Arsenic in Water

The short answer

Arsenic is a natural element in rocks and soil. The Minnesota Department of Health explains that small amounts can dissolve into groundwater that people use for drinking water. Drinking water with arsenic over many years raises the risk of cancer and other serious health effects, so the useful question is how much is in your water.

Two numbers frame every arsenic result:

• The legal limit for public water is 0.010 mg/L^[1]. That is EPA's maximum contaminant level (MCL), in force since January 23, 2006^[1].
• The health goal is 0 mg/L^[1]. EPA's maximum contaminant level goal (MCLG) is a health target, not a legal limit.

Your utility must meet the MCL. A private well has no such duty: EPA says private wells are not regulated by the federal government under the Safe Drinking Water Act, nor by most state governments. Around 15 percent^[2] of the U.S. population relies on a private well, and nobody tests that well unless the owner does.

The only way to know your level is a test by a state-certified lab. Our arsenic water test guide explains who should test and how to take the sample, and our water testing cost guide shows verified lab prices. If the lab doesn't detect arsenic, no treatment is needed based on that result.

Arsenic levels and what to do

The table below lines up the federal arsenic benchmarks with what each kind of result means for your household. The values come from our versioned benchmark data, which records EPA's figures and the date we last checked them.

How the arsenic limit works

EPA defines the MCL as the highest level of a contaminant allowed in drinking water, set as close to the health goal as feasible using the best available treatment technology and taking cost into consideration. It applies to public water systems. For arsenic, the MCL and the health goal are far apart. The Minnesota Department of Health puts it plainly: drinking water with arsenic below the EPA standard over many years can still increase your risk of cancer. That is why the health goal is zero, and MDH notes that EPA's health goals do not take the cost of treatment into account.

So read your result in two steps. First, compare it with the MCL of 0.010 mg/L^[1]. A result at or above it is above the legal limit for public water. Second, remember that a result below the MCL is under the legal limit, not a sign that the water has no arsenic at all. Some households with a detected result below the MCL choose to retest and track it over time.

Check the units on your report

Arsenic numbers appear in more than one unit. The limit above is shown in milligrams per liter, while the Minnesota Department of Health, for example, writes the federal standard in micrograms per liter. A microgram is one thousandth of a milligram. Before you compare your number with the limit, check which unit your report uses. If you're not sure, ask the lab to give you the result in the same unit as the limit. Our guide on how to read your lab report explains the other units and terms you will see on it.

Health effects of arsenic in drinking water

Arsenic in drinking water is a long-term concern. EPA's drinking water regulations list the possible effects of long-term exposure above the MCL as skin damage or problems with circulatory systems, and an increased risk of getting cancer.

The Minnesota Department of Health adds detail from more recent research. It says drinking water with low levels of arsenic over a long time is associated with diabetes and with a higher risk of cancers of the bladder, lungs, liver and other organs. Contact with arsenic can also contribute to cardiovascular and respiratory disease, reduced intelligence in children, and skin problems such as lesions, discoloration and corns. MDH notes that these effects may not appear right away and can develop after many years.

ATSDR, the federal agency for toxic substances, reports that the Department of Health and Human Services and EPA have determined that inorganic arsenic is a known human carcinogen. It also lists some evidence that long-term exposure in children may result in lower IQ scores.

Arsenic is mainly a concern when you swallow it. MDH says your skin does not easily absorb arsenic, so everyday uses such as washing dishes and clothes, showering and bathing are not the main route of exposure. Drinking water and water used for cooking are.

A water test measures arsenic in your water, not in anyone's body. It can't tell you whether a person has been exposed or how much. If you have been drinking water with arsenic above the MCL, MDH suggests letting your medical provider know. If you are worried about your health, talk to a health professional and bring your lab report.

Arsenic in well water: where it comes from

Most arsenic in well water comes from the ground. EPA lists the sources of arsenic in drinking water as erosion of natural deposits, runoff from orchards, and runoff from glass and electronics production wastes. ATSDR adds that arsenic occurs naturally in soil and minerals and can get into water from runoff and leaching.

