Is Los Osos, CA Tap Water Safe to Drink?
Graded B+ — but Copper, Chromium-6 and 1 more were detected above EPA limits. Here's what's in the water and how to remove it. What to do next ↓
84.8/100
waterverge.com Los Osos, CA — Water Quality Report
Los Osos's drinking water received a grade of B+ (84.8 out of 100), indicating good water quality. The city's 2 water systems serve approximately 7,677 residents using groundwater.
Lead levels were measured at 1.1 ppb (90th percentile), well within EPA limits. UCMR 5 testing detected 9 PFAS compounds, with levels exceeding EPA maximum contaminant levels in the water supply.
The system has 5 violations on record, including 0 health-based violations. 1 remains unresolved.
What to know about Los Osos's water
Los Osos ranks #275 out of 694 cities in California for water quality, placing it mid-range in the state.
Los Osos 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.
Of particular concern: PFAS "forever chemical" levels exceed the 2024 EPA maximum contaminant levels. These synthetic compounds don't break down naturally and require specialized filtration such as reverse osmosis or granular activated carbon.
Hexavalent chromium (chromium-6) was detected at 10.55 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.
Water quality score
See methodology →Is Los Osos, CA water safe to drink?
Los Osos's tap water meets most EPA standards but has areas that warrant attention. With a grade of B+ (84.8/100), some contaminant levels or compliance issues suggest that residents may benefit from additional filtration. The city's 2 water systems serve approximately 7,677 residents using groundwater (wells).
Recent water quality updates for Los Osos
A timeline of significant water quality events, violations, and data updates.
PFAS levels exceed EPA maximum contaminant levels. Reverse osmosis or activated carbon filtration recommended.
Latest EPA compliance and testing data incorporated into Los Osos's water quality assessment. Grade: B+ (84.8/100).
Federal disaster declaration (FEMA DR-3592). Flood event — may have impacted local water infrastructure.
Federal disaster declaration (FEMA DR-4683). Flood event — may have impacted local water infrastructure.
Contaminants: Lead and Copper Rule.
Contaminants: Lead and Copper Rule.
Key contaminant findings
Based on the most recent EPA sampling data for Los Osos's water supply.
Well within EPA limits.
Exceeds EPA action level. Copper can leach from household plumbing — flush taps for 30 seconds before drinking.
PFAS "forever chemicals" exceed EPA maximum contaminant levels. Reverse osmosis or granular activated carbon filtration strongly recommended.
The "Erin Brockovich" chemical. There is no federal MCL, but California has set a limit of 10 µg/L. Reverse osmosis filtration is effective at removing hexavalent chromium.
PFAS "forever chemicals" detected
UCMR 5 testing found 9 PFAS compounds in Los Osos's water supply. PFAS are synthetic chemicals that persist indefinitely in the environment and the human body.
Violation history
Los Osos's water system has 5 total violations on record, including 0 health-based violations. 1 remain unresolved.
Flood & environmental risk
San Luis Obispo County has experienced 10 federally declared disasters since 1973. 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 Los Osos's water come from?
Los Osos's drinking water comes from groundwater (wells), supplied by 2 water systems serving approximately 7,677 people. Groundwater is generally less susceptible to surface contamination but can contain naturally occurring contaminants like arsenic, radon, and nitrate.
What Los Osos residents can do
Recommended: Reverse osmosis system. This addresses the specific contaminants found in Los Osos'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.
