Arsenic in Drinking Water: Testing and Treatment

Arsenic: A Natural Contaminant in Groundwater

Arsenic is a naturally occurring element found in rock formations across the United States. When groundwater flows through arsenic-bearing rock, it dissolves the element and carries it into wells and aquifers. Unlike many water contaminants, arsenic is not caused by human activity in most cases; it is a geological hazard.

The EPA Maximum Contaminant Level (MCL) for arsenic is 10 ppb (parts per billion), also expressed as 0.010 mg/L. This standard has been in effect since 2006, when it was lowered from the previous limit of 50 ppb. The WHO also sets its guideline at 10 ppb. Long-term exposure above this level increases the risk of several cancers and other serious health effects.

Who Is Most at Risk

Private well owners

Private wells are the primary risk group because they are unregulated. The EPA\'s Safe Drinking Water Act applies only to public water systems serving 15 or more connections. If you are on a private well, no one is testing your water for arsenic unless you do it yourself. The USGS estimates that about 2.1 million people in the US are served by private wells with arsenic above 10 ppb.

Municipal water users in high-arsenic regions

Public water systems must test for arsenic and treat if levels exceed 10 ppb. However, some smaller systems struggle with compliance, and levels just below the MCL still pose health risks according to many public health researchers. The EPA\'s MCL balances health protection with treatment feasibility; it is not a "safe" threshold.

States with the Highest Arsenic Levels

State	Primary Source	Affected Areas
Nevada	Volcanic rock, geothermal activity	Statewide, especially rural wells
Arizona	Desert rock formations, mining legacy	Central and southern regions
Maine	Metamorphic bedrock	Statewide, especially private wells
New Hampshire	Granite and metamorphic rock	Bedrock wells statewide
Michigan	Glacial deposits, Marshall Sandstone	Southeastern and central regions
New Mexico	Volcanic rock, basin-fill aquifers	Rio Grande corridor
California	Alluvial aquifers, volcanic deposits	Central Valley, eastern Sierra
Oregon	Volcanic rock	Southern and eastern regions

The USGS National Water-Quality Assessment found that approximately 7% of all domestic wells in the US exceed the arsenic MCL of 10 ppb. In the states above, that percentage is significantly higher. Check your city\'s water to see arsenic levels in your area.

Health Effects of Arsenic Exposure

Arsenic is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC), meaning there is sufficient evidence that it causes cancer in humans.

Short-term (acute) exposure

Very high levels (above 500 ppb) can cause nausea, vomiting, diarrhea, and abdominal pain. Acute poisoning is rare from drinking water but can occur from contaminated well water in extreme cases.

Long-term (chronic) exposure

Chronic exposure to arsenic above 10 ppb is associated with:

• Skin cancer, bladder cancer, and lung cancer
• Cardiovascular disease, including increased risk of heart attack and stroke
• Skin changes including darkening, thickening, and small lesions
• Peripheral neuropathy (numbness and tingling in extremities)
• Developmental effects in children, including cognitive impairment
• Increased risk of type 2 diabetes

The risk increases with both concentration and duration of exposure. Even levels between 5-10 ppb carry elevated risk according to some studies, though they are below the regulatory limit.

Two Forms of Arsenic in Water

Arsenic exists in two forms in water, and the distinction matters for treatment:

Form	Name	Characteristics	Treatment Ease
Arsenic III	Arsenite	More toxic, uncharged, harder to remove	Must be converted to Arsenic V first
Arsenic V	Arsenate	Less toxic, charged molecule, easier to remove	Removed by most treatment methods

Most treatment systems include an oxidation step (using chlorine, ozone, or air injection) to convert Arsenic III to Arsenic V before filtration. A lab test can determine which form predominates in your water, which helps in choosing the right treatment approach.

Testing for Arsenic

Arsenic is colorless, odorless, and tasteless. You cannot detect it without testing. If you are on a private well, especially in one of the high-risk states listed above, testing is essential.

• Lab testing: Submit a water sample to a state-certified lab. Cost is $20-50 for arsenic analysis. Request speciation (Arsenic III vs. Arsenic V) if your level is above 10 ppb, as it affects treatment selection
• When to test: Test at least once; retest every 3-5 years or after any change in well construction, nearby land use, or water appearance
• Home test kits: Arsenic test strips exist but have limited accuracy at the low concentrations that matter (around 10 ppb). Lab testing is strongly preferred

For broader testing guidance, see our water testing guide.

Treatment Options

Point-of-use reverse osmosis

An under-sink RO system removes 95%+ of arsenic (both forms if the system includes an oxidation pre-filter). This is the most practical and affordable option for most homes. RO systems cost $150-400 and treat drinking and cooking water at a single tap. They also remove lead, nitrate, and PFAS.

Adsorptive media (iron-based)

Specialty filtration media made from iron oxide or iron hydroxide has a strong affinity for arsenic. These systems are designed specifically for arsenic removal and can be configured as whole-house point-of-entry systems. Brands include Bayoxide E33 and ADI media. Costs range from $500-2,000 for whole-house installation. Media replacement is needed every 1-4 years depending on arsenic levels and water usage.

Oxidation + filtration

For wells with high Arsenic III levels, an oxidation step (chlorine injection, ozone, or aeration) followed by an iron-based adsorptive filter provides comprehensive treatment. This is a more complex and expensive setup ($1,500-4,000) but handles the most difficult arsenic scenarios.

What does NOT work

• Standard carbon filters (Brita, PUR) do not remove arsenic
• Water softeners do not remove arsenic
• Boiling does not remove arsenic; it concentrates it
• UV disinfection does not affect arsenic

If Your Well Exceeds 10 ppb

Take these steps immediately:

• Switch to bottled water for drinking and cooking until treatment is installed
• Install a certified point-of-use RO system or arsenic-specific filter at the kitchen tap
• Retest after installation to confirm the system is reducing arsenic below 10 ppb
• Set a maintenance schedule for filter replacement
• Consider testing other wells in your neighborhood; arsenic contamination is often regional

Check your city\'s water for arsenic data. If you are on a private well, contact your state health department or cooperative extension service for testing resources and guidance specific to your area.