Mercury Contamination and Bioaccumulation from Historical

Gold Mining in the Sierra Nevada – Site Characterization and

Remediation

Charles N. Alpers, Ph.D.

U.S. Geological SurveyCalifornia Water Science Center

Placer Hall6000 J Street

Sacramento, CA

Cooperating AgenciesFederal

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:

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

(Churchill, 2000)

box

USGS Fact Sheet 2005-3014

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

• 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

UNDERCURRENT

SLUICE BOX Sluice and undercurrent,Oro Fino mine,

Siskiyou County, CACirca 1855

Sluice–undercurrent system,

Spring Valley mine, Butte County, CA,

Feather River watershed

Hg beads in sediment

South Fork American River, Lotus Camp (near Coloma)

Photo by R. Humphreys

USGS Fact Sheet 2005-3014

Mercury Loss to the Environment in Hydraulic Mining

Cleaning amalgam from stamp mill, Empire Mine, Cleaning amalgam from stamp mill, Empire Mine, Nevada County, California, 1900Nevada 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 The Mercury Cycle in Aquatic SystemsSystems

AIR

WATER

SEDIMENT

CH3Hg+

Hg(II)

Hg(II)

Particles

Hg0

HgCl2

HgCl42-

Runoff

Hg(II)

CH3Hg+

Hg0

DOM

Hg0

Hg(II)CH3Hg+ Hg0

Particles

Hg0

atmospheric transport

phytoplankton

zooplankton

lightlightmicrobes

microbes microbes

abiotic rxn.

Graphic: Mark Marvin-DiPasquale (USGS)SRB, FeRB

SAMPLING SITES, BEAR-YUBA, 1999

Source: May et al. (2000)

USGS OFR 00-367

• (δ15N) − MeHg slope similar

other studies

• similar rate of biomagnification of MeHg with increasing trophic level.

Stewart et al. (2008) CJFAS

Food Web Study, Camp Far West Reservoir, CA

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) CJFASCamp 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

AL

Source: Alpers et al. (2005) USGS SIR 2004-5251

DW

AL DW

Total mercury in sediment

Boston Mine

Source: Alpers et al. (2005) USGS SIR 2004-5251

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)

Clean-up Scenes – Polar Star Tunnel

Mercury vapor monitoringStabilizing the entrance

Washing the floor Finished product Ph

oto

s: R

. H

um

ph

reys,

SW

RC

B

Clean-up Scenes – Sailor Flat

Tunnel and pit areas restored

Tunnel before excavation

Tunnel during excavation Photos: R. Humpheys, SWRCB

Clean-up Scenes – Boston Mine

Tunnel outlet During remediation

Slusher

Trommel and concentrator bowl

Spiral concentratorPanning mercury Ph

oto

s: R

. H

um

ph

reys,

SW

RC

B

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