Frogs living high in Sierra Nevada national parks such as Lassen Volcanic and Sequoia are slowly being "contaminated" by agricultural pesticides, according to a study that found the contamination even in "the most remote mountain locations."

Some frog species in the Sierras have faced other problems. Yellow-legged frogs in Sequoia and Kings Canyon national parks have been prey for non-native trout first introduced to the lakes in the 1850s, and chytrid fungus has been another blight on the population.

Pesticides used in California's Central Valley long have been suspected of contaminating these lakes, and now researchers with the U.S. Geological Survey have documented the connection.

Writing in Environmental Toxicology and Chemistry, they researchers confirm they've found pesticides in Pacific Tree Fogs in remote mountain areas, including national parks.

California’s Central Valley is one of the most intensely farmed regions in North America, producing 8 percent of U.S agricultural output by value, according to the USGS. While the use of pesticides such as triazines, endosulfan and organophosphates is common across the United States, California uses more pesticides than any other state.

“Our results show that current-use pesticides, particularly fungicides, are accumulating in the bodies of Pacific chorus frogs in the Sierra Nevada,” said Kelly Smalling, a research hydrologist from the USGS. “This is the first time we’ve detected many of these compounds, including fungicides, in these remote locations."

The Pacific Chorus frog Pseudacris Regilla can be found in abundance across California’s Sierra Nevada mountain range. As with other amphibians, agrochemicals potentially pose a threat to Chorus frogs, as exposure to pesticides can decrease their immune system, thereby increasing the risk of disease.

The USGS researchers collected frogs, as well as water and sediment samples, from seven ponds ranging from Lassen Volcanic National Park at the northern most point of Central Valley, to the Giant Sequoia National Monument in the valley’s southern extent. All sites were downwind of agricultural areas.

“The samples were tested for 98 types of pesticides, traces of which were found in frog tissues from all sites,” said Smalling. “We found that even frogs living in the most remote mountain locations were contaminated by agricultural pesticides, transported long distances in dust and by rain.”

Two fungicides commonly used in agriculture -- pyraclostrobin and tebuconazole -- and one herbicide, simazine, were the most frequently detected compounds. According to the researchers, this is the first time these compounds have ever been reported in wild frog tissue. Another commonly detected pesticide was DDE (Dichlorodiphenyldichloroethylene), a breakdown product of DDT that was banned in the United States in 1972. The continued presence of a DDT byproduct reveals how long this banned chemical can impact biodiversity, the USGS release pointed out.

Documenting the occurrence of these compounds is an important first step in figuring out the health consequence associated with the exposures.

“Very few studies have considered the environmental occurrence of pesticides, particularly fungicides which can be transported beyond farmland,” concluded Smalling. “Our evidence raises new challenges for resource managers; demonstrating the need to keep track of continual changes in pesticides use and to determine potential routes of exposure in the wild.”