The use of arsenic as a poison is widely documented. As a result, many people are alarmed when they hear that their drinking water, either from a public or private water system, may contain an amount of arsenic.

What do you do if your water contains arsenic, and can it be removed? This Q&A addresses these questions and more.

Arsenic in Well Water: What You Need to Know

Table of Contents

What is arsenic?

Arsenic is a semi-metallic element with the chemical symbol “As”. It occurs naturally in rocks, soils, and waters that come in contact with these rocks and soils. Arsenic in solution is odorless and tasteless.

It most commonly occurs in combination with other elements in minerals such as arsenopyrite (FeAsS), the most common and widespread mineral form. As-mineral forms can strongly bind to and coprecipitate with iron mineral forms, both sulfides such as FeS and Fe oxides such as ferrihydrite. Both are abundant in groundwater and wells.

Arsenic can combine with other elements to form inorganic and organic arsenicals. Some of these have been used as antibiotics and pesticides. In general, inorganic derivatives are regarded as more toxic than the organic forms. While food can contain both inorganic and organic arsenicals, primarily inorganic forms are present in water.

Exposure to arsenic at high levels poses potential serious health effects as it is a known human carcinogen, or cancer-causing agent. It also has been reported to affect the vascular system in humans and has been associated with the development of diabetes.

Arsenic enters the human body principally through the mouth, according to the Centers for Disease Control and Prevention (CDC), and inhaled arsenic also is absorbed through the lungs into the blood stream. “Small amounts of arsenic may enter the body through the skin, but this is not usually an important consideration,” reads the CDC’s Public Health Statement on arsenic.

Is my private water well at risk of arsenic contamination?

Exposure to arsenic in drinking water has been identified as a health concern in regions of the United States where bedrock contains unusu­ally high levels of arsenic, such as areas of New Hampshire, Maine, Michigan, Ohio, and regions in the Southwest and the Rockies.

Your state’s Department of Natural Resources, Department of Environmental Protection or similar agency, or Geological Survey Office may have information on any areas of your state that may be prone to the presence of high levels of arsenic. As may occur in any classification of well, private or otherwise.

Private well water should be tested annually for bacteria, nitrates, and anything else identified that should be of concern to the well owner, such as arsenic. Authoritative test­ing should be conducted by a certified drinking water laboratory. Visit to find your state drinking water testing laboratory certification officer who can provide a list of certified labs in your area, or call the Private Well Owner Hotline at (855) 420-9355.

If you do have an arsenic level in your water that is higher than is safe or tolerable to you, there are water treatment technologies available to address the problem.

How can I remove arsenic from well water? What treatment solutions are available?

NSF International is a not-for-profit organization that develops stand­ards, product testing procedures, and certification services for products including water treatment devices. NSF has certified point-of-use reverse osmosis and distillation devices for the reduction of arsenic in drinking water. Pretreating water through chlorination or oxidation and filtration may be necessary to make reverse osmosis devices effective for arsenic re­moval.

For more information or a list of NSF-certified devices, contact the organization at (800) 673-8010, or visit the Web site at

Because of arsenic’s tendency to bind to and precipitate with iron oxide minerals, water treatment installed specifically to oxidize and filter out iron can also remove arsenic to safe levels. Cartridge filters are not sufficient for this purpose. If iron is being removed with water treatment filtration, a treatment device specific to arsenic removal may not be needed. Where iron is present along with arsenic, As levels may fluctuate up and down with Fe content, effective Fe removal is highly recommended upstream of arsenic removal devices such as RO.

Some of the treatment technologies may not be amenable to point-of-entry, or whole-house, treatments. In these cases, point-of-use units, which treat water at the tap, may be the best option.

Following installation of a treatment device, water quality should again be tested to verify the operation of the device. After that, water should be tested at least annually to confirm treatment effectiveness. A maintenance agreement for such devices is highly recommended.

Again, since water quality varies greatly, be sure to have your water tested and consult a local water professional for advice before purchasing a water treatment system.

 How does arsenic get into well water?

A primary source of arsenic to drinking water wells is from water flowing through arsenic-rich rocks and soil. It can be further released into the environment through natural activities such as volcanic action and forest fires, as well as through human actions.

Arsenic is used in paints, dyes, metals, drugs, soaps, and semiconductors, as well as pesticides around the property. Agricultural applications, mining, and smelting also contribute to arsenic releases in the environment. These can enter the groundwater system by gradually moving with the flow of groundwater from rains, melting of snow, etc.

Biofouling build up in wells may collect and concentrate low levels of As as it binds to Fe sulfides and Fe oxides, and release it into the water system as biofilm-bound minerals slough off well or pump surfaces.

Testing water for arsenic in areas where arsenic is a concern is an important strategy for private water well owners to safeguard the health and well-being of their family. Working with a water professional to monitor and maintain the quality of the well and water supply is an important responsibility of the private water system owner.

Your groundwater contractor or a hydrogeologist familiar with your area are your central source of information about caring for your system.

What is the measurement of arsenic?

The U.S. EPA established the current maximum contaminant level (MCL) for arsenic, 10 micrograms per liter (or parts per billion). The EPA does not regulate private water wells, but its drinking water rules provide a good standard by which to measure your water quality.

What are the symptoms of overexposure to arsenic?

Observable symptoms of arsenic poisoning are thickening and discol­oration of the skin, stomach pain, nausea, vomiting, diarrhea, numbness in hands and feet, partial paralysis, and blindness.

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