Remediation of Contaminated Sediments

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Remediation of Contaminated Sediments

• Danny Reible1, Louis Thibodeaux1, Don Hayes2
• 1Louisiana State University
• 2University of Utah

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Common Sediment Contaminants

• Metals As, Pb, Cd, Cr, Ni, Zn, Hg
• Organics
• Polychlorinated biphenyls - PCBs
• Polynuclear aromatic hydrocarbons PAHs
• Chlorinated benzenes HCB, HCBD
• Petroleum hydrocarbons, undifferentiated oil and
  grease

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Selecting Remedial Options

• NAS Committee On PCB Contaminated Sediments
• All decisions should be made within a risk-based
  framework
• Recommended Presidential and Congressional
  Commission on Risk Assessment and Management

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Requirements for Risk

• Accessibility
• Can receptors come in contact with the
  contaminants?
• Are the contaminants within the biologically
  active zone?
• Availability
• Can the chemical desorb from the associated solid
  phase into a phase that is ingested by the
  receptor?
• Can the organism encourage the desorption to an
  available form?
• Assimilative capacity
• Can the receptor accumulate or degrade the
  contaminant in the available form?
• Are there effects to the primary receptor or to a
  secondary receptor?

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Processes Controlling Exposure

• Bioturbation
• Normal life cycle activities of benthic organisms
  leading to sediment mixing and transport
• Dominated by deposit feeders that ingest sediment
  
• Densities up to 100,000 worms/m2
• Organisms may process 10-20 times their wt/day
• Effects
• Moves sediment and associated contaminants
• Allows oxygen and nutrients deeper into sediments
• Contributes to accumulation of contaminants in
  food chain
• Turnover of upper layers of sediment at 0.3-30
  cm/yr
• Depth of influence 5-15 cm (90 of observations)
• Estimated via movement of radionuclides of
  different half-lives

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Moisture Content ()
Worm
Control
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Depth (mm)
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Pyrene (mg/kg dry)
Worm
Control
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Depth (mm)
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Contaminant Availability

• Observation
• Locations exhibiting lower toxicity than might be
  expected from reversible partitioning of sediment
  contaminants
• Potential Cause
• Reduced availability of contaminants
• Metals in reduced, insoluble form
• Organics strongly sorbed to solid phase

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Metals Mobility and Availability

• Present in one of three forms
• Chemically fixed (largely unavailable)
• Physically sorbed (exchangeable)
• Soluble (available)
• Significant fraction unavailable
• Tied up in insoluble precipitates (e.g. sulfides)
• Simultaneously extracted metals to acid volatile
  sulfides-SEM/AVS
• lt 1 Certain metals unavailable
• gt 1 Metals may or may not be available
• SEM/AVS ratio useful for Cadmium, Copper, Lead,
  Nickel, Zinc

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Organic Contaminants

• Hydrophobic and strongly sorbing
• Or they wouldnt be sediment contaminants!
• Equilibrium accumulation in lipids governed by
  porewater concentrations
• Key characteristics desorption rate and extent
• kdesorption
• Ksw
• Biphasic desorption rates and/or equilibrium
  commonly observed
• Linear, reversible compartment
• Nonlinear, desorption resistant compartment
  (often Langmuir in shape)

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Organic Desorption Resistance

• Various Models
• Fast, Slow desorbing compartments
• J. Pignatello
• Soft (young) and hard (aged) carbon
• W. Weber
• Natural organic carbon and soot
• R. Luthy
• Reversibly sorbed and sorbed w/ conformational
  changes
• M. Tomson
• Conclusion
• Some contaminants desorb rapidly and reversibly
• Some contaminant desorption limited in rate or
  extent

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Desorption Resistant Model
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Resistant
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Reduced Availability

• Metals
• Simultaneously extracted metals (SEM)/Acid
  volatile sulfides (AVS)
• lt 1 Certain metals unavailable
• gt 1 Metals may or may not be available
• Organics
• Equilibrium accumulation in lipids governed by
  porewater concentrations
• Biota Sediment Accumulation Factor (BSAF)
• Accumulation normalized by lipid content and
  organic carbon normalized sediment concentration
• O(1) for reversibly sorbed contaminants in
  benthic community
• lt 1 for desorption resistant contaminants?

