“Results of this study may provide a basis for the identification and evaluation of remediation activities related to mercury,” said lead author and USGS scientist David Naftz. “Findings indicate that more limited areas within Lake Powell, such as side canyons and river arms, could be targeted by resource managers for remediation strategies, if deemed necessary.”

Lake Powell serves as a critical Western U.S. reservoir and source of power generation. It attracts more than 3 million visitors a year, many of whom are striped bass anglers. Findings from an earlier study resulted in a 2012 fish consumption advisory for striped bass for the lower portion of Lake Powell.  Striped bass in the lower half of the lake consistently exceeded the U.S. Environmental Protection Agency human consumption advisory limits for mercury.

In the recent study, USGS scientists collected water, plankton, and sediment samples from Lake Powell in May/June 2014 and August 2015. Fish tissue samples were collected during November 2014 by the Utah Division of Wildlife Resources and analyzed by the USGS Mercury Research Lab. Data were compared to previous fish surveys to gain a better understanding of mercury dynamics within the reservoir.

“We know mercury concentrations differ in certain parts of the lake. Our focus was to better understand the conditions that tend to promote higher or lower levels of toxic methylmercury in sediment, water, fish and plants,” said Mark Marvin-DiPasquale, a leading USGS mercury scientist working on the project.

Mercury is a naturally-occurring metal that can pose a threat to humans, fish and wildlife if they are exposed to elevated levels of its most toxic form, methylmercury. In aquatic systems, methylmercury is created from inorganic mercury by microorganisms under particular environmental conditions, such as low oxygen. Therefore, the movement of inorganic mercury from the atmosphere or land to the water does not always result in equivalent levels of methylmercury in fish and wildlife unless environmental conditions are favorable for methylmercury production.

Results show levels of methylmercury generally increased from the upper reservoir to the lower reservoir. Narrow side canyons in the lower half of Lake Powell also showed higher levels of methylmercury. Mercury concentrations were found to be higher in tissue samples from striped bass collected from Wahweap Bay, located at the lower end of Lake Powell, than bass from Good Hope Bay at the upper end. Results also indicated that fish tissue samples collected in the San Juan Arm of Lake Powell contained higher mercury concentrations than fish tissue samples collected from the lower reservoir. The same factors that influence higher mercury concentrations in the lower reservoir likely influence higher mercury concentrations in the arms compared to the main stem. 

Conditions in the lower part of the reservoir were found to be more conducive for methylmercury production. Findings show that inflowing water from the Colorado River at the upper end of Lake Powell carries suspended sediment, which blocks light from penetrating very deep into the water. This limits the production of microscopic algae, or phytoplankton, in the upper portion of Lake Powell. As water moves farther downstream, the flow slows and suspended sediment settles to the bottom, allowing more light penetration. Increased light enhances phytoplankton production, resulting in oxygen consumption by microorganisms that feed on phytoplankton, which creates favorable low-oxygen conditions for methylmercury production.

Critical work remains to be done in Lake Powell related to mercury trends during different times of the year, geographic areas and habitat types as well as gaining a better understanding of the relative contribution of sediment versus water-column mercury methylation.

This study was funded by the USGS/National Park Service Water Quality Partnership program.
The goal of scientific studies associated with this program is to provide results to guide policy and management actions by the NPS that are aimed at protecting and improving water quality within NPS-administered lands.