Nitrate is a chemical compound composed of nitrogen and oxygen that serves as a necessary nutrient for plant growth. While naturally present, elevated concentrations in drinking water are a significant public health concern. The primary risk is methemoglobinemia, or “blue baby syndrome,” which severely affects an infant’s ability to carry oxygen in the blood. Regulatory bodies like the Environmental Protection Agency set a Maximum Contaminant Level (MCL) for nitrate at 10 milligrams per liter (mg/L) as nitrogen, primarily to protect vulnerable populations.
The Natural Nitrogen Cycle and Baseline Levels
Nitrate exists naturally as an outcome of the planet’s continuous nitrogen cycle, a biological process involving the transformation of nitrogen compounds. The cycle begins with the decomposition of organic matter, such as dead plants and animal waste, releasing nitrogen as ammonia or ammonium. Specialized soil bacteria then convert this ammonium into nitrite, which other bacteria quickly oxidize into the more stable form of nitrate through nitrification. These natural processes mean water sources inherently contain some nitrogen, though natural background concentrations in groundwater are often less than 2 mg/L.
Agricultural Inputs: Fertilizers and Manure
The largest source of excess nitrate entering water systems globally comes from modern agricultural practices, specifically the use of synthetic fertilizers and the management of animal waste. Synthetic nitrogen fertilizers are manufactured to provide crops with concentrated, readily available nutrients. When these fertilizers are applied in excess of what the crops can immediately absorb, or when the timing of application is poor, the surplus nitrate remains highly soluble in the soil. This surplus nitrogen can eventually migrate out of the root zone and leach toward water tables.
Concentrated animal feeding operations (CAFOs) also generate enormous volumes of manure, which is rich in organic nitrogen compounds. This waste is often stored in large lagoons or spread onto fields as fertilizer, contributing a massive nitrogen load to the local environment. If manure application rates are not precisely managed based on the soil’s capacity and the crop’s uptake rate, the excess nitrogen converts to nitrate. This high-density input from both chemical fertilizers and organic waste overwhelms the soil’s natural ability to process or retain the compound.
Localized Sources: Septic Systems and Wastewater
Nitrate contamination can originate from localized sources, such as individual septic systems and municipal wastewater discharges. Household wastewater contains nitrogen compounds, largely from human waste and detergents. Bacteria in the septic drain field convert this nitrogen in the effluent into nitrate through nitrification. Although this process is part of the system’s design, the resulting nitrate is very mobile and is not efficiently removed by the surrounding soil. If a septic system is failing, old, or poorly sited, the nitrate-rich effluent can move directly into groundwater. Furthermore, even modern municipal wastewater treatment plants cannot remove all nitrogen, and the residual nitrate in the treated effluent contributes to the overall load when discharged into rivers and streams.
Pathways of Contamination: Runoff and Leaching
Once excess nitrate is introduced into the environment from any source, two primary physical mechanisms transport it into water bodies: leaching and surface runoff. Leaching is the process responsible for moving nitrate into groundwater, where water percolates downward through the soil profile. Because the nitrate ion is negatively charged and highly soluble, it does not bind to the negatively charged soil particles, allowing it to be carried easily with the infiltrating water. The rate of leaching is heavily influenced by soil type and water volume. Sandy soils, with their large pore spaces and high permeability, allow water to move quickly and carry nitrate rapidly to the water table.
Conversely, surface runoff occurs when the intensity of rainfall or irrigation exceeds the soil’s capacity to absorb the water. This excess water flows across the land surface, picking up dissolved nitrate and nitrogen-containing soil particles, and channeling them directly into nearby streams, lakes, and rivers.