The findings come from a study linked to the U.S. Geological Survey, which scientists published in the journal Environmental Toxicity and Chemistry. According to USGS research ecologist and lead author Dr. David Walters, "Managing exposure risks in the Grand Canyon will be a challenge, because sources and transport mechanisms of mercury and selenium extend far beyond Gran Canyon boundaries."
Scientists gathered data from six areas running down the Colorado River, which cuts through the canyon at the very bottom. They recorded high concentrations of mercury and selenium in minnows, insects, and larger fish, which means bad news for the mammals and birds that eat these organisms.
The problem represents biomagnification, or the process by which organisms low on the food web soak up ambient pollution and concentrate it over time inside of their predators higher up. If a beaver eats ten fish with unsafe mercury levels, it's worse for the beaver than for the fish.
There was a glimmer of good news, however. Rainbow trout, one of the most commonly caught game fish in the Canyon, were largely below the toxic threshold for mercury and selenium. Humans who ate the fish, however, were likely still at risk.
Selenium pollution can come from runoff from farms and mines, but levels in the Grand Canyon's soil are already naturally high. Most mercury pollution comes from burning coal at distant power plants, though the scientists suspect that much of the mercury in the Grand Canyon comes from algae in Lake Powell, which feeds the Colorado River.