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Environmental Health - Toxic Substances

Toxics Program Remediation Activities

Assessing Bottom Sediments as a Source of Mercury Contamination

  • Feasibility Studies
  • Site Characterization

Camp Far West Reservoir, California


To Be Determined

  • Mercury
  • Methylmercury
benthic flux bank view
View of the bank of the Camp Far West Reservoir, California
Click on image for larger version

The Challenge

The Sierra Trinity Abandoned Mine Lands Agency Group is faced with the difficult challenge of what to do about fish contaminated with mercury in the Camp Far West Reservoir and the Bear River and South Yuba River Watersheds, California. The fish are bioaccumulating mercury from contaminated sediments washed down from long-abandoned gold mines in the Sierras (see inset). The contaminated sediments have accumulated along river banks and flood plains, and on the bottom of reservoirs. Several reservoirs and rivers receiving drainage from abandoned gold-mining areas in the foothills of the Sierras do not meet water-quality standards. Resource managers are tasked with remediating the sources of contamination to comply with Total Maximum Daily Load (TMDL) allocations. The dual objectives of this task are to decrease concentrations and loads in rivers and streams and to reduce the bioaccumulation of mercury in fish consumed by humans and wildlife.

Mercury and Gold Mining

Miners used mercury to recover gold from gold ore and gold-containing sediments (placer deposits). Mercury was commonly mixed with a slurry of gold-bearing rocks and sediments to form gold-mercury amalgam. The loss of mercury at recovery operations was often as high as 30 percent, and the mercury-laden waste from this process was washed into nearby streams in most cases. A fact sheet is available that has more information on mercury contamination from historic gold mining.

Not All Mercury Is Alike

Mercury occurs in several forms, such as pure elemental mercury and the extremely toxic methylmercury. Methylmercury is of great concern in rivers and reservoirs because minute amounts of it can bioaccumulate in fish to levels that are toxic to humans. With each step in the food chain, concentrations of methylmercury can increase by ten times, which can result in worrisome concentrations of methylmercury in the game fish that are at the top of the food chain. This process is call biomagnification. Fact sheets on mercury in aquatic ecosystems and fish are available.

Mercury-Contaminated Sediments

Several man-made reservoirs in the foothills of the Sierras have been accumulating mercury-contaminated sediments. USGS scientists have demonstrated that mercury and other metals in the bottom sediments of reservoirs, lakes, and estuaries can be moved into the overlying water through chemical and biological activities. The rate that solutes, such as methylmercury, move in and out of sediments, known as the benthic flux, can be positive (into the water from bottom sediments) or negative (out of the water into bottom sediments). The magnitude and variability of the benthic flux determines whether or not methylmercury in the bottom of reservoirs represents a significant source of this toxic form of mercury to the environment. Understanding the magnitude of this flux is a critical part of the equation for determining remediation objectives and setting allocations for the TMDL process.

The Question

How significant are sources and sinks of dissolved total mercury and dissolved methylmercury associated with the bottom sediments of reservoirs relative to major surface-water inputs? This was the question posed to USGS scientists for the Camp Far West Reservoir, California. The Bear River has transported mercury-contaminated sediments into the reservoir and these sediments are stored in the bottom of the reservoir. The bottom sediments have been known to be a potential source of mercury, but the magnitude of the source was not known. USGS scientists applied methods developed by the USGS’s Toxic Substances Hydrology Program to estimate the magnitude of the bottom sediments in the reservoir as a source of mercury by quantifying the benthic flux of mercury.

boat on the lake
USGS scientists on a boat used for collecting cores from the bottom of the Camp Far West Reservoir, California, for benthic flux analysis. A davit and winch used for core sampling is on the right. A davit is a small crane used on ships to lower objects over the side.
(Click on image for a larger version)

The scientists collected sediment cores from three locations in the reservoir. The cores were taken to a laboratory where incubation experiments were conducted. During the incubations, concentrations of mercury species in water overlying the cores were monitored over time. Linear regressions of the time-series data were used to calculate the benthic flux of the solutes.

Results from the experiment showed that the benthic flux of mercury and methylmercury into the reservoir varied with location and time of year. A significant (and possibly predominant) percentage of dissolved total mercury in the reservoir’s water presently comes from the bottom sediments in the reservoir. The flux of total dissolved mercury was in the same range as the total dissolved mercury that rivers were discharging into the reservoir. Methylmercury flux was consistently lower than the load of methylmercury from the rivers. Understanding this variability will be key to developing and assessing the feasibility of any potential plans for remediation of contaminated sediments in the reservoir.

No Free Lunch: Balances in Environmental Systems

graph of benthic flux
A comparison of the amount of mercury moving out of bottom sediments (symbols) to the amount of mercury flowing into the reservoir from rivers (bars), Camp Far West Reservoir, California.
(Click on image for a larger version)

One of the options for resource mangers to reduce the mercury contamination in fish is to control or reduce the amount or load of mercury the Bear River and others discharge into the reservoir. This would be an advantageous action even if the reservoir was not a concern; however, the decrease in mercury loads from the rivers would most likely be compensated initially by increases in the benthic flux of mercury from the bottom sediments. This is because the management-induced change would result in a greater concentration difference between the water in the reservoir and the bottom sediments at the sediment-water interface. The magnitude of the benthic flux would be enhanced by larger concentration differences at the interface. The long-term outlook for this option is that concentrations of mercury in the reservoir are expected to decrease slowly after loads from rivers are controlled due to the burial of contaminated sediment with sediments containing fewer contaminants.

Remediation: What is Being Done

The USGS has been a participant in the Sierra Trinity Abandoned Mine Lands Agency Group. A major focus of the group is to determine the distribution, transport, and fate of mercury and methylmercury in the vicinity of historic hydraulic placer-deposit gold mines. The Group is coordinating its efforts to remediate or remove contamination sources in the watershed that pose the greatest risk and implement management systems that reduce the load of mercury in the watershed’s rivers and streams so water-quality objectives can be met. The results from the study of mercury in the sediments of the Camp Far West Reservoir, California, are being used to guide remediation efforts of the Group.

Technology Transfer

At Camp Far West Reservoir the USGS is applying methods that were developed by the Toxic Substances Hydrology Program for an investigation of the cycling of toxic metals in the southern part of San Francisco Bay. The methods have also been applied at investigations of contaminated sediments in Lake Coeur d'Alene, Idaho, and Lahontan Reservoir, Nevada.

map of California
Location of coring sites at Camp Far West Reservoir, California.
(Click on image for a larger version)

More Information
  • James Kuwabara, USGS, National Research Program, Menlo Park, CA,
  • Brent R. Topping, USGS, National Research Program, Menlo Park, CA,
  • Mark Marvin-Dipasquale, USGS, National Research Program, Menlo Park, CA,
  • Paul F. Woods, USGS, Idaho Distric, Boise, ID,
  • Charles N. Alpers, USGS, California Water Science Center, Sacramento, CA,
  • A. Robin Stewart, USGS, National Research Program, Menlo Park, CA,

Other Studies Using the Same Methods to Determine the Benthic Flux of Metals

USGS Studies Related to Mercury Remediation

Mercury Information

San Francisco Bay Information

Back to Toxics Program Remediation Activities Index

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Page Last Modified: Wednesday, 23-Nov-2016 12:28:20 EST