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Diablo WTR Sys - Seattle City Light

What EPA’s and Washington’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
53 people
Connections
41
Water source
Groundwater
Based in
MT Vernon, WA
County
Whatcom
Owner
Local government
Hardness
Soft
The ZIP code it serves

98283

The short answer

1 result is above EPA’s legal limit. The worst: Lead, above the action level. 1 health-based violation since 2016.

What handles it

What was found

Lead and copper: EPA SDWIS monitoring periods ending 2002 to 2024.

Nitrate, total trihalomethanes (TTHM) and haloacetic acids (HAA5): Washington’s own results, samples taken from Aug. 18, 2020 to Jan. 20, 2026, with EPA’s 2012 to 2019 file for earlier years.

Arsenic, chromium (total) and fluoride: EPA’s compliance monitoring file, samples taken from June 5, 2018 to June 5, 2018. Newer results are in the utility’s annual water quality report.

Above the legal limit1

Lead

Comes from pipes, often your own

Above the action level

15.5ppb

90th percentile, 2022 to 2024

In the latest period, 9 in 10 homes sampled were at or below 15.5 ppb, above EPA’s action level of 15 ppb. The action level drops to 10 ppb on Nov. 1, 2027, and this result is at or above that. EPA’s health goal for lead is zero.

What handles it

Below EPA’s limits2

Nitrate

Comes from the source

Below EPA’s limits

0.1ppm

Highest sample; found in 3 of 4

2026: not found

The highest sample, in 2023, was 0.1 ppm, below EPA’s legal limit of 10 ppm. In 2026, the latest year on file, it wasn’t found in its one sample. Earlier, from 2012 to 2019, EPA’s file shows a highest sample of 0.24 ppm, in 2016.

What handles it

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

Total trihalomethanes (TTHM)

Forms or is added in treatment

Below EPA’s limits

0.3ppb

Highest yearly average, 2020 to 2024

Averaged by year, the highest was 0.3 ppb, in 2024, below EPA’s legal limit of 80 ppb. Its highest single sample was 0.6 ppb. That was also the latest year on file.

What handles it

  • NSF/ANSI 53 look for “Trihalomethanes (TTHM) Reduction”
  • NSF/ANSI 58 look for “Trihalomethanes (TTHM) Reduction”
4 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
  • Arsenic
  • Chromium (total)
  • Fluoride
  • Haloacetic acids (HAA5)

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.”

At the tap, or for the whole house?

Lead and copper usually come from pipes and fixtures inside or near your home. A whole-house system sits where water enters, so it cannot catch what your own plumbing adds after it. For those, the filter that counts is the one at the tap. 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 water filters for lead, ranked

Hardness

Soft, typically 56 mg/L (3.2 grains per gallon), across 2 sampling points, in 2025.

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 3.2 grains per gallon, salt includedSoftener or salt-free conditioner?Washington water hardness, utility by utility

Violations since 2016

Contaminant or ruleTypePeriodMeasuredStatus
Lead and Copper RuleA required study or plan for corrosion control was not done on time.EPA: OCCT/SOWT Study/RecommendationOct. 23, 2025—Resolved
Health-based violations only. Resolved and archived mean the utility returned to compliance or the state closed the record. 5 monitoring or reporting violations not shown.

What this report can’t tell you

  • Everything else. Nitrate, total trihalomethanes (TTHM) and haloacetic acids (HAA5) results here are Washington’s own from 2020 on, with EPA’s national file for 2012 to 2019. Arsenic, chromium (total) and fluoride come from EPA’s file alone, which ends in 2019. Diablo WTR Sys - Seattle City Light’s own annual report, which it must send you every year, has newer results for those and 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; Washington’s results run from Aug. 18, 2020 to Jan. 20, 2026; EPA’s compliance samples are from June 5, 2018 to June 5, 2018.

Testing your own tap

Diablo WTR Sys - Seattle City Light’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 Washington certifies.

The 3 labs Washington certifies closest to the area Diablo WTR Sys - Seattle City Light serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.

  • Freshwater Analytical Labs, LLC

    Arlington, WA · about 51 miles

    Certified for bacteria

    (360) 913-7542

  • Eurofins DW and WW West, Washington Laboratory

    Burlington, WA · about 54 miles

    Certified for bacteria, nitrate, minerals and other inorganics, disinfection byproducts

    (360) 757-1400

  • Exact Scientific Services, Inc.

    Ferndale, WA · about 62 miles

    Certified for bacteria, nitrate, lead and copper, metals, minerals and other inorganics, organic chemicals, disinfection byproducts

    (360) 733-1205

All 84 certified labs for Washington

Sources

PWSID WA5319200. Rebuilt from EPA data on Oct. 18, 2026.

Also in Whatcom County

All Washington utilities

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