TSF responds to Senator Gaines: Sound science explains the Sierra mercury problem

By:  The Sierra Fund CEO Elizabeth “Izzy” Martin, and Science Director Carrie Monohan, Ph.D.

We would like to provide some information in response to Senator Ted Gaines’s statements about mercury in his June 12 letter in the Sacramento Bee, “Another View: Dredging ban steals property rights” (June 12, 2011), which was written in response to the Bee’s editorial,”A smart budget trim will help water quality” (June 3, 2011).  The behavior of mercury in our water ways is complex, and we hope this information from established studies helps reach a resolution that will protect the health of all Californians.

Contrary to the senator’s column, mercury is only in the water of the Sierra Nevada rivers and streams because of Gold Mining. In fact, millions of pounds of mercury were brought to the Sierra Nevada to be used during 19th Century gold mining operations. Dumping mercury into sluice boxes was a standard and also a messy process with an estimated 10-30% “lost to the environment” (Alpers et al. 2005, http://pubs.usgs.gov/fs/2005/3014/). The result is that liquid elemental mercury is entrained in the river gravels of the streams and rivers today where historic gold mining took place 150 years ago. It can be seen with the naked eye, and it can be sucked up with a turkey baster (no joke). This is how polluted our Sierra rivers and streams are as a result of the Gold Rush, the State’s longest neglected environmental disaster.


Mercury is “naturally” attracted to Gold. This is the physical property that allowed miners to used mercury to extract fine grain gold, commonly referred to as amalgamation. Not only do gold and mercury bind together, they travel the same way down rivers.  This property accounts for its historical usefulness and its present day nuisance. The recreational suction dredge miners are out looking for nooks and crannies where gold gets stuck behind rocks in rivers, and those are the same places that mercury get stuck because it is heavy like gold and falls out in the same areas where gold does. What this means is that looking for gold often means finding nuisance mercury.  59% of the suction dredge miners in a recent survey by California Department of Fish and Game reported finding mercury as part of their operation.

The elemental mercury present in our rivers from historic mining can be converted by bacteria to methyl mercury, which is the form most harmful to human health. Recent peer-reviewed research confirmed that recreational dredgers don’t just encounter mercury when they are out dredging for gold, they disturb, alter and disperse some of it in a form that is more harmful, because it is more biologically available, more likely to methylate, and more likely to be incorporated into the aquatic food chain (Fleck et al 2010, http://pubs.usgs.gov/of/2010/1325A, Humphreys et al. 2005). This is different than what winter storm conditions do, because recreational suction dredging happens in the summer when biological activity is higher (which means there are more active methylating bacteria due to warmer temperatures and anoxic environments).  Additionally,  unlike winter storms that move everything, recreational dredgers are targeting pay dirt, areas where gold and mercury have accumulated and are concentrated.

Finally and most importantly, the physical agitation of the dredge “flours” or atomizes liquid elemental mercury, breaking it down into small fine particles, like a spray bottle does to water. The small particles of mercury are what come out of the dredge effluent and are deposited back into the river. This form of mercury is called reactive mercury.  Because its surface area is increased (small surface area to volume ratio) and because the oxygenated coating has been broken off, reactive mercury is more likely to methylate and contaminate the aquatic food chain  (Marvin-DiPasquale et al. 2010, http://pubs.usgs.gov/of/2010/1325B/pdf/ofr20101325b.pdf).

Once mercury is methylated by sulfur-reducing bacteria at the bottom of the food chain, it then moves up the food chain and bioaccumulates in large predatory fish. Why do we care? Because people eat large predatory fish like bass, and so do birds, otters and other wildlife.
Mercury is a powerful neurotoxin that can cause developmental delays in fetuses.  As noted by US Senator Collins (R-Maine) when she introduced a bill this month to establish mercury monitoring nationally,  “Mercury is one of the most persistent and dangerous pollutants that threatens our health and environment today. This powerful toxin affects the senses, the brain, spinal cord, kidneys and liver. It poses significant risks to children and pregnant women, causing an elevated risk of birth defects and problems with motor skills.”

Mercury pollution is a national issue, but here in California, in the historic Gold Country, we have an especially atrocious contamination problem.  Suction dredge miners encounter and disperse mercury in the aquatic environment in a way that degrades the aquatic ecosystems.  We must take responsibility not to make the problem worse by allowing a recreational activity like suction dredging to take place in areas where there is legacy mercury contamination.


Alpers, C.N., Hunerlach, M.P., May, J.T., and Hothem, R.L., 2005, Mercury contamination from historical gold mining in California: U.S. Geological Survey Fact Sheet 2005–3014, version 1.1, 6 p. (http://pubs.usgs.gov/fs/2005/3014).

Fleck, J.A., C.N. Alpers, M. Marvin0DiPasquale, R.L. Hothem, S. A. Wright, k. Ellett, E. Beaulieu, J. L. Agee, E. Kakouros, L. H. Kieu, D.D. Eberl, A. E. Blum, and J.T. May, 2010, The Effects of Sediment and Mercury  Mobilization in the South Yuba River and Humbug Creek Confluence Area, Nevada County, California: Concentrations, Speciation, and Environmental Fate-Part 1: Field Characterization. U.S. Geological Survey Open-File Report 2010-1325A, 103 p. (http://pubs.usgs.gov/of/2010/1325A).

Humphreys, R. 2005. Mercury losses and recovery during a suction dredge test in the South Fork of the American River: Sacramento, Califor., California State Water Board Staff Report, 6 p.

Marvin-DiPasquale, M., J. L. Agee, E. Kakouros, L.H. Kieu, J.A. Fleck, and C.N. Alpers. 2010. The Effects of Sediment and Mercury Mobilization in the South Yuba River and Humbug Creek Confluence Area, Nevada County, California: Concentrations, Speciation, and Environmental Fate-Part 2: Laboratory Experiments. U.S. Geological Survey Open-File Report 2010-1325B, 53 p. (http://pubs.usgs.gov/of/2010/1325B/pdf/ofr20101325b.pdf).