Biodegradation and Natural Attenuation Cive 7332 Lect 5

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Biodegradation and Natural AttenuationCive
7332Lect 5
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Natural Attenuation

• The biodegradation, dispersion, dilution,
• sorption, volatilization, and/or chemical and
• biochemical stabilization of contaminants to
• effectively reduce contaminant toxicity,
• mobility, or volume to levels that are protective
• of human health and the ecosystem
• (US EPA ORD, OSWER)

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Natural Attenuation

• Naturally occurring processes in soil and ground
  water that act without human intervention to
  reduce the mass, toxicity, mobility, volume or
  concentrations of contaminants
• Biodegradation, dispersion, dilution, absorption,
  volatilization, and abiotic reactions

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Evidence of Natural Attenuation

• Plume Length Should Be
• Seepage Velocity
• x Time
• Retardation Factor
• Plume Length Is .....
• Shorter
• Thinner
• Appears not to be moving

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Natural Attenuation
A Do-Nothing Approach? Requires quantitative
assessment of a plumes behavior - amount,
extent, and rate of travel, as well as long-term
evidence of attenuation
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Requirements

• Site assessment - hydrogeology, geochemistry,
  microbiology
• High tech approaches - sampling, analytical,
  modeling techniques
• Prediction of plume behavior
• May be combined with source/hot spot control
• Containment of dissolved plume
• A risk management strategy

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Fate of Organic Contaminantsin the Subsurface
Volatilization
Sorption
Abiotic Transformations
Hydrolysis
Biodegradation
Advection
Dilution
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Advection

• Contaminants transported with ground water flow
• No effect on contaminant concentration
• No net loss of contaminant mass

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Dispersion

• Mechanical and hydraulic mixing
• Decreases contaminant concentration in center of
  plume increases concentration on edges
• No net loss of contaminant mass

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Sorption

• Partitioning of contaminants between aqueous
  phase and solid aquifer matrix
• Decreases dissolved contaminant concentration
  until sorption capacity is reached
• No net loss of contaminant mass

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Volatilization

• Movement of contaminants from aqueous phase in
  saturated zone to vapor phase in unsaturated zone
• Can result in net loss of contaminant mass from
  the aquifer
• Rarely a significant attenuation mechanism except
  in capillary zone

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Dissolution (Leachability)

• Transfer of contaminants from NAPL phase to
  aqueous phase
• Most significant physical process controlling
  extent of the plume
• Plume stable or expanding until residual (source)
  contaminants removed
• No net loss of contaminant mass

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Abiotic Transformations

• Reactions such as hydrolysis and dehalogenation
• Reduces aqueous concentrations of contaminants
• Reduction of contaminant mass

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Biotic Transformations

• Aerobic and anaerobic biodegradation
• Reduces aqueous concentrations of contaminant
• Reduction of contaminant mass
• Most significant process resulting in reduction
  of contaminant mass in a system

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When to Enhance?

• Bioremediation enhancement of natural
  attenuation processes
• Conditions in the subsurface not conducive for
  degradation
• Dissolved oxygen concentration - too high or too
  low
• Low electron donor concentration
• Too much mass for Natural Attenuation
• Short time frame
• Many processes available for bioremediation

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When Not to Enhance?

• Use Monitored Natural Attenuation (MNA)
• MNA is reliance on natural attenuation processes
  to achieve site-specific remedial objectives in a
  time frame that is reasonable compared to other
  methods.
• Appropriate only when demonstrated capable of
  meeting objectives in acceptable timeframe
• Often used in conjunction with other active
  measures

OSWER Directive 9200.4-17, 1997
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Observation of Natural Attenuation
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MNA Advantages

• Reduce potential for waste generation and human
  exposure during ex situ treatment
• Less intrusive
• Can be used with, or after, other remediation
  approaches
• Reduction in cost

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MNA Disadvantages

• Increased time frame for remediation and
  monitoring (institutional controls)
• Requires more complex and costly site
  characterization
• Incomplete attenuation may result in increased
  toxicity (especially chlorinated solvents)
• Potential for continued contaminant migration and
  cross-media transfer
• Public acceptance