Two forms. ATSDR explains that arsenic combined with oxygen, chlorine or sulfur forms inorganic compounds, while arsenic in plants and animals combines with carbon and hydrogen to form organic compounds. The Minnesota Department of Health says inorganic arsenic is the type found in contaminated drinking water, and the most harmful type.

Human uses. Some arsenic in the environment comes from people. MDH notes that arsenic was an ingredient in some pesticides and was used as a wood preservative for decks, wood foundations and children's play structures. ATSDR says the arsenic wood preservative used for pressure-treated lumber is no longer used in the U.S. for residential uses.

Where it shows up in wells. The U.S. Geological Survey studied about 2,100^[5] domestic wells sampled in 1991–2004^[5]. Arsenic was above the EPA MCL in 6.8 percent^[5] of wells nationwide, and in more than 10 percent^[5] of wells in several aquifer types: crystalline-rock aquifers in New England, basin-fill aquifers in California, Nevada, Arizona, New Mexico, Texas and Nebraska, and the Snake River basaltic-rock aquifers in Idaho.

Your well, not your neighbor's. MDH says arsenic levels can vary between wells, even within a small area. In Minnesota, about 10 percent^[6] of private wells are above the EPA standard. A neighbor's result is a reason to test your own well, not a substitute for it.

Food counts too. MDH notes that for most people, food and water are the biggest sources of exposure to arsenic. Rice and cereal grains contain inorganic arsenic, while fish and shellfish mostly contain the less harmful organic form.

If you are on a well, arsenic is one test among several. Our well water testing guide covers the yearly well check, and you can build a test plan that fits your water source and household.

When to retest for arsenic

The Minnesota Department of Health recommends testing every well for arsenic at least once. It says that if tests do not find arsenic, further testing is not necessary. Plan to retest in these situations:

• To confirm a result before acting. If arsenic was detected at or above the MCL, retest with a state-certified lab before you decide anything. MDH's advice on treatment starts from a result that repeat sampling confirms.
• After a new well is drilled. MDH says test results taken when a new well is built may not measure long-term arsenic levels, and that the level may rise or fall in the first few months. If arsenic was detected in a new well, MDH suggests retesting about six months^[7] after construction.
• After work on the well. CDC lists replacing or repairing any part of your well system among the times to have your well water tested again. Test the new setup rather than relying on old results.
• After you install treatment. MDH says to test the treated water after installing a unit to make sure it works, and to keep up the maker's maintenance. A unit that isn't maintained loses its effectiveness over time.
• When your household changes. CDC also names the times when you or someone you live with becomes pregnant, or a child starts living with you.

If your earlier result showed no arsenic and nothing about the well has changed, there is no need to keep testing for it often. Keep the report as a baseline.

How to lower arsenic in your water

Start with a confirmed result from a state-certified lab. What you do next depends on what it shows, and "no treatment needed" is a real outcome.

If arsenic was not detected, no treatment is needed based on that result. Keep the report.

If arsenic was detected below the MCL, no treatment is needed based on the federal limit. Because the health goal is zero, some households still choose to act. The Minnesota Department of Health suggests considering a treatment unit or a different drinking water source if arsenic is detected at any level.

If a confirmed result is at or above the MCL, MDH recommends using a different source of drinking water or installing a treatment system that reduces arsenic.

In summary, MDH lists these kinds of units that reduce arsenic: reverse osmosis, distillation, adsorptive media and anion exchange, as well as oxidation and filtration systems. Which one fits depends on your water. MDH explains that arsenic in Minnesota groundwater comes in two types, and some water needs a pre-oxidation step so the unit removes the type present. Other parts of your water chemistry can affect the choice too, so MDH recommends working with a water treatment professional.

A few rules apply whatever you choose:

1. Choose a certified unit. MDH advises choosing a unit certified by NSF, UL or the Water Quality Association to remove the contaminant you are concerned about.
2. Decide how much water to treat. MDH describes point-of-use units, which treat water at one tap and are a good option for drinking and cooking water, and point-of-entry units, which treat all the water in your home.
3. Retest the treated water with a certified lab after installation, and again as the maker's maintenance schedule comes due.

Weigh the options against your confirmed result, not a strip kit reading.