How Agricultural Practices Affect Groundwater Nitrate Levels

The effect of agricultural practices on groundwater quality is an increasing concern globally, particularly in highly intensive farming areas.

Groundwater nitrate levels are significantly elevated in regions such as the Midwest and the Central Valley of California, as well as various countries in the European Union, India, New Zealand, and the North China Plain.

This increase is primarily attributed to intensive agricultural practices, resulting in groundwater nitrate concentrations that surpass the maximum contaminant level (MCL) of 10 mg/l of NO3-N established by the U.S. Environmental Protection Agency for safe drinking water.

In regions in California where intensive vegetable production occurs, soils retain 40% of nitrogen from fertilisers fed to the plants.

The study “Assessing Nitrate Leaching During Drought and Extreme Precipitation: Exploring Deep Vadose-Zone Monitoring, Groundwater Observations, and Field Mass Balance”, published in AGU Journal in October 2024, assessed the effect of crop production on nitrate contamination of groundwater in a site in Yolo County, California.

The study site is a 34-ha field cropped in a processing tomato and cucumber rotation. Researchers conducted the study from 2021 to 2023, when the state was experiencing droughts followed by atmospheric rivers.

They used three different methods of monitoring nitrate leaching when nitrate from fertilisers moves into the groundwater: deep vadose-zone monitoring, shallow groundwater intensive monitoring, and field-level mass balance during the growing and rainy seasons.

They found that nitrate leaching accelerates during atmospheric rains or intense rainfall events in the winter following the dry summer period.

In California, where groundwater is the primary source of drinking water in some areas, high nitrate contamination levels can increase health risks.

Some studies show that growers are adopting improved irrigation and fertigation practices to comply with these regulations. This follows a recent life cycle assessment that identified the potential for improved resource use efficiencies in processing tomato production in California.

Research has identified specific management practices, such as fertigation methods that supply fertiliser and irrigation amounts at the appropriate rate, place, and time, that can be highly cost-effective in mitigating nitrogen pollution, including nitrate leaching. Applying these practices can help make farming more sustainable and reduce environmental impacts, particularly in drinking water sources.

Climate change is also projected to increase the severity and frequency of extreme events like droughts and atmospheric rivers, so addressing climate change through climate adaptation measures and mitigation can also address the problem.