Typically, when people hear about nitrates, it involves runoff from farm fields, sewage systems and cities that can fuel harmful algae blooms and raise the risk of a serious illness in infants known as “blue baby syndrome.”

But a river can also run through a sparsely populated landscape with little farming and still contain surprisingly high levels of this form of nitrogen that can harm aquatic life, drinking water and human health.

In a new study, my colleague and I found a reason why that happens: Dry climate conditions are associated with high river nitrate concentrations, even in places with little farming or urban development.

We found nitrate hot spots in some unexpected places, including the Rocky Mountains of the western United States and northern Africa’s Sahara Desert. Where water is scarce, we found that nitrates – which can naturally leach into surface water and groundwater from the soil as microbes break down organic material – were able to accumulate and become concentrated rather than being diluted or flushed away.

That raises health concerns for regions where nitrates in rivers and well water might not be given much thought. It also raises questions about how nitrate-related health risks will increase as global temperatures rise.

To see how widespread this pattern is, we combined global measurements from more than 7,000 monitoring sites and additionally extracted information about climate, soils, land use and other landscape features. We then trained machine learning models to estimate long-term average nitrate concentrations across nearly 2.9 million river segments worldwide.

The resulting map suggests that nitrates deserve attention well beyond the urban and agricultural regions where they are usually monitored.

Why it matters

High nitrate exposure in water and food is particularly dangerous for infants because it can interfere with the blood’s ability to carry oxygen. Blue baby syndrome, or methemoglobinemia, can result in serious illnesses and be caused by nitrate poisoning.

The U.S. drinking water standard of 10 milligrams of nitrate-nitrogen per liter was established primarily to guard against this acute risk.

We estimate that about 37,000 people worldwide live in areas where rivers exceed the 10 mg/L drinking water standard, predominantly in the U.S. Corn Belt.

That doesn’t mean 37,000 people are drinking water at those concentrations, since river water in many regions is treated before it is supplied for drinking. But it’s a concerning level they should be aware of, particularly those using well water.

Research has also raised concerns about possible effects of long-term exposure to nitrate at lower concentrations, including certain cancers and adverse birth outcomes, such as babies born preterm or with a low birth weight, although those risks are still being studied.

We estimate that about 480 million people worldwide live in areas where nearby rivers have elevated long-term average nitrate concentrations, above 2.5 milligrams per liter.

Many of the rivers our models identified as having elevated nitrate levels are small headwater streams. Many headwater streams cease to flow for some periods of time every year, and nitrates can accumulate to high concentrations under dry or low-flow conditions. These small streams make up most of the global river networks, have long been overlooked and feed larger rivers that supply drinking water to large populations and help shape the quality of water downstream.

Too much nitrate in rivers and lakes can also fuel algal growth and disrupt aquatic ecosystems.

What we still don’t know

A global map of nitrate concentrations in river water cannot show what comes out of someone’s tap. Their water may be mixed with other sources or treated before they drink it. Nitrate concentrations can also rise and fall over time, while our map shows long-term averages.

To understand a person’s actual exposure will require connecting river measurements with drinking water sources, treatment practices and tests of water delivered to households.

Many rivers with nitrate concentrations above 2.5 milligrams per liter flow through sparsely populated regions where access to treated drinking water may be limited. Even where treatment facilities exist, conventional methods remove little nitrate. Removing it requires specialized technologies that can be costly to operate.

What comes next

Our study shows why more drylands and small headwater streams need to be monitored for nitrate. It also raises a question for a warming world: If a region becomes drier, how much will nitrate concentrations rise even if people do not add more nitrogen to the landscape?

Answering that will require measurements through wet and dry periods, paired with studies of how water moves through soils, groundwater and rivers. For now, the message is clear: Protecting water quality requires paying attention to the climate as well as to the human activities that put nitrogen into the environment.

Li Li (李黎) does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.