Arsenic in well water

Best for

Homes with arsenic above 10 µg/L, or detectable and below it, who want safe drinking and cooking water at one tap.

Wrong fit

Homes with arsenic above 500 µg/L need whole-house treatment and health department advice. Wells with arsenic III need pre-oxidation ahead of RO.

Tradeoff

An under-sink RO system protects one faucet only, needs pre-oxidation for arsenic III, and must be maintained and retested to stay effective.

Why every well should be tested once

Arsenic occurs naturally in many rocks and sediments, and groundwater can dissolve it. You cannot see, taste or smell it, and the Minnesota Department of Health notes there is no way to know a well’s arsenic level before it is drilled. Levels can differ sharply between wells in the same area.

It is not rare. In a national study, the USGS found arsenic at 1 µg/L or more in nearly half of the drinking-water wells it sampled, and about 7% exceeded the 10 µg/L limit. Concentrations were generally higher in the West than in the East. The USGS estimates about 2.1 million people use private wells high in arsenic. In Minnesota, arsenic has been found in about 40% of new wells, and about 10% of private wells exceed 10 µg/L.

Is it a health risk?

Yes, over the long term. Minnesota says drinking water with even low levels of arsenic over many years is associated with diabetes and increased risk of cancers of the bladder, lungs, liver and other organs, and can contribute to heart and respiratory disease, reduced intelligence in children and skin problems. These effects can take years to develop.

The EPA’s enforceable limit for public water is 10 µg/L (0.010 mg/L), and its health goal is zero. Private wells are not regulated, so the 10 µg/L figure is the practical line most health departments use for well owners. Arsenic in drinking water is mainly a risk when you swallow it. Minnesota says the skin does not easily absorb arsenic, so showering, bathing, laundry, brushing teeth and watering gardens are fine unless the level is above 500 µg/L.

How to test

Use a state-certified laboratory. Ask for:

• Total arsenic. Labs usually report in µg/L (ppb). 10 µg/L is the same as 0.010 mg/L.
• Arsenic speciation (III and V), if the lab offers it. This tells you whether pre-oxidation is essential.
• Iron, manganese, pH and sulfate, which affect which arsenic treatment works.
• Total coliform bacteria and nitrate, the basic safety tests.

Minnesota recommends testing for arsenic at least once beyond the initial test when a well is built. If arsenic was detected in a new well, Minnesota suggests retesting about six months after construction, since levels can rise or fall in the first months. If arsenic was not detected in the first sample, Minnesota says it is unlikely to show up later.

The guide to reading a well water test explains units and detection limits. The test results tool builds a plan from your numbers.

If your result is X, do Y

Do not boil arsenic water to treat it. Minnesota warns that boiling concentrates the arsenic.

Arsenic III, arsenic V and why pre-oxidation matters

Groundwater carries arsenic in two forms, arsenic III and arsenic V, and a system that handles one form well may miss the other. Minnesota’s treatment comparison says that including pre-oxidation (chlorination, aeration or ozonation) with your treatment ensures it reduces both types, and it lists reverse osmosis, adsorptive media and anion exchange each “with preoxidation.”

In practice that means asking two questions before you buy:

1. Does my lab report show arsenic III, arsenic V or both?
2. Does the system include, or need, a pre-oxidation step?

If you cannot get speciation, assume some arsenic III is present and plan for pre-oxidation.

Treatment options

For most homes, arsenic is a drinking-water problem, and an under-sink reverse osmosis system at the kitchen tap is the simplest fit. SpringWell’s MOEN reverse osmosis system is one option; SpringWell lists arsenic among the contaminants it reduces. Be clear about two limits. First, RO is point-of-use, so it protects only the faucet it serves; above 500 µg/L you need more than that. Second, the product listing does not state how it handles arsenic III, so plan for pre-oxidation if your water has it, and test the treated water after installation. Minnesota recommends choosing treatment certified by NSF, UL or WQA for arsenic reduction. The reverse osmosis guide covers what to check.

Minnesota also points to other routes: a new well into a different water-bearing formation, connection to a public water system where available, or bottled water. Bottled water standards vary by type, so check arsenic levels on the label or with the bottler.

Order of treatment

If arsenic is one of several problems:

1. Sediment filter, if needed.
2. pH correction and iron, manganese or sulfur removal. See iron in well water.
3. Pre-oxidation, if arsenic III is present and not already handled by step 2.
4. Arsenic treatment at the kitchen tap or whole house.
5. UV, last, if needed.

Arsenic is often found with other natural contaminants. Wells in the same geology can carry uranium or radon; see uranium and radon in well water.

Maintenance

• Replace prefilters, membranes and media on the maker’s schedule.
• Test the treated water after installation and then at least once a year. Arsenic gives no warning when treatment fails.
• Retest the raw well water every few years, since levels can change.

When to call the health department

Call your state health department if arsenic is above 10 µg/L and children or a pregnant woman live in the home, if it is above 500 µg/L, or if you need help choosing treatment. Find your state’s well program under states.