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UC Berkeley Press Release

Pyrethroid pesticides found at toxic levels in California urban streams

– A group of commonly used pesticides recently found at toxic levels in stream sediments in many agricultural areas around California is also a problem in urban streams, according to a new study by researchers from the University of California, Berkeley, Southern Illinois University and the Central Valley Regional Water Quality Control Board.

Collecting water and sediments in Pleasant Grove Creek
Collecting water and sediments in Pleasant Grove Creek as it flows by a subdivision in Roseville, Calif. (Photo by Donald Weston/UC Berkeley)

Pyrethroid pesticides, widely used on crops like cotton, fruits and lettuce, and the main residential pesticide now that the once-dominant organophosphates, diazinon and chlorpyrifos, have been phased out, were found in three streams in Roseville, Calif., a largely residential community north of Sacramento, at levels toxic to organisms that live in the sediment.

Though toxicity tests were conducted on tiny, shrimp-like animals called amphipods, a species recommended by the Environmental Protection Agency (EPA) when testing sediment toxicity, the pesticides could be affecting other bottom-dwelling animals as well, according to the researchers.

"Amphipods are a relatively sensitive group of animals, so this study is telling us that pyrethroids at these test sites are reaching levels where they can be lethal to sensitive species," said study leader Donald Weston, adjunct professor of integrative biology at UC Berkeley. Future studies should focus on aquatic insect larvae that commonly live in creeks, such as stoneflies and mayflies, which also tend to be sensitive to pesticides.

Weston and his colleagues reported their findings in the Oct. 19 online issue of Environmental Science & Technology (ES&T), a publication of the American Chemical Society. The article will appear in the Dec. 1 print edition.

The California State Water Resources Control Board funded the project to investigate whether the switch to pyrethroids from organophosphates, which as of 2004 were eliminated for most urban uses, may pose risks to aquatic life, according to state biologist Robert Holmes, one of the authors.

Pyrethroids have been on the market for more than 20 years, and only now are people looking at potentially toxic effects on sediment-dwelling creatures, Weston noted. The new study demonstrates that a potential toxicity problem only recently recognized in agricultural areas is now spreading into urban areas as pyrethroid pesticides have taken over the residential market. Bifenthrin, the pyrethroid that contributed most to the toxicity measured in the study, is available widely and sometimes mixed with fertilizer to spread as granules over lawns.

"Pesticides are often cheap, but most people don't recognize the environmental costs of using them," Weston said. "It is my feeling that a lot of urban pesticide use may be unnecessary, and the answer lies in changing practices and changing the mentality of pesticide applications in the urban environment."

The findings come at a critical time, when the EPA is beginning the process of re-registration for most pyrethroids. When pyrethroids were first registered in the 1970s and 1980s, data indicated that they bind to sediment and do not stay in the water column. Now, more than 20 years after pyrethroids hit the market, the EPA is asking for studies of sediment toxicity.

"The presumption was that if it binds to sediment, that substance becomes unavailable to organisms and, from a toxicity standpoint, irrelevant. And we're showing that not to be a fair assumption," Weston said. "If binding to sediment was a solution, we wouldn't be worried about DDT, we wouldn't be worried about PCBs, and we wouldn't be worried about a half dozen other organochlorine pesticides now banned."

He noted, however, that having the pesticide bound to the sediment does offer some advantages. As compared to the organophosphates, which are mostly dissolved in the water, having the pyrethroids bound to the sediment may make it easier to minimize their release to creeks, and may mean that toxic levels do not travel as far downstream.

Though pyrethroid pesticides are generally considered safe for fish and other animals that live in the water, until last year there had been little testing of pyrethroid toxicity on insect larvae or shrimp-like amphipods that live in bottom sediments. In the April 8, 2004, online edition of ES&T, Weston and colleagues Michael J. Lydy and Jing You of Southern Illinois University in Carbondale reported that some 20 percent of streams in California's Central Valley contained sediments toxic to amphipods, a laboratory standard for sediment toxicity.

In the current follow-up study, Holmes, Weston, Lydy and You sampled three streams in a residential area of Roseville, a rapidly growing community of homes and subdivisions with little agricultural activity that could have confused the picture with respect to pyrethroid sources. Nearly all the creek sediments proved toxic to amphipods, the researchers found.

The main stream, Pleasant Grove Creek, was in pretty good shape, Weston said, with contamination only where tributary streams draining housing subdivisions entered the main creek. He and his team also sampled two of these tributaries - Kaseberg Creek and the south branch of Pleasant Grove Creek, which in the dry summers carry mainly runoff from overwatered residential lawns. Nearly all sediment samples from these tributaries were toxic to amphipods in lab tests and contained pyrethroid concentrations known to kill these crustaceans.

Weston and Holmes chose these creeks in part because they are in an area that harbors natural populations of the amphipod Hyalella azteca, the same species as the researchers used in their lab testing. While natural amphipods were found in the main creek where there were low levels of pyrethroids, no wild amphipods were found in areas of the tributaries with high levels of pyrethroids and where they had found the sediments to be toxic in their lab tests. This gives the researchers confidence that their laboratory findings are directly relevant to the organisms in the field.

Weston said that the most toxic of the pyrethroids - all of which are identified by the suffix -thrin - was bifenthrin, which could have gotten into the streams as runoff from homes treated by professional pest-control companies for pest such as ants, or from lawns treated with pesticides or popular fertilizer/pesticide combinations.

"Our work should be of broad public interest," Weston said, "because the source of the toxicity we are finding in the creeks is just residential pesticide use in a typical suburban community. When people apply pesticides to their yards, or hire exterminators to do it, they just assume the pesticides will stay there. I think our work will increase awareness of the possible environmental dangers of pesticide overuse and maybe help people think twice before using pesticides 'just to be safe' when they do not have a clear pest problem."

The study was funded by the Surface Water Ambient Monitoring Program (SWAMP) of the California State Water Resources Control Board.