Nitrates (NO3-) are a common contaminant in water supplies, particularly in groundwater. Nitrogen is necessary for all life, but when its compounds accumulate in water, they can pose a health risk. The U.S. Environmental Protection Agency (EPA) regulates the presence of nitrates in drinking water, setting the Maximum Contaminant Level (MCL) at 10 milligrams per liter (mg/L) measured as nitrogen.
Agricultural Sources and Practices
The extensive use of synthetic nitrogen fertilizers represents the largest human-influenced source of nitrate contamination. Farmers apply these fertilizers, such as potassium nitrate and ammonium nitrate, for optimal crop yields. The nitrogen in these products is highly water-soluble, making it readily available for plant uptake.
However, a significant portion of this applied nitrogen is not absorbed by the crop, especially if fertilizer is applied in excess or at times when plants are not actively growing. Studies estimate that around 30% of nitrogen applied to crops can be lost to the environment through leaching or runoff. This excess nitrogen is left in the soil, where it is easily converted to nitrate and becomes mobile.
Concentrated Animal Feeding Operations (CAFOs) and the widespread application of animal manure are a significant agricultural source. Manure contains high concentrations of organic nitrogen, which converts quickly to nitrate in the soil. The sheer volume of waste produced at CAFOs often exceeds the capacity of the local land to absorb it effectively.
When manure is spread on fields, particularly on frozen ground or immediately before heavy rainfall, a substantial amount of the nitrogen can run off the surface. This concentrated waste is a major contributor to both surface water and groundwater contamination in agricultural regions.
Waste Disposal Systems and Infrastructure
Human and domestic animal waste disposal systems are a major source of nitrogen compounds. Septic systems, which treat wastewater in areas without centralized sewage, release effluent into a drain field. This effluent contains high levels of ammonium, derived from urine and feces, with concentrations typically ranging from 50 to 60 mg/L.
Aerobic bacteria in the drain field perform nitrification, converting ammonium into nitrate. While a properly functioning system can reduce some nitrogen, the resulting nitrate is highly mobile. It can easily flow through the soil to contaminate groundwater, especially if the system is old, poorly maintained, or sited too close to a well.
Centralized municipal sewer systems also contribute to nitrate contamination through leaks and discharges. Cracks or breaks in aging sewer pipes can allow raw sewage to seep into the surrounding soil and groundwater. Furthermore, treated wastewater discharged from municipal treatment plants, while significantly cleaner, still contains residual nitrate that enters surface waters.
Natural Soil Processes and Atmospheric Input
Nitrate is a naturally occurring compound, and a baseline level exists in the environment even without human activities. This background level is primarily established through the decomposition of organic matter. This process, called mineralization, releases organic nitrogen as ammonium.
Soil microbes then convert this ammonium to nitrate through a two-step process known as nitrification, which occurs readily in warm, moist, and well-aerated soil. Natural background concentrations in groundwater are low, often less than 3 mg/L.
Atmospheric deposition also contributes to the nitrate load in water systems. Nitrogen oxides released into the air from sources like vehicle exhaust and industrial emissions can be converted into nitric acid. This acid is then carried down to the earth’s surface and into water bodies by rain and snow, a process commonly referred to as acid rain.
How Nitrates Move into Water Supplies
Nitrate is an anion, meaning it carries a negative electrical charge. Soil particles, particularly clay and organic matter, also typically carry a negative charge, causing them to repel the nitrate ion.
This repulsion means that nitrate is not readily bound to the soil and is highly soluble in water, making it exceptionally mobile. The primary pathway for groundwater contamination is leaching, the downward movement of the dissolved nitrate as water percolates through the soil profile. This process is particularly efficient in areas with sandy, porous soils or shallow water tables, allowing the contaminant to quickly reach the aquifer.
The second major pathway is surface runoff, where water carries nitrate across the land surface into streams, rivers, and reservoirs. This occurs when rainfall or snowmelt washes over saturated or frozen ground, picking up excess fertilizer or manure before draining directly into surface water bodies.