Water Quality Lookup / California / San Mateo County

EPA SDWIS · San Mateo County, California · 113 providers profiled

San Mateo County, CA Water Quality Report

San Mateo County is served by 113 major public water systems on file with the EPA.

Data current as of July 2026

🔍Straight from EPA

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Results for San Mateo County, CA

113 water providers on file for this county, sorted by population served.

Quick answer: San Mateo County has 113 water providers profiled, sorted by population served.

Violations on file

Mercury

Older/archived health-based violation

Found at 6 of 113 providers

Nitrate

Older/archived health-based violation

Found at 4 of 113 providers

Commonly from agricultural fertilizer runoff or septic systems. Above the legal limit, it's an immediate concern especially for infants.

Endrin

Older/archived health-based violation

Found at 1 of 113 providers

Total Haloacetic Acids (HAA5)

Older/archived health-based violation

Found at 1 of 113 providers

TTHM

Older/archived health-based violation

Found at 1 of 113 providers

A group of disinfection byproducts formed when chlorine reacts with organic matter. Long-term exposure above the legal limit has been linked to increased cancer risk in some studies.

Surface Water Treatment Rule

Currently open health violation

Found at 2 of 113 providers

Lead And Copper Rule Revisions

Currently open health violation

Found at 1 of 113 providers

Contaminants found

Metals

Naturally occurring or corrosion-related elements, e.g. lead, arsenic, copper

Lead Summary

Detected: 0.071 mg/L (ppm)

Found at 47 providers

Lead

Detected: 130 UG/L (ppb)

Found at 16 providers

A metal that can leach from old pipes, solder, and fixtures. Current medical guidance holds there is no truly safe level of lead exposure, especially for children and pregnant women.

Copper, Free

Detected: 2870 UG/L (ppb)

Found at 20 providers

Lithium, Total

Detected: 48 UG/L (ppb)

7/2/2025

Strontium

Detected: 800 UG/L (ppb)

7/2/2025

Iron

Detected: 11800 UG/L (ppb)

Found at 17 providers

Mostly an aesthetic issue (metallic taste, staining) rather than a health concern at typical levels.

Manganese

Detected: 689 UG/L (ppb)

Found at 17 providers

Mainly a taste/staining concern at typical levels; long-term neurological research at high chronic exposure is ongoing.

Chromium, Hex

Detected: 6 UG/L (ppb)

Found at 12 providers

Barium

Detected: 1400 UG/L (ppb)

Found at 10 providers

Zinc

Detected: 241 UG/L (ppb)

Found at 3 providers

Vanadium, Total

Detected: 7.8 UG/L (ppb)

Found at 3 providers

Copper Summary

Detected: 2.68 mg/L (ppm)

Found at 6 providers

Arsenic

Detected: 8.8 UG/L (ppb)

Found at 6 providers

Occurs naturally in some groundwater. Long-term exposure above the legal limit (10 ppb) has been linked to increased cancer risk and cardiovascular disease.

Aluminum

Detected: 507 UG/L (ppb)

Found at 8 providers

Selenium

Detected: 4.8 UG/L (ppb)

Found at 5 providers

Chromium

Detected: 13 UG/L (ppb)

Found at 2 providers

Occurs in two forms — trivalent (an essential nutrient in trace amounts) and hexavalent (linked to increased cancer risk).

Nickel

Detected: 26.4 UG/L (ppb)

Found at 3 providers

Manganese, Dissolved

Detected: 27 UG/L (ppb)

2/5/2025

Iron, Dissolved

Detected: 35 UG/L (ppb)

8/12/2025

Cadmium

Detected: 0.8 UG/L (ppb)

6/12/2025

Antimony, Total

Detected: 11.5 UG/L (ppb)

9/9/2025

Disinfection Byproducts

Formed when chlorine used to disinfect water reacts with organic matter

Totalhaloaceticacids (HAA5)

Detected: 58.9 UG/L (ppb)

Found at 31 providers

TTHM

Detected: 251 UG/L (ppb)

Found at 34 providers

A group of disinfection byproducts formed when chlorine reacts with organic matter. Long-term exposure above the legal limit has been linked to increased cancer risk in some studies.

