Fish in the Grand Canyon show levels of mercury and selenium that exceed risk thresholds for wildlife
FRISCO — Pollution runs deep in the Colorado River, according to U.S. Geological Survey scientists who recently documented traces of mercury and selenium contamination in fish living in the Grand Canyon.
Similar studies have documented mercury contamination in fish in Rocky Mountain National Park. In the bigger picture, the USGS has also documented mercury contamination in 25 percent of U.S. streams. In the Arctic, polar bears are being exposed to similar contaminants.
The Grand Canyon may be one of the most remote ecosystems in the lower 48 states, but the new study shows it’s not immune to exposure from toxic chemicals such as mercury. The findings are published in Environmental Toxicology and Chemistry.
After sampling fish in the Grand Canyon, the researchers said levels of mercury and selenium regularly exceeded risk thresholds for fish and wildlife.
“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, explaining that, in the modern world, nearly every place is vulnerable to long-range transport and bioaccumulation of contaminants.
Studies like this continue to educate the public and highlight the threats that face the park and its resources,” said Grand Canyon National Park chief David Uberuaga.
The study examined food webs at six sites along nearly 250 miles of the Colorado River downstream from Glen Canyon Dam within Glen Canyon National Recreation Area and Grand Canyon National Park in the summer of 2008. The researchers found that mercury and selenium concentrations in minnows and invertebrates exceeded dietary fish and wildlife toxicity thresholds.
Although the number of samples was relatively low, mercury levels in rainbow trout, the most common species harvested by anglers in the study area, were below the EPA threshold that would trigger advisories for human consumption.
The researchers explained their findings in a news release:
“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 transport and deposition — with much of it coming from outside the country — is most commonly identified as the mechanism for contaminant introduction to remote ecosystems, and this is a potential pathway for mercury entering the Grand Canyon food web.
Also, long-range downstream transport from upstream sources can deliver contaminants to river food webs. This is the case for selenium in this study, where irrigation of selenium-rich soils in the upper Colorado River basin contributes much of the selenium that is present in the Colorado River in Grand Canyon.
Exposure to high levels of selenium and mercury has been linked to lower reproductive success, growth, and survival of fish and wildlife. No human consumption advisories are currently in place for fish harvested from the study area. However, to assess potential risks to humans that may consume fish from Grand Canyon or Glen Canyon National Recreation Area, additional studies are planned.
Research partners in this study include the Cary Institute of Ecosystem Studies, Montana State University, and Idaho State University.