Extreme weather accelerates nitrate pollution in groundwater

University of California, Davis (WeatherFarm)– Extreme weather spurred by climate change, including droughts and heavy rains, may increase the risk of nitrates from fertilizers ending up in groundwater, said a recent study from researchers at the University of California, Davis. The study found heavy rains after a drought caused nitrates to seep 33 feet under farm fields in as little as 10 days.

“The conventional wisdom was that it could take several weeks to years for nitrates to move from the crop root zones to reach groundwater,” said UC Davis Prof. Isaya Kisekka. “We found these extreme events, such as California’s atmospheric rivers, are going to move nitrate more quickly.”

In this study, different methods were used to measure how much nitrate, a component of nitrogen fertilizer, was seeping down through the soil in a tomato and cucumber crop near Esparto, Cal. Scientists conducted their research from 2021 until 2023 when the state experienced periods of drought followed by atmospheric rivers. They measured nitrate during both the growing seasons and the rainy seasons.

Previous studies have shown about 40 per cent of nitrogen fertilizer used for vegetables isn’t absorbed by the plants but remains in the soil. During droughts, crops don’t use nitrogen efficiently, leading to excess nitrogen in the soil. This study found that if a drought is then followed by heavy rainfall, that sudden burst of water causes nitrate to seep into groundwater more quickly. The nitrate concentration in the shallow groundwater exceeded the United States Environmental Protection Agency maximum contaminant level of 10 milligrams per liter for drinking water.

“In California, we often say we swing between droughts and floods,” said Kisekka. “These extreme events that come with climate change are going to make the risk of these chemicals ending up in our drinking water much more severe.”

In California’s Central Valley, farmers are required to report to the Regional Water Board how much nitrogen they applied to their field and how much was removed as part of the crop’s yield. The study compared different ways of monitoring when nitrate from fertilizers seep into groundwater. Kisekka said the results highlight the need for affordable, real-time soil nitrate monitoring tools to help farmers manage fertilizer use efficiently.

By using conservation practices that limit leftover nitrates in the crop’s root zone after harvest, farmers can help reduce nitrate contamination in groundwater.