Skip to content
TapBuff
ZIP

Monte Sereno Mutual Benefit Water

What EPA’s and California’s records show in this utility’s water: nitrate, arsenic and other regulated chemicals; lead and copper; and violations since 2016, with the certification that handles each result.

Serves
65 people
Connections
28
Water source
Groundwater
Based in
San Luis Obispo, CA
County
San Luis Obispo
Owner
Private
Hardness
Very hard
The ZIP code it serves

93420

The short answer

1 result is above the health goal. The worst: Arsenic, above the health goal. No health-based violations on file since 2016.

What handles it

What was found

Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), nitrate, chromium (total), fluoride and uranium: California’s own results, samples taken from Jan. 22, 2020 to June 9, 2026, with EPA’s 2012 to 2019 file for earlier years.

Above the health goal1

Arsenic

Comes from the source

Above the health goal

1.0ppb

Highest yearly average, 2021 to 2024

2024: not found

Averaged by year, the highest was 1.0 ppb, in 2021: below EPA’s legal limit of 10 ppb, but above its health goal of zero. Its highest single sample was 2.0 ppb. In 2024, the latest year on file, it wasn’t found in any of its two samples.

What handles it

  • NSF/ANSI 58 look for “Arsenic (Pentavalent)<=50 ppb Reduction”
  • NSF/ANSI 53 look for “Arsenic (Pentavalent)<=50 ppb Reduction”

Below EPA’s limits5

Total trihalomethanes (TTHM)

Forms or is added in treatment

Below EPA’s limits

62.8ppb

Highest yearly average, 2020 to 2026

2026 average: 55.3 ppb

Averaged by year, the highest was 62.8 ppb, in 2023, below EPA’s legal limit of 80 ppb. Its highest single sample was 99 ppb. In 2026, the latest year on file, its six samples averaged 55.3 ppb. EPA recorded no violation for total trihalomethanes (TTHM) at this utility since 2020. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 105 ppb, in 2019.

What handles it

  • NSF/ANSI 53 look for “Trihalomethanes (TTHM) Reduction”
  • NSF/ANSI 58 look for “Trihalomethanes (TTHM) Reduction”

Haloacetic acids (HAA5)

Forms or is added in treatment

Below EPA’s limits

29ppb

Highest yearly average, 2020 to 2025

2025, one sample: 22 ppb

Averaged by year, the highest was 29 ppb, in 2021, below EPA’s legal limit of 60 ppb. In 2025, the latest year on file, its one sample was 22 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 31 ppb, in 2019.

What handles it

No certification claim covers this compound yet.

Nitrate

Comes from the source

Below EPA’s limits

2.7ppm

Highest sample; found in 4 of 18

2026 average: 2.3 ppm

The highest sample, in 2026, was 2.7 ppm, below EPA’s legal limit of 10 ppm. That was also the latest year on file, and its two samples averaged 2.3 ppm.

What handles it

  • NSF/ANSI 58 look for “Nitrate/Nitrite Reduction”
  • NSF/ANSI 53 look for “Nitrate/Nitrite (30 ppm challenge)”

Chromium (total)

Comes from the source

Below EPA’s limits

13.5ppb

Highest yearly average, 2021 to 2024

2024 average: 3.8 ppb

Averaged by year, the highest was 13.5 ppb, in 2021, below EPA’s legal limit of 100 ppb. Its highest single sample was 15 ppb. In 2024, the latest year on file, its two samples averaged 3.8 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 2.0 ppb, in 2013.

What handles it

Fluoride

Forms or is added in treatment

Below EPA’s limits

0.3ppm

Highest yearly average, 2021 to 2024

2024 average: 0.25 ppm

Averaged by year, the highest was 0.3 ppm, in 2021, below EPA’s legal limit of 4.0 ppm. Its highest single sample was 0.31 ppm. In 2024, the latest year on file, its two samples averaged 0.25 ppm. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 0.28 ppm, in 2016.

What handles it

2 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Lead
  • Uranium

What handles it

A certification covers specific claims, not a whole standard. Look for these claims on the certifier’s listing, not the box. Wording varies a little: NSF writes “Lead Reduction,” WQA just “Lead.”

  • NSF/ANSI 53

    Claim: Arsenic (Pentavalent)<=50 ppb Reduction

    For: Arsenic

    Other certifiers’ wording: Arsenic (III) (50 ppb influent level)

  • NSF/ANSI 58

    Claim: Arsenic (Pentavalent)<=50 ppb Reduction

    For: Arsenic

    Other certifiers’ wording: Arsenic (Pentavalent)<=300 ppb Reduction; Arsenic (V) (300 ppb influent level)

At the tap, or for the whole house?

These come with the water itself, so a whole-house system certified for the same claims treats every tap and shower. A filter at the tap treats drinking water for much less. How whole-house systems compare.

Filters with those claims on a certifier’s listing

Disclosure: TapBuff earns a commission if you buy through some links on this page. It does not change which standard or filter we name. As an Amazon Associate, TapBuff earns from qualifying purchases. How TapBuff makes money

Compare five-year costs

The best reverse osmosis systems, ranked

Hardness

Very hard, typically 585 mg/L (34.2 grains per gallon), across 2 sampling points, 2019 to 2024.

Hardness is calcium and magnesium. EPA sets no limit for it: it shows up as scale on fixtures and in water heaters, and as soap that won’t lather. USGS calls water above 60 mg/L moderately hard, above 120 hard and above 180 very hard.

Softeners compared at 34.2 grains per gallon, salt includedSoftener or salt-free conditioner?California water hardness, utility by utility

Violations since 2016

No health-based violations on file with EPA since 2016.

What this report can’t tell you

  • Everything else. Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), nitrate, chromium (total), fluoride and uranium results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Monte Sereno Mutual Benefit Water’s own annual report, which it must send you every year, has the contaminants this page does not show.

  • Your own plumbing. Lead results are the 90th percentile across homes the utility samples. Your home’s service line, solder and fixtures decide yours. A certified lab test is the only way to know.

  • What it means for your health. This page reports what the records show and EPA’s limits and goals; it is not medical advice. If you are worried about a health effect, talk to a doctor or your local health department.

  • Anything after the latest data. SDWIS data is from 2026 Q3; UCMR 5 didn’t sample this utility; California’s results run from Jan. 22, 2020 to June 9, 2026.

Testing your own tap

Monte Sereno Mutual Benefit Water’s results are for the water it sends out. What your own service line, solder and fixtures add shows up only in a test of your tap, by a lab California certifies.

The 3 labs California certifies closest to the area Monte Sereno Mutual Benefit Water serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • County of San Luis Obispo Public Health Laboratory

    San Luis Obispo, CA · about 12 miles

    Certified for bacteria

    (805) 781-5507

  • Fruit Growers Laboratory, Inc.

    San Luis Obispo, CA · about 12 miles

    Certified for bacteria

    (805) 783-2940

  • Abalone Coast Analytical, Inc.

    San Luis Obispo, CA · about 12 miles

    Certified for bacteria

    (805) 595-1080

All 145 certified labs for California

Sources

PWSID CA4000808. Rebuilt from EPA data on Oct. 18, 2026.

Also in San Luis Obispo County

All California utilities

What to do next