01The verdict: is San Luis Obispo water safe?
San Luis Obispo's drinking water has significant quality concerns based on EPA testing data. With a grade of F (39.4/100), the system has issues across multiple categories. A water filter is recommended for all residents. The city's 21 water systems serve approximately 69,712 residents using surface water (rivers, lakes, or reservoirs).
Copper is over its legal limit
Measured at 1.80 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 2.8 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 10.0000 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 Luis Obispo's drinking water received a grade of F (39.4 out of 100), indicating failing water quality. The city's 21 water systems serve approximately 69,712 residents using surface water.
Lead levels were measured at 2.8 ppb (90th percentile), well within EPA limits. UCMR 5 testing detected 1 PFAS compound in the water supply. The system has 110 violations on record, including 27 health-based violations. 32 remain unresolved.
San Luis Obispo ranks #677 out of 690 cities in California for water quality, placing it among the lowest-rated in the state. The city draws from surface water sources, which are more susceptible to seasonal runoff and agricultural contamination, requiring extensive multi-barrier treatment including coagulation, filtration, and disinfection. 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. Hexavalent chromium (chromium-6) was detected at 13.00 µg/L, above California's 10 µg/L limit. There is no federal MCL, but the EPA is reviewing evidence linking long-term exposure to cancer risk. The system has seen 35 violations in the past five years, suggesting a pattern of compliance challenges that residents should monitor closely.
02What's in San Luis Obispo's water
Every contaminant flagged in San Luis Obispo, 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. | 2.8 | 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. | 1.80 | 1.3 | mg/L | Inorganic | Over Limit |
| 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. | 10.000 | 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. | ND | HI | µg/L | PFAS | Not 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 California
Cities ranked by compounds detected Cu Copper in California
Cities ranked by 90th-percentile copper
03San Luis Obispo'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 110 violations on record, including 27 health-based violations. 32 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 5 ppb since 1992. Latest reading: 0 ppb (2025) — 0% of the action level. Sampled at higher-risk homes (90th percentile).
1.8mg/L(1993)
One sampling round on record — 138% of the 1.3mg/L EPA action level. Sampled at higher-risk homes (90th percentile).
04Where San Luis Obispo's water comes from
San Luis Obispo's drinking water comes primarily from surface water sources such as rivers, lakes, or reservoirs.
Surface Water
Rivers, lakes, or reservoirs — more exposed to runoff, agriculture, and seasonal swings than groundwater.
Local treatment
Multi-barrier treatment: coagulation, sedimentation, filtration, and disinfection to meet EPA standards.
21 systems
21 water systems (4 wholesalers), operated under local government ownership.
69,712 people
Residents served across San Luis Obispo's service area.
Surface water systems require multi-stage treatment including coagulation, sedimentation, filtration, and disinfection to meet EPA Safe Drinking Water Act standards. These sources can be impacted by seasonal changes, stormwater runoff, upstream agriculture, and industrial discharge. The system is operated by local government ownership and serves approximately 69,712 people through 21 water systems.
| System name | PWSID | Population | Source |
|---|---|---|---|
| SAN LUIS OBISPO WATER DEPARTMENT | CA4010009 | 47,788 | Surface Water |
| CALIFORNIA MENS COLONY | CA4010830 | 15,000 | Surface Water |
| LOPEZ PROJECT | CA4010022 | 1,569 | Surface Water |
| SLO CWWD NO. 23 - SANTA MARGARITA | CA4010024 | 1,291 | Groundwater |
| SLOCSA #10A - CAYUCOS | CA4010901 | 1,200 | Purchased Surface Water |
| SLO CSA NO. 16 - SHANDON | CA4010028 | 1,085 | Purchased Surface Water |
| LAUREATE WATER COMPANY | CA4000206 | 700 | Groundwater |
| SANTA YSABEL RANCH MWC | CA4000802 | 275 | Groundwater |
| IRISH HILLS MUTUAL WATER CO | CA4000203 | 135 | Groundwater |
| CALLENDER GROVE MUTUAL WATER | CA4000805 | 126 | Groundwater |
| LAS VENTANAS RANCH | CA4000815 | 108 | Groundwater |
| BARON CANYON MUTUAL WATER CO. | CA4000214 | 81 | Groundwater |
| AFUERA DE CHORRO WATER COMPANY | CA4000744 | 79 | Groundwater |
| AVILA VALLEY MUTUAL WATER CO | CA4000716 | 75 | Purchased Surface Water |
| MONTE SERENO MUTUAL BENEFIT WATER | CA4000808 | 65 | Groundwater |
| ANZA VINEYARD ESTATES MUTUAL WATER CO | CA4000834 | 60 | Groundwater |
| BEAR VALLEY WATER COMPANY | CA4000660 | 45 | Groundwater |
| HIGUERA APARTMENTS | CA4000563 | 30 | Groundwater |
| SLO CWD NO. 10 - CAYUCOS WTP | CA4010025 | — | Surface Water |
| CHORRO VALLEY PIPELINE - SLO COUNTY | CA4010030 | — | Purchased Surface Water |
| NACIMIENTO WATER PROJECT | CA4010080 | — | Surface Water |
Water bodies near San Luis Obispo
As a surface-water system, these water bodies may directly influence San Luis Obispo's drinking water supply.
05Pressure on San Luis Obispo'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.
Industrial discharge
5 facilities within 10 miles reported 0 lb released to surface water in the latest EPA Toxics Release Inventory. Proximity doesn't equal tap contamination — treatment and hydrology decide — but it's pressure on the watershed.
Flood & storm history
10 federal disaster declarations on record for San Luis Obispo County. Flooding and severe storms can overwhelm treatment plants, cause sewage overflows, and push runoff into supplies.
06How San Luis Obispo compares
Scores for nearby California cities on one line, with the average marked.
dashed line = California average (57/100)
San Luis Obispo scores 39.4/100 — below the California average of 57/100. It outscores 3 of 10 nearby cities. Full California rankings →
42.8 / 100FSan Jose, CA
28 / 100BOakland, CA
77.9 / 100FSan Diego, CA
39.7 / 100FSacramento, CA
31.2 / 100ABurlingame, CA
89.2 / 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 Obispo's water — not generic advice.
Filter drinking & cooking water
Reverse osmosis system. 1 PFAS compound detected
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 Obispo 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 Obispo, CA tap water safe to drink?
San Luis Obispo's water quality earned a grade of F (39.4/100). Significant issues have been found. A water filter is strongly recommended. The city ranks #677 out of 690 cities tested in California.
What contaminants are in San Luis Obispo's water?
Lead was measured at 2.8 ppb (90th percentile). 1 PFAS compound was detected. 110 violations are on record.
How is San Luis Obispo'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 Obispo?
PFAS compounds have been detected. A filter with activated carbon can help reduce exposure.
Where does San Luis Obispo's water come from?
San Luis Obispo's water is sourced from Surface water. The city has 21 water systems serving approximately 69,712 residents.
What health violations has San Luis Obispo's water system had?
San Luis Obispo has 27 health-based violations on record. The most recent violation was recorded in October 2025. Health-based violations mean the water exceeded EPA maximum contaminant levels (MCLs) for a regulated substance. 32 violations remain unresolved.
How does San Luis Obispo's water compare to other cities?
San Luis Obispo ranks #677 out of 690 cities in California (better than 2% of state cities) and #14911 out of 15382 cities nationally (3th percentile). The grade of F reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.