Pesticides can harm aquatic ecosystems and human health, so scientists need to understand how they move from farm fields into streams.
A management tool commonly implemented is riparian buffers-- strips of vegetation, like shrubs or grasses, bordering streams-- that the U.S. Department of Agriculture (USDA) advises can reduce the amount of nutrients, sediment and pesticides getting into waterways.
But it’s unclear how effective buffers actually are at stopping pesticides from entering streams, according to a multidisciplinary team led by Penn State researchers.
To find out, the researchers conducted a study on a small agricultural stream, finding that adding buffers likely reduces the amount of specific pesticides from reaching the stream, but not others.
The researchers, who published their findings Feb. 24 in the Journal of Environmental Quality, focused on Halfmoon Creek [in Centre and Huntingdon counties] and its 24-square-mile watershed in central Pennsylvania characterized by a karst landscape-- an area underlain by soluble bedrock with lots of cracks, sinkholes and underground water flow.
The study is part of the team’s three-year investigation into the overall mitigation ability of riparian buffers, funded by a $750,000 grant from the USDA’s National Institute of Food and Agriculture.
“In this research, we wanted to understand whether vegetative buffers can reduce the amount of various pesticides getting into streams in agricultural areas,” said team leader Heather Preisendanz, professor of agricultural and biological engineering in the College of Agricultural Sciences. “We have seen evidence that buffers can protect streams from nutrients and sediment, but pesticides are a different story because they move with water in different ways, depending on the type.”
Pesticides can reach streams by surface runoff-- rain washes them over land-- or in groundwater if they soak into soil and move underground, explained Preisendanz, who is the director of the Institute for Sustainable Agricultural, Food, and Environmental Science at Penn State.
In karst watersheds, she added, groundwater can move fast and far through cracks and underground channels-- often seeping directly into streams. That can bypass buffers completely.
The team monitored five stream locations and sampled water biweekly during the 2023 growing season.
They tested for two herbicides often sprayed on fields to control weeds-- Atrazine and Simazine-- and four insecticides widely used by suppliers to coat and protect corn and soybeans seed-- Clothianidin, Imidacloprid, Thiacloprid and Thiamethoxam.
The researchers analyzed how concentrations of those pesticides in stream water varied with flowrate to identify transport pathways.
The most commonly detected pesticides in stream water were Simazine, in 93% of samples; Atrazine, in 92% of samples; and Clothianidin, in 75% of samples.
These results suggest that the chemicals are almost always present in the stream’s water during the growing season, researchers said.
Atrazine and clothianidin showed strong links between high stream flow and concentration, the researchers found, meaning they’re mainly moved by surface runoff.
Because of that, riparian buffers can help trap or slow them down, Preisendanz pointed out.
But Simazine did not behave like a runoff-driven chemical. It moved with groundwater, even during periods of low flow, traveling through underground fractures and conduits under karst fields directly into streams.
Riparian buffers don’t help much with this pathway, she said.
According to the researchers, this study highlights the need to consider transport pathways to mitigate pesticide pollution, particularly in karst watersheds, because water can travel long distances underground and pesticides applied far upstream can show up downstream, often with very little dilution.
In karst landscape, pollution can enter streams below the buffer zone, so even well-designed local buffers may miss a lot of the contamination.
“Our results here show that buffers are helpful, but only for pesticides that move in surface runoff-- groundwater-transported pesticides, especially in karst watersheds, can bypass buffers entirely,” she said. “Before we choose how to reduce pesticide pollution, we need to consider how pesticides actually move through the landscape, because we can’t greatly reduce pesticide pollution with buffers alone if the pesticides are slipping underground.”
First author Henry Kibuye, a doctoral degree candidate in the Department of Agricultural and Biological Engineering; Tyler Groh, assistant research professor and watershed management extension specialist; and Tameria Veith, agricultural engineer with the USDA Agricultural Research Service, contributed to the research.
(Photos: Heather Preisendanz, Professor of Agricultural and Biological Engineering; and Henry Kibuye, a doctoral degree candidate in the Department of Agricultural and Biological Engineering.)
(Reprinted from Penn State News.)
Related Articles This Week:
-- Penn State Extension Hosts 5-Part Advanced Stream Repair Webinar Series Starting March 27 [PaEN]
-- Penn State Extension Hosts In-Person Stream Health Investigation Educator Workshops March 18 In Butler County, March 31 In Cumberland County [PaEN]
[Posted: February 24, 2026] PA Environment Digest