Chloroform

Detected: 134.5 UG/L (ppb)

Found at 29 providers

One of the individual trihalomethane disinfection byproducts, formed when chlorine reacts with organic matter.

Bromodichloromethane

Detected: 80 UG/L (ppb)

Found at 29 providers

Dibromochloromethane

Detected: 55 UG/L (ppb)

Found at 32 providers

Bromoform

Detected: 130 UG/L (ppb)

Found at 16 providers

Inorganics

Common dissolved minerals and salts, e.g. nitrate, fluoride, chloride

Nitrate

Detected: 8.6 MG/L (ppm)

Found at 14 providers

Commonly from agricultural fertilizer runoff or septic systems. Above the legal limit, it's an immediate concern especially for infants.

Chloride

Detected: 310 MG/L (ppm)

Found at 15 providers

Sulfate

Detected: 158 MG/L (ppm)

Found at 15 providers

Fluoride

Detected: 1.9 MG/L (ppm)

Found at 17 providers

Often added deliberately for dental health; can also occur naturally.

Boron, Total

Detected: 82 UG/L (ppb)

Found at 2 providers

Nitrate-Nitrite

Detected: 3.7 MG/L (ppm)

Found at 4 providers

Minerals

Everyday minerals that mainly affect taste and water hardness

Magnesium

Detected: 43 MG/L (ppm)

Found at 16 providers

Calcium

Detected: 107 MG/L (ppm)

Found at 16 providers

Sodium

Detected: 120 MG/L (ppm)

Found at 16 providers

Potassium

Detected: 7.2 MG/L (ppm)

Found at 13 providers

Properties

General water characteristics like pH, hardness, and cloudiness (turbidity)

Langelier Index (pH(s))

Detected: 1.69 LANG

Found at 9 providers

Hardness, Total (ASCACO3)

Detected: 359 MG/L (ppm)

Found at 15 providers

pH, Field

Detected: 8.14 PH

Found at 2 providers

Orthophosphate

Detected: 0.6 MG/L (ppm)

9/29/2025

Gross Alpha Particle Activity

Detected: 2.68 PCI/L

Found at 4 providers

pH

Detected: 8.3 PH

Found at 13 providers

A measure of how acidic or basic the water is (7 is neutral). Affects taste and plumbing, not a direct health measure.

PFAS & Emerging Contaminants

Human-made "forever chemicals" and other substances not yet fully regulated

PFPEA

Detected: 0.0041 µg/L (ppb)

12/4/2023

PFHPA

Detected: 0.0043 µg/L (ppb)

12/4/2023

PFHXS

Detected: 0.0034 µg/L (ppb)

12/4/2023

PFOA

Detected: 0.0096 µg/L (ppb)

12/4/2023

One of two PFAS with an enforceable federal limit (4 ppt) since 2024.

PFHXA

Detected: 0.0042 µg/L (ppb)

12/4/2023

PFOS

Detected: 0.0083 µg/L (ppb)

12/4/2023

One of two PFAS with an enforceable federal limit (4 ppt) since 2024.

Lithium

Detected: 33 µg/L (ppb)

Found at 3 providers

Occurs naturally in some groundwater. Not currently federally regulated in drinking water.

VOCs (Volatile Organic Compounds)

Industrial solvents and fuel-related chemicals that evaporate easily

1, 2, 3-Trichloropropane

Detected: 0.006 UG/L (ppb)

9/11/2025

Cis-1, 2-Dichloroethylene

Detected: 2.3 UG/L (ppb)

2/3/2025

Bromide

Detected: 0.5 MG/L (ppm)

Found at 4 providers

Calculated Parameters

Values worked out mathematically from other test results, not measured directly

Alkalinity, Bicarbonate

Detected: 374 MG/L (ppm)

Found at 15 providers

Alkalinity, Carbonate

Detected: 10 MG/L (ppm)

8/28/2025

Other

Additional substances tested that don't fit neatly into the categories above

Turbidity

Detected: 45 NTU

Found at 15 providers

A measure of water cloudiness from suspended particles.

