Arsenic in Private Well Water FAQs

What you need to know about the possibility of arsenic in your well water.

Arsenic is a naturally occurring contaminant in some groundwater in Massachusetts, usually in bedrock aquifers in the central part of the state and in the Merrimack River Valley. Drinking water from bedrock wells, also called drilled or artesian wells, and from shallow or dug wells, may contain arsenic.

Table of Contents

Where does arsenic come from?

Arsenic (chemical symbol As) occurs naturally in soil and bedrock in parts of Massachusetts. During the 1800s there was commercial arsenic mining in New Hampshire, but since 1985 arsenic used in the U.S. has been imported. Activities that could have left arsenic residuals include:

  • apple orchard spraying
  • coal ash disposal
  • use of some pressure treated wood

Arsenic has no smell, taste, or color when dissolved in water, even in high concentrations, so only laboratory analysis can detect its presence and concentration.

How can arsenic affect my health?

Arsenic ingestion can result in both chronic (long-term) and acute (short-term) health effects.

Acute effects can include:

  • nausea
  • vomiting
  • neurological effects such as numbness or burning sensations in the hands and feet
  • cardiovascular effects
  • decreased production of red and white blood cells, which may result in fatigue

Chronic effects include:

  • changes in skin coloration
  • skin thickening
  • small corn-like growths, especially on hands and feet

Chronic exposure to arsenic is also associated with an increased risk of skin, bladder, and lung cancer. There is also evidence that long-term exposure to arsenic can increase risks for kidney and prostate cancer. The following factors determine health risks:

  • the concentration of arsenic in your water,
  • the amount of water you consume each day,
  • the length of time you have been consuming the water,
  • your dietary intake of arsenic (in the foods that you eat), and
  • your individual sensitivity to arsenic.

What is the regulatory standard for arsenic in drinking water?

The current drinking water standard, or Maximum Contaminant Level (MCL), from the U.S. Environmental Protection Agency (EPA) is 0.010 mg/L or parts per million (ppm). This is equal to 10 ug/L (micrograms per liter) or 10 ppb. In 2001, EPA reduced the regulatory MCL from 50 ppb to 10 ppb on the basis of bladder and lung cancer risks. Long term exposure to drinking water containing arsenic at levels higher than 10 ppb increases the chances of getting cancer; for lower levels the chances are less.

If your water has arsenic levels above 10 ppb, you should get drinking water from another source or install a home treatment device. Levels above 10 ppb will increase the risk of long-term or chronic health problems. The higher the level and length of exposure, the greater the risk. It is especially important to reduce arsenic water concentrations if you have children or are pregnant. Children are at greater risk because of their greater water consumption on a per unit body weight basis.

Pregnant women should reduce their arsenic exposures, because arsenic may occur in mother's milk and will cross the placenta, increasing exposure and risk for the fetus.

If your water has arsenic levels above 200 ppb, you should immediately stop drinking the water until you can either get water from another source or install treatment.

What about bathing/showering, or other uses?

Unless your arsenic level is over 500 ppb, showering, bathing and other household uses are safe. Arsenic is not absorbed through the skin and does not evaporate into the air.

Do I have arsenic in my water?

If you have a private well constructed in bedrock, you should have the water tested to determine if arsenic is present. For a searchable listing of MassDEP Certified Laboratories, see the link under "Additional Resources" below. The cost for a homeowner to test for arsenic may range from $15 to $30.

Where can I have my well water tested for arsenic?

The online database of certified labs is located here:

To use this tool, select "arsenic" in the box labeled "analyte" and "potable" from the dropdown list labeled "Matrix", then click "search". The results, including contact information, will appear at the bottom of the page. 

Some of the labs listed in the search result may not test water for individuals. Please contact the lab(s) before arranging for a test. See also "Find a Certified Laboratory for Water Testing", linked below.

Please note that inclusion in this list does not mean that MassDEP endorses any of these labs or their products. MassDEP certification simply means that they meet Massachusetts' regulatory criteria for testing water samples.

Additional Resources

What can I do if my water has high arsenic levels?

If the arsenic level in your well water is above 10 ppb, there are several treatment methods available. Before selecting a treatment method, consider these factors:

  1. There are two varieties, or species, of arsenic in water: "arsenic 3" and "arsenic 5." This is significant because "arsenic 3" is very difficult to remove from water and must be "oxidized" to "arsenic 5" before it can be removed. A laboratory can determine how much of each kind of arsenic is in your water, at extra cost. Ask the laboratory what they need for this process as it may need extra samples for them to test. Oxidants that can convert arsenic 3 to 5 include: liquid chlorine (bleach), hydrogen peroxide, and ozone. Chlorine is the most readily available oxidant for home water treatment.
  2. You must decide whether you want point-of-use (at the tap) treatment, installed under the kitchen sink with a special tap for drinking water, or whole house treatment (point-of-entry) that treats all the water entering the house.
  3. Other possible constituents in the water, such as iron and manganese, might hinder the effectiveness of arsenic removal and will need to be removed before the arsenic treatment. Arsenic removal methods or systems include anion exchange, reverse osmosis, activated alumina, and other types of adsorptive media filters. Each method has its advantages and disadvantages. Information on these treatment methods follows:
    • Anion exchange units operate using the same principle as a water softener. In this case, the arsenic is exchanged for chloride. The systems are generally used to treat water for the entire house and generally need little maintenance.
    • Reverse osmosis (RO) is a point-of-use treatment system and usually requires pre-filtration to remove sand and grit that might foul the system. RO is not effective at removing arsenic 3.
    • Both point-of-use and whole-house treatment systems use adsorptive filter media. There are several varieties of adsorptive media available, including activated alumina (AA) and other types, including some with proprietary ingredients. Activated alumina and most of the other adsorptive media will either not remove arsenic 3, or are not very efficient. The efficiency of removal depends on the pH of the water, and may need pre-treatment to adjust pH. For significant arsenic 3 removal, a typical installation would use a single adsorptive media cartridge with a pre-oxidation cartridge ahead of it. There is an ongoing cost of replacing the adsorptive media cartridges about every six months. You can discard used media as non-hazardous waste.
    • Point-of-use systems generally use reverse osmosis and adsorptive media. A recent survey by the New Hampshire Department of Environmental Services (NHDES) indicated that the median cost to install a point-of-use treatment system was $1,200, with median annual maintenance costs of $343. RO treatment systems are less expensive than adsorptive media systems.
    • Whole-house treatment systems generally use adsorptive media and anion exchange. The same NHDES survey indicated a median cost of $3,000 to install a whole house treatment system, with median annual maintenance costs of $550. Anion exchange treatment systems are generally less expensive than adsorptive media systems.

The NHDES survey did not include costs for the installation and maintenance of pre-oxidation treatment, which arsenic 3 concentration may require.

Who can I contact for more information or questions?

Contact Joe Cerutti of the Boston Drinking Water Program at 617-292-5859 or email

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