SUCTION DREDGING AND THE ENVIRONMENT
The Western Mining Alliance and our members are strong supporters of a clean environment and for
over thirty years suction dredgers have been removing toxic mercury from California watersheds.
According to a survey conducted in 2009 by the California Department of Fish and Game suction
dredgers are removing nearly 14,000 ounces of mercury a year [Ref 9]. In addition to mercury the dredges
are removing lead and other heavy
metals from the watershed. The
results are proven in the US EPA
Report to Congress which shows that
California trout and other fish are
some of the cleanest fish in the nation
[Ref 8]. The trout in California streams
are far below US EPA thresholds for
consumption and are in fact less than
one third the level of mercury of trout
in streams that have never seen
suction dredging.
The river shown in the picture to the
right is very typical of gold mining
rivers and streams. This river has
Typical Rugged Gold Country Stream
been continuously dredged for over
thirty years and is still a pristine
alpine river with no affects from dredging. Studies by the US Geological Survey show that a typical
dredge spends about 90% of the time in clean, loose packed gravels. The same study measured the
mercury output from an operating three inch dredge in 2007 and found that mercury output was below
the threshold detection limits – in other words no mercury could be measured from the dredge while it
was operating [Ref 5].
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OVERVIEW OF SUCTION DREDGING
A suction dredge utilizes an engine, typically the same size as a lawn mower engine, a hose and a
recovery system known as a sluice box. This motorized equipment vacuums gravel from the bottom of a
river and processes it over the sluice box then returns the cleaned gravel to the riverbed free from heavy
metals including mercury, iron and lead.
A suction dredge is engineered to
recover the heavy material from the
streambed while returning the lighter
materials such as gravels back to the
stream. These lighter materials are the
natural gravels of the riverbed. Dredges
are found on rivers in the mountains
where historically gold has been found.
Gold Dredges operate on Federal mining
claims. A Federal mining claim allows a
miner the right to recover valuable
minerals from a parcel of land. Claims
only exist on Federal land. A claim only
reserves the recovery of minerals by the
miner. The use of the land and river is
open to the public for fishing, hiking,
camping or any other outdoor activity.
A Modern Gold Dredge at Work
The public is being misled on the impact of suction dredging. A typical mining claim is a quarter mile in
length or twenty acres. Today, typical claims are about a half mile of river. Miners sometimes hold
multiple claims meaning that up to a mile of river may be claimed by a single miner. This is an important
point. It is uncommon to have a density of miners that exceeds one suction dredge per quarter mile of
river. That's very few dredges actually in the streams and on a typical day there may be only five
hundred total dredges operating across the entire State.
MERCURY
No one is pro-mercury, and miners are no exception. Today's miners didn't put the mercury in the rivers
but we are removing the mercury with 98% efficiency – a higher efficiency than any other method
currently in use on the rivers [Ref 4]. Mercury is both naturally occurring and introduced into gold bearing
rivers of California. The early miners in the 1800's used a significant amount of mercury in their
recovery boxes to catch the fine gold. Mercury is naturally attracted to gold and amalgamates with it.
This property of mercury made it widely used in the recovery of gold during the hydraulic mining days.
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Mercury is no longer used to recover gold due to its toxic nature. Today's recovery systems are far more
efficient than the crude, wooden sluice boxes of the historical mining period.
Ironically, today's miners are recovering this toxic legacy from the rivers. In fact, today's miners are the
only people removing this mercury from the watershed and at no cost to the taxpayers.
Environmentalists distort this beneficial aspect of suction dredging by claiming that suction dredges are
releasing mercury "locked" in the sediments. This is a misunderstanding of how the sediments in a river
work as well as how mercury behaves. Mercury is a liquid metal with a specific gravity of 13.6 which
means it is 13.6 times as heavy as water. Lead has a specific gravity of 11.3 and gold has a specific
gravity of 19.3. This means relative to water these heavy materials will be captured by the sluice box on
the suction dredge which is designed to capture material with a specific gravity of at least 6. The sluice
box is designed to prevent the heavy material from escaping. In practice the dredge will capture nearly
all the mercury it encounters in the stream [Ref 4].
Some groups want you to believe that gold and mercury behave the same at the bottom of a river and
we should leave the mercury to travel naturally without interception by miners. Gold moves only during
exceptional flood events which disrupt the entire bottom of the stream bed with boulders breaking up
bedrock and releasing previously trapped gold particles and mercury. On average these flood events
happen only once every ten years. During a major flood event gold will be released by the action of the
river and will travel until it finds another low velocity position in the river and resettles. Mercury does
not behave this way. Mercury can be displaced from its position in the river by much lighter material.
Mercury is a liquid and travels quite readily by gravity. It is this movement by gravity, and not
necessarily floods that creates the problem with mercury. In the cold mountain streams where suction
dredges operate mercury remains in its natural form. As the mercury travels down the watershed it can
reach warmer, slower water where a process called methylation can transform the mercury into a form
that is absorbed into the food chain. The displacement and movement of mercury by gravity and low
flow events is of concern to miners and we argue it is far better to recover this mercury at its source
where it is relatively harmless than to allow it to continue to move downstream.
