Nitrate levels in soil suggest extreme weather events due to climate change are accelerating groundwater contamination

Extreme weather events caused by climate change, such as droughts and heavy rains, can increase the risk of nitrates from fertilizers entering groundwater, according to a recent study by researchers at the University of California, Davis. The study found that drought caused heavy rains that allowed nitrates to seep 33 feet below the field in just 10 days. The study was published in the journal Water Resources Research.

“Conventional wisdom has held that it can take weeks to years for nitrates to migrate from crop root zones and reach groundwater,” said Corresponding author, Land, Air and Water Resources; said Professor Isaya Kisekka of the Department of Bioagricultural Engineering. . “We found that extreme events, such as rivers in California’s atmosphere, cause nitrates to move faster.”

The study used various methods to measure how much nitrate, a component of nitrogen fertilizer, penetrates into the soil in tomato and cucumber crops near Esparto, California. Scientists conducted the study from 2021 to 2023, during which time California experienced a period of drought followed by a decline in atmospheric rivers. They measured nitrate during both the growing and rainy seasons.

Drought can leave more nitrogen in the soil

Previous research has shown that about 40% of nitrogen fertilizers used on vegetables remains in the soil without being absorbed by the plants. During droughts, crops are unable to use nitrogen efficiently, resulting in excess nitrogen in the soil.

The study found that when a drought is followed by heavy rain, that sudden burst of water causes nitrates to seep into groundwater faster. Nitrate levels in shallow groundwater exceeded the U.S. Environmental Protection Agency’s maximum drinking water contaminant level of 10 milligrams per liter.

“California is often described as oscillating between drought and flood,” Kisekka said. “Extreme events associated with climate change will further exacerbate the risk of these chemicals entering drinking water.”

Groundwater is the primary source of drinking water in most areas of California’s Central Valley. In some areas, such as the Tulare Lake Basin, nearly one-third of drinking and irrigation wells exceed EPA safe nitrate levels. High nitrate levels in drinking water can pose increased health risks, especially for young children. It may also increase your risk of colorectal cancer.

The need for real-time soil nitrate monitoring

Central Valley farmers are required to report to regional water boards how much nitrogen they apply to their fields and how much nitrogen is removed as part of the crop yield. This study compared different methods of monitoring nitrate infiltration from fertilizers into groundwater. Kisekka said the results highlight the need for affordable, real-time soil nitrate monitoring tools to help farmers efficiently manage fertilizer use.

Farmers can reduce nitrate contamination in groundwater by implementing conservation practices that limit nitrates remaining in crop root zones after harvest.

Data from this study will also help improve a model called SWAT used to track nitrate seepage into groundwater throughout California’s Central Valley. The effort is part of the Central Valley Water Commission’s program to regulate irrigated agricultural land.

Other authors at the University of California, Davis include Iael Raisi Hoffmann, Thomas Harter, and Helen Dahlke.