Is San Luis, CO Tap Water Safe to Drink?
Graded D — but Copper was detected above EPA limits. Here's what's in the water and how to remove it. What to do next ↓
46/100
waterverge.com San Luis, CO — Water Quality Report
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.
What to know about San Luis's water
San Luis ranks #195 out of 246 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.
Water quality score
See methodology →Is San Luis, CO water safe to drink?
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).
Recent water quality updates for San Luis
A timeline of significant water quality events, violations, and data updates.
Latest EPA compliance and testing data incorporated into San Luis's water quality assessment. Grade: D (46/100).
Contaminants: Public Notice.
Contaminants: Consumer Confidence Rule.
Contaminants: Public Notice.
Federal disaster declaration (FEMA DR-3224). Coastal Storm event — may have impacted local water infrastructure.
Federal disaster declaration (FEMA DR-396). Flood event — may have impacted local water infrastructure.
Key contaminant findings
Based on the most recent EPA sampling data for San Luis's water supply.
Well within EPA limits.
Exceeds EPA action level. Copper can leach from household plumbing — flush taps for 30 seconds before drinking.
Violation history
San Luis's water system has 242 total violations on record, including 9 health-based violations. 35 remain unresolved. 26 violations were issued in the last 5 years.
Flood & environmental risk
Costilla County has experienced 3 federally declared disasters since 1970. Flooding and severe storms can overwhelm water treatment plants, cause sewage overflows, and introduce agricultural runoff, bacteria, and sediment into drinking water supplies.
Where does San Luis's water come from?
San Luis's drinking water comes from groundwater (wells), supplied by 4 water systems serving approximately 1,275 people. Groundwater is generally less susceptible to surface contamination but can contain naturally occurring contaminants like arsenic, radon, and nitrate.
What San Luis residents can do
Recommended: NSF-certified water filter. This addresses the specific contaminants found in San Luis's water.
Your water utility is required to publish an annual Consumer Confidence Report (CCR) with detailed testing results. Ask for the latest copy or check your utility's website.
San Luis's area has a history of flooding. After severe weather, watch for boil water advisories from your local utility.
Data: EPA SDWIS, UCMR 5 (PFAS), FEMA, NOAA. Last updated May 2026.
Top contaminants to know
View all ↓Violation summary
Violations & advisories
Drought conditions
D4 — exceptional droughtCostilla County is currently in D4 (exceptional drought) per the U.S. Drought Monitor (week of May 5, 2026). 12.8% of the county is in D4 (exceptional) drought. Drought can elevate disinfection-byproduct (TTHM/HAA5) levels and taste/odor issues as utilities draw from lower reservoirs.
Source: U.S. Drought Monitor, updated weekly by NDMC, USDA, and NOAA.
Flood & disaster history
Costilla County has experienced 3 federally declared disasters since 1970. Flooding and severe weather can compromise water treatment infrastructure and introduce contaminants into drinking water supplies.
Recommended water filters
Based on contaminants detected in San Luis's water supply, we recommend the following filter types.
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 |
Lead level trend (90th percentile)
Copper level (90th percentile)
Latest reading: 1.622 mg/L (2007)
EPA action level: 1.3 mg/L
See how San Luis compares by contaminant
Explore where San Luis ranks among all Colorado cities for specific contaminants.
Water source & infrastructure
Where San Luis's water comes from
San Luis's drinking water is drawn from underground aquifers through wells.
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.
Water systems serving San Luis
| System Name | PWSID | Population | Source |
|---|---|---|---|
| SAN LUIS WSD | CO0112900 | 629 | GW |
| COSTILLA COUNTY WS | CO0112300 | 561 | GW |
| VIEJO SAN ACACIO | CO0112810 | 45 | GW |
| SAN ACACIO DOMESTIC WA | CO0112850 | 40 | GW |
How San Luis compares
Full Colorado rankings →San Luis's score of 46/100 is on par with the average of 45/100 among major Colorado cities. It outscores 7 of 10 nearby cities.
Nearby cities
View Colorado rankings →About San Luis, CO
Learn more about your water
Guides and resources related to San Luis's water quality findings.
Explore water quality across Costilla
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 #195 out of 246 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 #195 out of 246 cities in Colorado (better than 21% of state cities) and #14048 out of 15744 cities nationally (11th percentile). The grade of D reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.