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USGS Studies the Impact of Insecticides on Northwestern Clackamas County Streams

A new study found high concentrations of commonly used insecticides in streams running through the highly urbanized portion of Clackamas County.

Water running down a creek.
 A turbid Lost Dog Creek tumbles down steep slopes before feeding Lake Oswego.Public domain

PORTLAND, Ore. — A new study by the U.S. Geological Survey, published today in the Journal of Environmental Monitoring and Assessment, found high concentrations of commonly used insecticides in streams running through the highly urbanized portion of Clackamas County.

The levels found in streams flowing through the greater Portland metropolitan area during a September 2013 storm were above U.S. Environmental Protection Agency benchmarks to protect aquatic-life. The active ingredients of the insecticides detected included the chemicals bifenthrin, fipronil, malathion, breakdown products of DDT, and others.

The chemical appearing to cause the most toxicity was bifenthrin, a broad-spectrum insecticide used to kill insect pests around homes and businesses. Bifenthrin attaches tightly to sediments contained in stormwater, traveling from the areas where it was applied through storm drainage systems to streams. When even small amounts of this chemical are mobilized, beneficial insects may be affected when the sediments and associated chemical settle out in streams.

“Because aquatic insects provide food for fish, birds and other wildlife, it is critical to better understand the full impact from this insecticide, including whether it is entering the food chain,” said Kurt Carpenter, USGS hydrologist and co-lead on the study. “The strong negative effect of bifenthrin on aquatic invertebrates seen in our study is consistent with national USGS findings that in urban streams, of the many contaminants examined, bifenthrin in bed sediments was the single best predictor of observed toxicity.”

More than 600 products contain bifenthrin; its high use and persistence in the environment result in its frequent detection. Bifenthrin was detected in all five stormwater outfalls sampled, and 73 percent of streams sampled during the storm, sometimes at levels well above EPA benchmarks. Seventy-one percent of streams contained bifenthrin in streambed sediments at levels likely to affect aquatic insects.

Streams with no or low levels of bifenthrin in their sediment had significantly more aquatic insects such as mayflies, caddisflies and stoneflies than streams with high concentrations of bifenthrin. Streams with high sediment levels contained mostly pollution-tolerant organisms, primarily non-insects.

Tanner Creek outfall
Tanner Creek outfall delivers stormwater runoff with insecticides to Tanner Creek, tributary of the Willamette RiverPublic domain

The USGS study was conducted, in part, to comply with a 2012 Oregon Department of Environmental Quality “MS4” (Municipal Separate Storm Sewer System) Permit issued to 13 local governments in Clackamas County. The permit’s primary purpose is to limit pollution discharges to creeks, wetlands and rivers from urban areas.

"I'm proud of the fact that a dozen separate units of local government worked together to design and fund this monitoring study in partnership with the USGS,” said Andrew Swanson, water quality analyst with Clackamas County's Water Environment Services. “Each of these local governments could have chosen to implement separate, small monitoring efforts without assistance from the USGS, but by working together, the cost for each participating local government was reduced, potential duplication of effort was avoided, and the quality of the final product was substantially increased."

Results of the study were published in the Journal of Environmental Monitoring and Assessment.

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