1. Biodegradation and Natural Attenuation

2. 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)

3. 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

4. Evidence of Natural Attenuation
Plume Length Should Be =
Seepage Velocity
x Time
÷ Retardation Factor
Plume Length Is .....
Shorter
Thinner
Appears not to be moving

5. Natural Attenuation A Do-Nothing Approach?
Requires quantitative assessment of a
plume’s behavior -
amount, extent, and rate of travel,
as well as long-term evidence of attenuation

6. 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

7. Fate of Organic Contaminants in the Subsurface
Volatilization
Sorption
Abiotic Transformations
Hydrolysis
Biodegradation
Advection
Dilution

8. Advection Contaminants transported with ground water flow
No effect on contaminant concentration
No net loss of contaminant mass

9. Dispersion Mechanical and hydraulic mixing
Decreases contaminant concentration in center of plume – increases concentration on edges
No net loss of contaminant mass

10. 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

11. 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

12. 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

13. Abiotic Transformations
Reactions such as hydrolysis and dehalogenation
Reduces aqueous concentrations of contaminants
Reduction of contaminant mass

14. 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

15. 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

16. 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 , 1997

17. Observation of Natural Attenuation

18. 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

19. 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

20. 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

21. Multi-Site Studies (Newell and Connor, API)
0 ft
200 ft
400 ft
600 ft
800 ft
1000 ft
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

22. 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

23. 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

24. Shrinking Ground Water Plume
Affected Ground Water
“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
TIME 1
PLAN VIEW
MW-9
MW-5
MW-1
TIME 2
TIME 3
WHEN ? Mass loading rate < attenuation rate, resulting in reduced plume mass in water-bearing unit.
WHY ? Shrinking plume is evidence of natural attenuation.

25. Stable Groundwater Plume
Affected Ground Water
“ 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
TIME 1
PLAN VIEW
MW-5
MW-9
TIME 2
MW-1
TIME 3
WHEN ? Mass loading rate = attenuation rate, resulting in plume stabilization.
WHY ? Stable plume is evidence of natural attenuation.

26. Expanding Plume TIME 1 TIME 2 TIME 3
Affected Ground Water
“Solute plume margin is continuing to move outward or down-gradient from the source area.”
CROSS SECTION
TIME 1
PLAN VIEW
MW-9
MW-1
MW-5
TIME 2
TIME 3
WHEN ? Mass loading rate > 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.

27. Effort for Site Characterization and Data Interpretation
NRC,2000

28. Conceptual Models of Source Distribution
Must determine source information during site characterization to use MNA
a) Aqueous Phase Release to Saturated Zone
b) NAPL Release in Vadose Zone Only
c) LNAPL Release to Water Table
d) DNAPL Release to Saturated Zone

29. Natural Attenuation Footprints
NRC, 2000

30. 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.

31. 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

32. What We Don’t 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?