Mercury, Selenium Accumulating In Grand Canyon Stretch Of Colorado River

Aug 24, 2015

The Colorado River rushes through the Grand Canyon in a powerful pulse. Trips down the remote river are legendary for knock-down rapids and gorgeous grottos. Being far from civilization, though, apparently doesn’t offer much help when it comes to keeping out pollution.

A study released in the journal Environmental Toxicology and Chemistry found high levels of two serious contaminants in the Colorado River through the Grand Canyon.

The two contaminants tested for were mercury and selenium. In high concentrations, both can damage developing animals and have negative health effects. Humans are also vulnerable to the pollutants, although the study did not find high levels of mercury in rainbow trout, the primary fish that people would be eating from that stretch of rivers.

David Walters, an ecologist with the U.S. Geological Survey in Fort Collins who led the study, said it is notable that such a remote river had higher levels of contaminants than many other large U.S. rivers.

“You’d think ‘oh, the Grand Canyon, it’s far from everything, there is nobody living there.’ But these remote ecosystems are really vulnerable.”

The reason? Mercury is everywhere, mostly in the atmosphere. A lot of it comes from burning coal in power plants. While Walters and his team didn’t trace the mercury back to specific sources, he said once it gets into the atmosphere, it can travel far, and be deposited by rain, or even in dry conditions.

The selenium problem is more site specific. The upper part of the Colorado River runs through areas that were once part of an ancient ocean, said Walters.

“So basically those oceans have dried out now and we are farming on those and they are naturally high in selenium,” he said. “The problem is when you irrigate on those kind of lands.”

Irrigation flushes the selenium out of the soil and into tributaries of the Colorado River -- and eventually down into the Grand Canyon. In Colorado, selenium levels exceed water quality standards in over 7,000 miles of streams, making it the most widespread water pollutant in the state, according to Environmental Protection Agency data

The study find high levels of selenium and mercury in organic matter floating in the river as well as in small fish species like speckled dace and fathead minnows. That pollution can get transferred up the ecosystem as bigger fish and birds eat small animals with high concentration of those metals, said Walters.

Walters and other scientists plan to continue researching how the contaminants are affecting other animals and the greater Grand Canyon ecosystem.