A recent study from researchers at the University of California, Davis, reveals that extreme weather events linked to climate change—like droughts followed by heavy rainfall—could heighten the risk of nitrates from fertilizers contaminating groundwater. The research found that heavy rains, following a drought, led to nitrates traveling down 33 feet beneath farm fields in just 10 days. This study was published in Water Resources Research.
Isaya Kisekka, a professor in the Departments of Land, Air and Water Resources and Biological and Agricultural Engineering and the study’s corresponding author, noted, “It was previously believed that it could take several weeks to even years for nitrates to migrate from crop root zones to groundwater. Our findings show that extreme weather events, such as California’s atmospheric rivers, accelerate the movement of nitrate.”
In this study, researchers employed various techniques to gauge the amount of nitrate—a nitrogen fertilizer component—reaching groundwater through the soil in a tomato and cucumber crop located near Esparto, California. The research was conducted from 2021 to 2023 during periods of drought followed by atmospheric river events, measuring nitrate levels in both the growing and rainy seasons.
Drought can leave a nitrogen surplus in soil
Past research has indicated that roughly 40% of nitrogen fertilizers applied to vegetables aren’t taken up by the plants and remain in the soil. During drought conditions, crops struggle to effectively use nitrogen, resulting in an accumulation of excess nitrogen in the soil. The current study observed that when heavy rains follow a drought, the sudden influx of water facilitates rapid nitrate movement into groundwater. The concentration of nitrate in shallow groundwater surpassed the U.S. Environmental Protection Agency’s maximum contaminant level of 10 milligrams per liter for drinking water.
Kisekka commented, “In California, we often experience a cycle of droughts and floods. The increasing frequency of these extreme events due to climate change will intensify the risk of these chemicals contaminating our drinking water.”
Groundwater serves as the main source of drinking water for the majority of California’s Central Valley. In certain areas, such as the Tulare Lake Basin, around one-third of drinking and irrigation wells exceed the safe nitrate level set by the EPA. High nitrate concentrations in drinking water pose health risks, especially for young children, and may even raise the likelihood of colorectal cancer.
The necessity for real-time soil nitrate monitoring
Farmers in California’s Central Valley are mandated to report their nitrogen application rates to the Regional Water Board, along with the quantities removed through crop yields. The study evaluated different monitoring methods to track nitrate infiltration into groundwater. Kisekka emphasized the findings underline the urgent need for cost-effective, real-time soil nitrate monitoring tools to aid farmers in optimizing fertilizer usage.
By adopting conservation practices that minimize leftover nitrates in the crop’s root zone after harvest, farmers can play a significant role in reducing nitrate pollution in groundwater.
The data generated from this study will contribute to enhancing a model called SWAT, which tracks nitrate seepage into groundwater throughout California’s Central Valley. This initiative is part of the Central Valley Water Board’s program aimed at regulating irrigated farmlands.
Additional contributors from UC Davis include Iael Raij Hoffman, Thomas Harter, and Helen Dahlke.
The study received support from the USDA Natural Resource Conservation Service via its Conservation Effects Assessment Project, which aims to evaluate the effectiveness of conservation strategies across various watersheds. Additional backing was provided by the USDA National Institute of Food and Agriculture.
