Mercury in the Hells Canyon Reservoirs of the Snake River

Preparing water samples in the Hells Canyon Complex; samples are taken at set depths at many sites to track mercury, methylmercury and other constituents. USGS.

The Hells Canyon Complex

The Hells Canyon Complex (HCC) is a hydroelectric project built and run by the Idaho Power Company (IPC): three dams on the Snake River along the Oregon–Idaho border, creating Brownlee, Oxbow and Hells Canyon reservoirs. Together they hold more than 1.5 million acre-feet and span about 90 miles of river.

• Long-standing problems: the Snake above and through the HCC has been impaired by excess nutrients, algae, sediment and other pollutants.
• Mercury: data since the 1960s show high mercury in fish and sediment, and recent studies find elevated mercury and methylmercury in the water, bottom sediments and wildlife.
• Listings: Idaho lists Brownlee and Hells Canyon reservoirs as impaired for mercury; Oregon lists the river from the Oregon–Idaho border through the HCC to the Oregon–Washington border.

The three dams and reservoirs of the Hells Canyon Complex. USGS.

Why study it now

Operators are weighing different ways to release water from Brownlee Reservoir, which could change water temperatures downstream — a priority because of species listed under the Endangered Species Act, such as bull trout and fall Chinook salmon. But with mercury in the HCC poorly understood, nobody could say whether changing releases would raise downstream fish's exposure to methylmercury.

So in 2014 the USGS, with IPC and the Idaho Department of Environmental Quality (IDEQ), began studying what controls mercury's transport, cycling and bioaccumulation in Hells Canyon. Its goals:

1. find where each form of mercury occurs in water, sediment and living things;
2. learn what controls its variation in space and time;
3. uncover how it's taken up, passed through the food web and accumulated.

How mercury cycles

Mercury (Hg) is a global pollutant that reaches every aquatic ecosystem through the water cycle, with serious potential effects on human health. It occurs naturally, but industrialization has raised its levels — likely causing high mercury in fish worldwide.

• Sources: coal burning, artisanal mining and incinerators send mercury into the air, where it travels long distances before settling and washing through watersheds. Atmospheric deposition probably supplies most mercury to most waters; geologic sources and point discharges can add locally.
• Methylation: deposited mercury is mostly inorganic — Hg(II). In water, bacteria can quickly convert it to methylmercury (MeHg), which matters because MeHg is much more toxic, enters the base of the food web easily, and biomagnifies efficiently.
• Where it happens: the exact mechanisms aren't fully understood and probably vary, but microbes that thrive where oxygen is scarce — including sulfate- and iron-reducing bacteria — do it, often at the sediment–water boundary, in low-oxygen water, and where organic matter decays.
• Undoing it: demethylation turns MeHg back into inorganic or elemental mercury — in the water mainly by sunlight, after which mercury can escape to the air, and in sediment by bacteria.

Mercury's pathways in water: the key step is inorganic mercury becoming methylmercury. It ends up in sediment, fish and wildlife, or returns to the air. USGS.

Into the food web

Phytoplankton likely absorb MeHg directly from the water, and the next level — likely zooplankton — eats MeHg-laden particles. Once in the food web, MeHg:

• bioaccumulates, because organisms take it up faster than they shed it;
• biomagnifies, rising two- to five-fold with each step up the food chain;
• builds up over a lifetime, so large, long-lived predatory fish and wildlife carry the most.

Eating fish has well-established health benefits, but it's also the main way people are exposed to MeHg — a concern when fish caught for commercial, subsistence or recreational eating carry high levels.

Zooplankton and fish-tissue samples show how mercury moves up the food web. USGS.

Mercury in Hells Canyon

The river's water quality, combined with the dams, may make the HCC efficient at producing methylmercury. Recent data show MeHg — and the share of mercury that is MeHg — in bottom sediments and deep water well above other natural waters and reservoirs in Idaho, Oregon and Washington, and elevated in the food web too.

Smallmouth bass of similar size collected by IPC in 2013: MeHg rose from the Brownlee inflow (river mile 348) down through the HCC, then fell below Hells Canyon Dam toward the Salmon River confluence (river mile 188). Of 198 fish:

Mercury in smallmouth bass 250–300 millimeters long, from the Brownlee inflow to above the Salmon River; data from Idaho Power Company. USGS.

Why the reservoirs may favor methylmercury

Mercury buildup isn't simply a matter of how much comes in: sites with similar inputs can yield fish whose MeHg differs by more than tenfold. But the HCC has traits known to raise susceptibility:

1. River becomes lake: in the upstream reach the Snake turns lacustrine (lake-like), dropping abundant nutrients, organic matter and sediment in the transition zone.
2. Summer layering: warming water forms a warm top layer (epilimnion) over a cold, dense bottom layer (hypolimnion); warm water, sun and nutrients fuel phytoplankton growth.
3. Oxygen runs out: sinking algae and organic matter decay, using up oxygen. The bottom layer and deep transition zones turn anoxic, and anaerobic bacteria — including mercury methylators — take over.
4. Seasonal pulse: MeHg in the bottom water rises through summer and autumn as Brownlee and Hells Canyon reservoirs stratify. In Brownlee, oxygen loss starts in spring near river mile 310, where the river becomes lake-like; by late summer the whole bottom layer is typically anoxic.
5. Turnover: in late autumn and early winter the layers mix, spreading MeHg through the water where it can be carried downstream. In spring the cycle begins again.

How organic enrichment and decay drive mercury cycling in Brownlee Reservoir. USGS.

Dissolved oxygen in Brownlee Reservoir, April 2015 (A) and September 2015 (B). USGS.

The study

Much of what's known about mercury in fresh water comes from natural lakes, so rivers and reservoirs are an important research gap. The HCC lets scientists ask:

• How does the inflowing river's water quality shape mercury in the HCC?
• How do stratification and turnover move mercury into and out of the complex?
• Are bottom sediments a store of mercury that can be methylated and released?
• Where in the water, and when, does MeHg enter the food web?
• Does food-web complexity make mercury build up more in the reservoirs than above or below them?

Routine work: regular water-column profiles; chemistry sampled at set depths in Brownlee and Hells Canyon reservoirs; inflow and outflow sampling for a mercury mass balance; and zooplankton and fish sampling.

Intensive assessments, twice a year: wider coverage of methylation rates in sediment and water, photodegradation and volatilization, the roles of organic matter and chemistry, and biomagnification through the food web.

Sediment cores test whether reservoir bottoms feed methylmercury into the deep water. USGS.

The study will likely take 5 to 8 years, adapting as results come in. It aims to give managers a better grasp of the hydrologic, geochemical and ecological processes behind mercury in the HCC; in the long run, to build a predictive model of methylmercury for weighing future operational and water-quality changes; and to inform other large reservoirs with mercury problems.

Sources

Based on Mercury Cycling in the Hells Canyon Complex of the Snake River, Idaho and Oregon, by Gregory M. Clark, Jesse Naymik, David P. Krabbenhoft, Collin A. Eagles-Smith, George R. Aiken, Mark C. Marvin-DiPasquale, Reed C. Harris and Ralph Myers, USGS Fact Sheet 2016–3051, U.S. Geological Survey (DOI), citing Harris and Beals (2013), Clark and Maret (1998), Essig (2010), Fosness and others (2013), Scudder and others (2009) and Wentz and others (2013, 2014); a work of the United States government in the public domain. Figures and photographs come from the fact sheet's PDF, which also restores its scrambled text.