A UNSECO World Heritage site and the 15th oldest national park in the U.S. the Grand Canyon is about as far removed from the congestion and pollution associated with our nation’s industrial and urban centers. And yet, according to a study by the United States Geological Survey, even the mighty Grand Canyon (and the Colorado River that forms it) isn’t immune to pollution: The levels of mercury and selenium now regularly exceed the “risk threshold” for fish and local wildlife.

Should the toxic elements enter the food supply, they could be harmful to fish and wildlife that eat them. It seems that the Grand Canyon’s remote location is of little import – the pollutants are airborne, and come from as far away as entire other countries.

In examining six food webs along the Canyon in 2008, the researchers found that mercury and selenium concentrations in both minnows (small fish) and invertebrates were above the EPA’s recommended toxicity thresholds for dietary fish and wildlife. Surprisingly, rainbow trout (the species most commonly harvested by fishermen) were not yet affected to a degree that makes them unsafe for human consumption.

“The good news is that concentrations of mercury in rainbow trout were very low in the popular Glen Canyon sport fishery, and all of the large rainbow trout analyzed from the Grand Canyon were also well below the risk thresholds for humans,” said Dr. Ted Kennedy, USGS researcher and co-author of the study.

“We also found some surprising patterns of mercury in rainbow trout in the Grand Canyon. Biomagnification usually leads to large fish having higher concentrations of mercury than small fish. But we found the opposite pattern, where small, 3-inch rainbow trout in the Grand Canyon had higher concentrations than the larger rainbow trout that anglers target. This inverted pattern likely has something to do with the novel food web structure that has developed in Grand Canyon.”

Airborne pollution accounts for much of the problem, but in the case of selenium, irrigation of selenium-rich soils far upstream is the main factor. While at certain levels it can harm wildlife, the USGS says that as of now there are no selenium advisories for any fish or wildlife caught in the area.

While the Grand Canyon’s rainbow trout are presently safe to eat, mercury is a big problem for the world’s fisheries. Popular fish like marlin, swordfish and ahi tuna are on the Natural Resource Defense Council’s “Do Not Eat Ever” list, and American staples like cod, mahi mahi and lobster are not recommended for consumption more than six times per month.

“Managing exposure risks in the Grand Canyon will be a challenge, because sources and transport mechanisms of mercury and selenium extend far beyond Grand Canyon boundaries,” said Dr. David Walters, USGS research ecologist and lead author of the study.