Mercury Cycling in the Hells Canyon Complex

Science Center Objects

The Hells Canyon Complex (HCC) is the largest privately owned hydroelectric power complex in the United States. Upstream are millions of acres of irrigated agricultural lands and Idaho’s largest metropolitan area clustered around the state capital Boise. Downstream lie confluences with the Salmon and Clearwater Rivers, critical habitat for threatened bull trout and fall chinook salmon.

Elevated concentrations of mercury and methylmercury in the water column, bottom sediments, and biota in this reach have resulted in two of the reservoirs, Brownlee and Hells Canyon, being listed as impaired for mercury by the state of Idaho, and the entire reach being listed as impaired for mercury by the state of Oregon. The objective of this study is to determine key processes controlling mercury transport, cycling, and bioaccumulation in the Hells Canyon Reach of the Snake River.

Getting into position to collect samples in Hells Canyon Reservoir

Getting into position to collect samples in Hells Canyon Reservoir

(Credit: Austin Baldwin, Idaho Water Science Center. Public domain.)

Although concentrations of inorganic mercury entering the HCC are typical in comparison to other sites in the U.S., biogeochemical conditions in Brownlee Reservoir, and to a lesser extent, Hells Canyon Reservoir, effectively promote the conversion of some of the mercury into the more toxic and bioaccumulative form, methylmercury. Fish tissue samples regularly exceed human health criteria for methylmercury set by states, such as Oregon and Idaho, and by the U.S. Environmental Protection Agency. Exposure to too much methylmercury can affect the human nervous system and can hamper a child’s development both in the womb and in early life.

A proposal to use cold water from Brownlee Reservoir’s lower layer to cool the Snake River downstream of the HCC raised concerns about exposing sensitive aquatic species to methylmercury-rich waters from deep in the reservoir. In 2012, the Idaho Water Science Center, with help from the Wisconsin Water Science Center, confirmed elevated mercury concentrations in water column and bed sediment core samples. In 2014, Idaho Power Company, the owner and operator of the HCC, asked the USGS for help to better understand the mercury dynamics in the reservoirs and how the proposed reservoir operations might affect downstream transport of methylmercury.

The scale of the study area and the multitude of research questions required a multidisciplinary approach. USGS Idaho Water Science Center hydrologist Greg Clark, who has since retired, invited Dave Krabbenhoft, leader of the USGS Mercury Research Laboratory, to help form a study team. As Dave puts it, “When Greg Clark and I started this effort, we had the opportunity to hand pick an all-star team, and we did!” Today, that team of USGS hydrologists, biologists, ecologists, chemists, and technicians from five states works alongside colleagues from the Idaho Power Company, Reed Harris Environmental, the University of Wisconsin, and Portland State University.

The hydrologic component of the study focuses on the mass loading of mercury and methylmercury into, within, and out of the HCC to better understand the hydrologic processes promoting methylmercury production and the fate of that methylmercury as the large reservoirs stratify and destratify seasonally. This part of the research team is composed primarily of scientists from the USGS Idaho and Wisconsin Water Science Centers and the Water Mission Area (WMA).

John Pierce, James Willacker, and Collin Eagles-Smith (USGS FRESC Corvallis, OR) on Hells Canyon Reservoir

John Pierce, James Willacker, and Collin Eagles-Smith (USGS Forest and Rangeland Ecosystem Science Center) on Hells Canyon Reservoir. 

(Credit: Austin Baldwin, Idaho Water Science Center. Public domain.)

The biological component of the study is examining how methylmercury is bioaccumulating—and biomagnifying—through the food web from phytoplankton to predator sportfish such as largemouth bass. The biological research is led by Collin Eagles-Smith of the USGS Forest and Rangeland Ecosystem Science Center.

The geochemical component of the study is seeking to pinpoint mercury sources and to peer inside the processes of mercury methylation. This portion of the study is led by Dave Krabbenhoft and by Mark Marvin-DiPasquale and Brett Poulin of the USGS Water Resources Mission Area.

The team is also developing a simulation model to help understand the present-day behavior of mercury in Hells Canyon Complex, especially factors leading to increases in fish mercury levels in the complex and predict the response of the system (and downstream) to future reservoir management scenarios. Mercury cycling is being added to CE-QUAL-W2, a well-established model originally developed by the US Army Corps of Engineers and maintained by Portland State University, that simulates hydrodynamics and water quality.  The model will simulate mercury in water, sediments, and the food web.

Mike Tate (USGS Upper Midwest Water Science Center) retrieving a sample from a sediment trap.

Mike Tate (USGS Upper Midwest Water Science Center) retrieving a sample from a sediment trap.

(Credit: Austin Baldwin, Idaho Water Science Center. Public domain.)

As the Hells Canyon study continues, the project serves as a prime example of how integrated USGS science can bring nationwide expertise to bear on a significant issue of regional concern. It also shows how the USGS can collaborate effectively with academia and the private sector to help solve economic, environmental, and societal challenges.