Nitrates in water: origin, risks, standards, and treatment

Nitrates are nitrogen compounds naturally present in the nitrogen cycle, but their concentrations can increase significantly due to fertilizers, livestock effluent, wastewater, and sanitation facilities. Highly soluble, they migrate easily into groundwater. The main recognized health risk concerns infants, in whom the conversion to nitrites can cause methemoglobinemia.

Formula: NO₃⁻
Common origin: Agriculture
Sensitive population: Infants
EU value: 50 mg/L

What are nitrates?

Nitrates are ions with the formula NO₃⁻, consisting of one nitrogen atom and three oxygen atoms. They represent a highly oxidized and generally highly soluble form of nitrogen in the environment.

They are a natural part of the nitrogen cycle. Microorganisms transform organic nitrogen and ammonium into nitrites, and then into nitrates. This transformation, called nitrification, occurs primarily in soils and water containing oxygen.

Nitrates are essential for plant growth. However, their presence in water becomes a concern when it reflects an excess of agricultural inputs, sewage infiltration, or the vulnerability of the groundwater table.

Key point: nitrate and nitrate-nitrogen are not expressed using the same unit. 50 mg/L of nitrate corresponds to approximately 11.3 mg/L of nitrogen as nitrate.

How do nitrates fit into the nitrogen cycle?

Nitrogen circulates between the atmosphere, soil, plants, animals, water, and microorganisms. Several biological transformations determine the chemical form present in water.

Process	Transformation	Main conditions
Fixation	Atmospheric nitrogen to assimilable nitrogen	Bacteria, legumes, or industrial processes.
Ammonification	Organic nitrogen to ammonium	Decomposition of organic matter.
Nitrification	Ammonium to nitrite then nitrate	Oxygenated environment and microbial activity.
Assimilation	Nitrate to plant matter	Growth of plants and algae.
Denitrification	Nitrate to gaseous nitrogen	Low-oxygen environment and available carbon.

In well-oxygenated soils, nitrates are not easily retained by mineral particles because they carry a negative charge. They therefore migrate easily with infiltration water into groundwater.

What are the main sources of nitrates in water?

Nitrogen fertilizers

Mineral fertilizers provide ammonium, urea, or nitrate to crops. When inputs exceed plant needs or occur before heavy rains, some of the nitrogen can be leached into groundwater.

Livestock effluent

Manure, slurry, and animal waste contain a lot of nitrogen. Defective storage, excessive spreading, or poor timing with crop needs can increase losses into water.

Individual sanitation

Septic tanks and non-collective sanitation systems can release ammonium, which is then transformed into nitrate in the soil. Nearby, shallow, or poorly protected wells are particularly vulnerable.

Wastewater and sewage networks

Leaking pipes, insufficiently treated discharges, and wastewater infiltration can locally enrich groundwater and streams with nitrogen.

Natural organic matter

The mineralization of plants, crop residues, and soil organic matter naturally produces ammonium and then nitrates. This source can become significant after clearing land, tilling meadows, or changing land use.

Atmospheric deposition

Nitrogen oxides from combustion and agricultural ammonia can be transported in the atmosphere and then deposited onto soil and water.

Why is groundwater vulnerable?

Nitrates are highly soluble and poorly adsorbed by soil. They can pass through the unsaturated zone and reach a water table, sometimes years or decades after their surface application.

highly permeable sandy or gravelly soils;
low water table depth;
fissured or karst rocks;
heavy rain after fertilization;
high density of livestock or individual sanitation;
lack of winter plant cover;
poorly protected or poorly sealed collection points.

Some oxygen-poor aquifers can naturally denitrify a portion of the nitrates. This capacity is neither unlimited nor uniform and may produce other chemical changes, notably the mobilization of iron or manganese.

What are the effects of nitrates on surface water?

In rivers, lakes, estuaries, and coastal areas, nitrates contribute to nutrient enrichment. Along with phosphorus, they can promote excessive production of algae and aquatic plants.