DREDGING AND WILDLIFE
Do suction dredges harm wildlife? The answer as provided in two separate environmental impact
reports is clearly no. Suction dredging is one of the most extensively studied outdoor activities and
numerous reports have been prepared that find no effect from dredging on wildlife. There is thirty
years of dredging history that shows no harm to fish, or aquatic organisms [Ref 9]. The current state of the
Sierra Yellow Legged Frog and the Foothill Yellow Legged Frog are not the result of suction dredging.
According to a leading expert and researcher of the Yellow Legged Frog, Dr. Roland Knapp, the near
extinction of this species is the result of the stocking of non-native trout into California watersheds [Ref 1].
This stocking of trout, coupled with a fatal fungus have brought the frog to the brink of extinction. Yet
nearly half the rivers in the State are proposed for closing ostensibly due to the frog. According to
multiple research papers the best hope for recovery of the frogs is simply the removal of the trout [Ref 2].
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Multiple studies, including a USGS study conducted in 2007 and
2008 measured the turbidity and sediment plumes from suction
dredges [Ref 5]. Every study conducted has concluded the same –
the sediment plume from a dredge returns to normal levels
within 100 meters of the operating dredge. In fact, it is
common for schools of trout to hover at the tail end of the
dredge eating invertebrates deposited by the dredge.
Miners, and the Western Mining Alliance, care about the
environment. We don't want mercury in the waters anymore
than you do, and every year we are removing significant
amounts of toxic mercury from California's rivers. We are the
All That Glitters is not Gold
only people removing mercury from the rivers. The banning of
miners from the rivers will allow the mercury to continue
unabated to lower elevations where it can be absorbed into the food chain.
Don't let the scare tactics of misguided environmental groups misrepresent the truth. No one knows
rivers better than suction dredgers. We spend years underwater learning how a river moves and where
the heavy metals accumulate. We maintain trails that are used by hikers and fishermen; we maintain
the remote roads that access areas for recreation and every year we are removing tons of trash left
behind by thoughtless individuals.
TODAYS MINERS
Suction Dredging is not destructive mining. A single spring flood moves more material and destroys
more stream bed than a thousand dredgers working a
hundred years could possibly do.
Mining is a California heritage; we are fighting to
maintain this spirit of California. The solitary miner
working his claim is still very real. The difference is
today's miners are often equipped with college
degrees, thousands of dollars of equipment and an
appreciation of both the history of mining and the
preservation of our environment.
The rugged
individualism of the men and women who built
California is alive in these present day miners. Don't
let a small group of misinformed individuals kill the
last of the gold miners in California.
A Solitary Miner on His Way to Work
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DREDGING AND THE ENVIRONMENT FACTS
Suction Dredges are 98% efficient at removing mercury from the watershed
Source: CAL EPA/RWQCB, Humphreys Report, 2005 and US Geological Survey, Fleck, 2007
Suction Dredgers remove nearly 14,000 ounces of mercury from the watershed per year
Source: Draft Subsequent Environmental Impact Report, California Department of Fish and Game, 2009 Survey of
Suction Dredgers
Mercury levels in trout in California streams are far below the national averages and well below
health concerns
Source: US EPA Report to Congress and California Regional Water Quality Board Study 2007
Turbidity from suction dredges returns to normal levels within 100 meters of the dredge
Source: Draft Subsequent Environmental Impact Report, California Department of Fish and Game, 2011
No evidence of suction dredging one year after the event
Source: Multiple studies cited in the Draft Subsequent Environmental Impact Report, California Department of
Fish and Game, 2011
Introduced trout are a major cause of the Mountain Yellow Legged Frogs current status (not dredging)
Source: Dr. Roland Knapp, multiple studies
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REFERENCES
1. 1. Knapp, R.A. et al, 1996 "Non Native Trout in Natural Lakes of the Sierra Nevada: an analysis of
their distribution and impacts on native acquatic biota."
2. Knapp, R.A. et al, 2007. "Removal of nonnative fish results in population expansion of declining
amphibian (mountain yellow-legged frog, Rana muscosa)
3. Knapp, R.A. et al, 2003. "Developing probabilistic models to predict amphibian site occupancy in a
patchy landscape."
4. Humphreys et al, 2005 "Mercury Losses and Recovery during a suction dredge test in the South Fork
of the American River."
5. Fleck, et al, 2010, USGS Report 2010-1325A, "The effects of sediment and mercury mobilization in the
South Yuba River and Humbug Creek Confluence Area, Nevada County California."
6. Mercury Report, August 2002, California Department of Toxic Substance Control
7. California Environmental Protection Agency, Sacramento – San Joaquin Delta Estuary TMDL for
Methylmercury, Staff Report Draft, February 2008
8. US Environmental Protection Agency, Mercury Report to Congress, EPA-452/R-97-003, 1997.
9. California Department of Fish and Game, Draft Subsequent Environmental Impact Report, 2011
10. California Department of Water Resources – Northern Region, Mercury Contamination in Fish from
Northern California Lakes and Reservoirs, July 2007
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