Dichloroaceticacid

Detected: 33.1 UG/L (ppb)

Found at 29 providers

Trichloroaceticacid

Detected: 33 UG/L (ppb)

Found at 29 providers

Perfluoroctanoicacid (PFOA)

Detected: 3 NG/L (ppt)

8/5/2025

One of two PFAS with an enforceable federal limit (4 ppt) since 2024.

Perfluorohexanoicacid (PFHXA)

Detected: 2.3 NG/L (ppt)

8/5/2025

Aggressive Index

Detected: 13 AGGR

Found at 12 providers

Alkalinity, Total

Detected: 306 MG/L (ppm)

Found at 15 providers

TDS

Detected: 1300 MG/L (ppm)

Found at 16 providers

Conductivity (at 25°C)

Detected: 6720 UMHO/CM

Found at 14 providers

Color

Detected: 250 UNITS

Found at 9 providers

Monochloroaceticacid

Detected: 14 UG/L (ppb)

Found at 9 providers

Dibromoaceticacid

Detected: 13 UG/L (ppb)

Found at 17 providers

Trans-1, 2-Dichloroethylene

Detected: 0.69 UG/L (ppb)

2/3/2025

Perchlorate

Detected: 2 UG/L (ppb)

8/27/2025

Can occur naturally or from industrial/military sources; may interfere with thyroid function at elevated levels.

Carbon, Total

Detected: 2.47 MG/L (ppm)

Found at 5 providers

Langelier Index (Source Temp.)

Detected: 0.86 LANG

Found at 6 providers

Odor

Detected: 2 TON

Found at 3 providers

Radium-228

Detected: 0.92 PCI/L

Found at 3 providers

Combined Radium (-226 & -228)

Detected: 0.965 PCI/L

Found at 2 providers

A naturally occurring radioactive element found in some groundwater.

Gross Beta Particle Activity

Detected: 4.62 PCI/L

Found at 2 providers

Monobromoaceticacid

Detected: 2 UG/L (ppb)

Found at 5 providers

Foamingagents (SURFACTANTS)

Detected: 0.05 MG/L (ppm)

6/19/2025

Bromate

Detected: 1.1 UG/L (ppb)

8/6/2025

Filtration considerations: General system types, not a specific product recommendation.