Eutrophication

Eutrophication can reduce water transparency, alter habitats, encourage certain algal blooms, and cause oxygen depletion during biomass decomposition.

Dead zones

In marine and estuarine environments, excess nutrients can contribute to hypoxic zones where oxygen concentration becomes insufficient for many species.

Cost of water treatment

Contaminated collection sites sometimes require blending with water that has lower nitrate levels, relocating the site, or using specific treatment, all of which increase costs for municipalities.

What are the health effects of nitrates?

Nitrate is relatively unreactive. After ingestion, a portion can be converted into nitrite by bacteria in the mouth and digestive tract. Nitrite can then oxidize hemoglobin into methemoglobin.

Methemoglobinemia

Methemoglobin transports oxygen less effectively. A high proportion can cause a bluish skin discoloration, shortness of breath, fatigue, neurological issues, and, in severe cases, a life-threatening emergency.

Infants

Infants under six months are more sensitive due to a greater ease of converting nitrate to nitrite, more easily oxidizable hemoglobin, and an immature enzymatic capacity.

Gastrointestinal infections

A digestive infection can increase nitrite production and the risk of methemoglobinemia. Microbiological contamination of a well can therefore aggravate the risk associated with nitrates.

Water exceeding the applicable value should not be used to prepare infant formula. Boiling water does not reduce nitrates and may concentrate them.

Nitrates, nitrites, and N-nitroso compounds

Under certain conditions, nitrites can participate in the endogenous formation of N-nitroso compounds. Several of these compounds are carcinogenic in animals, which explains the scientific interest in chronic exposure.

The formation depends on many factors: quantity of ingested nitrate, presence of amino precursors, intake of vitamin C and antioxidants, digestive inflammation, microbiota, and other individual characteristics.

Vegetables often represent an important source of dietary nitrate but also provide vitamins, polyphenols, and fiber. The benefits of a diet rich in vegetables should therefore not be confused with the risk associated with contaminated water.

Epidemiological data on cancers and other chronic effects remain less conclusive than those regarding infantile methemoglobinemia. Regulatory values for drinking water remain primarily based on protecting infants.

Which populations are most sensitive?

infants under six months old;
babies fed with powder formulas reconstituted with water;
people suffering from disorders that promote methemoglobinemia;
people with severe gastrointestinal infections;
users of uncontrolled private wells;
people exposed simultaneously to certain medications or oxidizing products.

Pregnant women and individuals with chronic diseases should seek medical advice in the event of confirmed exceedance or significant exposure.

How to convert nitrate and nitrate-nitrogen?

Results can be expressed either as the mass of the nitrate ion NO₃⁻, or as the mass of nitrogen contained in this ion, denoted NO₃-N.

Expression	Conversion	Example
Nitrate to nitrate-nitrogen	Multiply by 14/62, i.e., 0.226	50 mg/L NO₃ ≈ 11.3 mg/L NO₃-N.
Nitrate-nitrogen to nitrate	Multiply by 62/14, i.e., 4.43	10 mg/L NO₃-N ≈ 44.3 mg/L NO₃.

A US limit of 10 mg/L expressed as nitrogen is close to 45 mg/L expressed as nitrate. It should not be compared directly to 50 mg/L without checking the unit.

What are the reference values for nitrates?

Organization or regulation	Value	Expression
World Health Organization	50 mg/L	Nitrate NO₃⁻.
European Union – Directive (EU) 2020/2184	50 mg/L	Nitrate, with combined nitrate/nitrite condition.
France	50 mg/L	Quality limit for drinking water.
Canada	45 mg/L	Equivalent to 10 mg/L of nitrate-nitrogen.
United States – EPA	10 mg/L	Expressed as nitrate nitrogen, i.e., approximately 45 mg/L of nitrate.
Australia	50 mg/L	Guideline value to protect infants.

