Nelson Issue Brief: Nitrate contamination in drinking water and groundwater

What Drives Nitrate Leaching Into Groundwater?

Contact the authors

Eric Booth, Department of Agronomy, Department of Civil and Environmental Engineering, UW-Madison,

Christopher Kucharik, Department of Agronomy, UW-Madison,

Randall Jackson, Department of Agronomy, UW-Madison,

“Nitrate leaching” is the process of nitrate moving from the land surface into groundwater, and it is the primary cause of nitrate contamination in groundwater. The amount of synthetic fertilizer and manure applied to the land is the most important driver of nitrate leaching.

We know through fertilizer sales data that fertilizer nitrogen application rates have increased since the 1950s and have continued to rise more slowly since 2002. The limited data on manure nitrogen application reveals that nitrogen from manure applications is similar in magnitude to nitrogen from synthetic fertilizers.

Does changing nitrogen application change nitrate leaching? Changing the land cover can have a large impact because different amounts of nitrogen are taken up by different plant communities. For instance, our simulations of Midwestern cropping systems show higher levels of nitrate leaching under corn than under soybeans. Simulations of the Yahara watershed in south-central Wisconsin show that increased coverage of perennial grasses and decreasing nitrogen applications result in decreased nitrate leaching.

Nitrate research in the Central Sands

Student preparing a water collection instrument
Liam Dangeur, an undergraduate student at UW-Stevens Point, prepares the water collection instrument. Photo by Kevin Masarik, UW-Stevens Point.

The Wisconsin Central Sands (WCS) is a major vegetable producer, but due to the region’s sandy soil, irrigation is required in combination with applications of nitrogen fertilizer. This makes the region prone to leaching of nitrate and groundwater contamination. Irrigation water drawn from wells that tap into contaminated groundwater is already high in nitrate, but it is unclear how nitrate levels in groundwater change across space and time.

Thus we want to know how to account for the nitrogen applied through irrigation water, as opposed to the nitrogen directly applied through fertilizer, in farmers’ mandatory nitrogen management plans. By accounting for irrigation-water nitrogen, farmers may be able to reduce synthetic fertilizer applications, saving money while reducing nitrate losses to the groundwater system. Additionally, crops’ efficiency in using water and nitrogen will shift with a changing climate, which may change how we can reduce nitrate leaching to groundwater.

UWLandLab and Grassland 2.0

Managed grazing using perennial grass systems across Wisconsin can reduce nitrate leaching while also providing a sustainable livelihood for dairy and beef producers in a challenging economic environment. In the UWLandLab we (along with Claudio Gratton, Michael Bell, and Bradford Barham) work with farmers, scientists, distributors, processors, and consumers to incentivize moving livestock production toward perennial grassland grazing. UWLandLab aims to serve as a roadmap for those interested in moving from input-intensive annual cropping systems towards perennial grassland-dominated landscapes, a vision we call Grassland 2.0.

Through both field-testing and modeling, we hope to provide the tools to improve groundwater quality and reduce water use. Both will be important as increasing weather variability makes water resources management and nitrogen applications to crops more challenging.

Learn more about nitrate contamination