Is San Marcos, CA Tap Water Safe to Drink?
Graded D — but Manganese was detected above EPA limits. Here's what's in the water and how to remove it. What to do next ↓
49/100
waterverge.com San Marcos, CA — Water Quality Report
San Marcos's drinking water received a grade of D (49 out of 100), indicating poor water quality. The city's 2 water systems serve approximately 113,596 residents using purchased surface water.
Lead levels were measured at 0.0 ppb (90th percentile), well within EPA limits. UCMR 5 testing detected 1 PFAS compound in the water supply.
The system has 96 violations on record, including 27 health-based violations. 4 remain unresolved.
What to know about San Marcos's water
San Marcos ranks #613 out of 694 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 0.09 µg/L in UCMR 3 testing. While below California's 10 µg/L limit and with no federal MCL set, residents sensitive to this contaminant may consider reverse osmosis filtration.
The system has seen 67 violations in the past five years, suggesting a pattern of compliance challenges that residents should monitor closely.
Water quality score
See methodology →Is San Marcos, CA water safe to drink?
San Marcos's drinking water has significant quality concerns based on EPA testing data. With a grade of D (49/100), the system has issues across multiple categories. A water filter is recommended for all residents. The city's 2 water systems serve approximately 113,596 residents using surface water (rivers, lakes, or reservoirs).
Recent water quality updates for San Marcos
A timeline of significant water quality events, violations, and data updates.
Detected at levels within current EPA limits. PFAS persist indefinitely in the environment.
Latest EPA compliance and testing data incorporated into San Marcos's water quality assessment. Grade: D (49/100).
Contaminants: LEAD AND COPPER RULE REVISIONS.
Contaminants: Nitrate.
Contaminants: Fluoride, Nitrate.
Federal disaster declaration (FEMA DR-4758). Flood event — may have impacted local water infrastructure.
Key contaminant findings
Based on the most recent EPA sampling data for San Marcos's water supply.
Well within EPA limits.
Detected but within current EPA limits. PFAS do not break down in the environment and can accumulate in the body over time. An activated carbon filter can reduce exposure.
Violation history
San Marcos's water system has 96 total violations on record, including 27 health-based violations. 4 remain unresolved. 67 violations were issued in the last 5 years.
Flood & environmental risk
San Diego County has experienced 10 federally declared disasters since 1980. 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 Marcos's water come from?
San Marcos's drinking water comes from surface water (rivers, lakes, or reservoirs), supplied by 2 water systems serving approximately 113,596 people. Surface water sources are more susceptible to contamination from runoff, industrial discharge, and algal blooms, requiring extensive treatment.
What San Marcos residents can do
Recommended: Activated carbon or reverse osmosis filter. This addresses the specific contaminants found in San Marcos'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 Marcos'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 San Marcos
Industrial polluters nearby
Reported releases to surface water by facilities near San Marcos, ranked by pounds discharged annually.
| Facility | Top chemical | To surface water (lbs/yr) | Distance |
|---|---|---|---|
QUIDELORTHO CARLSBAD, CA92008 | — | — | 6.0 mi |
VERSUM MATERIALS US LLC CARLSBAD, CA92011 | — | — | 6.6 mi |
MERCOTAC INC CARLSBAD, CA92011 | — | — | 6.0 mi |
IONIS PHARMACEUTICALS INC. CARLSBAD, CA92008 | — | — | 6.2 mi |
CEMEX ESCONDIDO PLANT DBA PALOMAR TRANSIT MIX ESCONDIDO, CA92025 | — | — | 4.3 mi |
Source: EPA Toxic Release Inventory (TRI) 2023
Flood & disaster history
San Diego County has experienced 10 federally declared disasters since 1980. 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 Marcos'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 |
| 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. | 52.000 | 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 |
Lead level trend (90th percentile)
See how San Marcos compares by contaminant
Explore where San Marcos ranks among all California cities for specific contaminants.
Water source & infrastructure
Where San Marcos's water comes from
San Marcos's drinking water comes primarily from surface water sources such as rivers, lakes, or reservoirs.
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 113,596 people through 2 water systems.
Water systems serving San Marcos
| System Name | PWSID | Population | Source |
|---|---|---|---|
| VALLECITOS WD | CA3710002 | 113,500 | SWP |
| OCOTILLO OASIS, LLC | CA3700291 | 96 | GW |
How San Marcos compares
Full California rankings →San Marcos's score of 49/100 is below the average of 57/100 among major California cities. It outscores 5 of 10 nearby cities.
Nearby cities
View California rankings →About San Marcos, CA
Wikipedia →San Marcos is a city in the North County region of San Diego County, California, United States. As of the 2020 census, the city's population was 94,833. It is the site of California State University San Marcos and Palomar College. The city is bordered by Escondido to the east, Encinitas to the southwest, Carlsbad to the west, and Vista to the northwest. Lake San Marcos is an enclave, or county island, within the southwestern part of the city and its sphere of influence but is an unincorporated community.
Learn more about your water
Guides and resources related to San Marcos's water quality findings.
Explore water quality across San Diego
Frequently asked questions
Is San Marcos, CA tap water safe to drink?
San Marcos's water quality earned a grade of D (49/100). Significant issues have been found. A water filter is strongly recommended. The city ranks #613 out of 694 cities tested in California.
What contaminants are in San Marcos's water?
Lead was measured at 0.0 ppb (90th percentile). 1 PFAS compound was detected. 96 violations are on record.
How is San Marcos'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 Marcos?
PFAS compounds have been detected. A filter with activated carbon can help reduce exposure.
Where does San Marcos's water come from?
San Marcos's water is sourced from Purchased surface water. The city has 2 water systems serving approximately 113,596 residents.
What health violations has San Marcos's water system had?
San Marcos has 27 health-based violations on record. The most recent violation was recorded in July 2025. Health-based violations mean the water exceeded EPA maximum contaminant levels (MCLs) for a regulated substance. 4 violations remain unresolved.
How does San Marcos's water compare to other cities?
San Marcos ranks #613 out of 694 cities in California (better than 12% of state cities) and #13284 out of 15744 cities nationally (16th percentile). The grade of D reflects the combined assessment of violation history, lead and copper levels, PFAS contamination, and regulatory compliance.