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Updated 2026-10-15
County water report

San Luis Obispo County Water Quality: 7 Cities Ranked by the Records

San Luis Obispo County, California: 7 city reports built from EPA and utility records. A county has many suppliers; each city shows the system that serves it.

Find my systemOpen on the mapPrivate wells in San Luis Obispo County
Across the county's systems

Highest result for each contaminant

One row per contaminant: the highest result any system in the county reported, against its limit. Open a row for the systems behind it.

LithiumDetected in 5 systems of 7 tested10 µg/L EPA health reference level (screening, not a legal limit) · US EPAHighest60 µg/L
Perfluorooctanesulfonic acid (PFOS)Detected in 3 systems of 7 tested4.0 ng/L federal MCL (2024) · US EPAHighest22.3 ng/L
Perfluorooctanoic acid (PFOA)Detected in 4 systems of 7 tested4.0 ng/L federal MCL (2024) · US EPAHighest14.7 ng/L
Copper (90th percentile)Detected in 1 system1.3 mg/L federal action level · US EPAHighest1.5 mg/L
Perfluorohexanesulfonic acid (PFHxS)Detected in 4 systems of 7 tested10 ng/L federal MCL (2024) · US EPAHighest8.4 ng/L
Lead (90th percentile)Detected in 1 system of 7 tested15 µg/L federal action level · US EPAHighest1.1 µg/L
Perfluorobutanesulfonic acid (PFBS)Detected in 4 systems of 7 tested2,000 ng/L EPA health advisory (screening, not a legal limit) · US EPAHighest18 ng/L
Perfluorohexanoic acid (PFHxA)Detected in 3 systems of 7 tested400,000 ng/L Michigan EGLE state MCL (strictest US state) · Michigan EGLEHighest20 ng/L
3 more contaminants detectedShow
Perfluorobutanoic acid (PFBA)Detected in 3 systems of 7 testedNo federal or state limit; shown for transparencyHighest10 ng/L
Perfluoroheptanoic acid (PFHpA)Detected in 2 systems of 7 testedNo federal or state limit; shown for transparencyHighest6.3 ng/L
Perfluoropentanoic acid (PFPeA)Detected in 3 systems of 7 testedNo federal or state limit; shown for transparencyHighest22 ng/L
21 PFAS tested, none detected in any systemShow

Tested in these 7 systems: ATASCADERO MUTUAL WATER CO, CITY OF ARROYO GRANDE UTILITIES DIVISION, CITY OF GROVER BEACH PUBLIC WORKS DEPT, CITY OF PASO ROBLES WATER DIVISION, LOS OSOS COMMUNITY SERVICES DISTRICT, MORRO BAY PW DEPT - WATER DIVISION, SAN LUIS OBISPO WATER DEPARTMENT

  • 11-Chloroeicosafluoro-3-oxaundecane-1-sulfonic acid (11Cl-PF3OUdS)
  • 1H, 1H, 2H, 2H-perfluorodecane sulfonic acid (8:2 FTS)
  • 1H, 1H, 2H, 2H-perfluorohexane sulfonic acid (4:2 FTS)
  • 1H, 1H, 2H, 2H-perfluorooctane sulfonic acid (6:2 FTS)
  • 4,8-Dioxa-3H-perfluorononanoic acid (ADONA)
  • 9-Chlorohexadecafluoro-3-oxanone-1-sulfonic acid (9Cl-PF3ONS)
  • Hexafluoropropylene oxide dimer acid (HFPO-DA) (GenX chemicals)
  • N-ethyl perfluorooctanesulfonamidoacetic acid (NEtFOSAA)
  • N-methyl perfluorooctanesulfonamidoacetic acid (NMeFOSAA)
  • Nonafluoro-3,6-dioxaheptanoic acid (NFDHA)
  • Perfluoro (2-ethoxyethane) sulfonic acid (PFEESA)
  • Perfluoro-3-methoxypropanoic acid (PFMPA)
  • Perfluoro-4-methoxybutanoic acid (PFMBA)
  • Perfluorodecanoic acid (PFDA)
  • Perfluorododecanoic acid (PFDoA)
  • Perfluoroheptanesulfonic acid (PFHpS)
  • Perfluorononanoic acid (PFNA)
  • Perfluoropentanesulfonic acid (PFPeS)
  • Perfluorotetradecanoic acid (PFTA)
  • Perfluorotridecanoic acid (PFTrDA)
  • Perfluoroundecanoic acid (PFUnA)
7 cities

Cities ranked by the records

Showing 6 of 7 cities · scores are system-level, not tap tests

Ranked by the Purity Score of each city's main water system, highest first. Scores belong to water systems; there is no county score. The score is computed only from contaminants measured in the published EPA record, so this ranks the record, not which city's water is safest. How the score works.

PFAS in San Luis Obispo County4 of 7 city reports had a PFAS sample above a 2024 EPA limit
  1. Atascadero22.3 ng/L
  2. Los Osos11.7 ng/L
  3. Arroyo Grande6.8 ng/L
4.0 ng/L federal MCL · PFOS, ng/LPFAS in California
Lead

0of 7 cities above the 15 µg/L action level

Based on each system's 90th-percentile result. Lead from household plumbing isn't in this record.

Lead in California
Who supplies the county

Largest water systems

  1. San Luis ObispoSAN LUIS OBISPO WATER DEPARTMENT · CA4010009
    49,534 served
  2. AtascaderoATASCADERO MUTUAL WATER CO · CA4010002
    33,307 served
  3. Paso RoblesCITY OF PASO ROBLES WATER DIVISION · CA4010007
    31,176 served
  4. Arroyo GrandeCITY OF ARROYO GRANDE UTILITIES DIVISION · CA4010001
    17,963 served
  5. Grover BeachCITY OF GROVER BEACH PUBLIC WORKS DEPT · CA4010004
    12,701 served
  6. Morro BayMORRO BAY PW DEPT - WATER DIVISION · CA4010011
    10,757 served
  7. Los OsosLOS OSOS COMMUNITY SERVICES DISTRICT · CA4010016
    7,086 served
Questions

Frequently asked

How does San Luis Obispo County tap water compare across cities?

PurityMap ranks 7 cities in San Luis Obispo County by the Purity Score of each city's main water system, computed only from contaminants measured in the published EPA record. Scores belong to water systems, not to cities. The highest published score is San Luis Obispo's main water system: SAN LUIS OBISPO WATER DEPARTMENT · PWSID CA4010009 · Purity Score 100 (Excellent) · Confidence: moderate · Assessed at the water-system level. The ranking compares the records, not which city's water is safest, and your own tap can differ with household plumbing.

How many San Luis Obispo County cities had a PFAS sample above a 2024 EPA limit?

4 of 7 San Luis Obispo County city reports had a PFAS sample above a 2024 EPA limit in EPA's UCMR 5 testing, including Atascadero (PFOS 22.3 ng/L), Los Osos (PFOA 14.7 ng/L), Arroyo Grande (PFOS 6.8 ng/L). That limit is judged on a yearly average, and water systems have until 2029 to meet it, so a single sample is not a violation. PFAS were detected at any level in 4.

Is there lead in San Luis Obispo County tap water?

Utilities report lead as a 90th-percentile result from home taps. 1 of 7 San Luis Obispo County cities reported a value above zero, and none was above the 15 µg/L federal action level. Lead usually comes from a building's own pipes and fixtures, so only a test at your tap tells you what you drink.

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