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Normalized Accumulation in Oligochaetes
Reversibly Sorbed Phenanthrene
BSAF
Desorption Resistant Phenanthrene
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ObservedPredicted
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Choosing Remedial Options

• Contaminant sources?
• Processes influencing exposure and risk?
• How do the prospective remedial approaches
  interact with these processes to reduce exposure
  and risk?
• There are no default options!
• There are no options that do not leave a residual
  risk!

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Conceptual Model

• Defines primary sources of risk
• Defines mechanisms that may translate that risks
  to effects
• Defines requirements of remedial approaches and
  opportunities for control of processes/mechanisms
  leading to risk
• Provides framework for comparative evaluation of
  risks

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Evaluation of Options

• Develop options consistent with conceptual model
• Consider entire life cycle of options
• Transportation and disposal of dredged material
• Repair/replacement of in-water or on-land
  disposal sites
• Risks associated with time required for approval
  or implementation
• Assess residuals and associated risk
• Compare options on an equal basis!

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Comparative Evaluation Metrics

• Primary metric Risk
• Secondary metrics
• Link to appropriate conceptual model of system
• Indicator species concentrations (e.g. fish)
• Contaminant mass (dynamic environment)
• Surficial average concentrations (stable
  environment)
• When risk due to diffuse contamination (not hot
  spots)
• SWAC surface area weighted average
  concentration
• Integral measures (allows incorporation of time)

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Contaminated Sediments
What are the Options?
P Natural Recovery P Passive In-Situ Treatment
or Containment P Removal and Upland Treatment or
Containment P Removal and Subaqueous Treatment
or Containment
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Sediment Management

• Risk defined by contamination in relatively small
  well defined areas (hot spots) in dynamic
  sediment environment with defined on-shore
  disposal options?
• Encourages removal options
• Risk defined by diffuse contamination in stable
  sediment environment?
• Encourages in-situ management options
• What about other sites?
• Requires site specific assessment and conceptual
  model development

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Natural Recovery

• What is it?
• Use of natural processes to contain, dilute or
  degrade contaminants
• Typically associated with deposition of clean
  sediment
• When is it appropriate?
• Large volumes of contaminated sediment with
  marginal level of contamination
• Low energy, depositional environments
• Dredging for navigation not required
• Sources adequately controlled
• What are key assessment needs?
• Contaminant and sediment processes adequate
  conceptual model of system
• Potential for reversal of natural attenuation by
  storm events/use changes
• Potential for rate reductions due to control of
  sediment loads to watershed
• Long term monitoring of system recovery is
  required
• Example - Kepone in James River
• Expected to eliminate fish advisories faster than
  active remedial alternatives

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Natural Attenuation

• Goal Reduce bioavailability and potential
  transport.
• Positive processes
• Natural capping with clean soils from watershed.
• Degradation in the bed these include
  biotransformation, redox reactions, and other
  abiotic reactions.
• Burial at depth due to bed accretion from clean
  soils.
• Adsorption/sequestration on to cleaner soils.
• Bioturbation mixes cleaner particles downward
  into the bed.
• Solubilization processes at the S/W interface
  releases contaminant quantities to the water
  column at a slow rate and deplete, thereby, the
  surface layers.
  
  

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Natural Attenuation

• Negative Processes
• Scour/ resuspension. Although some resuspension
  may be acceptable any site where net scour occurs
  is unacceptable.
• Bioturbation is a 2-edged sword in this case
  it moves contaminated particles at depth in the
  bed and places them directly on the surface.

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Other Considerations

• If the bed at a site has been stable for the past
  30-50 years it may be a good candidate for
  natural recovery.
• Normally when selected it is combined with
  on-going site monitoring and institutional
  controls.
• Since it is continuously on-going, even at sites
  undergoing more aggressive remediation
  technologies, its contribution must be quantified.

