Soil Mercury Sequestration
and Re-Emissions
D. Obrist, C. Moore, Y. Agnan, T. LeDantec, C. Pearson, C. Hedge
Desert Research Institute, Reno
D. Helmig, J. Hueber, University of Colorado, Boulder
Introduction
Semi-volatile atmospheric pollutants: a unique environmental cycling
Long-range Transport /
Atmospheric Chemistry
Wet Deposition / Dry Deposition
Controlled by policies
(national/global)
Primary
Emissions
Secondary
Emissions
Controlled by land
use/climate, etc.
Accumulation in
Environmental
Reservoirs (soil,
ocean, snow, ice)
Ecosystem/
Human Impacts
Introduction
Semi-volatile atmospheric pollutants: a unique environmental cycling
Long-range Transport /
Atmospheric Chemistry
Wet Deposition / Dry Deposition
Controlled by policies
(national/global)
Primary
Emissions
Secondary
Emissions
Controlled by land
use/climate, etc.
Accumulation in
Environmental
Reservoirs (soil,
ocean, snow, ice)
Ecosystem/
Human Impacts
Terrestrial Accumulation
Ecosystem distribution patterns: averaged across 14 U.S. forests
Hg/C ratio
Obrist et al., 2011 (ES&T); Obrist 2012 (ES&T)
Foliage/branches/wood
Bark
Bole
Soils
Terrestrial Accumulation
Estimated soil Hg mass (top 40 cm):
United States: 15,230 metric tons
Globally: > 300,000 tons (??)
Hararuk et al., 2013 (Biogeosciences)
mg Hg m-2
Smith et al. U.S. Geological Survey Data Series 801, 19 p., http://pubs.usgs.gov/ds/801/.
Introduction
Semi-volatile atmospheric pollutants: a unique environmental cycling
Long-range Transport /
Atmospheric Chemistry
Wet Deposition / Dry Deposition
Controlled by policies
(national/global)
Primary
Emissions
Secondary
Emissions
Controlled by land
use/climate, etc.
Accumulation in
Environmental
Reservoirs (soil,
ocean, snow, ice)
Ecosystem/
Human Impacts
Introduction
Semi-volatile atmospheric pollutants: a unique environmental cycling
Long-range Transport /
Atmospheric Chemistry
Wet Deposition / Dry Deposition
Controlled by policies
(national/global)
Primary
Emissions
Secondary
Emissions
Controlled by land
use/climate, etc.
Accumulation in
Environmental
Reservoirs (soil,
ocean, snow, ice)
Ecosystem/
Human Impacts
Re-emissions of the volatile Hg0
A comprehensive database of global Hg0 flux measurements
(Agnan et al., SSSA 433-3)
Do Hg0 emissions originate in the soil matrix ?
Conceptual diagram of hypothesized diffusion profiles
Photochemical Reduction:
Atmosphere
Hg0
Hg0 (?)
Soil
Hg0 (?)
Soil Depth
Litter
HgII
h 
Hg0
Microbial/abiotic reduction:
HgII-Cl2
Hg0
II
Hg -O/N
HgII-SR
Hg0 Diffusion Profile
In-situ Hg0 soil diffusion gradients
•
•
•
•
•
2 forest soils in Sierra Nevada
(California)
Sandy arid soils (Entisols)
No significant surface litter layers
Low organic carbon content (1.0 –
3.3% C)
Hg content: 25-43 µg kg-1
•
•
•
•
•
Northern Arctic tundra (Alaska)
Histic Pergelic Cryaquepts
Deep, organic litter (25 to 40 cm)
horizons
Organic carbon content (O/Ah: 3043% C; B: up to 5%)
Hg content: 70 – 140 µg kg-1
In-situ Hg0 soil diffusion gradients
Sierra Nevada forest soils
Trace gas system 1:
• 6 semi-permeable PTFE-tubes, placed at
different soil depths
• Flow rate: 50 mL min-1; sampling
duration: 45 min
• Total sampling volume of 2.25 L
Synchronized Port Sampler
Atmos
phere
Tekran 2537 Mercury
& LICOR 7000 CO2
Analyzers
7 cm
20 cm
40 cm
High Density Porous
PTFE Tubing
In-situ Hg0 soil diffusion gradients
2.0
Atmosphere
1.5
1.0
7 cm
0.5
20 cm & 40 cm
0
Aug 8
Aug 18
Aug 28
Date
Sep 7
Mineral Soil
Hg0 concentration (ng m-3)
Sierra Nevada forest soils
In-situ Hg0 soil diffusion gradients
Sierra Nevada forest soils
Obrist et al., 2014 (ES&T)
• 14 months of measurements, spanning summer and winter
• Consistent soil Hg0 sink
In-situ Hg0 soil diffusion gradients
Sierra Nevada forest soils (mineral soil horizons)
Atmosphere
Photochemical Reduction:
Hg0
Soil Depth
Soil
Hg0
HgII
h 
Hg0
• No Hg0 production in
soil matrix!
• Immobilization of Hg0
• But…only measured in
mineral horizons in
arid, aerobic conditions
1.5
Hg0 concentration
In-situ Hg0 soil diffusion gradients
Arctic tundra soils (Toolik Field Station)
In-situ Hg0 soil diffusion gradients
Arctic Tundra Soils
Trace gas system 2:
• Six Teflon® soil wells
• Volume: 4.2 liters
• 65 perforated holes,
covered with Gore-Tex®
membranes and Teflon
screens
• Fully water proof and
sturdy
• Sampling rate: 1 l min-1,
2.5 l sampling volume
In-situ Hg0 soil diffusion gradients
Testing of Teflon® soil gas wells
Ambient air measurements: in- and outside of gas wells
In-situ Hg0 soil diffusion gradients
In-situ Hg0 soil diffusion gradients
In-situ Hg0 soil diffusion gradients
Tundra soils Hg0 patterns
In-situ Hg0 soil diffusion gradients
Tundra Soil Diffusion Profiles:
Photochemical Reduction:
Atmosphere
Hg0
Hg0
Soil Depth
Organic
Horizon
HgII
Mineral
Soil
Hg0
h 
Hg0
Microbial/abiotic reduction:
HgII-Cl2
HgII-O/N
Hg0
HgII-SR
Oxidation/Sorption/Dissolution
HgII-Cl2
HgII-O/N
Hg0
HgII-SR
HgII-FeS
Hg0/IIaq
Hg0 Diffusion Profile
Conclusions and Outlook:
• Worldwide, there are substantial pools of Hg sequestered in
terrestrial ecosystems
• The fate and re-emissions of Hg0 from these terrestrial pools have a
direct impact on atmospheric Hg loads and transport patterns
• Measurements of Hg0 concentrations in the soil profile show lower
concentrations compared to atmospheric levels, suggesting a sink
(not a source) of Hg0 in the soil matrix
• This observations so far is consistent in an arid, dry soils and in an
Arctic tundra soil (incl. in highly organic layers)
• Measuring periods of high water saturation (and reducing conditions)
and freeze/thawing cycles to assess in-soil Hg0 production will be of
high interest
In-situ Hg0 soil diffusion gradients
Testing of Teflon® soil gas wells
Zero air and spike tests: in- and outside of gas wells
Ambient air measurements: in- and outside of gas wells
In-situ Hg0 soil diffusion gradients
Tundra soils CO2 patterns
Re-emissions of the volatile Hg0
Importance of surface re-emissions
Global chemical transport model (GEOS-Chem Global 3-D) simulations
Amos et al. 2013, Global Biogeochemical Cycles