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Critical Questions in MNA

• How long will the plume extend?
• How long will it take for the contamination to
  disappear?
• Mass transfer vs. fate processes
• Future land use
• Natural resource damage assessment

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Multi-Site Studies (Newell and Connor, API)
BTEX plumes at 42 retail LUST sites
213 ft x 150 ft
Chlorinated ethene (PCE, TCE, DCE, or VC) plumes
at 88 sites
1000 ft x 500 ft
Other chlorinated solvent plumes (TCA, DCA) at
29 sites
500 ft x 350 ft
Chloride, salt water plumes at 25 sites
700 ft x 500 ft
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EPA Lines of Evidence

• Historical groundwater and/or soil chemistry data
  that demonstrate clear trends of decreasing
  contaminant mass (concentration) that is not the
  result of migration
• Hydrogeologic and geochemical data that are
  indirect indicators of attenuation mechanisms
• Data from field and microcosm studies that
  directly demonstrate certain attenuation
  mechanisms

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Assessment of MNA Potential

• Source evaluation
• components
• distribution
• loading rate
• Plume evaluation
• contaminants
• groundwater chemistry
• metabolic products
• aquifer characteristics

• Site characterization and data evaluation
• Site conceptual model
• Footprint analysis
• Lab Studies and Modeling

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Shrinking Ground Water Plume
Solute plume margin is receding back toward the
source area over time and the concentrations at
points within the plume are decreasing over time.
CROSS SECTION
PLAN VIEW
MW-5
MW-9
MW-1
WHEN ? Mass loading rate lt attenuation rate,
resulting in reduced plume mass in water-bearing
unit.
WHY ? Shrinking plume is evidence of natural
attenuation.
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Stable Groundwater Plume
Solute plume margin is stationary over time and
concentrations at points within the plume are
relatively uniform over time or may decrease over
time.
CROSS SECTION
PLAN VIEW
MW-5
MW-9
MW-1
WHEN ? Mass loading rate attenuation rate,
resulting in plume stabilization.
WHY ? Stable plume is evidence of natural
attenuation.
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Expanding Plume
Solute plume margin is continuing to move
outward or down-gradient from the source area.
CROSS SECTION
PLAN VIEW
MW-9
MW-1
MW-5
WHEN ? Mass loading rate gt attenuation rate,
resulting in increased plume mass in
water-bearing unit.
WHY ? Expanding plume may be evidence of natural
attenuation if expansion is less than expected
based on ground water flow.
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Effort for Site Characterization and Data
Interpretation
NRC,2000
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Conceptual Models of Source Distribution
Must determine source information during site
characterization to use MNA
a) Aqueous PhaseRelease to Saturated Zone
b) NAPL Release in Vadose Zone Only
c) LNAPL Releaseto Water Table
d) DNAPL Release to Saturated Zone
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Natural Attenuation Footprints
NRC, 2000
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Lab Studies and Modeling

• Lab studies or microcosms
• Must mimic site conditions
• Data may be useful for rate estimates
• Requires appropriate expertise throughout

• Modeling
• Budget analysis on electron donor and electron
  acceptor
• Screening models
• BIOCHLOR, etc.
• Complex models
• MODFLOW, etc.

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MNA Observations

• Attenuation occurs at all sites
• Effectiveness dominated by mass reduction
  mechanisms, usually biodegradation
• Rate and extent of biodegradation controlled by
  site specific conditions
• Acceptance of MNA requires considerable analysis
  and monitoring
• Tools for incorporating natural attenuation into
  groundwater management strategies continue to
  improve

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What We Dont Know

• When do you give up on natural attenuation?
• What do you pump into an aquifer and why?
• In what form do you add supplements to enhance
  bioattenuation? Liquid, gas or solid?
• How long should you wait to see a response after
  enhancement of bioattenuation?
• Are there compounds, classes of compounds, or
  aquifers that require bioaugmentation?