Post on 01-Apr-2021
Remediating mercury-contaminated sediment sitesChris S. Eckley, US EPA Region-10
Todd P Luxton, US EPA ORDCindy Gilmour, Smithsonian Environmental Research CenterSarah Janssen, USGS Upper Midwest Water Science CenterPaul M Randall, US EPA ORDLindsay Whalin, San Francisco Bay Water Board
Collaborators:
Mercury Releases
Hg2+Hg-P
Methylmercury (MeHg)
Mercury Exposure
Hg0
Photo: Eckley
Photo: Eckley
Introduction: Mercury Pollution
Transport, Methylation, Bioaccumulation
Mercury in the Environment
Source: USGS
Sediment processes are important—they are often the primary zones of MeHg production
Introduction: Methylmercury (MeHg)
Where is Hg methylated? Occurs under anoxic conditions
Source: Watras et al, 1995
How is Hg methylated? Methylation is a microbial processhgcAB+
Firmicute• Many Sulfate Reducing Bacteria
• Some Iron Reducing Bacteria• Some Methanogens
dissolved Hg(II)
Hg-DOM, Hg(HS)2
BioavailableInorganic Hg
amorphous, hydratednanostructuredweakly sorbed
well-crystallinemacrostructuredstrongly sorbed
Polymerization & Sorption: Sulfide, NOM
ripening or
aging
dissolution and
desorption
aggregation
Hg InputsSulfate (Sulfide)Organic carbon
Adapted from: Hsu-Kim et al, 2018
Introduction: MeHg production in sediment
Anaerobic Microbiome
Carbon substratesElectron acceptors
Nutrients
Biogenic sulfide, organic carbon, redox gradients
hgcAB+d-Proteobact.
hgcAB+
Firmicute
hgcAB+methanoge
nhgcAB+
microbial community composition
hgcAB+
MeHg
Release to Ecosystem:Biomagnification
TransportExposure
Interactive Content ideaSelect 4 parameters below that most likely identify areas of elevated methylmercury (MeHg) concentrations:
1) Elevated bulk sediment total mercury 2) Elevated sediment porewater total mercury3) Sediment with a positive redox potential (oxic conditions)4) Sediment with a negative redox potential (anoxic conditions)5) Low organic matter sediment6) High organic matter sediment7) Low sulfate concentrations8) Moderate sulfate concentrations9) High sulfate concentrations and buildup of sulfideAnswer: 2, 4, 6, 8
Commonly remediation options:• Reduce loading to the sediment• Sediment excavation/dredging• Sediment containment/capping
Photo: EPA/CDM
Photo: EPA/CDM
Photo: Dune Technologies
Optimized by focusing on areas:• Elevated Hg—specifically
bioavailable Hg• Higher MeHg production• Preferential uptake into foodweb
Remediation of Mercury Contaminated Sediments
Addressing Spatial Variability of Hg Pollution
Identify spatial distribution of Hg contamination
Example from Puget Sound Naval Shipyard, WA:
Opportunities for optimization:• Identifying erosion/ deposition zones
Initial Source Area
Sedi
men
t Hg
conc
.
• Areas of MeHg production
Permanent pool
Seasonal mudflats
Sample Locations
Wetlands
Addressing Spatial Variability of Hg PollutionSite Assessment (Example from Black Butte Mine, OR): Optimization potential: targeted remediation on area of high MeHg production
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Perm. Pool Seasonal Mudflats Seasonal WetlandsTH
g ug
g-1
; MeH
g ng
g-1 THg MeHg
Remediation options may include controlling variables other than total-Hg
Eckley et al., 2017
Factors Controlling Methylmercury Production
Source Attribution Using Stable Isotopes• Downstream/wind of contaminated sites the source of Hg pollution can be more difficult to
discern, especially when there are multiple potential sourcesHg Source #1
Downstream transport
Downstream
Deposition
Atmospheric
Deposition
Hg Source #2
Downstream
transport
Downstream
Deposition
MeHg
Source attribution using Hg stable isotopesWhat are stable isotopes?Forms of the same element that contain equal numbers of protons but different numbers of neutrons and as a result have different atomic massesMercury Isotopes:7 stable isotopes with range in mass from 196 to 204 amuMass dependent fractionation:Lighter isotopes react faster and become enriched in the products
Source attribution using Hg stable isotopes• Hg stable isotope analysis has provided insights into different sources of Hg—requires
unique end-members—and minimal post-source transformation
Black Butte Mine
Outer/Downstream Mining Area
Central Mining/Tailings Area
~40-70% of Hg in reservoir sediment shows clear mine signature Sediment Core Sample Locations
Remediations can be optimized by using stable isotopes to identify the portion of Hg in that originated from a contaminated site
Mercury Speciation Measurements
Inorganic Hg speciation impacts its mobility, toxicity & availability for methylation
Types of factionation measurements: • X-ray absorption fine structure (XAFS) spectroscopy: direct measure of Hg speciation
Ø Requires relatively high Hg concentrations (typically > 1 mg/kg)Unprocessed ore
Processed ore
Yin et al, 2016
• Chemical extractions: Ø Environmental Mobility: SPLP, TCLP, SSEØ Inorganic Hg bioavailability: IVBA (human ingestion); HgR (bacteria methylation)
• Pyrolysis Method (thermal desorption monitored over a temperature range)
Cleveland, 2018Brooks Applied Labs
Mine site
Downstream
Mercury Speciation Measurements
• Methods to identify the fraction Hg in the sediment that is more available for methylation
Porewater Measurements
Ndu et al, 2018
Diffuse gradient in thin film (DGT) samplers
Eckley et al, 2017
Remediation can be optimized by prioritizing areas where inorganic Hg is more mobile and/or bioavailable
Controlling Hg availability using In Situ Amendments• In situ amendments to sediments compete for Hg or MeHg against natural sorbents• Common types: biochar, activated carbon (AC), material modified with S ligands, Fe
Gilmour et al, 2013
• Lab and field tests with amendments:• Shown reductions in porewater THg & MeHg• Effectiveness impacted by amendment type, sediment properties, and DOM
Variables Controlling Methylation
MeHg production is a function of: bioavailability of Hg + microbial community/activity
Mitchell et al, 2008
Factorial incubation/mesocosm experiments:
• Varying sulfate, DOC, Hg, etc• Varying redox conditions• Inhibiting microbial populations
Longer-term mesocosm plotsshort-term isotope additionincubations
Remediation actions can focus on reducing variables enhancing MeHg production
Conclusions:
Site assessments and remediation can be optimized by:• Identify Hg forms/speciation that are mobile and available for methylation• Using stable isotope fractionation to identify sources of contamination• Identifying opportunities to reduce MeHg that may or may not require changes in THg
Requires significant investments in research aimed at understanding the system
Next Steps:• Novel approaches to addressing contaminated sites have been identified at the laboratory
and test plot scale;• However, more examples of large-scale applications are needed to encourage broader
adoption of these methods