Chemicals sprayed on farm fields are reaching nearby streams not only through surface runoff but also via hidden underground channels, according to new research from Penn State University.
The study, published in the Journal of Environmental Quality, found that while vegetative streamside buffers can reduce some pesticide pollution, they are ineffective against chemicals that travel through groundwater especially in landscapes shaped by porous, fractured bedrock.
For years, the U.S. Department of Agriculture has promoted riparian buffers strips of grass, shrubs, and trees planted along waterways as a natural way to protect streams from agricultural runoff. These vegetative zones slow surface water, trap sediment, and reduce nutrient pollution.
But pesticides behave differently depending on their chemical properties and how they move through soil and water.
“In this research, we wanted to understand whether vegetative buffers can reduce the amount of various pesticides getting into streams in agricultural areas,” said Heather Preisendanz, professor of agricultural and biological engineering at Penn State.
The research team focused on Halfmoon Creek in central Pennsylvania, a 24-square-mile watershed characterized by karst geology. In such landscapes, soluble bedrock forms cracks, sinkholes, and underground channels that allow water to move rapidly below ground.
Scientists monitored five sites along the creek during the 2023 growing season, collecting water samples every two weeks. They tested for common herbicides Atrazine and Simazine as well as four insecticides frequently used to coat corn and soybean seeds: Clothianidin, Imidacloprid, Thiacloprid, and Thiamethoxam.
The findings were striking. Simazine appeared in 93% of samples, while Atrazine showed up in 92%. Clothianidin was detected in 75% of samples.
Atrazine and Clothianidin concentrations rose sharply after rainfall, suggesting they were carried into the stream by surface runoff a pathway that buffers can partially intercept.
Simazine, however, followed a different route. It was detected even during periods of low stream flow, indicating that it was traveling primarily through groundwater. In karst landscapes, groundwater can move long distances and enter streams beneath the buffer zones, bypassing vegetation entirely.
“Our results show that buffers are helpful, but only for pesticides that move in surface runoff,” Preisendanz said. “Groundwater-transported pesticides, especially in karst watersheds, can bypass buffers entirely.”
The study highlights the need for more targeted pollution-control strategies that account for how individual chemicals move through the landscape. While riparian buffers remain an important conservation tool, they may not be sufficient on their own in regions with complex underground water systems.
As agricultural production continues to rely on chemical weed and pest control, researchers say farmers and policymakers must consider both surface and subsurface water pathways when designing water-protection measures.
The findings underscore a broader lesson: protecting streams requires more than a single solution. In landscapes where water flows unseen beneath the soil, pollution can follow slipping past even well-established environmental safeguards.