Nitrite (NO₂⁻) is a simple chemical compound that exists naturally in the environment. However, its presence in drinking water, even at low levels, signals a potential water quality problem. This compound is an intermediate step in the breakdown of nitrogen-containing substances and is typically unstable in oxygenated water. Elevated concentrations often indicate contamination from human or animal waste, or agricultural runoff. Nitrite is a serious concern for public health due to its ability to interfere with the body’s ability to transport oxygen, making its monitoring and control a significant priority for water systems worldwide.
Nitrite vs. Nitrate: Understanding the Nitrogen Cycle
Nitrite and its chemical cousin, nitrate (NO₃⁻), are both forms of nitrogen transformed through the natural nitrogen cycle. This cycle is a fundamental biogeochemical process where nitrogen compounds are converted by microbial actions in soil and water. Nitrite is the unstable intermediate product in nitrification, a two-step process where ammonia or ammonium is converted into more stable forms.
In the first step, specialized bacteria known as Nitrosomonas convert ammonia or ammonium into nitrite (NO₂⁻). Immediately following, a different group of bacteria, primarily Nitrobacter, rapidly oxidizes the nitrite into the more stable nitrate (NO₃⁻). Because this conversion happens quickly, nitrite is rarely found in high concentrations in healthy, oxygen-rich surface or groundwater. Its presence suggests a recent or incomplete conversion process, which can occur in water distribution systems or contaminated wells.
The chemical distinction is significant: nitrite contains two oxygen atoms bonded to a single nitrogen atom, while nitrate contains three. Nitrite is more toxic to humans and aquatic life than nitrate, leading regulatory bodies to set a much lower limit for it in drinking water. The detection of nitrite serves as a warning sign of ongoing pollution or a system imbalance.
Primary Sources of Nitrite Contamination
High levels of nitrite almost always originate from an excessive input of nitrogen-rich precursor compounds into the environment. Agricultural practices are a major contributor, as synthetic fertilizers and animal manure are rich in nitrogen that can be washed into surface water or leach into groundwater during heavy rainfall. This agricultural runoff introduces massive amounts of ammonia and ammonium, which then enter the nitrification process.
Leaking or malfunctioning private septic systems and municipal sewage lines are another common source of contamination. Human waste contains high levels of nitrogenous compounds that can infiltrate the surrounding soil and groundwater. If a septic system is poorly maintained or too close to a well, the resulting high concentration of nitrogen can lead to elevated nitrite levels.
Industrial wastewater discharge and the breakdown of organic materials also contribute to the problem. While natural decomposition of plant and animal matter releases nitrogen, this usually results in low, manageable levels. The excessive amounts introduced by human activity overwhelm the environment’s capacity to process these compounds, leading to an accumulation of both nitrite and nitrate.
Health Risks Associated with Nitrite Exposure
The most serious health risk linked to ingesting nitrite-contaminated water is a condition called methemoglobinemia, commonly known as Blue Baby Syndrome. This life-threatening disorder occurs when nitrite interferes directly with the function of hemoglobin in the blood. Nitrite oxidizes the iron atom within the hemoglobin molecule from its normal ferrous state (Fe²⁺) to the ferric state (Fe³⁺).
This change converts functional, oxygen-transporting hemoglobin into methemoglobin, which cannot effectively bind or release oxygen to the body’s tissues. The result is a lack of oxygen delivery, causing the affected individual’s skin and lips to take on a noticeable blue-gray tint. Infants under six months of age are especially vulnerable because their digestive systems allow a greater proportion of nitrate to be converted into the highly reactive nitrite.
Additionally, infants have a less active enzyme system to convert methemoglobin back to functional hemoglobin. While infants are the most susceptible, other vulnerable populations include pregnant women, the elderly, and individuals with certain enzyme deficiencies. Nitrite is also highly toxic to aquatic life, even at low concentrations, because it impairs the ability of fish and amphibians to take up oxygen from the water.
Testing, Regulation, and Removal Methods
The United States Environmental Protection Agency (EPA) has established a Maximum Contaminant Level (MCL) for nitrite in public drinking water systems at 1 milligram per liter (mg/L), measured as nitrogen. This standard is set to protect infants from methemoglobinemia. Public water systems must regularly test and report their levels, but private well owners are responsible for their own testing.
Homeowners can utilize home testing kits, but these are generally considered screening tools. For accurate results, a sample should be sent to a state-certified laboratory for analysis. Routine testing for nitrate is recommended, as high nitrate levels indicate a persistent nitrogen contamination problem and a high potential for nitrite presence.
If testing reveals levels exceeding the MCL, several treatment methods are effective for removal. Reverse osmosis (RO) systems, typically installed at a single tap, force water through a semi-permeable membrane that filters out the nitrite and nitrate molecules. Ion exchange systems operate similarly to a water softener, using a specialized resin to swap the nitrogen compounds for less harmful ions, like chloride. Distillation is also effective, as it boils the water and collects the purified steam, leaving the contaminants behind. Boiling water is not a solution; since nitrite does not evaporate, boiling only concentrates the contaminant as the volume of water decreases.