Monitored Natural Attenuation
of Inorganics
USEPA Scientist - to - Scientist Meeting
Las Vegas, Nevada
June 14 - 15, 2000
Robert W. Puls, Ph.D.
National Risk Management Research Laboratory
OSWER Directive on MNA
Recognizes monitored natural attenuation
of inorganics in ground water and soils
Requires knowledge of specific
mechanisms
Most applicable when contaminant is
irreversibly immobilized
Presentation Topics
Monitored Natural Attenuation of Metals
Processes controlling MNA
Site Characterization Issues
Research Questions
Factors Affecting MNA of Metals
NRMRL/SPRD Research
Natural Attenuation of Inorganics
PROCESSES
Adsorption-Desorption …..… Immobilization-Mobilization
Precipitation-Dissolution …… Immobilization-Mobilization
Oxidation-Reduction ………. Change in chemical state -
speciation; may affect
mobility and toxicity
Volatilization ………. Change in phase
Colloid, Sediment Transport .. Facilitated transport, sediment loading
Site Characterization -
Inorganics
Monitored Natural Attenuation
1.1 Identity and Extent Contaminant
of Contamination Flux
1.4 Temporal & Spatial
Ground Water Geochemistry
Contaminant
1.3 Ground Water
Attenuation
Hydrogeology
Capacity
1.5 Mechanism and Stability
of Attenuation
1.2 Aquifer Composition Geochemical
and Heterogeneity Buffering
Capacity
Research Questions for MNA of
Inorganics
What are the important processes to measure
for natural attenuation for metal contaminants?
How do you make these measurements?
How do you integrate these data to understand
the behavior of the system?
Factors Affecting MNA of
Inorganics
Speciation (e.g. Cr(III), Cr(VI), As(III), As(V)
Geochemical factors
pH
Eh
DO
Water chemistry
Microbial activity
Sorbents
Mineralogy
Natural Attenuation Definition:
Naturally occurring processes in the
environment that act without human
intervention to reduce the mass, toxicity,
mobility, volume or concentration of
contaminants.
Source: Waste Research Strategy
Office of Research and Devolopment
Washington, DC 20460
EPA/600/R-98/154
www.epa.gov/ORD
NRMRL Research on MNA of
Inorganics
Laboratory studies of the role of mineralogy for
MNA
Arsenic and Iron (Hydr)oxides (6400)
Solid phase speciation/extractions (4279)
Arsenic and Iron sulfides (7060)
Field studies of the natural attenuation of
arsenic in an urban watershed (6401)
MNA of As in an Urban
Watershed
Aberjona watershed in Woburn, MA
(industriplex site); pesticide manufacturer and
tannery
Cooperative effort between ORD/NRMRL and
region 1
Involves ground water, surface water, soils and
sediments
Provide part of the science needed to develop
guidance on MNA for metals
Industriplex Site, Woburn, MA
Holding Area Halls Brook
Plume Discharge into HBHA
Halls Brook Holding Area
THE POND
Hall’s Brook
Cross-Section
Picnocline (turbidity high)
Fe S As organics
Project Goal
“ … to understand the hydrogeochemical and
biogeochemical mechanisms controlling arsenic
mobilization, immobilization, and bioavailability in
subsurface and surface aquatic environments, and
develop practical and cost-effective ecosystem restoration
measures to manage risk to human and environmental
health.”
Note: Results should be applicable to a variety of sites
including both industrial and mining sites
Objectives
Site-specific assessment of potential for natural
attenuation of arsenic
Preliminary guidance for monitoring
requirements to assess the potential of MNA
Develop/evaluate sampling protocols arsenic
speciation in ground and surface water samples.
