Climate change and extreme events promote nitrogen contamination

In California, the interaction between drought and subsequent rainfall promotes the dispersion of nitrogen that can reach surface waters, explains a U.S. study

by Matteo Cavallito

 

Extreme weather events caused by climate change, such as droughts and heavy rains, could increase the risk of nitrogen contamination in groundwater. This is suggested by a study, published in the journal Water Resources Research, which focused on the movement of nitrates from fertilizers used in crops in California’s Central Valley. The research was conducted by the University of California Davis.

The study revealed that precipitation followed by drought, can cause nitrates to move into groundwater much faster than previously thought, that is, in as little as 10 days after the rainfall event.

The study

Conducted between 2021 and 2023, the investigation focused on an agricultural field near Esparto, California, during drought and heavy rainy seasons. Three methods were used: “deep vadose-zone monitoring, shallow groundwater intensive monitoring, and field-level mass balance,” the study details. The main objective was to assess changes in nitrate leaching between periods.

“The study found that all methods showed nitrate leaching despite some uncertainties,” the authors point out.

After drought, in other words, when crops fail to use nitrogen effectively, excess fertilizer remains in the soil. Later, heavy rainfall promotes the dispersion of nitrates into the soil, which can also reach groundwater. In some cases, the concentration of nitrogen compounds in the latter has even exceeded the maximum contamination limit set by the U.S. Environmental Protection Agency (EPA), which is 10 milligrams per liter.

The role of climate change

Nitrogen is an essential nutrient for soil and plants and, not surprisingly, is a key component of many fertilizers. Its overuse, however, can have very negative consequences. Excessive nitrate levels, in fact, can damage water quality, cause eutrophication (the over-enrichment of nutrients) and pose health risks to animals and humans. In this scenario, climate change represents a major challenge.

Rising temperatures, in fact, increase the incidence which, in turn, increase the risk of contamination of water resources. Groundwater, the researchers explain in a statement from the U.S. university, is the main source of drinking water for much of California’s Central Valley. Nitrogen contamination poses a significant risk.

Nitrogen monitoring needed

The study, the scientists point out, “contributes to knowledge and ongoing research by providing insights into effective monitoring strategies for nitrate leaching into groundwater from intensive cropping systems.” The highlighted results also underscore “the importance of continuous monitoring and assessment in understanding nitrate dynamics and groundwater contamination risks.”

In this context, data from the study will particularly contribute to the development of a new model called SWAT (Soil and Water Assessment Tool), which is used to monitor nitrate infiltration into groundwater across the Central Valley as part of a government program to regulate irrigation of agricultural lands. Finally, the research contributes to the evaluation of the United States Department of Agriculture’s conservation practices related to water quality and land management.