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