The study also found that among the water sources tested, Sacramento’s regional wastewater treatment plant is the single largest source of pyrethroid pollution in the Delta. The plant discharges treated sewage into the Sacramento River near Freeport.

The reason for this contamination is less clear. People dumping unused pesticides into sink drains may be the cause of the problem. It could also come from consumer products, such as shampoos made to kill lice and fleas.

There is no evidence pyrethroids are harmful to people at typical consumer exposure levels. But they are proving harmful to aquatic life at very low concentrations.

The chemicals attach easily to soil. They can remain toxic in creek beds or landscaping for months, then hitch a ride downstream when over-watering or a storm washes topsoil into storm drains. Pyrethroid-based pesticides dominate the shelves at grocery and hardware stores and are commonly used in landscapes. Urban areas appear to be a much bigger source, with Sacramento by far the largest among the areas sampled in the Delta.

“Ultimately, if we took better steps to keep pests from entering our homes, or redefined what our landscapes should look like, we wouldn’t have to use these chemicals,” said Paul Towers, state director of Sacramento-based Pesticide Watch.