Protecting drinking water from nitrate pollution

boy drinking from kitchen faucet

Wisconsin has a vast source of water underground, and many people, communities, and industries rely on groundwater for their needs.

While for many people, private wells that draw groundwater are a reliable and safe source of water, for many others, groundwater pollution poses a major challenge.

There are many contaminants from many sources that pollute groundwater in Wisconsin.

However, nitrate plays an outsized role in groundwater pollution problems around the state.

What is nitrate?

Nitrate is one chemical form of the element nitrogen and is a critical component of life. Nitrogen is found in everything from humans to plants, and follows a natural cycle of moving from the atmosphere to the ground, similar to the water cycle.

Nitrate is the most widespread groundwater pollutant in Wisconsin.

How does nitrate get into groundwater?

 In an agricultural state like Wisconsin, nitrogen in the form of fertilizer and manure is spread on farm fields to help promote and accelerate the growth of crops.

An overabundance of nitrogen use in agriculture has led to nitrate leaching into groundwater. While other sources, like septic systems, also contribute to nitrate pollution in drinking water, estimates suggest that 90% of the nitrate contaminating groundwater come from agricultural sources.

Local geology and the type of soil present both influence how susceptible an area is to groundwater pollution.  


The nitrogen cycle

Source: WDNR

In areas with cracked bedrock or sandy soils, contaminated water on the earth’s surface can easily and quickly make its way into the groundwater. Aquifers, the rock layers containing groundwater, made up of sandstone or dolomite contain many fissures or cracks, making it easy for water to travel through without removing contaminants.

Areas with sandy soils are particularly vulnerable to nitrate pollution.  The sand might filter out bacteria, but nitrates move quickly with the water through the sand and down into the aquifer. 


Percentage of people in WI who get their drinking water from a private well

Upper estimate of private wells in WI contaminated with unsafe levels of nitrate


Estimated amount of nitrate pollution in groundwater from agriculture sources

What’s at risk?

Our health. Nitrogen is a critical elemental building block of life, but too much of it in certain forms, like nitrate, can pose significant health risks to everyone, not just pregnant women or small children.

Nitrate is linked to Blue Baby Syndrome, a condition that reduces oxygen in the bloodstream in young babies, causing them to turn a blue-gray color. It can also cause neural birth defects in fetuses, which can be life threatening.

In adults, excessive nitrate exposure is linked to increased risk ofa thyroid disease, colon cancer and non-Hodgkin’s lymphoma.

But there are also risks to our economy. A study conducted by Clean Wisconsin and Environmental Working Group estimates that the health consequences of nitrate pollution in drinking water costs Wisconsin residents up to $80 million in annual medical expenses.

Nitrate pollution in drinking water is linked to:

Blue Baby Syndrome

Neural birth defects

Thyroid disease

Colon cancer

non-Hodgkin’s lymphoma

Nitrate pollution in drinking water is estimated to cost Wisconsin up to


annually in health care costs

Source: Mathewson, P.D. et al. (2020)

“Vicious Cycle: The story of Wisconsin’s most common contaminant”

Case study: Southwest Wisconsin


Southwest Wisconsin is a part of the state with extensive fractured and porous bedrock and shallow soils, making the groundwater susceptible to contamination. While it was widely assumed that this area had groundwater contamination from nitrates and bacteria, no conclusive study had been conducted.

Clean Wisconsin had encouraged state and local leaders for over two years to fund a study of the three-county region in the southwest corner of the state to determine the extent and sources of contamination to private wells.


Contour Strip Farming

In the first round of testing in late 2018, the study found that the percentage of wells in Grant, Iowa, and Lafayette Counties that tested positive for coliform bacteria or above the safe level for nitrate was higher than the state average, with 42 percent of wells tested contaminated with one or both pollutants.

These results prompted Assembly Speaker Robin Vos to form the Speaker’s Task Force on Water Quality.

This study is ongoing, but it has already made clear that groundwater pollution is a serious challenge in Southwest Wisconsin that must be addressed through robust policy and increased funding.

What’s being done about it?

Clean Wisconsin has been a leading voice in calling for state efforts to reduce nitrate pollution in groundwater.

After years of work to encourage state officials to tackle the nitrate pollution issue, Gov. Evers announced in 2019 the start of work  to develop new rules for how nitrogen is applied to farm fields. These new administrative rules will stipulate how nitrates from commercial fertilizer and manure are applied to farm fields.

This rulemaking process is an important step for protecting rural drinking water across the state, but much more needs to be done. Increased funding for programs to address water pollution, more support for people with contaminated wells, and investment in county conservation staff is needed to make significant strides to tackle and start solving this public health crisis.

Learn more.

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