1. California Water Science Center
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

2. Cooperating Agencies Federal State Local
Hydraulic mining, Placer County, CA

3. 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

4. HISTORICAL MINING: Gold & Mercury box
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
(Churchill, 2000)
USGS Fact Sheet

5. 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

6. Hydraulic mining, Malakoff Diggins, Nevada County, CA, circa 1880
Hydraulic mine,
ground sluice system,
Scott Valley mine,
Siskiyou County, CA
circa 1870s

7. Sluice Tunnels 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.
Photos: Rick Humphreys, SWRCB

8. Sluice and undercurrent, Oro Fino mine, Siskiyou County, CA Circa 1855
SLUICE BOX
UNDERCURRENT
Sluice–undercurrent system,
Spring Valley mine,
Butte County, CA, Feather River watershed

9. Hg beads in sediment
Photo by R. Humphreys
South Fork American River, Lotus Camp (near Coloma)

10. Mercury Loss to the Environment
in Hydraulic Mining
USGS Fact Sheet

11. Cleaning amalgam from stamp mill, Empire Mine, Nevada County, California, 1900

12. Abandoned bucket-line dredge, Yuba Goldfields, CA

13. 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

14. The Mercury Cycle in Aquatic Systems
Hg0
Hg(II)
Hg0
atmospheric
transport
Particles
AIR
Runoff
phytoplankton
HgCl42-
Hg(II)
CH3Hg+
Hg0
HgCl2
zooplankton
light
light
CH3Hg+
Hg(II)
Hg0
microbes
Simplified Hg model in aquatic system
Illustrates that the Hg cycle is complex
Hg takes many forms (organic & inorganic)
Hg(II) is readily absorbed to particles and transported to sediments
Human and ecosystem health concerns when MeHg is formed and incorporated into the food chain
We will focus on sediment processes
Hg-methylation and MeHg degradation under anoxic conditions
Particles
DOM
WATER
microbes
microbes
CH3Hg+
Hg(II)
Hg0
SEDIMENT
abiotic rxn.
SRB, FeRB
Graphic: Mark Marvin-DiPasquale (USGS)

15. SAMPLING SITES, BEAR-YUBA, 1999

16. Source: May et al. (2000)
USGS OFR

17. 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

18. Camp Far West Reservoir, CA
Data from:
Alpers et al. (2008) USGS SIR
Camp Far West Reservoir, CA

19. Camp Far West Reservoir, CA
Data from:
Alpers et al. (2008) USGS SIR
Camp Far West Reservoir, CA

20. Camp Far West Reservoir, CA
Stewart et al. (2008) CJFAS

21. 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

22. Source: Alpers et al. (2005) USGS SIR 2004-5251
DW = Drinking water std.
AL = Aquatic life std. (CTR) DW DW AL
Source: Alpers et al. (2005) USGS SIR AL DW

23. Total mercury in sediment
Boston Mine
Source: Alpers et al. (2005) USGS SIR

24. Remediation of mercury-contaminated 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)

25. Clean-up Scenes – Polar Star Tunnel
Stabilizing the entrance
Mercury vapor monitoring
Photos: R. Humphreys, SWRCB
The tunnel entrance in the pit was located just below the pit high wall. The area was stabilized and the tunnel sediment and remenats of the sluice were brought out using mining equipment from the upstream end. After the removal was completed, the tunnel floor was washed with high pressure hoses and the floor was them cemented over. The mercury-rich sediment sluice lumber was hauled to a landfill. Cementing over the sluice floor isolated mercury left in bedrock crevices.
Washing the floor
Finished product

26. Clean-up Scenes – Sailor Flat
Tunnel before excavation
Tunnel and pit areas restored
Tunnel during excavation
Photos: R. Humpheys, SWRCB

27. Clean-up Scenes – Boston Mine
Trommel and concentrator bowl
Photos: R. Humphreys, SWRCB
Tunnel outlet
During remediation
Spiral concentrator
Slusher
Panning mercury

28. 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

29. 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