Arsenic in Drinking Water in Washington
What residents of Washington need to know about arsenic in drinking water — including natural geological sources, private well risk, which utilities have documented violations, and how to remove arsenic from tap water.
Source: EPA SDWIS, Washington Department of Health, USGS · Last reviewed: 2025-01-01
Quick Answer
Is arsenic in drinking water a concern in Washington?
Yes. Washington State has significant arsenic occurrence from its volcanic geology — the Cascade Range and Columbia River Basalt formation create conditions for naturally elevated arsenic in groundwater across eastern Washington and some parts of western Washington. Multiple Washington public water systems have documented arsenic above the MCL, and private well owners throughout the state face risk from volcanic aquifer geology.
Where does arsenic come from in Washington's water?
Volcanic geology is the dominant arsenic pathway in Washington. The Columbia River Basalt — which underlies most of eastern Washington — has documented arsenic in the regional basalt aquifer system. The Cascade volcanic range contributes arsenic to groundwater on both the west and east sides of the mountains. Eastern Washington communities (Yakima Valley, Walla Walla, Columbia Basin) relying on basalt aquifer groundwater face the most documented risk.
What should Washington residents know?
Eastern Washington communities relying on Columbia River Basalt aquifer groundwater, Cascade foothills private well users, and communities throughout Washington's volcanic terrain should test for arsenic. Washington DOH monitors public water systems and has worked with multiple eastern Washington communities on arsenic compliance. Private well owners — approximately 15% of Washington residents — have no regulatory protection.
Key Facts
| EPA MCL | 10 µg/L (10 ppb) |
| MCLG | Zero |
| Primary source | Columbia River Basalt aquifer (eastern WA); Cascade volcanic geology (statewide) |
| Highest-risk area | Eastern Washington — Yakima Valley, Columbia Basin, Walla Walla area relying on basalt aquifer |
| State regulator | Washington Department of Health (DOH) |
| Health effects | Bladder, lung, skin cancer; cardiovascular; diabetes risk |
| Effective treatment | Reverse osmosis (NSF/ANSI 58) or activated alumina |
Why Arsenic Matters in Washington
Washington State sits within one of North America's most active volcanic regions, and the geochemical legacy of Cascade volcanism has distributed arsenic across the regional groundwater system. The Columbia River Basalt aquifer, which supplies water to large portions of eastern Washington, has documented arsenic in multiple areas. Communities in the Yakima Valley, Tri-Cities area, and Walla Walla region have faced arsenic compliance challenges. Washington DOH has been active in providing guidance and compliance assistance, and the state's environmental health program has produced detailed arsenic occurrence data for public water systems.
Washington Arsenic Program
Washington DOH monitors arsenic under federal SDWIS and has documented elevated arsenic in eastern Washington's Columbia River Basalt geology. Multiple small water systems in eastern Washington required treatment after 2006. Private well owners in Adams, Franklin, Grant, Lincoln, and other eastern Washington counties face the highest arsenic risk due to basalt-derived groundwater.
Largest Washington Water Utilities
No arsenic violations on record in EPA SDWIS for Washington utilities in our database. Browse the largest utilities to review their full water quality record.
How Does Arsenic Get Into Drinking Water?
Arsenic in drinking water is almost always naturally occurring — it leaches from arsenic-bearing rocks and minerals into groundwater over time. New England granite, Southwest volcanic geology, and Upper Midwest glacial aquifers are the primary high-risk formations. It has no taste, odor, or color.
Full arsenic overview — geology maps, health effects, all 50 statesWho Should Pay Closest Attention
Eastern Washington communities in the Yakima Valley, Columbia Basin, Walla Walla area, and other Columbia River Basalt aquifer-dependent regions; Cascade foothills private well users; and private well owners throughout Washington's volcanic terrain face the most relevant arsenic risk.
Private well owners near mining districts or agricultural areas
Residents in states with documented volcanic or geothermal geology
Long-term consumers of water from small groundwater systems
Households in homes built before 1960 with older well casings
Residents whose well water has never been tested for arsenic
Anyone living in a state where bedrock wells are common
How to Check Your Situation in Washington
- 1
Identify your water source. If you use a public utility, use the ZIP lookup on this page to find your system and check its compliance record.
- 2
If on public water, review your utility's most recent Consumer Confidence Report (CCR) for arsenic monitoring data. The MCL is 10 ppb — your report should show recent test results.
- 3
If on a private well, order an arsenic test from a Washington Department of Health-certified laboratory. A basic arsenic test costs $15–$40. The state agency website maintains a certified lab list.
- 4
Test your well at the tap — not just at the wellhead. The entire water distribution system within your home can affect water quality.
- 5
If your test shows arsenic above 5 ppb, install certified treatment immediately. If above 10 ppb, do not use the water for drinking or cooking until treatment is installed.
- 6
Retest after installing treatment to confirm it is working as certified. Replace filter media on the manufacturer's schedule — an exhausted filter may not perform as rated.
Treatment Options for Arsenic
Boiling does not remove arsenic — it concentrates it. Standard activated carbon filters (Brita, etc.) do not effectively remove arsenic. Certified treatment is required.
NSF/ANSI Standard 58 — Reverse Osmosis
RO systems certified to NSF/ANSI 58 remove 85–95% of arsenic. Under-sink installation. Most effective for removing multiple contaminants simultaneously. Replace membranes and pre-filters on schedule.
Activated Alumina Filters
Activated alumina is specifically designed for arsenic and fluoride removal. Point-of-use or whole-house options available. Must be certified by NSF International or WQA for arsenic reduction. Requires periodic media regeneration or replacement.
Iron/Manganese Oxidation Filters
Effective for arsenic in iron-rich well water, which is common in the Midwest and New England. Oxidation converts dissolved iron and arsenic to a form that can be filtered out. Best when arsenic is co-occurring with high iron levels.
What does NOT work for arsenic
Standard activated carbon filters (Brita, refrigerator filters, most pitcher filters) do NOT effectively remove arsenic. Boiling concentrates arsenic. Water softeners do not remove arsenic. Only use products with NSF certification specifically for arsenic reduction.
Take Action Now
If you use a private well in Washington, test for arsenic — especially if you are in a region with granite, volcanic, or sedimentary geology. A basic arsenic test costs $15–$40 at a state-certified lab.
Public water users: check your utility's Consumer Confidence Report (CCR) for arsenic results. The EPA MCL is 10 ppb — any detection warrants attention.
If arsenic is detected above 10 ppb (or even below it, given MCLG is zero), install an NSF/ANSI 58-certified reverse osmosis system for drinking and cooking water.
Standard carbon filters do NOT remove arsenic — do not rely on a Brita or refrigerator filter for arsenic protection.
Frequently Asked Questions
Related Pages
Arsenic — National Overview
All U.S. utilities with arsenic records
Washington State Overview
All utilities and water quality data
Washington Well Water Guide
Testing, risks & certified labs for private wells
Reverse Osmosis Guide
Removes 85–95% of arsenic
Best Arsenic Filter Guide
What actually works (carbon doesn't)
PFAS in Drinking Water
The 'forever chemical' contamination overview
All Contaminants
Complete reference library
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
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Washington Department of Health ↗