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Reducing off-site movement of pyrethroids not difficult

Sampling of various agricultural water bodies in 17 Central Valley counties by University of California researchers has shown that roughly one-quarter to one-third of the sites have toxic levels of pesticides or other substances.

Using Hyalella azteca, a small amphipod commonly used to text toxicity, Donald Weston, adjunct professor, University of California, Berkeley, determined that pyrethroids were largely responsible for presence of toxicity among the 127 sites.

Another study along the Salinas Creek also found problems with pyrethroids. Researchers looked at the different levels of toxicity in various land use areas adjacent to the Salinas Creek.

“Stretches of that creek move in and out of urban and agricultural areas,” Weston says. “We found there was toxicity regardless of land use. There was a little bit different fingerprint, depending on whether it agricultural or urban, but it was still there.

“The pyrethroids commonly used in urban settings differ somewhat from those commonly used in agriculture. We could pick that up in the sampling, but the levels of toxicity didn't vary much.

Reported non-ag use of pyrethroids in California is about 700,000 pounds per year, with estimated retail sales of approximately 100,000 pounds more. Reported agricultural use of pyrethroids in the state is approximately 300,000 pounds per year.

In terms of ag use of pyrethroids, there are mitigation practices that are fairly easy to implement, which can reduce off-site movement substantially, according to Weston.

“Pyrethroids are extremely particle-associated and insoluble in water,” he said at the Irrigation and Nutrient Management Seminar conducted by Monterey County Cooperative Extension. “Anything that reduces the movement of sediment, particularly the smallest particles, through the water should reduce the movement of pyrethroids.”

To test that theory, Weston set up a series of irrigation trials at Salinas, Davis, and Chico, using three types of runoff ditches and polyacrylamide (PAM) to determine impact on pyrethroid movement. The three ditch designs included a simple dirt ditch, a sediment trap, and a vegetative ditch. The ditch designs were also compared to the practice of adding PAM to the irrigation water.

Polyacrylamide is frequently used as a soil conditioner for erosion control on farmland and construction sites.

In Salinas, the test site was planted to lettuce, sprinkler irrigated, and treated with Pounce. At Davis, tomatoes were treated with Warrior and flood irrigated, while the Chico site had beans treated with Mustang, also under flood irrigation.

“We looked at the quality of the water at the entry point, compared to the quality at the exit point,” Weston says. “The sediment trap is supposed to allow sediment to settle out before the water continues through the system. Basically, it was worthless in stopping the movement of pyrethroids.”

The simple ditch results varied considerably, depending on the location, with the best efficiency at the Davis site. However, even at that site, the reduction in pyrethroid from the entry to the exit point was only about 60 percent. At Chico, the simple ditch only stopped about 10 percent of the pyrethroid movement.

“The vegetative ditch was a mixed bag, depending on the soils,” Weston says. “It was fairly effective at the Davis site, stopping almost 80 percent of the pyrethroid, but it was only a little over 20 percent effective at the Salinas location.”

Compared to the different ditch designs, PAM was by far the most effective method of reducing pyrethroid off-site movement.

“PAM is extremely effective at containing pyrethroids,” Weston says. “It is usually very striking — you can see the difference just looking at the water, because you have very little sediment movement where it is used. Since pyrethroids are so highly particle-associated, stopping the movement of the sediment stops the movement of the pyrethroid.”

The results of adding PAM to the irrigation water ranged from about 80 percent reduction of pyrethroid runoff at the Salinas site to almost 100 percent reduction at the Chico site.

“Either PAM or the vegetative ditch appear to be good mitigation practices to reduce pyrethroid runoff,” Weston says. “Neither is cost-prohibitive, and PAM is extremely easy to use.”

There some concern about the relative toxicity of PAM itself, and Weston is planning to conduct additional studies to determine what, if any, impact it might have on water quality.

“We certainly don't want to solve one problem only to create another,” he says. “The literature suggests that PAM will not create a problem, but we're going to look at that issue. We'll have an answer within a few months.”

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