ARGOS

NITRATE CONTAMINATION

Nitrate in groundwater: the 10 mg/L standard, agricultural sources and well risk

Why nitrate is one of the most widespread groundwater contaminants, what the 10 mg/L drinking water standard means, how fertilizer, manure and septic systems drive it, and which wells are most at risk.

By Argos Tellus · Updated

KEY TAKEAWAYS

  • The federal drinking water standard (MCL) for nitrate is 10 mg/L measured as nitrogen; for nitrite it is 1 mg/L as nitrogen.
  • USGS found nitrate above 10 mg/L in more than 20% of shallow domestic wells (less than 100 feet below the water table) in agricultural areas during its 1992–2004 national assessment.
  • Most nitrate in groundwater comes from nitrogen fertilizer, animal manure, and septic systems; it is highly soluble and moves readily with recharge.
  • Private domestic wells are the most exposed: they are shallow, near sources and not regulated, while public-supply wells are tested and treated.
  • Because nitrate travels with groundwater, today’s concentrations reflect land use years to decades ago — so trends, not single samples, show where a well is heading.

What nitrate is and why it matters

Nitrate (NO₃⁻) is a form of nitrogen that plants use as a nutrient. Applied nitrogen that crops don’t take up — from fertilizer or manure — can convert to nitrate in soil, where it dissolves easily and leaches down to the water table with rain and irrigation water.

In drinking water, high nitrate is a risk mainly for infants: it can reduce the blood’s ability to carry oxygen (methemoglobinemia, or “blue baby syndrome”). That is the basis of EPA’s maximum contaminant level of 10 mg/L as nitrogen. Utilities that exceed it must treat or blend water; private well owners are responsible for their own testing.

ContaminantFederal MCL
Nitrate (as N)10 mg/L
Nitrite (as N)1 mg/L

Where nitrate in groundwater comes from

  • Nitrogen fertilizer on cropland — the largest source in many agricultural regions.
  • Animal manure — from feedlots, dairies and land application; concentrated animal operations can create local hotspots.
  • Septic systems — dense rural or suburban septic use can raise nitrate in shallow aquifers.
  • Wastewater and biosolids land application, and some natural geologic sources.

Which wells are most at risk

USGS’s national assessment of nutrients in streams and groundwater (1992–2004) found nitrate concentrations highest in shallow groundwater beneath agricultural land. More than 20% of shallow domestic wells in agricultural areas exceeded the 10 mg/L standard. Public-supply wells, which are typically deeper and regularly tested, exceed it far less often.

  • Shallow wells in permeable, well-drained soils under cropland or livestock operations.
  • Older or poorly sealed wells near septic systems, barnyards or fertilizer storage.
  • Aquifers with oxygen-rich groundwater, where nitrate is not naturally broken down (denitrified).

Why nitrate trends lag land use

Water infiltrating today can take years to decades to reach a well, depending on depth and aquifer properties. Nitrate measured now often reflects farming practices of the past, and improvements at the surface take years to show up in wells. Conversely, deeper public-supply wells may see rising nitrate in the future as older, cleaner water is replaced by younger recharge.

That lag is why nitrate is best managed with long monitoring records: concentration trends per well, compared against the standard, show which wells are approaching 10 mg/L before they exceed it — the same forecasting logic used for other groundwater contaminant plumes.

Reducing nitrate risk

  • Nutrient management — matching fertilizer and manure rates and timing to crop needs.
  • Manure and wastewater controls at livestock operations, including lined storage and setback distances.
  • Well construction and siting — sealed casings, distance from septic systems and feedlots, and deeper completions where appropriate.
  • Treatment for affected supplies — ion exchange, reverse osmosis or biological denitrification, or blending with low-nitrate water.
  • Regular testing of private wells, especially in agricultural areas.

Frequently asked questions

What is the safe level of nitrate in drinking water?

The EPA maximum contaminant level is 10 mg/L of nitrate measured as nitrogen. The limit protects infants, who are most sensitive to nitrate.

What causes nitrate in well water?

Mostly nitrogen fertilizer, animal manure and septic systems. Nitrate is very soluble, so it leaches from the soil into shallow groundwater with rain and irrigation.

How common is nitrate contamination in wells?

In USGS’s 1992–2004 national assessment, more than 20% of shallow domestic wells in agricultural areas had nitrate above 10 mg/L. Public-supply wells exceed the standard much less often.

How do you remove nitrate from water?

Common treatments are ion exchange, reverse osmosis and biological denitrification. Utilities may also blend high-nitrate water with low-nitrate sources.

HOW ARGOS HELPS

Data Vault — environmental data

Sampling history, MCL exceedance flagging and anomaly detection across every well and site.

TERRA — environmental research agent

Ask about a site’s chemistry trends and regulatory limits and get cited answers.

SOURCES

  1. USGS Circular 1350 — Nutrients in the Nation’s Streams and Groundwater, 1992–2004
  2. USGS Circular 1350 — Frequently asked questions
  3. USGS — Nutrients in the Nation’s Waters: Too Much of a Good Thing?
  4. U.S. EPA — National Primary Drinking Water Regulations

Figures are taken from the primary sources above and dated as of the update shown. Concentrations are expressed in mg/L. Spot an error? Email support@argostellus.com.

RELATED GUIDES

Need a site-specific answer?

Argos models groundwater availability, drawdown and contamination for individual sites and portfolios.

CONTACT ARGOS