Toxic Pesticides In Two County Creeks

Contra Costa — Grayson Creek and Dry Creek contain toxic amounts of pyrethroid pesticides, according to a new water-monitoring study by the Contra Costa Clean Water Program.

The Contra Costa Clean Water Program conducted Stressor/Source Identification Studies (SSID) in 2014 to determine the causes of toxicity in the two local creeks. The study found that pyrethroid levels in Dry Creek are two to four times higher than the amount considered safe. In Grayson Creek, the levels are more than three times the acceptable levels.

Pyrethroids are a class of synthetic pesticides used widely to control ants, fleas and roaches in homes, businesses and agriculture. Studies have found that pyrethroids are the primary cause of toxicity in urban water bodies throughout California. Researchers suspect the source of the pesticide pollution in Contra Costa County is lawns, gardens, buildings and homes.

“Pyrethroids applied to impervious surfaces like driveways, sidewalks, foundations or any asphalt or concrete surface are a particular problem,” says environmental scientist Armand Ruby, who contributed to the study. “When it rains, the pesticides run directly into the storm drains and out into the creeks.”

The study also found increased levels of other pesticides and compounds, including fipronil, which is commonly used to control ants, beetles cockroaches, fleas and termites, as well as DDT. The United States banned the use of DDT in 1972; however, the insecticide remains highly persistent in the environment today.

In the study, researchers sampled creek water during wet weather and sediment during dry weather months from both upstream and downstream in the creeks. Across the board, pesticide concentrations were higher in Grayson Creek than in Dry Creek, but pesticides were found in high amounts in both creeks.

  • Six different pyrethroids (bifenthrin, cyfluthrin, cypermethrin, deltamethrin, lambds-cyhalothrin, permethrin), as well as fipronil and four DDT breakdown products were found.
  • Pyrethroid content in water samples taken from Dry Creek was 1.4 – 1.9 times higher than acceptable levels.
  • Pyrethroid content in water samples from Grayson Creek was 2.0 – 3.5 times higher than acceptable levels.
  • Pyrethroids in Dry Creek’s sediment was 4.3 times higher than acceptable levels, while Grayson Creek’s sediment was not acutely toxic.

“Pyrethroids are creating a serious environmental problem in Contra Costa County and throughout California,” says Tom Dalziel, program manager at Contra Costa County Clean Water. “It doesn’t stop with Dry Creek or Grayson Creek. The impact of these pesticides spreads downstream to the San Joaquin Delta and out into the San Francisco Bay. The only way to turn this around is to curb pesticide use at the source—in our homes and on our lawns and gardens.”

The Contra Costa Clean Water Program is running three public education campaigns to educate residents on the dangers of toxic pesticides and offer effective alternatives.

The three campaigns aim to address all residents, whether they tackle pest control on their own, hire pest control operators or are gardeners tending their own garden. The first campaign helps do-it-yourself folks consider less toxic options when shopping for pest control ( The second promotes hiring local eco-certified pest control operators (, and the third focuses on sharing ways to combat pests naturally in the garden (


The Contra Costa Clean Water Program is a coalition of 19 cities in Contra Costa dedicated to keeping harmful pollutants from entering local creeks, rivers, the Delta and the Bay.


For more information on the study or to schedule an interview, please contact

Rachel Dowd at 510-224-5086 or [email protected].