Mercury Contamination and
Bioaccumulation from Historical Gold
Mining in the Sierra Nevada – Site
Characterization and Remediation
Charles N. Alpers, Ph.D.
U.S. Geological Survey
California Water Science Center
Placer Hall
6000 J Street
Sacramento, CA
Cooperating Agencies
Federal
State
Local
Hydraulic mining, Placer County, CA
Outline of Presentation
• Background
– Review of mining history and mercury use in gold mining
• Environmental geochemistry of mercury in the Bear,
Yuba, and American River watersheds, California
–
–
–
–
Water Quality
Sediment
Biota
Importance of seasonality in Hg cycle
• Remediation of 3 Hg-contaminated
placer mine sites
• What have we learned?
• What information gaps remain?
Casci Creek, Nevada Co., CA
HISTORICAL
MINING:
box
Gold & Mercury
• More than 100,000,000 kg
mercury (Hg) produced from
239 mines in California
• Approx. 33,000,000 kg Hg
lost to atmosphere from
furnaces at Hg mines
• Approx. 12,000,000 kg Hg
used in Calif. gold mining
USGS Fact Sheet 2005-3014
(Churchill, 2000)
GOLD MINING AND
MERCURY USE IN THE
NORTHERN SIERRA
NEVADA
• Highest intensity of hydraulic
mining (placer gravel deposits)
in Bear-Yuba watersheds
• Approx. 5,000,000 kg of
mercury lost during gold
processing in Sierra Nevada
(USGS, 2000; Churchill, 2000)
• Significant gold dredging in all
rivers draining Sierra Nevada
USGS Fact Sheet 2005-3014
Hydraulic mining,
Malakoff Diggins,
Nevada County, CA,
circa 1880
Hydraulic mine,
ground sluice system,
Scott Valley mine,
Siskiyou County, CA
circa 1870s
Sluice Tunnels
Photos: Rick Humphreys, SWRCB
• Sluices recovered
gold.
• Mercury was used to
amalgamate fine gold.
• Mercury was lost
during sluicing.
• Mercury is still found
in sluices and their
foundations today.
SLUICE BOX
UNDERCURRENT
Sluice–undercurrent
system,
Spring Valley mine,
Butte County, CA,
Feather River
watershed
Sluice and undercurrent,
Oro Fino mine,
Siskiyou County, CA
Circa 1855
Hg beads
in sediment
Photo by R. Humphreys
South Fork American River, Lotus Camp (near Coloma)
Mercury Loss to the Environment
in Hydraulic Mining
USGS Fact Sheet 2005-3014
Cleaning amalgam from stamp mill, Empire Mine,
Nevada County, California, 1900
Abandoned bucket-line dredge, Yuba Goldfields, CA
TRANSPORT AND
TRANSFORMATION
OF MERCURY
ENVIRONMENTS:
• Hydraulic and hardrock gold
mines – Sierra Nevada
• Mercury mines – Coast
Ranges
• Mountain streams above
reservoirs
• Foothill reservoirs
• Rivers below reservoirs –
gold dredging environments
• Floodplain deposits
• San Francisco Bay-Delta
estuary
USGS Fact Sheet 2005-3014
The Mercury Cycle in Aquatic Systems
atmospheric
transport
Hg0
Particles
AIR
Runoff
Hg(II)
phytoplankton
HgCl42HgCl2
zooplankton
CH3Hg+
CH3
Hg0
light
microbes Hg(II)
Hg+
Particles
WATER
SEDIMENT
Hg0
Hg(II)
CH3
light
Hg0
DOM
microbes
Hg(II) abiotic rxn.Hg0
SRB, FeRB
Hg+
microbes
Graphic: Mark Marvin-DiPasquale (USGS)
SAMPLING SITES, BEAR-YUBA, 1999
Source: May et al. (2000)
USGS OFR 00-367
Food Web Study, Camp Far West Reservoir, CA
• (δ15N) − MeHg
slope similar
other studies
•
similar rate of
biomagnification
of MeHg with
increasing
trophic level.
Stewart et al. (2008) CJFAS
Data from:
Alpers et al. (2008)
USGS SIR 2006-5008
Camp Far West
Reservoir, CA
Data from:
Alpers et al. (2008)
USGS SIR 2006-5008
Camp Far West
Reservoir, CA
Stewart et al. (2008) CJFAS
Camp Far West
Reservoir, CA
Principal Findings – Seasonal Cycles
in Camp Far West Reservoir
• Fall-Winter phytoplankton bloom is triggered by
phosphorus in inflowing water
• Spring is the key season for zooplankton growth
and MeHg bioaccumulation
• Mass load of MeHg inflow exceeds in-reservoir
production (benthic flux and hypolimnion)
• MeHg bioaccumulation in upper trophic levels
(fish, invertebrates) dependent on MeHg uptake in
plankton, which have strong seasonal cycles
DW = Drinking water std.
AL = Aquatic life std. (CTR)
DW
DW
AL
Source: Alpers et al. (2005) USGS SIR 2004-5251
AL
DW
Total mercury in sediment
Boston Mine
Source: Alpers et al. (2005) USGS SIR 2004-5251
Remediation of mercurycontaminated placer gold mines
• 2000: Polar Star Tunnel, Dutch Flat Mining
District (USEPA), $1.4M, 150 m tunnel (~$9K/m)
• 2003: Sailor Flat Tunnel, Tom and Jerry Mining
District (USFS), $300K, 130 m tunnel (~$2K/m)
• 2006: Boston Mine Tunnel, Red Dog Mining
District (BLM), $250K, 60 m tunnel (~$4K/m)
Stabilizing the entrance
Washing the floor
Mercury vapor monitoring
Finished product
Photos: R. Humphreys, SWRCB
Clean-up Scenes – Polar Star Tunnel
Clean-up Scenes – Sailor Flat
Tunnel before excavation
Tunnel and pit areas restored
Tunnel during excavation
Photos: R. Humpheys, SWRCB
Trommel and
concentrator bowl
Tunnel outlet
During remediation
Slusher
Panning mercury
Spiral concentrator
Photos: R. Humphreys, SWRCB
Clean-up Scenes – Boston Mine
What have we learned?
• Mercury “hot spots” occur in Sierra Nevada
– Tunnels and ground sluices at hydraulic mines
– Stamp mill sites (and downstream) at lode mines
• From limited post-remediation monitoring:
– At Polar Star and Boston mine tunnels, persistent
contamination from upstream sources
– Difficult to demonstrate benefits of remediation
• Bioaccumulation depends on seasonal
dynamics involving food web
– Critical to sample seasonally for water and biota
What information gaps remain?
• Baseline data on Hg and MeHg loads in mining-affected watersheds
– Quantify potential benefits from mine remediation
– Seasonal variability
– Information needed for TMDLs
• Data on Hg and MeHg in reservoir sediments
– Dam removal issues
– Potential sites for Hg removal, sand-gravel-gold extraction
• Studies of Hg methylation and bioaccumulation
– Controls on what makes reactive Hg(II) available to microbes
– Controls on microbial methylation: S, C, Fe, nutrients
– Food web studies
– Effects of wetland restoration, wet/dry cycles
– Effects of agricultural amendments (esp. S on rice and other crops)
• Wildlife health effects
– Effects of MeHg exposure on salmon and steelhead
– Very little information on mammals, reptiles, many bird species
• Modeling of mercury cycling in rivers and reservoirs
– Improved understanding of biogeochemical and hydrologic processes
– Management tools for testing scenarios, confirming results