Arsenic in Private Well Water

Arsenic is not noticeable in water because it has no taste or odor. Arsenic is naturally occurring in bedrock, therefore, wells drilled into bedrock may contain arsenic. Depending on local conditions it can leach from mineral deposits into groundwater. There are locations across Connecticut with private wells that have high levels of arsenic (see USGS report). Levels that have been found vary across the State.
The only way to find out if your well has high arsenic is to have your private well water tested. Homeowners should test their private well at least once for arsenic.

There are two predominant types of arsenic in groundwater: arsenic III (also known as arsenite) and arsenic V (also known as arsenate). These two types of arsenic have different characteristics. Arsenic III has no charge (neutral), and arsenic V has a negative charge. Laboratory test results will show you the total arsenic level present in your well water, which is the sum of arsenic III plus arsenic V.

Potential Health Effects of Arsenic in Private Well Water:

The U.S. Environmental Protection Agency (EPA) and expert scientific committees have classified arsenic as a human cancer-causing chemical. Research indicates that people living in areas where arsenic concentrations in drinking water are very high are more likely to have bladder, lung, liver or skin cancer. Long term exposure to high levels of arsenic in drinking water can also cause other health effects including cardiovascular and skin problems.

Sources of Arsenic in Private Well Water:

Arsenic is a naturally occurring metal that has no odor or taste. It can enter the groundwater supply from natural deposits in the bedrock. Arsenic can also contaminate groundwater from past or present industrial waste or agricultural pollution (i.e. fertilizers, pesticides, and herbicides).

To determine if arsenic is present in your private well water, arrange to have your well tested through a state certified laboratory. A current list of certified labs that are approved to test drinking water can be obtained from the DPH Environmental Laboratory Certification Program’s website. Be sure to discuss proper collection and handling procedures with the laboratory before sample collection. Follow the laboratory's instructions very carefully to obtain a good sample. Be sure to follow transportation, handling instructions, and holding times for the samples once they’ve been collected.

Testing Frequency:

As of October 1, 2022, testing for arsenic is required for all new private and semipublic wells. Refer to Connecticut General Statutes Section 19a-37 for more information. You should also test for arsenic when you buy a house with a private well or at least once if your existing private well has never been tested for arsenic. It is possible for arsenic levels in well water to fluctuate. If your well water has a detectable level of arsenic, it is a good idea to test every 5 years. If you have a treatment system to remove arsenic from your water, you should test the treated water at least annually to be sure your treatment system is working properly.

If Water Tests Indicate the Presence of Arsenic:

The EPA establishes standards for contaminants in drinking water, which are referred to as Maximum Contaminant Levels (MCLs). The MCL for arsenic is 0.01 milligram per liter (mg/L), which is equivalent to 10 micrograms per liter (µg/L). MCLs are enforceable for public water and newly constructed private wells. The CT DPH has set a health-based guideline (Action Level) for arsenic, which is the same as the MCL. Refer to the Action Level List for Drinking Water for more information. If your well water has arsenic at a level greater than the MCL and Action Level (0.01 mg/L or 10 µg/L) you should use an alternate source of water or install a treatment system.

Alternative sources of water may include bottled water or by connecting to a public water system, if it is available in your area. Because arsenic gets into your body primarily through ingestion (and not through the skin or through inhalation), the installation of a point-of-use treatment (POU) system at a tap frequently used for drinking and cooking is usually all that is needed. A water/ice dispenser on a refrigerator should also be considered as a location for POU treatment. However, if your well water has arsenic levels greater than 0.5 mg/L (500 µg/L), you should contact CT DPH for advice on treatment.
Even though some treatment technologies will reduce both forms of arsenic, their effectiveness at reducing arsenic III is significantly lower than arsenic V. If your total arsenic level is above the MCL and you are considering installation of a treatment system you should consider testing for arsenic III. There are out-of-state drinking water laboratories that offer this special testing. Please refer to the listing of out-of-state Commercial Environmental Laboratories on the Department of Public Health Environmental Laboratory Certification Program website.

Medical Testing for Arsenic:

Although there are medical tests that can measure the amount of arsenic in your urine, hair, and blood, results from these tests are difficult to interpret. The best way to find out if you are exposed to excessive amounts of arsenic from your drinking water is to test the private well water you drink.

Corrective Action and Treatment Options:

Removal of arsenic from well water can be a complicated process. If your water has high arsenic, you should consult with a knowledgeable water treatment professional. There are several treatment technologies that can remove arsenic from well water. These technologies include reverse osmosis, metal oxide (adsorptive) filters, and ion exchange. The chemical parameters of your well water and the chemistry of the arsenic present in your well water will dictate which treatment technology will most effectively remove arsenic for your situation. Arsenic III (arsenite) is harder to remove than arsenic V (arsenate). In most cases treatment needs can be met by installing a point-of-us treatment system on a water line supplying fixtures that are commonly used for drinking and cooking only.

Reverse Osmosis:

Reverse osmosis (RO) is effective at removing approximately 60% - 65% of arsenic III and 95% - 98% of
arsenic V. The percent removal of both forms of arsenic by RO can vary based on the RO manufacturer and other water quality characteristics, such as pH and mineral content. An oxidizing agent can be installed in front of the RO unit to convert Arsenic III to Arsenic V. This will increase the removal rate of both forms of arsenic. If you are installing a POU RO treatment system for arsenic removal you should consider either using specific oxidizing agents such as chlorine or permanganate to convert arsenic III to arsenic V, or testing for arsenic III to determine if this is necessary for adequate removal. This type of testing can be done through an out-of-state drinking water laboratory that offers the testing. Consult with the treatment manufacturer, your water treatment professional, or the CT Department of Public Health, Private Well Program for further detail or to discuss your options.

POU RO treatment devices are effective and are readily available; however, they do waste approximately 3-4 gallons of water for every gallon of water treated. Refer to our guidance on Reverse Osmosis Treatment of Private Well Water Systems for more information.

Metal Oxide Filters:

Metal oxide filters may be favorable because of the way arsenic binds to the media inside the filter and because in some cases metal oxide filters do not need to backwash. Metal oxide filters can be used to treat both at a POU or whole house (point of entry). As with RO, if arsenic III is present, pre-oxidation should be installed to extend the filter life and to make removal of arsenic III more effective. The filter and its ability to effectively remove arsenic can be affected by the pH and mineral content of your water. Consult with your treatment manufacturer or water treatment professional to determine if pre-oxidation or another pretreatment is necessary for your situation.

Ion Exchange:

Anion exchange is a type of ion exchange treatment that can be used for whole house (point of entry)
treatment of arsenic. It effectively removes arsenic V, but not arsenic III because it has no charge. Consult with the treatment manufacturer or your water treatment professional to determine which pre-oxidation or other pretreatment is required or recommended for your situation.

Backwash Wastewater Generated:

Ion exchange requires the filter media to backwash. Refer to the DPH Technical Standards for Subsurface Sewage Disposal, Section X - Water Treatment Wastewater, for more information on filter media backwash disposal.

Testing to Evaluate the Effectiveness of Treatment:

Once you've installed a treatment system, you should establish a routine testing schedule during the first year of operation to verify the effectiveness of the treatment system that was installed. Testing will also help to determine maintenance needs for your treatment media or filter. For information regarding purchasing treatment for your home, refer to our guidance on Questions to Ask When Purchasing Water Treatment Equipment for Your Home.

For More Information:

For health questions regarding arsenic in your private well water please contact the CT DPH Toxicology Program.

For all other questions please contact the CT DPH, Private Well Program.