In the European Union, the following condition must also be met: [nitrate]/50 + [nitrite]/3 ≤ 1, with concentrations expressed in mg/L of nitrate and nitrite.

How are nitrates in water analyzed?

Common methods include ion chromatography, spectrophotometry after reduction, ion-selective electrodes, and certain automated methods.

Ion chromatography

It separates and measures several anions simultaneously, notably nitrate, nitrite, chloride, sulfate, and fluoride. It offers good specificity and is suitable for drinking water analysis laboratories.

Spectrophotometry

Nitrate can be reduced to nitrite and then quantified by a colorimetric reaction. Interferences and the quality of reagents must be controlled.

Test strips and photometers

Rapid tests are useful for screening and monitoring, but any result near or above the limit must be confirmed by a competent laboratory.

Sampling

collect according to the laboratory's instructions;
avoid taps equipped with devices that do not represent the raw water;
indicate the source: network, spring, borehole, or well;
report any installed treatment;
adhere to storage conditions and analysis deadlines.

When should a well's water be analyzed?

A private well must be checked regularly, especially if it provides water for infants, is located in an agricultural area, or is near an individual sewage system.

upon commissioning the well;
before using the water for an infant;
after a flood or heavy rainfall;
after work on the water intake;
in the event of a change in nearby agricultural land use;
if the color, taste, or odor changes, even though nitrates themselves are invisible;
after installation or replacement of a treatment system.

A microbiological analysis is also important, as the same sources can contaminate the water with both bacteria and nitrates.

Which technologies reduce nitrates?

Anion exchange

Anion resins replace nitrates with other ions, generally chlorides. Selective resins may be preferred when sulfate levels are high.

The resin must be regenerated or replaced before saturation. Improper management can lead to nitrate breakthrough. The regeneration brine constitutes a concentrated waste stream.

Reverse osmosis

A suitable membrane can effectively reduce nitrates at the point of use. Pressure, membrane quality, pretreatment, and maintenance determine performance.

Electrodialysis

Electrodialysis uses an electric field and ion-exchange membranes to separate dissolved salts. It is primarily used at a municipal or industrial scale.

Biological denitrification

Bacteria transform nitrates into nitrogen gas under controlled conditions. The process may use an organic carbon source or hydrogen. A final treatment step is required to ensure microbiological quality.

Distillation

Distillation can retain nitrates, which are non-volatile compounds. However, it consumes energy and requires regular maintenance.

Water blending

Networks may blend water high in nitrates with water that has a lower load to ensure compliance with the limit. This solution does not eliminate pollution at the source.

Activated carbon and mechanical filtration

Standard activated carbon, sediment filters, and ceramics are generally not designed to retain dissolved nitrates.

Boiling

Boiling does not destroy nitrates. Evaporation may increase their concentration.

Comparison of treatment technologies

Technology	Reduction potential	Points of attention
Anion exchange	High	Competing sulfates, saturation, chlorides, and brine.
Reverse osmosis	High with validated membrane	Pressure, maintenance, water discharge, and post-membrane monitoring.
Electrodialysis	High	Cost, operation, and management of concentrate.
Biological denitrification	High	Operation, by-products, and microbiological safety.
Distillation	High	Energy, flow rate, and maintenance.
Standard activated carbon	Low or unproven	Do not extrapolate from chlorine or pesticide performance.
Mechanical filtration	Ineffective	Nitrate is dissolved.
Boiling	Ineffective	May concentrate nitrates.

How to evaluate a filter for nitrates?

The presence of a filter medium does not guarantee nitrate reduction. You must verify a specific test conducted up to the stated volume and in representative water.

Element to check	Why is it important?
Nitrate explicitly tested	Avoids extrapolation from other contaminants.
Unit used	Distinguishes mg/L of nitrate from mg/L of nitrate nitrogen.
Inlet concentration	Allows judgment of test difficulty.
Outlet concentration	Must remain below the applicable value.
Volume treated	Saturation can cause breakthrough.
Sulfates and chlorides	They influence ion-exchange resins.
Flow rate and pressure	They influence membranes and contact time.
End-of-life testing	An initial measurement does not demonstrate longevity.

