Arsenic Rule


Arsenic occurs naturally in soil and rock and can dissolve into groundwater. For most people, food and water are the biggest sources of exposure to arsenic. There are two forms of arsenic:

  • Inorganic arsenic is the type found in contaminated drinking water, and is the most harmful type of arsenic. It is also found in rice, cereal grains, and other foods. 
  • Organic arsenic is the most common type of arsenic found in food. It is common in fish and shellfish and is less harmful to health than inorganic arsenic. Some arsenic in the environment comes from human activity. Arsenic was an ingredient in some pesticides and was used as a wood preservative for wood foundations, decks, and children's outdoor play structures.


Both community water systems (CWSs) and non-transient, non-community water systems (NTNC) are required to reduce the arsenic concentration in their drinking water systems to 10 µg/L.
The arsenic rule also clarifies compliance requirements for inorganic contaminants (IOC), volatile organic contaminants (VOC), and synthetic organic contaminants (SOC).

Health Effects

Drinking water with low levels of arsenic over a long time is associated with diabetes and increased risk of cancers of the bladder, lungs, liver, and other organs. Arsenic can also contribute to cardiovascular and respiratory disease, reduced intelligence in children, and skin problems, such as lesions, discoloration, and the development of corns.  Health impacts of arsenic may take many years to develop, especially if you are in contact with arsenic at a low level over a long time.


Maximum Contaminant Level (MCL)

The MCL for arsenic is 10 µg/L. The map above shows the areas where arsenic is over the MCL in Montana.

Maintaining Compliance

Compliance is achieved by monitoring quarterly, annually or one sample every three years.  If a sample exceeds the MCL, quarterly monitoring is triggered for four quarters.  If the running annual average for four quarters is reliably and consistently below the MCL, the system will qualify for reduced monitoring.


Diane Jordan
Chemical and Radiological Rule Manager
PO Box 200901
Helena, MT 59620
Phone: (406) 444-6741
Fax: (406) 444-1374
DEQ: (406) 444-4400