It’s one of the most recognizable features in the United States, and pollution may be killing it before our very eyes: mercury is threatening the Grand Canyon.

The U.S. Geological Survey has published a study in the journal Environmental Toxicity and Chemistry that determines that concentrations of the toxic substances of mercury and selenium, which is impacting so-called “food webs” in the canyon. This means that fish and wildlife may be getting unsafe doses of these substances, according to a Discovery News report.

USGS research ecologist Dr. David Walters, the study’s lead author, said according to the report that it will be a challenge the manage the exposure risks in the Grand Canyon because the sources of mercury and selenium come from far beyond the boundaries of this geological wonder.

The research team collected data from six sites on the Colorado River, which carved out the Grand Canyon many years ago and today serves as the main source of water snaking its way through the canyon. Scientists found concentrations of mercury and selenium that were above what minnows, fish, and invertebrates can consume and still be considered within safe levels. In addition, the mercury concentrations were above what would be safe for humans to consume.

Mercury is an extremely deadly substance, a neurotoxin that can damage the central nervous system for both humans and wildlife. Selenium is also dangerous in that it can result in symptoms like hair and tooth loss or the development of tumors. Interestingly, the selenium is cancelling out the mercury levels in rainbow trout so there haven’t been many deformities — that doesn’t make it safe for humans, however.

The rainbow trout had mercury levels that were below federal thresholds, which was a silver lining int he report.