with the independent data on this page.
How to read a CCR →Watch for San Luis 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 Luis, CO tap water safe to drink?
San Luis's water quality earned a grade of D (46/100). Significant issues have been found. A water filter is strongly recommended. The city ranks #193 out of 244 cities tested in Colorado.
What contaminants are in San Luis's water?
Lead was measured at 0.0 ppb (90th percentile). 242 violations are on record.
How is San Luis'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 Luis?
Based on current data, basic filtration should suffice for additional peace of mind.
Where does San Luis's water come from?
San Luis's water is sourced from Groundwater. The city has 4 water systems serving approximately 1,275 residents.
What health violations has San Luis's water system had?
San Luis has 9 health-based violations on record. The most recent violation was recorded in September 2025. Health-based violations mean the water exceeded EPA maximum contaminant levels (MCLs) for a regulated substance. 35 violations remain unresolved.
Is San Luis's groundwater at risk of contamination?
San Luis 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 242 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 Luis's water compare to other cities?
San Luis ranks #193 out of 244 cities in Colorado (better than 21% of state cities) and #13750 out of 15382 cities nationally (11th percentile). The grade of D reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.