Nitrate/Nitrite Rule

Health Concern

Infants less than six months old are the most vulnerable population to consuming high nitrate levels. In the human body, nitrates are converted to nitrites, which interfere with the oxygen carrying capacity of the blood. High levels of nitrates in drinking water have caused serious illness and sometimes death of infants less than six months old1. Symptoms of this condition, called Methemoglobinema, are shortness of breath and blueness of the skin.

For children and adults, long term exposure of nitrate above Maximum Contaminant Level (MCL) can lead to development of diuresis, increased starchy deposits, and hemorrhaging of the spleen.

Contaminant Source

Nitrate (NO3) and nitrite (NO2) are inorganic chemicals that are composed of nitrogen and oxygen. They are naturally found in soil, human and animal feces, and as natural deposits. Because nitrogen is a critical element for plant growth, nitrate is major component of synthetic fertilizer.


There are Public Water Systems (PWS) in Montana that have nitrate concentrations that have exceeded the MCL and half the MCL for nitrates.

Maximum Contaminant Level (MCL)

The MCL for nitrate is 10 mg/L.

The MCL for nitrite is 1 mg/L.

The MCL for Nitrate+Nitrite is 10 mg/L.

Protecting Your Water/Health

Nitrate is a water soluble molecule that can be carried with surface runoff into creeks, rivers, and lakes along with infiltrating into the soil and groundwater. Through these pathways both surface and groundwater can become contaminated with nitrate. Elevated nitrate levels in groundwater are an indication that surface water has a direct connection to groundwater.

Compared to other states in the USA, Montana has a lower risk of nitrate groundwater contamination.2 However, the source of nitrate contamination is often found locally (e.g. onsite leaking septic tank or neighboring agriculture field).

Preventing nitrate contamination of groundwater is an important element of source water protection. Blocking surface to groundwater pathways near the well can be done by ensuring the well casing and sanitary seal are secure and providing a 100 feet buffer around the well.

For those PWS that remove nitrate through a treatment process, ensuring the treatment system is functioning is vital for serving safe drinking water. Nitrates can be removed from water through reverse osmosis, ion exchange, and electrodialysis processes. All of these treatment processes require engineering design and persistent operations and maintenance to guarantee continual functionality.

Maintaining Compliance

The only way to know the nitrate level in water is to measure it through a laboratory analyzed sample. The foundation of the Safe Drinking Water Act is for PWS to sample their water as this is the most reliable way to know contaminate levels.

The level of nitrate concentration determines the frequency of nitrate (and nitrite) monitoring. PWS that have nitrate levels below the action level (<5 mg/L) are required to sample once per year. Whereas PWS that have historic nitrate samples above the action level (>5mg/L) are required to sample quarterly for at least four consequent quarters.

Understanding Your Samples

The required nitrate sample results are useful information for managing a PWS. It is recommended to graph all nitrate samples over time. This will highlight trends in nitrate levels. Nitrate levels can change over time in the same well. This can occur between wet and dry seasons, wet and dry years, and with changes in land use.

Nitrate Concentration Guide
The Nitrate Concentration Guide provides recommended action that PWS can take to proactively tackle nitrate contamination.

Example of nitrate level graph. Notice the increasing nitrate level over time.