Contaminant	Whole-House Carbon	Under-Sink RO	Under-Sink Carbon
Total Trihalomethanes (TTHM)	Yes	Yes	Yes
Haloacetic Acids (HAA5)	Yes	Yes	Yes
Chloroform	Yes	Yes	Yes
Chlorine / Taste	Yes	Yes	Yes
Sediment / Turbidity	Yes	Yes	Yes
Trichloroethylene	Yes	Yes	Yes
Tetrachloroethylene	Yes	Yes	Yes
cis-1,2-Dichloroethylene	Yes	Yes	Yes
1,1-Dichloroethylene	Yes	Yes	Yes
Bromoform	Yes	Yes	Yes
Bromodichloromethane	Yes	Yes	Yes
Dibromochloromethane	Yes	Yes	Yes
1,1,2-Trichloroethane	Yes	Yes	Yes
Vinyl Chloride	Yes	Yes	Yes
Carbon Tetrachloride	Yes	Yes	Yes
Benzene	Yes	Yes	Yes
Toluene	Yes	Yes	Yes
Xylene	Yes	Yes	Yes
Totalhaloaceticacids (HAA5)	Yes	Yes	Yes
1, 2, 3-Trichloropropane	Yes	Yes	Yes
Bromide	Yes	Yes	Yes
Lead	Partial	Yes	Partial
Arsenic	Partial	Yes	Partial
Chromium-6 (Hexavalent)	Partial	Yes	Partial
Copper	Partial	Yes	Partial
Combined Uranium	Partial	Yes	Partial
PFOA	Partial	Yes*	Partial
PFOS	Partial	Yes*	Partial
Nitrate	Partial	Yes	Partial
Combined Radium	Partial	Yes	Partial
Dichloroaceticacid	Partial	Yes	Partial
Trichloroaceticacid	Partial	Yes	Partial
Copper, Free	Partial	Yes	Partial
PFPEA	Partial	Yes*	Partial
PFHPA	Partial	Yes*	Partial
PFHXS	Partial	Yes*	Partial
PFHXA	Partial	Yes*	Partial
Perfluoroctanoicacid (PFOA)	Partial	Yes	Partial
Perfluorohexanoicacid (PFHXA)	Partial	Yes	Partial
Aggressive Index	Partial	Yes	Partial
Alkalinity, Total	Partial	Yes	Partial
TDS	Partial	Yes	Partial
Chloride	Partial	Yes	Partial
Sulfate	Partial	Yes	Partial
Fluoride	Partial	Yes	Partial
Lithium, Total	Partial	Yes	Partial
Strontium	Partial	Yes	Partial
Iron	Partial	Yes	Partial
Manganese	Partial	Yes	Partial
Chromium, Hex	Partial	Yes	Partial
Barium	Partial	Yes	Partial
Conductivity (at 25°C)	Partial	Yes	Partial
Color	Partial	Yes	Partial
Boron, Total	Partial	Yes	Partial
Zinc	Partial	Yes	Partial
Monochloroaceticacid	Partial	Yes	Partial
Lithium	Partial	Yes*	Partial
Vanadium, Total	Partial	Yes	Partial
Dibromoaceticacid	Partial	Yes	Partial
Trans-1, 2-Dichloroethylene	Partial	Yes	Partial
Perchlorate	Partial	Yes	Partial
Carbon, Total	Partial	Yes	Partial
Aluminum	Partial	Yes	Partial
Langelier Index (Source Temp.)	Partial	Yes	Partial
Nitrate-Nitrite	Partial	Yes	Partial
Selenium	Partial	Yes	Partial
Odor	Partial	Yes	Partial
Combined Radium (-226 & -228)	Partial	Yes	Partial
Gross Beta Particle Activity	Partial	Yes	Partial
Chromium	Partial	Yes	Partial
Nickel	Partial	Yes	Partial
Monobromoaceticacid	Partial	Yes	Partial
Manganese, Dissolved	Partial	Yes	Partial
Iron, Dissolved	Partial	Yes	Partial
Cadmium	Partial	Yes	Partial
Foamingagents (SURFACTANTS)	Partial	Yes	Partial
Bromate	Partial	Yes	Partial
Antimony, Total	Partial	Yes	Partial
Coliform Bacteria (indicator)	No	Yes	No
Langelier Index (pH(s))	No	No	No
Magnesium	No	Partial	No
Calcium	No	Partial	No
Sodium	No	Partial	No
Potassium	No	Partial	No
Alkalinity, Bicarbonate	No	No	No
Hardness, Total (ASCACO3)	No	No	No
pH, Field	No	No	No
Orthophosphate	No	No	No
Gross Alpha Particle Activity	No	No	No
pH	No	No	No
Alkalinity, Carbonate	No	No	No

Browse by city

Individual water providers

CITY OF DALY CITY No violations on file — pop. 104,901 — serves DALY CITY
CALIFORNIA WATER SERVICE - SAN MATEO No violations on file — pop. 102,348
CITY OF REDWOOD CITY No violations on file — pop. 92,819 — serves REDWOOD CITY
CALIFORNIA WATER SERVICE-S SAN FRANCISCO No violations on file — pop. 64,146
CALIFORNIA WATER SERVICE - BEAR GULCH No violations on file — pop. 61,431
CITY OF SAN BRUNO No violations on file — pop. 42,631 — serves SAN BRUNO
NORTH COAST COUNTY WATER DIST No violations on file — pop. 38,546
ESTERO MUNICIPAL IMPROVEMENT DISTRICT No violations on file — pop. 36,556
CALIFORNIA WATER SERVICE - SAN CARLOS No violations on file — pop. 35,344
CITY OF BURLINGAME No violations on file — pop. 31,451 — serves BURLINGAME
MID-PENINSULA WATER DISTRICT No violations on file — pop. 30,609
CITY OF EAST PALO ALTO No violations on file — pop. 29,519 — serves EAST PALO ALTO

