Evaluating Effects of Mineral Fouling on the Long-Term Performance of Permeable Reactive Barriers by Lin Li and Craig H. Benson Geoenvironmental Engineering University of Wisconsin-Madison RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003 Evaluating Effects of Mineral Fouling on the Long-Term Performance of Permeable Reactive Barriers by Lin Li and Craig H. Benson Geoenvironmental Engineering University of Wisconsin-Madison RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003
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BENSON~1.PPTEvaluating Effects of Mineral Fouling on the Long-Term
Performance of
Permeable Reactive Barriers
Geoenvironmental Engineering University of Wisconsin-Madison
RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003
Evaluating Effects of Mineral Fouling on the Long-Term Performance of
Permeable Reactive Barriers
Geoenvironmental Engineering
RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003
Effectiveness of PRB depends on: - ability for water to flow through the wall - adequate reactivity - adequate residence time
SourceSource
groundwatergroundwater
flowflow
PRBPRB
aquitardaquitard
Effectiveness of PRB depends on: - ability for water to flow through the wall - adequate reactivity
- adequate residence time
Issues
• Mineral fouling is a key concern in long- term performance of PRBs
– Mineral precipitation occurring as a result of changes in geochemistry due to iron corrosion
– Reduces porosity, reactivity, and hydraulic conductivity of iron media in PRBs
– Alters hydraulic characteristics, treatment ability, and life time of PRBs
• Flow heterogeneity causes unpredictable fouling in PRBs
Issues
• Mineral fouling is a key concern in long- term performance of PRBs
– Mineral precipitation occurring as a result of changes in geochemistry due to iron corrosion
– Reduces porosity, reactivity, and hydraulic conductivity of iron media in PRBs
– Alters hydraulic characteristics, treatment ability, and life time of PRBs
• Flow heterogeneity causes unpredictable fouling in PRBs
Objectives
• Estimate the degree of fouling that will occur in PRBs located in realistic heterogeneous aquifers
• Evaluate how fouling is influenced by flow heterogeneity
• Evaluate impact of fouling on long-term hydraulic performance of PRBs
Objectives
• Estimate the degree of fouling that will occur in PRBs located in realistic heterogeneous aquifers
• Evaluate how fouling is influenced by flow heterogeneity
• Evaluate impact of fouling on long-term hydraulic performance of PRBs
Mineral Precipitates
– Iron oxides: magnetite (Fe3O4), ferrous hydroxide (Fe(OH)2), ferric hydroxide (Fe(OH)3)
– Carbonates: aragonite (CaCO3), magnesite (MgCO3), siderite (FeCO3), dolomite (CaMg(CO3)2)
– Others: ferrous sulfide (FeS), brucite (Mg(OH)2), green rust
• Crystalline & amorphous mineral formation
– Iron oxides: magnetite (Fe3O4), ferrous hydroxide (Fe(OH)2), ferric hydroxide (Fe(OH)3)
– Carbonates: aragonite (CaCO3), magnesite (MgCO3), siderite (FeCO3), dolomite (CaMg(CO3)2)
– Others: ferrous sulfide (FeS), brucite (Mg(OH)2), green rust
• Crystalline & amorphous mineral formation
After 15 mos. operation, as reported Philips et al. (2000)
Mineral Precipitates in PRB at Oak Ridge, TN
Porosity reduction of 0.02-0.20 per year (Sarr 2001)
Mineral Precipitates in PRB at Oak Ridge, TN
After 15 mos. operation, as reported Philips et al. (2000)
PorePoreFeFe00
filingsfilings
FeOOHFeOOHCaCOCaCO33FeSFeS
PorePoreFeFe00
filingsfilings
Porosity reduction of 0.02-0.20 per year (Sarr 2001)
Mineral Precipitates in Iron Media: After 4 years of operation - PRB at Elizabeth City, NC,
Wilkin et al. (2002)
Mineral Precipitates in Iron Media: After 4 years of operation - PRB at Elizabeth City, NC,
Wilkin et al. (2002)
• MODFLOW – groundwater flow