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Examples of Natural Recovery Half-life
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Natural Recovery - Summary
Contaminated Sediments in Ports and Waterways,
Nat. Acad. Press, Wash. DC, 1997.
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In Situ Containment/Treatment

• What is it?
• Enhanced biodegradation
• In-situ solidification
• Containment - capping
• When is it appropriate?
• When removal may result in unacceptable or
  increased risks
• When hydraulic and other water body conditions
  are supportive
• When navigation dredging is not required
• What are the key assessment needs?
• Evaluation of effectiveness/engineering
  feasibility of approach
• Evaluation of long-term contaminant fate
• Evaluation of armoring needed to maintain
  stability of sediments during treatment

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In Situ Treatment Options

• Enhance deposition to ensure clean surficial
  sediments
• Enhance natural fate processes
• Capping to drive anaerobic conditions and (often)
  mobility reduction for metals
• Addition or capping with reactants/catalysts
  sources for sequestration and degradation
• Active in-situ treatment (Limited development)
• In-situ solidification technologies
• In-situ reactor (e.g. auger/reactive hood
  technologies)

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In Situ Capping Objectives

• Armor sediment for containment
• Design for stability in high flow conditions
• High confidence in describing dynamics of
  noncohesive, granular media
• Eliminates uncertainty of existing sediment
  dynamics
• Separate contaminants from benthic organisms
• Reduce dissolved release by increasing transport
  path/resistance
• Provide opportunities for habitat development

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Sediment Profiling Camera
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Cap Effectiveness

• Elimination of erosion and bioturbation as
  transport processes
• Leaves diffusion (always present)
• Advection if seepage significant
• Transient behavior after cap placement
• Consolidation of sediment and cap materials
• Advection diffusion retarded by sorption
• Ksw focKoc
• Rf e rb Ksw

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Cap Breakthrough

• Effective cap thickness
• Seepage dominated breakthrough
• Diffusion dominated breakthrough

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Example

• Pyrene
• Koc 105
• Ksw, Rf 1000 if foc 1
• Cap
• Leff 30 cm
• Dcap 10-6 cm2/sec
• Transient time (diffusion) 10,000 years

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Steady State Cap Performance

• Diffusion dominated system
• Flux prior to capping (bioturbation)
• NA/rbWs 1 cm/yr (without erosion)
• Flux after capping
• NA/ rbWs Dcap/Leff Rf
• For pyrene example 0.001 cm/yr
• Advection dominated system
• Flux before and after capping ultimately equal!

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Innovative Caps

• Sand caps easy to place and effective
• Greater effectiveness can be achieved with
  active caps
• Encourage fate processes such as sequestration or
  degradation of contaminants beneath cap
• Discourage recontamination of cap

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Potential Cap TechnologiesAnacostia
Demonstration

• Aquablok for control of seepage and advective
  contaminant transport
• Zero-valent iron to encourage dechlorination and
  metal reduction
• Phosphate mineral (Apatite) to encourage sorption
  and reaction of metals
• BionSoil to encourage degradation of organic
  contaminants
• Natural organic sorbent to encourage
  sorption-related retardation (reduction in
  advective-diffusive transport)

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Summary In-Situ Management

• Relies on development of accurate conceptual
  model
• What are the sources of the contaminants and the
  transport and fate processes that influence
  exposure?
• How can management options influence those
  sources, transport and fate processes and
  exposure?
• Depends on understanding of natural processes and
  employs remedial approaches that enhance and are
  consistent with those processes

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Removal Options

• What is it?
• Hydraulic or mechanical dredging
• Generally requires temporary holding facility
  -confined disposal facility (CDF)
• Upland landfill or treatment/destruction in
  incinerator, thermal desorber, biotreater, etc
• When is it appropriate?
• Dredging required for navigation purposes
• Dredging will not result in unacceptable or
  increased risks
• Dredging can be accomplished with minimal
  generation of contaminated water
• Upland treatment and containment options
  available and cost-effective
• What are the key assessment needs?
• Evaluation of contaminant losses during dredging
  and handling including residual suficial
  sediments
• Availability of adequate disposal facilities