Project Tasks
Collect soil cores from Industri-Plex site and
HBHA wetlands to assess arsenic levels
Collect ground water samples in suspected
source areas and adjacent to HBHA wetland
Collect surface water samples and sediment
samples in HBHA pond and tributaries
Establish monitoring wells to intercept flow into
HBHA pond (adjacent to pond and within pond
sediments)
Project Milestones and Products
Agency documents:
• Project Report on the Natural Attenuation of Arsenic
in Soils and Sediments (12/2001)
Peer-reviewed journal articles:
• Preservation methods for maintaining aqueous
arsenic speciation in ground water samples
(12/2000)
• Solid phase arsenic speciation as a function of
geochemical parameters in soils and sediments
(05/2001)
• Assessing natural attenuation of arsenic in soils and
sediments (10/2001)
Agency Needs
GPRA Goal 5: Improvements in Waste Management
Practice
ORD
• Remediation & Restoration => Natural Attenuation
NRMRL
• Provides Region I with scientifically valid data
• Remediation/watershed restoration for heavy metals
SPRD
• Research on chemical and physical processes of
subsurface environment
• Research on fluxes to other environmental media, i.e.
ground water to surface water
Connection to other in-house
research:
Task 6400 (Robert Ford)
Attenuation/Stabilization of Arsenic by Iron (Hydr)oxides in
Soils/Sediments: Laboratory Study
Task 4279 (Cynthia Paul)
Natural Attenuation of Inorganic Contaminants
Laboratory study, solid phase extraction methods
Task 7060 (Richard Wilkin)
Natural Attenuation of Metals During Sulfide Formation
Task 6403 (Richard Wilkin)
Monitoring Oxidation-Reduction Processes in Ground Water,
Sediments
Task 1579 (Guy Sewell)
Microbial Ecology of the Subsurface/Biomarker Development
Natural Attenuation of Metals During
Sulfide Formation
Task 7060 (Richard Wilkin)
Fe-O2-S-H2O (25 °C)
1.00
FeSO4+ O2
FeHSO4
2+
Oxidizing (6401)
0.75
environments:
0.50
FeSO4
0
-6 FeOOH
sorption/co-ppt
0.25 -4 with iron oxyhyrox.
Eh (V) 0.00
-0.25 Fe
2+
FeS2 Reducing (7060)
H2
environments:
-0.50
S=10
-3 sorption/co-ppt
-0.75 C=10 Fe7S8 with iron sulfides
-4
Fe=10 and10
-6 -4
-1.00
2 3 4 5 6 7 8 9 10 11 12
pH
Objectives (Task 7060)
Laboratory studies to identify mechanisms of
metal uptake and evaluate rates
Focus on As, Ni, Zn, Mn
Identify controls on attenuation capacity =>
reactive Fe, sulfate, organic C
Identify controls on formation of pure Me-
sulfide vs. co-precipitation with Fe
Develop practices for field monitoring
Monitoring Oxidation-Reduction Processes in
Ground waters, Sediments, and Soils
Task 6403 (Richard Wilkin)
Objectives
DO methods comparison
Winkler titrations, colorimetric, electrodes
Eh interpretation: H2S/S° equilibrium
Develop methods to partition S in the solid
phase
AVS, CRS, S°, S-organic, S-sulfate
EPA Workshop on Monitoring Oxidation-
Reduction Processes for Ground Water
Restoration
April 25-27, 2000 in Dallas
•About 50 experts from government, academia, and industry met to
discuss redox characterization, methods, and data interpretation as
they apply to the remediation of chlorinated hydrocarbons,
petroleum hydrocarbons, and inorganic contaminants
•Process: Plenary talks, Breakout Sessions, Summary
•Product: Workshop Document planned for early 2001
Task 4279 - Natural Attenuation of
Inorganic Contaminants (Lab-based)
Reliable/acceptable extractions to assess
speciation on the solid phase?
Reliable/acceptable methods to assess
oxidation capacities for soils/seds?
Reliable/acceptable methods to assess
reduction capacities for soils/seds?
Cheap, fast, reliable analytical methods for solid
phase speciation assessments?
Summary
NRMRL/SPRD MNA Research
Mineralogy role in MNA (arsenic)
Fe and S cycling
Solid phase analyses, site assessment
extraction methods
Sample collection, preservation methods
arsenic speciation
solids analyses
Development of MNA protocols for field application
NRMRL/SPRD MNA Research
& Mining
Arsenic a common contaminant of mine wastes
Fe and S cycling important processes for mine
waste site assessment and treatment
Solid phase analyses and extraction methods
important for mine waste assessments
MNA may be an important part of any mine
waste remedial measure