Approach
• MODFLOW – groundwater flow
Updating Hydraulic Conductivity of the PRB
Modeling SchemeModeling Scheme
Heterogeneous Aquifer GenerationHeterogeneous Aquifer Generation
Max Simulation Max Simulation Period ?Period ? PostPost--ProcessingProcessing
YesYes
(MODFLOW)(MODFLOW)
Updating Porosity andUpdating Porosity and Reactivity of the PRBReactivity of the PRB
NoNo
Reactive Transport Simulation Reactive Transport Simulation in the Vicinity of PRBin the Vicinity of PRB
(RT3D)(RT3D)
Iron Reactions
DODO
NitrateNitrate
SulfateSulfate
WaterWater
Mineral Precipitates
Fe2+ + 2OH- Fe(OH)2 (s)
Fe2+ + 2OH- Fe(OH)2 (s)
Reaction Kinetics • Iron Corrosion Rate
– Pseudo first-order rate law for iron corrosion by DO and NO3 -
(Mayer et al. 2001)
– Zero-order rate law for iron corrosion by water under anaerobic conditions (Reardon 1995)
• Mineral Precipitation Rate – Reversible rate law based on transition state theory (Lasaga 1998)
• Microbial Sulfate Reduction Rate – Monod equation (Gu et al. 2002)
Reaction Kinetics
(Mayer et al. 2001)
– Zero-order rate law for iron corrosion by water under anaerobic conditions (Reardon 1995)
• Mineral Precipitation Rate – Reversible rate law based on transition state theory (Lasaga 1998)
• Microbial Sulfate Reduction Rate – Monod equation (Gu et al. 2002)
Heterogeneous Hydraulic Conductivity FieldHeterogeneous Hydraulic Conductivity Field ((mmlnKlnK = = --10 m/s, 10 m/s, ss lnK lnK = 1, = 1, llxx = 3 m, = 3 m, ll yy = 1 m= 1 m))
Groundwater Groundwater flowflow
PRB PRB (1 m x 25 m x 10 m)(1 m x 25 m x 10 m)
Model Validation: Moffett Field PRB
• Data from Yabusaki et al. (2001)
• PRB 3 m x 3 m x 5.5 m used to remove chlorinated solvents from groundwater
• 1D Model
Model Validation: Moffett Field PRB
• Data from Yabusaki et al. (2001)
• PRB 3 m x 3 m x 5.5 m used to remove chlorinated solvents from groundwater
• 1D Model
Flow
Pea Gravel
3.0 m
Predicted and Measured pH - Moffett Field PRBPredicted and Measured pH - Moffett Field PRB
12
pH - field data
0.005
0.004
Distance into PRB (m)
0.005
Distance into PRB (m)
Fe(OH) 2
Sensitivity Analysis Sensitivity Analysis -- Key MineralsKey Minerals
CaCO 3
MgCO 3
FeCO 3
Fe(OH) 2
Total Reduction
Porosity Reductions: Averages and MaximaPorosity Reductions: Averages and Maxima
0.16
0.12
Distance into PRB (m)
Groundwater Flow
Groundwater Flow
P o
ro si
ty R
ed u
ct io
Porosity Reduction Related to Balance Between Porosity Reduction Related to Balance Between Seepage Velocity and Reaction RateSeepage Velocity and Reaction Rate
Average and Maximum Porosity Reduction at 10, 30, 50 yr
Average and Maximum Porosity Reduction at 10, 30, 50 yr
0.64 Initial Porosity in PRB = 0.60
Max
Avg
Distance in PRB (m)
Average and Maximum Conductivity Reduction at 10, 30, 50 yr
Average and Maximum Conductivity Reduction at 10, 30, 50 yr
R at
io o
f H
yd ra
u lic
C o
n d
u ct
iv it
y K
/K o
Distance in PRB (m)
Initial Condition
30 Years
10 Years
50 Years
Initial Condition
Inflow Darcy Velocity Over TimeInflow Darcy Velocity Over Time
lightlight--low velocity, dark high velocitylow velocity, dark high velocity
Darcy Velocities in PRB Over TimeDarcy Velocities in PRB Over Time D
ar cy
V el
oc iti
es in
P R
0.12
0.10
0.08
0.06
0.04
0.02
Reduction in high velocities Reduction in high velocities due to filling of preferential due to filling of preferential flow pathsflow paths
Initial 10 yr 30 yr 50 yr
Seepage Velocities and Residence Times
Darcy velocity (q) largely controlled by facies in aquifer, Elder et al. 2002
Seepage velocity (vs) increases because porosity decreases (vs = q/n)
Residence decreases as porosity fills because seepage velocity is increasing
Seepage Velocities and Residence Times R
es id
en ce
T im
e (d
Existing Existing InstallationsInstallations Future?Future?
