Keeping Your Well Water Safe: Understanding and Treating Arsenic

Ryan Lewis
Dec 5, 2025
3 min read

A cross-section diagram illustrating the natural occurrence of arsenic in groundwater specifically affecting Snohomish, Whatcom, Island, and Skagit Counties. The graphic demonstrates how geological leaching from arsenic-containing minerals contaminates the aquifer supplying a private well, emphasizing that arsenic is invisible, odorless, and tasteless and that concentration levels can vary seasonally.

For homeowners with private wells in Snohomish, Whatcom, Island, and Skagit counties, understanding the risks and solutions for arsenic contamination is crucial. Arsenic is a naturally occurring element in the local geology, and while it's odorless and tasteless, long-term exposure can pose serious health risks.

Here's what you need to know about arsenic in your drinking water and how we can help you tackle it.

What is Arsenic and Where Does it Come From?

Arsenic (As) is a semi-metallic element that exists naturally in the earth's crust. In our region, particularly in areas like Snohomish County, the presence of arsenic in groundwater is due to geological factors.

Natural Occurrence: The primary source of arsenic in private wells is the natural erosion and weathering of arsenic-containing rocks and minerals in the earth. As groundwater flows through these rock formations (aquifers), the arsenic dissolves and is released into the water.
- Fun Fact: Arsenic is typically found in two forms in water: arsenite AS3+and arsenate AS5+. The arsenite form is generally more mobile, toxic, and common in low-oxygen groundwater conditions, like those found in deep wells.
Local Concern: Elevated levels of naturally occurring arsenic have been found in groundwater throughout the Puget Sound region, including Snohomish, Whatcom, Skagit, and Island counties. Arsenic levels can vary significantly between different wells, and even seasonally within the same well.

An educational infographic titled 'Regulations & Testing: The 10 ppb Standard' illustrating water safety limits. A central gauge identifies the EPA Maximum Contaminant Level (MCL) of 10 parts per billion (ppb) as the Chronic Exposure Limit, differentiating between safe levels (green) and potential health risks (red). The bottom section contrasts Private Wells (which are not federally regulated and require the homeowner to test seasonally) against Public Systems (which are regulated and must meet the EPA standard)

Regulations: Testing and the EPA Standard

Unlike public water systems, private wells are not federally or state-regulated in Washington for water quality, which means the responsibility for testing and treatment falls entirely on the homeowner.

EPA Maximum Contaminant Level (MCL): The U.S. Environmental Protection Agency (EPA) has set the enforceable standard for public drinking water systems at 0.010 milligrams per liter (mg/L), which is equivalent to 10 parts per billion (ppb). This standard is designed to protect public health from the effects of long-term (chronic) exposure.
The Importance of Testing: Since arsenic is odorless and colorless, testing your well water is the only way to know if you and your family are at risk.
- We recommend testing for arsenic at least once, especially if you live in an area known for high arsenic levels (like parts of Snohomish and Whatcom counties) or if you are buying a new property with a well.
- Due to seasonal variations, a single test might not be enough. Testing both in the late summer and early spring is a good practice to check for seasonal differences.

Complete guide to arsenic testing and well water safety in Northwest Washington. See how geological erosion affects wells in Whatcom and Island Counties and learn how to interpret your lab results against the EPA Maximum Contaminant Level (MCL).

Effective Solutions: How to Treat Arsenic in Your Well Water

If your water tests above the 10 ppb standard, don't worry—there are effective, proven solutions available for private wells. The best system for your home depends on the arsenic concentration and your water usage needs.

1. Point-of-Use (POU) Treatment

These systems treat water at a single tap, typically the kitchen sink, and are usually sufficient if you only need to treat water for drinking and cooking.

Reverse Osmosis (RO):
- How it Works: Water is forced through a semi-permeable membrane that removes the vast majority of contaminants, including most forms of arsenic.
- Best For: Treating water for drinking and cooking. These are typically installed under the sink.

2. Point-of-Entry (POE) Treatment (Whole-House)

These systems treat all the water entering your home, ensuring all water used for showering, cooking, and drinking is safe.

Adsorptive Media Filtration:
- How it Works: Water passes through a tank containing media (often activated alumina or iron oxide) that chemically attracts and binds to the arsenic, pulling it out of the water.
- Best For: Whole-house treatment. The media eventually gets saturated and must be replaced or regenerated. This is often the most cost-effective whole-house solution for residential wells.
Anion Exchange:
- How it Works: Similar to a water softener, this system uses a resin that exchanges undesirable ions (like arsenate, AS5+ for safe ions (like chloride). Note that it is less effective against the arsenite form AS3+.

We strongly recommend that any system installed is certified by an independent body,, to ensure it meets safety and performance standards for arsenic removal.

If you have a well in Snohomish, Whatcom, Island, or Skagit counties and are concerned about arsenic, we can help with testing recommendations and professional installation of certified treatment systems.