Air & Water Quality, Inc.

Air & Water Quality, Inc.

Arsenic In Well Water



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). While the EPA does not regulate private well water quality, home owners are advised to test and, when necessary, treat their well water for the contaminants regulated by the EPA.Naturally-occurring arsenic is commonly found in Maine well water. Contamination from non-natural sources of arsenic is xtremely rare in Maine. Naturally-occuring 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.

Testing recommendations

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.
Treatment recommendations

In general, exposure to arsenic from water comes predominantly from drinking water and beverages made from that water and eating foods prepared with that water. 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 us to discuss your test result and to determine the best treatment method.
Treatment techniques

  1. Reverse osmosis. Reverse osmosis is a membrane technology which uses pressure to force water across a membrane while leaving contaminants behind. The contaminants are then flushed down the drain. Reverse osmosis systems can be purchased to treat a small amount of water for drinking and cooking at the 'point of use' (POU) or all the water at the point of entry (POE) to the house. This form of treatment has a number of advantages including:
    • The highest removal rate for both As (V) (nearly 100%) and As (III) (approximately 60% to 65%).
    • The arsenic concentration in the waste water is low enough for it to be disposed of in the septic system or sewer.
    • It is easy to monitor the functionality of the reverse osmosis system by testing the Total Dissolved Solids (TDS) of the input and output water using a TDS meter. Air & Water Quality standardly installs a TDS meter with reverse osmosis systems.
  2. Arsenic media filtration. This technique uses an iron-oxide media to trap the arsenic. There are several manufacturers of this type of media and they all have similar chemical properties and ability to remove arsenic. In general, when using this technique, we recommend using twin tanks. The first tank does the work of removing the arsenic and the second tank is called a guard tank. Regular testing of the water coming out of the first tank is necessary to determine when the media in the first tank has become saturated. When the first tank is saturated, the second tank is put into the first position and the first tank is removed and a tank with new media is put into the second position. Under the right water chemistry conditions and with proper maintenance and testing this can be an effective way to treat for 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 water treated by the first tank approximately every quarter and then arranging for replacement of the media when it becomes saturated.
  3. Anion filtration. 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.

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