Extreme weather is ramping up toxic nitrate pollution in groundwater: Study

Climate-driven extreme weather conditions, such as droughts and intense precipitation, may be accelerating the pace at which toxic nitrates are polluting groundwater, a recent study has found.

Heavy rains following a drought caused nitrates — byproducts of nitrogen-based fertilizers — to seep 33 feet under Northern California farm fields in as little as 10 days, scientists observed in the study, published in Water Resources Research.

“In California, we often say we swing between droughts and floods,” corresponding author Isaya Kisekka, a professor of agricultural water management at the University of California Davis, said in a statement.

“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," Kisekka warned.

From 2021 through 2023, when Californians were enduring periods of drought followed by atmospheric rivers, Kisekka and his team used a variety of methods to measure how much and how deep nitrates crept into a specific cropland's soil.

They focused on an 84-acre field in Yolo County, west of Sacramento, used to cultivate tomatoes and cucumbers — measuring nitrate levels during both growing and rainy seasons.

Previous studies have already demonstrated that about 40 percent of nitrogen fertilizer used for vegetables isn't absorbed by the plants and instead remains in the soil, the authors noted.

But the current study established that when facing drought, crops process nitrogen less efficiently and allow a surplus of nitrogen to leach into the soil. If a dry period is then followed by heavy rains, the resulting deluge of water accelerates the seepage of nitrates into the ground, the authors found.

As they monitored these shifts, the researchers discovered that nitrate concentrations in shallow groundwater exceeded the Environmental Protection Agency's drinking water safety thresholds.

Excess exposure to nitrates has been linked to an increased risk of pregnancy complications, certain types of digestive cancers and methemoglobinemia in infants, also known as "blue baby syndrome."

In light of the study’s findings, Kisekka expressed support for the distribution of affordable, real-time soil nitrate monitoring tools that could help farmers manage fertilizer use.

He also stressed the need to adopt conservation practices that restrict the amount of nitrates left in the crop's root area after harvest, so that farmers can minimize groundwater contamination.

"These results strengthen the need to develop better and inexpensive in-situ, real-time, soil pore water nitrate concentration monitoring technologies," the authors concluded.