Arsenic in well water: testing, health risks and treatment

Arsenic is one of the most important contaminants for private well owners to know about, because you can't detect it without a lab test. It occurs naturally in rocks and soil across much of the country, and it can dissolve into groundwater even where no one has ever used it. This guide covers the standard, the health concerns, who should test, and how treatment works.

Where arsenic comes from

USGS describes arsenic as a naturally occurring element. In groundwater, it usually comes from minerals in the surrounding rock and sediment that slowly dissolve. EPA's drinking water table also lists human-made sources: runoff from orchards, where arsenic-based pesticides were once used, and waste from glass and electronics production. In most private wells, though, arsenic is geologic. Two wells a few hundred feet apart can have very different levels.

How common it is

USGS estimated that about 2.1 million people in the U.S. may be using domestic wells with high arsenic levels. Some regions are known hotspots, but arsenic turns up in many states. That's why your local health department's knowledge of local geology is valuable.

The standard

EPA's maximum contaminant level (MCL) for arsenic is 0.010 mg/L, which equals 10 parts per billion (ppb) or 10 micrograms per liter. It has applied to public water systems since January 23, 2006. EPA's health goal (MCLG) is zero.

Private wells aren't regulated by EPA. CDC notes that EPA's public water rules don't apply to private wells. Well owners use the MCL as the benchmark when reading their results.

Reading units: labs may report arsenic in mg/L, µg/L or ppb.

• 0.010 mg/L = 10 µg/L = 10 ppb.
• A result of 0.004 mg/L is 4 ppb, which is below the MCL.
• A result of 25 µg/L is 25 ppb, which is above the MCL.

Health concerns

EPA's drinking water table says long-term exposure to arsenic above the MCL can cause skin damage or problems with the circulatory system, and may increase the risk of cancer. USGS also notes that long-term exposure can cause cancer. EPA's private well page lists arsenic among heavy metals that, at high levels, carry risks including organ damage and cancer.

Key points:

• The concern is mostly long-term drinking, over years.
• There are usually no immediate symptoms at typical well levels, which is why testing matters.
• Children may take in more relative to their body weight, so families with young children should be especially careful.

Who should test

CDC recommends asking your health or environmental department, or EPA, whether you should test for arsenic, along with lead, mercury, radium, VOCs and pesticides. Test for arsenic if:

• you've never tested your well for it;
• your health department says arsenic is common in your area;
• you live near old orchards or industrial sites;
• you're buying a home with a well; or
• someone in the home is pregnant or there are young children.

Arsenic in groundwater is usually fairly stable over time, but levels can change. Many health departments suggest retesting periodically even after a good result.

How to test

1. Use a state-certified lab. CDC recommends certified labs, and EPA says to use labs certified for drinking water testing.
2. Ask for total arsenic. If your result is high and you're choosing treatment, ask whether the lab can also speciate arsenic into arsenic III and arsenic V. Treatment works differently for each.
3. Sample from an untreated tap, such as the pressure tank spigot or an outside faucet before any treatment, to measure the raw well water.

Treatment options

Treatment depends on the arsenic level, the arsenic form and your other water chemistry, including iron, pH, phosphate and silica. Common approaches:

• Reverse osmosis (RO): often installed under the sink to treat drinking and cooking water. Works best on arsenic V.
• Adsorptive media filters: use iron-based or other media to capture arsenic. Available as point-of-use or whole-house systems. Media must be replaced periodically.
• Oxidation plus filtration: converts arsenic III to arsenic V, which is easier to remove, often combined with iron removal.
• Anion exchange: can remove arsenic V, but performance depends on sulfate and other ions.

Important: boiling doesn't remove arsenic. As water evaporates, the arsenic concentration rises.

Choosing a treatment provider

Water treatment is a market with a wide range of quality. Protect yourself:

• Get an independent lab test first, not just a vendor's in-home test.
• Ask for products certified to reduce arsenic by an accredited third-party certifier.
• Get the expected removal performance in writing, based on your actual water chemistry.
• Ask about ongoing costs: replacement media, membranes, filters and service visits.
• Check with your health department. Some offer guidance on treatment or keep lists of resources.

Point-of-use vs. whole-house

Because the main concern is drinking, many homeowners treat only the kitchen tap. That's often the most cost-effective choice. Whole-house treatment may make sense at very high levels, or when you also need to treat iron or other contaminants. Ask a water treatment professional and your health department.

After installing treatment

• Test the treated water soon after installation to confirm it's working.
• Follow the maintenance schedule for filters, membranes or media. Spent media can let arsenic through.
• Keep testing both raw and treated water periodically.

Frequently asked questions

Can I shower in water with arsenic? Arsenic is mainly a concern when swallowed. Ask your health department about very high levels.

Does a water softener remove arsenic? A standard softener isn't designed for arsenic.

Is bottled water a good short-term solution? Yes. Use it for drinking and cooking while you arrange testing and treatment.

Key takeaways

• Arsenic is usually natural, invisible and tasteless.
• EPA's limit is 0.010 mg/L (10 ppb), and the health goal is zero.
• Test with a certified lab, especially if arsenic is common locally.
• Treatment works, but it needs maintenance and follow-up testing.