Dredging Up the Past: The Danger of Remobilizing Heavy Metals

In 1898, the discovery of gold in Nome, Alaska triggered a new gold rush. Miners commonly used mercury to recover gold from ore and, over time, mercury and unrecovered gold accumulated in the aquatic environment and settled into the sediment below. Now, over 100 years later in Nome, summer gold prospectors seek their fortune in the mercury-laden sediments dredged up from the sea floor. Using shovels, sluice boxes, and boat-mounted excavators to dig up sediments, miners look for gold and remobilize toxic heavy metals in the process.

Heavy Metals

Heavy metals commonly enter aquatic environments as the result of natural processes, such as rock weathering and soil erosion. At low concentrations, these metals support metabolic activity in aquatic organisms and have little to no detrimental effect. However, at higher concentrations, these heavy metals can become toxic. Releases associated with anthropogenic activities such as mining, municipal wastewater treatment, manufacturing, and the use of fertilizers and organochlorine insecticides have significantly increased heavy metal concentrations in aquatic environments.

Sediments serve an important environmental role in absorbing heavy metals. Metals such as cadmium, chromium, copper, lead, and mercury precipitate out of the water over time and become concentrated in the sediments below. When remobilized, these sediments may act as a non-point sources of pollution, releasing high concentrations of heavy metals and directly impacting overlying waters, aquatic species, and other organisms dependent upon the contaminated water source for nourishment.

Heavy Metals in the Aquatic Environment

Water and sediment contamination has a significant correlation with the heavy metal content of aquatic organisms, such as fish. However, studies show that heavy metal levels are higher in aquatic organisms than in the surrounding environment. This is due to bioaccumulation, a process by which the amount of heavy metals present in an organism progressively increases over time because the rate of intake exceeds the rate at which the body can eliminate the substance.

Heavy metal contaminants in water sources have triggered significant fish and bird kills. In Montana, a once large, upstream copper mining and processing industry contaminated the upper Clark Fork River environment with toxic heavy metals. By the mid-1950s, high levels of copper and other metals caused significant fish kills, and most of the fish that once inhabited the area disappeared. Presently, efforts are in place to remove and clean up contaminated sediment, and fish have returned to the river. However, the negative impacts still linger. Osprey chicks in the area, identified as an indicator species because they eat fish from a specific area located very close to the nest, show blood mercury levels one hundred times the level considered problematic for humans. Further, blood mercury levels will increase over time as these birds mature and consume more heavy metals.

The Threat to Human Health

Remobilized heavy metals may also present a significant threat to human health. Consuming fish contaminated with heavy metals carries significant health risks, which may include, among other things, still-births and miscarriages; hypertension; severe damage to the body organs and the nervous, digestive, and immune systems; and even death.

For thousands of years, residents of Nome have relied upon fishing activities for “cultural and nutritional sustenance.” Today, these fishing activities continue in Nome’s public mining areas and other offshore locations where miners dredge up sediments and toxic metals. Although health officials in Alaska express concern with the risks miners face from exposure to mercury, the effects of dredging up heavy-metal rich sediments may be farther reaching. Where dredging and other releases of long-sequestered sediments occur in areas contaminated with heavy metals as a result of human activities, these operations have the potential to remobilize heavy metals, severely impacting the aquatic environment and local animal and human populations.


The title picture is of a gold mining dredge in Chatanika, Alaska. The picture is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license to Darren Giles. The use of this picture does not in any way suggest that Darren Giles supports this blog.



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Characterization of the Ashepoo-Combahee-Edisto (ACE) Basin, South Carolina: Sediment Contaminants, National Estuarine Research Reserve System, available at http://www.nerrs.noaa.gov/doc/siteprofile/acebasin/html/modules/watqual/wmsedcon.htm (last visited April 1, 2014).

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Yereth Rosen, Alaska, concerned about gold miners’ health, to test them for mercury, Reuters (Aug. 25, 2012), http://www.reuters.com/article/2012/08/25/us-usa-alaska-miners-idUSBRE87O09220120825.