Arsenic in Well Water


The EPA MCL for Arsenic

The United States Environmental Protection Agency sets a Maximum Contaminant Level (MCL) for contaminants in public drinking water. The MCL is the maximum concentration that is allowed for public water supplies. For arsenic, the MCL is 10 ug/L (which is the same as 0.01 mg/L). Arsenic in well water of private well owners is not regulated by the EPA. Home owners are advised to test and, when necessary, treat arsenic in their well water.


Naturally-occurring arsenic is commonly found in Maine well water. Contamination from non-natural sources of arsenic is extremely rare in Maine. Naturally-occurring arsenic exists in two forms, trivalent arsenic (also known as As (III) and “arsenite”) and pentavalent arsenic (also known as As (V) and “arsenate”). As (V) is the most common form found in Maine well water and is also the most easily removed from well water.


The only way to know if there is arsenic in your well water is to test. The State of Maine Center for Disease Control and Prevention recommends that home owners test their water every 3 to 5 years for arsenic. This is because the concentration can change over time.

We commonly hear from homeowners, “I tested my water for arsenic a few years ago and it was fine then.” The EPA’s MCL for arsenic used to be 50 ug/L but was changed to 10 ug/L a few years ago. This means that many wells that formerly had levels below the EPA MCL now have levels above the MCL.

In general, if you have arsenic levels in your water above 20 ug/L (the same as 0.020 mg/L) we recommend testing for both forms of arsenic prior to finalizing a treatment system. This is known as “speciation”. Some laboratories have special “speciation” sampling kits. Because a sample is exposed to oxygen the As (III) begins to oxidize and change to the As (V) form, it is important to properly sample the water on-site to accurately determine the amount of As (V) and As (III) in the water.


In general, the predominant exposure to arsenic is from drinking water and beverages made from the water.  Eating foods prepared with water containing high concentrations of arsenic also pose a risk. A much smaller level of exposure comes from dermal contact or accidental ingestion while brushing teeth or bathing. In most cases, properly treating a small amount of water for drinking and cooking virtually eliminates health risks due to arsenic exposure from the well water. Please call Air & Water Quality to discuss your test result and to determine the best treatment method.


Arsenic is well water
Kinetico K5 RO Drinking Water System

Reverse Osmosis (RO) -Reverse osmosis is a membrane technology that uses pressure to force water against a semipermeable  membrane. A portion of the water is forced through the membrane while leaving contaminants behind. The contaminants are then flushed down the drain with the remaining water. Reverse osmosis systems can be purchased to treat a small amount of water for drinking and cooking at a point of use (POU) or all the water at the point of entry (POE) to the house. Treatment with reverse osmosis  has a number of advantages including:

  • The high removal rate for both As (V) (nearly 100%) and As (III) (approximately 60% to 65%).
  • The system does not accumulate arsenic that might become a disposal issue.
  • The system performance can easily be monitored by the homeowner with a total dissolved solids (TDS) meter. Air & Water Quality installs TDS meters  on all of its  reverse osmosis systems.

 Arsenic Media –  This technique uses an iron-oxide media to adsorb arsenic. There are several manufacturers of this type of media and they all have similar capacities for arsenic removal. The arsenic media continues to remove arsenic until it has exhausted its ability to adsorb arsenic. Once exhausted, the media is replaced. These systems should be placed in a twin tank configuration. The two tanks should be placed in series. The first tank, called the lead tank, removes the arsenic and the second tank, called a guard tank, provides protection when the lead tank is exhausted. Regular testing of water from the lead tank is necessary to determine when the media is exhausted. When the lead tank is exhausted, the guard tank is put into the lead position. The exhausted lead tank with new media is placed in the guard tank position. Under the right water chemistry conditions and, with proper maintenance and testing this can be an effective way to treat arsenic. Under certain conditions, including high pH, high sulfate, silica and phosphate concentration, the media may not be practical because it can become saturated quickly. Maintenance required by the homeowner includes testing the lead tank effluent approximately every quarter and then arranging for replacement of the media when it becomes saturated.

Anion Exchange – This method is a water softener that has anion resin instead of cation resin. This system removes only As (V). Its main disadvantage is that the anion resin also dramatically lowers the pH of the water which can cause health and plumbing issues. This type of system can also bleed high concentrations of arsenic. We recommend anion treatment in commercially maintained systems only.

Water Resources