Los Osos'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 ↓Forever chemicals overview
National PFAS report →Violation summary
Violations & advisories
Top industrial polluters within 10 miles of Los Osos
Industrial polluters nearby
Reported releases to surface water by facilities near Los Osos, ranked by pounds discharged annually.
| Facility | Top chemical | To surface water (lbs/yr) | Distance |
|---|---|---|---|
PROMEGA CORP SAN LUIS OBISPO, CA93401 | — | — | 9.7 mi |
RANTEC POWER SYSTEMS INC LOS OSOS, CA93402 | — | — | 0.5 mi |
U.S. ARMY NATIONAL GUARD CAMP SAN LUIS OBISPO RANGES SAN LUIS OBISPO, CA93405 | — | — | 5.6 mi |
CALPORTLAND CO SAN LUIS OBISPO SAN LUIS OBISPO, CA93401 | — | — | 9.8 mi |
Source: EPA Toxic Release Inventory (TRI) 2023
Flood & disaster history
San Luis Obispo County has experienced 10 federally declared disasters since 1973. Flooding and severe weather can compromise water treatment infrastructure and introduce contaminants into drinking water supplies.
Recommended water filters
Based on contaminants detected in Los Osos'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. | 1.1 | 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. | 1.47 | 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. | 16.100 | 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. | 0.006 | HI | µg/L | PFAS | 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. | 0.018 | 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. | 0.005 | HI | µg/L | PFAS | 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. | 0.017 | HI | µg/L | PFAS | 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. | 0.008 | HI | µg/L | PFAS | 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. | 0.015 | 0.004 | µg/L | PFAS | Over MCL |
| 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. | 0.012 | 0.004 | µg/L | PFAS | Over MCL |
| 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. | 0.017 | HI | µg/L | PFAS | 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 |
Lead level trend (90th percentile)
Copper level trend (90th percentile)
See how Los Osos compares by contaminant
Explore where Los Osos ranks among all California cities for specific contaminants.
Water source & infrastructure
Where Los Osos's water comes from
Los Osos'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 7,677 people through 2 water systems.
Water systems serving Los Osos
| System Name | PWSID | Population | Source |
|---|---|---|---|
| LOS OSOS COMMUNITY SERVICES DISTRICT | CA4010016 | 7,086 | GW |
| S & T MUTUAL WATER COMPANY | CA4000523 | 591 | GW |
How Los Osos compares
Full California rankings →Los Osos's score of 84.8/100 is above the average of 57/100 among major California cities. It outscores 8 of 10 nearby cities.
Nearby cities
View California rankings →About Los Osos, CA
Wikipedia →Los Osos is an unincorporated town in San Luis Obispo County, California, United States. Located on the Central Coast of California, Los Osos had a reported population of 14,465 in 2020. For statistical purposes, the United States Census Bureau has defined Los Osos as a census-designated place (CDP).
Learn more about your water
Guides and resources related to Los Osos's water quality findings.
Explore water quality across San Luis Obispo
Frequently asked questions
Is Los Osos, CA tap water safe to drink?
Los Osos's water quality earned a grade of B+ (84.8/100). The water generally meets EPA standards and is considered safe for consumption. The city ranks #275 out of 694 cities tested in California.
What contaminants are in Los Osos's water?
Lead was measured at 1.1 ppb (90th percentile). 9 PFAS compounds were detected. 5 violations are on record.
How is Los Osos'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 Los Osos?
PFAS compounds exceed EPA limits — a reverse osmosis or activated carbon filter is recommended.
Where does Los Osos's water come from?
Los Osos's water is sourced from Groundwater. The city has 2 water systems serving approximately 7,677 residents.
Is Los Osos's groundwater at risk of contamination?
Los Osos 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 5 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.
Why does Los Osos have so many PFAS compounds in its water?
9 different PFAS "forever chemical" compounds were detected in Los Osos's water supply during UCMR 5 testing. PFAS contamination often originates from proximity to military installations (AFFF firefighting foam), airports, industrial manufacturing sites, or wastewater treatment facilities. Some levels exceed the 2024 EPA maximum contaminant levels — a reverse osmosis or NSF-certified activated carbon filter is strongly recommended.
How does Los Osos's water compare to other cities?
Los Osos ranks #275 out of 694 cities in California (better than 60% of state cities) and #5652 out of 15744 cities nationally (64th percentile). The grade of B+ reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.