CITY OF MILLBRAE No violations on file — pop. 22,795 — serves MILLBRAE
MENLO PARK MUNICIPAL WATER No violations on file — pop. 22,362 — serves MENLO PARK
COASTSIDE COUNTY WATER DISTRICT No violations on file — pop. 18,940
WESTBOROUGH WATER DISTRICT No violations on file — pop. 13,486
TOWN OF HILLSBOROUGH No violations on file — pop. 11,766
MONTARA WATER AND SANITARY DISTRICT No violations on file — pop. 6,078
CITY OF BRISBANE No violations on file — pop. 3,402 — serves BRISBANE
GUADALUPE MUNICIPAL IMPROVEMEN No violations on file — pop. 3,000
PALO ALTO PARK MUTUAL WATER COMPANY No violations on file — pop. 2,900
O'CONNOR TRACT CO-OPERATIVE WATER CO. No violations on file — pop. 2,789
CSP MT TAMALPAIS-SUMMIT No violations on file — pop. 2,049 — serves NC DAY USE
SAN MATEO COUNTY MEMORIAL PARK No violations on file — pop. 1,790
SKYLINE COUNTY WATER DISTRICT No violations on file — pop. 1,650
GUADALUPE VALLEY MID No violations on file — pop. 1,472
SAN MATEO HIGH SCHOOL No violations on file — pop. 1,000
PILLAR RIDGE MHP (FORMER EL GRANADA MHP) No violations on file — pop. 1,000
BEAN HOLLOW COMFORT STATION No violations on file — pop. 958
CUESTA LA HONDA GUILD INC No violations on file — pop. 915
MENLO COLLEGE No violations on file — pop. 850
DIMOND PUBLIC UTILITY DIST No violations on file — pop. 725
CSP - ANO NUEVO STATE PARK No violations on file — pop. 709
COUNTY SERVICE AREA 11 No violations on file — pop. 642
COSTANOA CAMPGROUND No violations on file — pop. 583
CAPITOL STATE PARKS No violations on file — pop. 504
S M CO PARKS & REC DEPT No violations on file — pop. 500
SKYLONDA MUTUAL Older violation on record — pop. 420
CAMP JONES GULCH No violations on file — pop. 357
COUNTY SERVICE AREA 7 No violations on file — pop. 300
CAMP LOMA MAR Older violation on record — pop. 208
OPTIMISTS VOLUNTEERS FOR YOUTH No violations on file — pop. 207
CONVENT OF THE SACRED HEART No violations on file — pop. 200
WOODSIDE MUTUAL No violations on file — pop. 195
PESCADERO HIGH SCHOOL Older violation on record — pop. 192
CSP - PORTOLA STATE PARK No violations on file — pop. 181
SYNANON HOME PLACE RANCH No violations on file — pop. 173
PACIFIC MUSHROOM FARM No violations on file — pop. 150
MENS CORRECTIONAL CENTER Older violation on record — pop. 150
BUTANO STATE PARK No violations on file — pop. 150
BUTANO CANYON MUTUAL No violations on file — pop. 150
REDWOOD GLEN CAMP & CONFERENCE CTR No violations on file — pop. 114
HARLEY FARMS GOAT DAIRY No violations on file — pop. 113
MAINTENANCE DISTRICT No violations on file — pop. 110
ST PATRICKS SEMINARY No violations on file — pop. 100
BUTANO STATE PARK - CSP No violations on file — pop. 95
PORTOLA IMPROVEMENT ASSOCIATION MWC Older violation on record — pop. 90
CUTTER SCOUT RESERVATION - INACTIVE No violations on file — pop. 77
PIGEON POINT HOSTEL Open violation — pop. 67
KINGS MOUNTAIN PARK MUTUAL Older violation on record — pop. 66
LOMA MAR MUTUAL Older violation on record — pop. 65
BIANCHI FLOWERS INC No violations on file — pop. 65
ST JOHN MUTUAL WATER COMPANY No violations on file — pop. 60