The performance of a filtration system must be evaluated based on specific tests carried out by the manufacturer. In the absence of published results regarding this contaminant, no numerical reduction can be claimed.

View Monderma certifications and analyses

How to prevent nitrate contamination?

Agriculture

match inputs to the actual needs of crops;
split fertilization applications;
avoid spreading before heavy rainfall;
maintain plant cover in winter;
protect riparian strips and wetlands;
improve storage of livestock effluents.

Sanitation

maintain individual installations;
respect distances between wells and sanitation systems;
repair leaking networks;
monitor wastewater treatment plant discharges.

Water source protection

Protection zones, agronomic monitoring, and reducing pressures at the source are often more sustainable solutions than treating the water alone.

Frequently asked questions about nitrates in water

What is the nitrate limit in France?

The quality limit is 50 mg/L expressed as nitrate.

What is the WHO value?

The WHO maintains a guideline value of 50 mg/L for nitrate.

Why do the United States indicate 10 mg/L?

The American value is expressed as nitrate nitrogen. It corresponds to approximately 45 mg/L of nitrate.

Do nitrates have a taste or odor?

No. Clear and pleasant water may exceed the limit.

Why are infants more sensitive?

Their physiology is more prone to the formation of methemoglobin, which carries oxygen poorly.

Can I prepare a baby bottle with water above 50 mg/L?

No. You must use compliant water and follow local health recommendations.

Does boiling the water eliminate nitrates?

No. Evaporation may even increase their concentration.

Does an activated carbon carafe remove nitrates?

Standard activated carbon is generally ineffective against dissolved nitrates.

Does reverse osmosis remove nitrates?

Yes, a validated and properly maintained membrane can achieve significant reduction.

Does a water softener remove nitrates?

A classic cationic softener is not designed for this. A suitable anion resin is required.

Do nitrates come only from fertilizers?

No. They can also come from livestock effluents, wastewater, septic tanks, and natural processes.

Do nitrates disappear over time?

Not necessarily. They can persist for a long time in aquifers and reflect older inputs.

What is the difference between nitrate and nitrite?

Nitrate is more oxidized and generally more stable. Nitrite is more reactive and directly responsible for the formation of methemoglobin.

Is a quick analysis sufficient?

It is useful for screening, but a result near the limit must be confirmed in a laboratory.

Should bacteria also be analyzed?

Yes for a well, as a sanitation source can contaminate water with both nitrates and microorganisms.

Are vegetables rich in nitrates comparable to contaminated water?

No. Vegetables also provide protective nutrients, and their overall benefit remains recognized.

What to do if the limit is exceeded?

Use compliant water, confirm the result, search for the source, and install a validated treatment if necessary.

Is a concentration of 10 mg/L always low?

It depends on the unit. 10 mg/L of nitrate is low, but 10 mg/L of nitrate nitrogen is equivalent to approximately 45 mg/L of nitrate.

Scientific and administrative sources

World Health Organization – Nitrate and nitrite
Consult the WHO chemical fact sheet
WHO – Guidelines for drinking-water quality, 2026 edition
Consult the WHO guidelines
European Union – Directive (EU) 2020/2184
Consult the directive on EUR-Lex
Health Canada – Nitrate and nitrite
Consult the technical document
Environmental Protection Agency – Drinking water standards
Consult EPA standards
Monderma – Certifications and laboratory analyses
View Monderma certifications and analyses

Contaminants and associated parameters

Nitrites Ammonium Fluorides Chlorides Sulfates Manganese Iron Arsenic

Health warning: this sheet is provided for informational purposes. It does not replace an analysis performed by a laboratory, the recommendations of the competent health authority, or the advice of a healthcare professional. Water exceeding the applicable value should not be used to prepare baby bottles.