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Last Edited: 3/12/2012

Drinking Water Contaminants: Nitrate

For more information on nitrates, view the Agency for Toxic Substances and Disease Registry (ATSDR) Nitrate/Nitrite page and the Environmental Protection Agency's (EPA) Nitrates fact sheet.  View data on nitrate levels in drinking water for California.


What is nitrate and how does it get in the water?

Nitrate is the most common contaminant in groundwater aquifers worldwide. Nitrates are nitrogen-oxygen chemical units which can combine with various organic and inorganic compounds. They do not evaporate, do not bind to soils, are very soluble in water, and can easily migrate to ground water. Because they do not evaporate, nitrates are likely to remain in water until consumed by plants or other organisms.

Nitrate gets into drinking water from nitrate-containing fertilizers, sewage and septic tanks, and decaying natural material such as animal waste. As a result of human activities and population growth, nitrates are increasing in water resources. The greatest use of nitrates is in fertilizers.

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What are the health effects of nitrate?

Nitrate was first identified as a public health threat in drinking water in 1945 when high nitrate levels from private wells were shown to cause methemoglobinemia or “blue baby” syndrome in infants who received formula mixed with well water.  When an individual is exposed to nitrate from drinking water, it converts to methemoglobin in the body, preventing oxygen from being transferred to tissues.  Thus, nitrate can starve the body of oxygen and cause a condition called cyanosis, where the lips and extremities turn gray or blue.

Short term exposures in infants

Infants below the age of six months could become seriously ill from intake of water with a concentration higher than 10 milligrams per liter (mg/L) of nitrate. If untreated, these infants may die. In the body, nitrate converts to nitrite, which then interferes with the oxygen-carrying capacity of blood. Symptoms include shortness of breath and blueness of the skin. Symptoms can occur rapidly over a period of days. EPA set levels of 10 mg/L for total nitrate/nitrite, 10 mg/L nitrate, and 1 mg/L nitrite as drinking water standards.

Long term exposures

Long term exposure to high nitrate levels in drinking water has been found in some studies to be a risk factor for several types of cancer (gastric, colorectal, bladder, urothelial, brain, esophagus, ovarian, and non-Hodgkins lymphoma). However, other studies have found no association with cancer.

There is also some evidence to suggest that exposure to nitrates in drinking water is associated with adverse reproductive outcomes such as spontaneous abortions, intrauterine growth retardation, and various birth defects. However, some studies have found no association. As studies are not conclusive at this time, health standards are focused on protecting infants.

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How much nitrate is allowed in drinking water?

All systems are required to monitor for nitrate at each entry-point to the distribution system; however, the frequency of monitoring varies based on source water type and the level of nitrate / nitrite observed in past samples.

Nationally, nitrate has been regulated since 1977. In California, the regulation became effective in 1994. The MCL for nitrate has been set at 10 mg/L.  Currently, there are three equivalent MCLs for nitrate: nitrate measured as nitrogen at 10 mg/L; nitrite measured as nitrogen at 1 mg/L; and total nitrate/nitrite measured as nitrogen at 10 mg/L.  In California, nitrate is measured as NO3 and its MCL is 45 mg/L (which is equal to 10 mg/L measured as nitrogen). Sampling requirements for nitrates vary among various water systems.

Routine required monitoring is annual for surface water, once every three years for ground water, or quarterly if a sample exceeds half the MCL.  Nitrate is monitored once a year during the quarter which previously had the highest nitrate result. If a water system’s samples are less than 0.5 mg/L nitrite, the State specifies the frequency of additional monitoring. Initially a water system samples quarterly for at least a year.

For more information on monitoring requirements, see the EPA's guide to the standardized monitoring framework (PDF).  See Drinking Water Monitoring for more information on how water samples are collected.

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How do I know if there is nitrate in my drinking water?

If nitrate levels consistently exceed the maximum contaminant level, the water system must notify the public via newspapers, radio, TV, and other means.  Additional actions, such as providing alternative drinking water supplies, may be required to prevent risks to public health.  The following treatment methods have been approved by EPA for removing nitrates/nitrites: ion exchange, reverse Osmosis, and electrodialysis.

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