Nitrate
The most widespread diffuse chemical pollutant of groundwater in intensively farmed regions. Nitrate is highly soluble and moves readily with water, so it accumulates in aquifers over decades and responds only slowly to changes in land management.
Key facts
- Contaminant group
- Nutrient
- World Health Organization — guideline
- 50 mg/l as NO₃
- European Union — legal limit
- 50 mg/l as NO₃
- United Kingdom — legal limit
- 50 mg/l as NO₃
- United States — legal limit
- 10 mg/l as NNumerically equivalent to 44.3 mg/l as nitrate ion — a slightly stricter standard than the WHO guideline, not a laxer one.
Where it comes from
- Agricultural nitrogen fertiliser and manure
- Ploughing of grassland, which mineralises stored soil nitrogen
- Sewage effluent and septic systems
- Atmospheric deposition
How it behaves in water
Nitrate is an anion and is not retained by soil, so it leaches downward with drainage water. Travel time from the soil surface to the water table in a thick chalk aquifer can be decades, which means groundwater nitrate today reflects farming practice a generation ago. Reversal is correspondingly slow.
How it is measured
Measured as milligrams per litre, but the units differ between jurisdictions: as nitrate ion (NO₃) or as nitrate-nitrogen (NO₃-N). The conversion factor is 4.43, so 50 mg/l as NO₃ equals 11.3 mg/l as N. Comparing the two without converting is a common and serious error.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| World Health Organization | 50 mg/l as NO₃ | guideline | — |
| European Union | 50 mg/l as NO₃ | legal limit | — |
| United Kingdom | 50 mg/l as NO₃ | legal limit | — |
| United States | 10 mg/l as N | legal limit | Numerically equivalent to 44.3 mg/l as nitrate ion — a slightly stricter standard than the WHO guideline, not a laxer one. |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
The limit is set to protect bottle-fed infants against methaemoglobinaemia, in which nitrate reduced to nitrite impairs the blood’s oxygen carrying capacity. The standard is protective for infants and therefore comfortably protective for adults.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Ion exchange with nitrate-selective resin | Boiling, which concentrates it |
| 2 | Reverse osmosis | Activated carbon |
| 3 | Biological denitrification | Standard sediment filtration |
| 4 | Blending with a low-nitrate source, the most common utility response | Water softeners |
Occurrence
Concentrations in shallow groundwater beneath arable land in parts of eastern England, the Paris basin and the North European Plain routinely approach or exceed the limit, requiring blending or treatment before supply.
Relationships
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted