Study Shows Higher Pyrethroid Contamination in Urban Runoff

A recent study published in Environmental Science and Technology reveals that urban runoff is a primary source for toxic pyrethroids in the Sacramento-San Joaquin Delta and the American River.

Pyrethroids are used in many pest control products in place of what were thought to be more toxic compounds--organophosphate pesticides, such as diazinon and malathion.

Pyrethroids are synthetic chemical compounds that are modified versions of natural chemicals (pyrethrins) that are produced by some chrysanthemums. There has been a rise in the urban use of pyrethroid insecticides since about 2000. In comparison to the pesticides they have replaced, pyrethroids are less toxic to humans and other mammals, but they are highly toxic to aquatic invertebrates.

The study, “Urban and Agricultural Sources of Pyrethroid Insecticides to the Sacramento-San Joaquin Delta of Sacramento” by biologists Donald P. Weston and Michael J. Lydy, showed that residential runoff to the Delta has higher pyrethroid contamination than agricultural runoff. Nearly all the residential runoff samples within the study were toxic to the amphipod Hyalella azteca, a common crustacean in aquatic ecosystems used as an indicator of environmental health and water quality. All samples contained pyrethroids at concentrations that exceeded known amphipod toxicity thresholds-some by as much as 10 times those levels.

“In the past, pesticides have often been approved for use with minimal data on potential environmental effects,” Weston, a biologist at the University of California, Berkeley, said. “For example, pyrethroids have been in use for decades, but it wasn’t until the past few years that we could measure their concentrations in the environment at levels that cause acute toxicity, and even today we can’t measure concentrations that would cause sublethal or chronic toxicity, such as slower growth or reproductive impairment. How is environmental risk to be mitigated when the concentrations that cause harm can’t be measured?”


“How is environmental risk to be mitigated when the concentrations that cause harm can’t be measured?”

--Donald P. Weston, UC Berkeley biologist


Urban runoff, municipal wastewater treatment plants and agricultural drains in the Sacramento-San Joaquin River Delta were sampled to understand their importance as contributors of these pesticides to surface waters.

Pyrethroids are used for agricultural pest control and also in urban settings for applications around structures. Possible sources of pyrethroids to municipal wastewater treatment plants include stormwater runoff that enters the sewer system, sewer disposal of household insecticides, lice control shampoos, pet products containing pyrethroids, and the laundering of permethrin-treated clothing used for mosquito protection. The researchers also found that pyrethroids pass through secondary treatment at municipal wastewater treatment facilities, and can be detected in the final discharge at levels that are toxic to Hyalella azteca.

The species used in the study, Hyalella azteca, is more sensitive to pyrethroids than some of the other more common species used for this type of test like Ceriodaphnia dubia (water flea.) The lethal concentration for Hyalella azteca is only 2 nanograms per liter (ng/L) as compared to C. dubia where the lethal concentration is over 50 ng/L. This made their toxicity tests more sensitive and also appropriate for the Delta since Hyalella azteca is a native species.

“Toxicity in standard tests provides an indicator of potential effects on residents, but with Hyalella, we are even one step further along in demonstrating ecological relevance,” Weston said. “It is a common species throughout the Delta so toxicity to it directly translates to toxicity to a resident species. But it would be desirable to have more information on other residents. Data on more resident species would be helpful in building an even stronger case for ecological relevance. That would include potential impacts to fish species of particular concern, possibly directly but more likely indirectly through the food web.”

“The occurrence of pyrethroids, especially bifenthrin, above toxic thresholds in most urban runoff and their previously unrecognized presence in municipal wastewater, indicate further investigation is needed into how their use or misuse has led to these results,” said the paper from Weston and Lydy.

“We have abundant data that pyrethroids are present at concentrations acutely toxic to the standard testing species, Hyalella azteca, throughout the Delta and California in general,” Weston said.

“We need to break the cycle of approving new pesticides with minimal environmental data or without appropriate analytical methods, and then forcing environmental chemists and toxicologists to play decades of catch-up,” Weston said.