Nitrites (\(\text{NO}_2^-\)) are chemical compounds existing in water as a negatively charged ion, formed by one nitrogen atom bonded to two oxygen atoms. They are a natural part of the global nitrogen cycle, but elevated concentrations in drinking water indicate contamination and pose a health concern. Nitrites are highly reactive and less stable than nitrate, making their accumulation a warning sign of disrupted environmental processes or pollution. Understanding the origins and behavior of this compound is paramount for safeguarding public water quality.
The Role of Nitrites in the Nitrogen Cycle
Nitrites function as a brief, intermediate step in the natural process known as the nitrogen cycle. The cycle begins with organic nitrogen, such as ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)), which is a byproduct of decaying matter and biological waste. Specialized microorganisms then perform a two-step biological oxidation process called nitrification, which requires dissolved oxygen.
The first stage involves ammonia-oxidizing bacteria, like Nitrosomonas, converting ammonia into the nitrite ion (\(\text{NO}_2^-\)). This initial step, nitritation, can sometimes limit the rate of the entire process. Immediately following this, nitrite-oxidizing bacteria, such as Nitrobacter, rapidly convert the nitrite into the more stable nitrate ion (\(\text{NO}_3^-\)).
Nitrate is the final oxidized form of nitrogen and is utilized by plants, completing the natural cycle. Since the second step of nitrification is usually efficient in oxygen-rich surface water, high concentrations of nitrite are unexpected in healthy environments. The presence of nitrite levels suggests that the microbial conversion process has been interrupted or that the water is contaminated by a point source.
Primary Sources of Water Contamination
The accumulation of nitrites in water sources stems from human activities that overwhelm the natural nitrogen cycle, usually by introducing excessive ammonia or nitrate. Agricultural runoff is a major non-point source, as fertilizers and animal waste are washed into surface water and seep into groundwater. While most nitrogen converts to nitrate in aerobic soil, anoxic (low-oxygen) conditions in deeper aquifers or saturated soils can reduce stable nitrate back to nitrite before denitrification to harmless nitrogen gas is complete.
Poorly maintained or failing septic systems and sewage leaks also introduce high concentrations of nitrogen-rich organic waste and ammonia directly into the soil and groundwater. In a properly functioning septic field, ammonia should undergo full nitrification to nitrate. However, if the system is overloaded or the surrounding soil lacks sufficient oxygen, the process stalls. This incomplete nitrification leads to the accumulation of nitrite in the groundwater supply.
Industrial wastewater discharge and municipal water distribution issues also contribute to nitrite contamination. Wastewater treatment plants using nitrification processes may discharge effluent with elevated nitrite if the balance of oxygen, temperature, or pH is disturbed, leading to incomplete conversion. In public water systems using chloramines for disinfection, the decay of chloramine can release ammonia. This ammonia initiates nitrification within the pipes, resulting in nitrite spikes in the distributed drinking water.
Health Risks Associated with Nitrite Exposure
The most severe health risk from consuming nitrite-contaminated water is methemoglobinemia, or “Blue Baby Syndrome,” affecting infants under six months of age. Infants are vulnerable because their less acidic stomach environment allows bacteria to convert ingested nitrate into nitrite. They also possess fetal hemoglobin, which is more susceptible to oxidation, and have lower levels of the enzyme needed to convert methemoglobin back to functional hemoglobin.
The mechanism of toxicity involves nitrite entering the bloodstream and interacting with hemoglobin, the protein responsible for transporting oxygen. Nitrite oxidizes the iron atom within the hemoglobin molecule from its ferrous state (\(\text{Fe}^{2+}\)) to the ferric state (\(\text{Fe}^{3+}\)), forming methemoglobin. Since methemoglobin cannot bind oxygen, increasing concentrations cause the infant’s blood to lose its capacity to carry sufficient oxygen to the tissues.
Symptoms include a bluish or grayish discoloration of the skin, particularly around the mouth, hands, and feet, due to the lack of oxygen. Untreated, severe methemoglobinemia can lead to brain damage, coma, and death. While adults are less susceptible due to robust enzyme systems, high exposure can still cause symptoms like rapid heart rate, low blood pressure, and reduced oxygen-carrying capacity.
Testing and Removal Methods
Protecting against nitrite contamination begins with vigilant testing, especially for households relying on private wells, which are not subject to mandatory regulations like public supplies. The U.S. Environmental Protection Agency (EPA) established a Maximum Contaminant Level (MCL) for nitrite in public drinking water at 1 milligram per liter (mg/L), measured as nitrogen. This enforceable standard is set to prevent adverse health effects, particularly methemoglobinemia.
Water testing is typically performed using colorimetric methods, such as the Griess assay, which reacts with nitrite to produce a visible color change proportional to the concentration. More precise laboratory analysis uses ion chromatography, which is accurate and can measure both nitrite and nitrate simultaneously. Regular testing is the only reliable way for well owners to ensure their water meets the safety standard.
For water sources confirmed to have high nitrite levels, several effective treatment technologies are available for removal at the point of use.
Treatment Technologies
Reverse osmosis (RO) systems use pressure to push water through a semipermeable membrane that filters out nitrite ions. Distillation is a viable option, involving boiling the water and collecting the purified steam, leaving the nitrite behind. Ion exchange units utilize a resin to swap nitrite ions for less harmful ions like chloride, providing an efficient method for remediation.