CORDILLERS MUTUAL WATER COMPANY No violations on file — pop. 59
REDWOOD TERRACE MUTUAL Older violation on record — pop. 56
NICASIO SCHOOL No violations on file — pop. 50
DINELLIS No violations on file — pop. 50
DUARTES Older violation on record — pop. 50
PESCADERO ELEMENTARY SCHOOL Older violation on record — pop. 50
LA HONDA PROPERTIES No violations on file — pop. 50
SKYLAWN MEMORIAL PARK (SKYLAWN INC) Open violation — pop. 50
MARCHI CENTRAL FARM No violations on file — pop. 50
MARTINS BEACH WATER SYSTEM No violations on file — pop. 49
DEARBORNE PARK WATER SYSTEM No violations on file — pop. 45
SPRING CREEK LANE M W C No violations on file — pop. 40
ANO NUEVO FLOWER GROWERS No violations on file — pop. 36
OKU INCORPORATED No violations on file — pop. 35
WEBB RANCH No violations on file — pop. 35
DURARTE'S WATER SYSTEM No violations on file — pop. 34
UC ELKUS RANCH No violations on file — pop. 34
LA HONDA CENTER No violations on file — pop. 33
GAZOS CREEK BEACH HOUSE No violations on file — pop. 33
SAN GREGORIO COMPANY No violations on file — pop. 32
AUSTIN CREEK - BLOCK 5 & 6 No violations on file — pop. 30
SILVA WHOLESALE FLORIST No violations on file — pop. 30
BUTANO GIRL SCOUT CAMP No violations on file — pop. 29
PUENTE HOUSING SOLUTIONS Open violation — pop. 28
AMESPORT MUTUAL WATER COMPANY No violations on file — pop. 25
CALIFORNIA GOLF CLUB No violations on file — pop. 25
WILLIAMSON STORE Older violation on record — pop. 25
LAGUNA JOINT SCHOOL No violations on file — pop. 25
LINCOLN SCHOOL No violations on file — pop. 25
UNION JOINT SCHOOL No violations on file — pop. 25
CSP INDIAN BEACH No violations on file — pop. 25 — serves NO PRESENT USE
NPS GOLDEN GATE NRA No violations on file — pop. 25 — serves NC FT FUNSTON
NPS GOLDEN GATE NRA No violations on file — pop. 25 — serves NC KIRBY COVE
STONE PINE MUTUAL WATER COMPANY No violations on file — pop. 25
BEAN HOLLOW S.B. RESIDENCE No violations on file — pop. 25
JACOBS FARMS No violations on file — pop. 25
LOG CABIN RANCH No violations on file — pop. 25
COAST WHOLESALE FLORISTS INC No violations on file — pop. 17
CABRILLO FARMS INC No violations on file — pop. 15
CROCKER CURVE WATER CO No violations on file — pop. 15
JAMES FOLAN No violations on file — pop. 12
VINCE MUZZI No violations on file — pop. 12
TOM PHIPPS No violations on file — pop. 11
ALEXANDER COZZOLINO No violations on file — pop. 8
ISAMU KUWAHARA No violations on file — pop. 7
ROBERT CERVASCO No violations on file — pop. 5
PETE MARCHI AND SONS No violations on file — pop. 5
CSP INVERNESS RIDGE No violations on file — pop. 2 — serves NC PARK RES
LOS TRANCOS WD No violations on file — pop. 0
CSP ANO NUEVO STATE RESERVE No violations on file — pop. 0 — serves NC PARK RES

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What is TTHM? Understanding EPA Violation Status Is Chromium-6 Dangerous? Whole-House vs. Under-Sink Filtration Lead and Copper in Drinking Water What is PFAS? Arsenic and Uranium Explained Nitrate and Fluoride Explained Water Hardness, pH, and Turbidity What is Coliform Bacteria?