Transcript MODFLOW - Rice University

Biodegradation and Natural
Attenuation
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)
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
Evidence of Natural Attenuation
• Plume Length Should Be =
Seepage Velocity
x Time
÷ Retardation Factor
• Plume Length Is .....
Shorter
Thinner
Appears not to be moving
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
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
Fate of Organic Contaminants
in the Subsurface
Volatilization
Sorption
Abiotic Transformations
Hydrolysis
Biodegradation
Advection
Dilution
Advection
• Contaminants transported with ground water flow
• No effect on contaminant concentration
• No net loss of contaminant mass
1.0
t1
C/C0
t2
0.5
0.0
Time
t3
Dispersion
• Mechanical and hydraulic mixing
• Decreases contaminant concentration in center of
plume – increases concentration on edges
• No net loss of contaminant mass
1.0
C/C0
0.5
0.0
t1
t2
Time
t3
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
1.0
t1
C/C0
t2
0.5
0.0
Time
t3
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
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
Abiotic Transformations
• Reactions such as hydrolysis and dehalogenation
• Reduces aqueous concentrations of contaminants
• Reduction of contaminant mass
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
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
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
Observation of Natural Attenuation
Source Removed
Distance
Flow
Direction
t0
Time
t1
t2
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
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
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
Multi-Site Studies (Newell and Connor, API)
0 ft
BTEX plumes at 42
retail LUST sites
Chlorinated ethene
(PCE, TCE, DCE, or VC)
plumes at 88 sites
200 ft 400 ft
600 ft 800 ft 1000 ft
213 ft x 150 ft
1000 ft x 500 ft
Other chlorinated solvent
plumes (TCA, DCA) at
500 ft x 350 ft
29 sites
Chloride, salt water
plumes at 25 sites
700 ft x 500 ft
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
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
Shrinking Ground Water Plume
CROSS
SECTION
PLAN VIEW
WHEN ?
WHY ?
“Solute plume
margin is receding
back toward the
Affected Ground Water
source area over
time and the
TIME 1 concentrations at
MW-5
MW-9
MW-1
TIME 2 points within the
plume are
TIME 3 decreasing over
time.”
Mass loading rate < attenuation rate, resulting
in reduced plume mass in water-bearing unit.
Shrinking plume is evidence of natural
attenuation.
Stable Groundwater Plume
CROSS
SECTION
Affected Ground Water
PLAN VIEW
MW-5
MW-9
MW-1
TIME 1
TIME 2
“ Solute plume margin is
stationary over time
and concentrations at
points within the
plume are relatively
uniform over time or
may decrease over
time.”
TIME 3
WHEN ?
WHY ?
Mass loading rate = attenuation rate, resulting
in plume stabilization.
Stable plume is evidence of natural
attenuation.
Expanding Plume
CROSS
SECTION
Affected Ground Water
PLAN VIEW
TIME 1
MW-9
MW-1
MW-5
TIME 2
“Solute plume
margin is
continuing to
move outward or
down-gradient
from the source
area.”
TIME 3
WHEN ?
WHY ?
Mass loading rate > attenuation rate, resulting
in increased plume mass in water-bearing unit.
Expanding plume may be evidence of natural
attenuation if expansion is less than expected
based on ground water flow.
Effort for Site Characterization
and Data Interpretation
Site Hydrogeolo gy
Simple flo w, unifor m geochemistry,
and low concentrations
Simple flo w, small scale physical or
chemic al heterogeneity , and
medium-hig h concentratio ns
Strongly transient flow, large-scale
physical or chemical heterogeneity,
or high concentrations
Likelih ood of Success of Natural
Attenuatio n for the Contaminant of
Concern
High
Moderate
Low
(e.g., BTEX,
Alcohols )
(e.g., MC,Pb)
(e.g., 3H,
MTBE, TCE)
1
2
2
2
2
3
2
3
3
NOTES: Level of effort refers to number and fre quency of samples taken,
parameters analyzed in site samples, and type of data analysis :
1= low effort 2 = moderate effort 3 = high effort
NRC,2000
Conceptual Models of
Source Distribution
a) Aqueous Phase
Release to
Saturated Zone
Must determine
source
information
during site
characterization
to use MNA
WELL
b) NAPL Release in
Vadose Zone
Only
c) LNAPL Release
to Water Table
d) DNAPL Release
to Saturated Zone
t =0
t = t1
t = t2
Natural Attenuation Footprints
Case Study
Contamin ant
Contamin ants
Footprints
Controlled?
Traverse City
BTEX
Yes
Borden AFB
Five chlorin ated
solvents
Partially
St. Joseph
Edwards AFB
Dover AFB
Depletion of O2, formation of
CH4 and Fe2+
Detection of metabolit es of
solvent degradation
No
Formation of CH4; detection
of degradatio n by-products
(vinyl chloride and ethene)
TCE
No
Documentation of high NO3and SO42- concentrations;
documentation of TCE
transport
TCE, 1,1,1-TCA
Yes
Formation of degradation
by-products
TCE
NRC, 2000
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.
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
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?