Nitrates are common chemical contaminants found in groundwater supplies. These compounds are a form of nitrogen, an element naturally present in the environment and necessary for the growth of all living organisms, particularly plants. While nitrogen-containing substances naturally enter the water cycle, human activities can drastically increase concentrations in drinking water. Elevated levels of nitrates pose a specific public health risk, especially for vulnerable populations.
Defining Nitrates, Nitrites, and Nitrogen Cycling
The natural nitrogen cycle is a biological process that converts nitrogen between various forms. Bacteria convert organic nitrogen and ammonia from decaying matter into nitrite (\(\text{NO}_2^-\)). Other bacteria quickly convert this nitrite into nitrate (\(\text{NO}_3^-\)), which is the most stable and common form found in water sources. Nitrate is an inorganic ion and a major nutrient, explaining its widespread use in agriculture.
Nitrate itself is generally considered less toxic than nitrite. The danger arises because the human body, or bacteria within it, can reduce ingested nitrate back into the more reactive nitrite. This distinction is significant because nitrite is the compound responsible for most acute health effects associated with contaminated water. In healthy water systems, nitrate levels are typically below 1 milligram per liter, with higher concentrations indicating contamination from external sources.
Primary Sources of Water Contamination
Problematic nitrate concentrations primarily result from the widespread use of nitrogen-based agricultural fertilizers. These fertilizers, often containing potassium nitrate and ammonium nitrate, are highly soluble and do not bind to soil particles. Excess nitrate easily leaches through the soil profile, infiltrating the groundwater aquifer and contaminating wells. This diffuse pollution is particularly pronounced in areas with coarse soils and shallow water tables.
Human and animal waste management is another major source of contamination. Septic systems and wastewater treatment plants discharge effluent containing nitrogenous compounds that break down into nitrates. Poorly maintained septic systems in rural areas allow sewage to seep directly into groundwater. Additionally, concentrated animal feeding operations (CAFOs) generate large amounts of manure, contributing substantial nitrogen loading to the environment.
Contamination also occurs from non-agricultural sources, including residential lawn fertilizers and atmospheric deposition. Airborne nitrogen compounds from industrial emissions and automobiles are deposited on the land via precipitation. However, the most significant risk remains agricultural runoff and the proximity of private wells to septic systems.
Health Risks Associated with Exposure
The most serious health threat linked to nitrate-contaminated drinking water is methemoglobinemia, commonly known as “Blue Baby Syndrome.” This condition primarily affects infants under six months of age. When an infant ingests nitrate, bacteria in their gastrointestinal tract convert the compound into nitrite.
The nitrite enters the bloodstream and oxidizes the ferrous iron (\(\text{Fe}^{2+}\)) in hemoglobin, the protein responsible for oxygen transport. This reaction creates methemoglobin, an altered form of hemoglobin that cannot effectively bind or carry oxygen. Because methemoglobin also causes the remaining normal hemoglobin to hold onto oxygen more tightly, the tissues are starved of oxygen, leading to symptoms like a bluish discoloration of the skin, known as cyanosis.
Infants are uniquely vulnerable because their stomach chemistry is less acidic than an adult’s, favoring the growth of nitrate-reducing bacteria. Newborns also have lower levels of the protective enzyme, NADH-dependent methemoglobin reductase, which converts methemoglobin back to functional hemoglobin. This enzyme system is not fully active until about four months of age, allowing methemoglobin to build up quickly. Prolonged exposure in adults may also be associated with certain cancers and thyroid dysfunction.
Testing, Regulation, and Treatment Options
Since nitrate is colorless, odorless, and tasteless, contaminated water requires specific chemical analysis. Private well owners should have their water tested annually for nitrates, especially if they live in an agricultural area or have a shallow well. Testing is particularly relevant during heavy rainfall or snowmelt when contaminant leaching is more likely.
The Environmental Protection Agency (EPA) established the Maximum Contaminant Level (MCL) for public water systems. This enforceable standard is set at 10 milligrams per liter (mg/L) for nitrate, measured as nitrate-nitrogen, and is designed to protect infants. Although this regulation does not apply to private wells, the 10 mg/L limit is the recognized threshold for determining safe drinking water.
If testing reveals nitrate levels above this guideline, homeowners have several treatment options. Simple methods like boiling water are ineffective and should be avoided, as boiling concentrates the nitrates. Effective removal technologies include distillation, reverse osmosis (RO), and ion exchange.
Reverse osmosis (RO) systems are a common point-of-use solution, forcing water through a semi-permeable membrane to separate contaminants for drinking and cooking water. For treating water entering the entire home, an anion exchange system is effective, exchanging nitrate ions for chloride ions, similar to a water softener. Distillation also removes nitrates by heating water to steam and collecting the condensation, leaving non-volatile compounds behind.