Darcy velocity (q) largely controlled by facies in aquifer, Elder et al. 2002
Seepage velocity (vs) increases because porosity decreases (vs = q/n)
Residence decreases as porosity fills because seepage velocity is increasing
Initial 10 yr 30 yr 50 yr
1 0.3
2
4
6
8
10
Hydraulic Head Distribution h (m) at y=25 m, Initial
P R
2
4
6
8
10
Hydraulic Head Distribution h (m) at y=25 m, After 50 years
P R
50 yrs50 yrs
Head DistributionsHead Distributions Initially and at 50 YrsInitially and at 50 Yrs
Gradient becomes Gradient becomes steeper, but dramatic steeper, but dramatic
head changes not head changes not apparentapparent
Effect on Flow PathsEffect on Flow Paths
50 yrs50 yrs
Increasing thickness ofIncreasing thickness of sacrificial zonesacrificial zone
6.0
6.5
7.0
7.5
8.0
8.5
9.0
0 5 10 15 20 25 30
No auger mixing Mixing every 15 yrs Mixing every 10 yrs Mixing every 5 yrs
M ed
ia n
R es
id en
ce T
im e
in P
R B
(d ay
Operational Time (year)
Effect of Auger Mixing on Residence TimeEffect of Auger Mixing on Residence Time
Summary
• Reductions are spatially variable due to flow heterogeneities (larger reductions where flow rates are higher)
• Impacts on hydraulic performance: – Re-distribution of flow paths – Reductions in residence time, largely after 10 yrs – Flow bypassing, largely after 10 yrs – Hydraulic gradient build-up, but subtle
• Auger mixing and sacrificial iron/gravel zones have modest effect on porosity reductions and changes in hydraulics
Summary
• Reductions are spatially variable due to flow heterogeneities (larger reductions where flow rates are higher)
Permeable Reactive Barriers
Geoenvironmental Engineering University of Wisconsin-Madison
RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003
Evaluating Effects of Mineral Fouling on the Long-Term Performance of
Permeable Reactive Barriers
Geoenvironmental Engineering
RTDF Permeable Reactive Barriers Action Team Meeting Niagara Falls, October 15-16, 2003
Effectiveness of PRB depends on: - ability for water to flow through the wall - adequate reactivity - adequate residence time
SourceSource
groundwatergroundwater
flowflow
PRBPRB
aquitardaquitard
Effectiveness of PRB depends on: - ability for water to flow through the wall - adequate reactivity
- adequate residence time
Issues
• Mineral fouling is a key concern in long- term performance of PRBs
– Mineral precipitation occurring as a result of changes in geochemistry due to iron corrosion
– Reduces porosity, reactivity, and hydraulic conductivity of iron media in PRBs
– Alters hydraulic characteristics, treatment ability, and life time of PRBs
• Flow heterogeneity causes unpredictable fouling in PRBs
Issues
• Mineral fouling is a key concern in long- term performance of PRBs
– Mineral precipitation occurring as a result of changes in geochemistry due to iron corrosion
– Reduces porosity, reactivity, and hydraulic conductivity of iron media in PRBs
– Alters hydraulic characteristics, treatment ability, and life time of PRBs
• Flow heterogeneity causes unpredictable fouling in PRBs
Objectives
• Estimate the degree of fouling that will occur in PRBs located in realistic heterogeneous aquifers
• Evaluate how fouling is influenced by flow heterogeneity
• Evaluate impact of fouling on long-term hydraulic performance of PRBs
Objectives
• Estimate the degree of fouling that will occur in PRBs located in realistic heterogeneous aquifers
• Evaluate how fouling is influenced by flow heterogeneity
• Evaluate impact of fouling on long-term hydraulic performance of PRBs
Mineral Precipitates
– Iron oxides: magnetite (Fe3O4), ferrous hydroxide (Fe(OH)2), ferric hydroxide (Fe(OH)3)
– Carbonates: aragonite (CaCO3), magnesite (MgCO3), siderite (FeCO3), dolomite (CaMg(CO3)2)
– Others: ferrous sulfide (FeS), brucite (Mg(OH)2), green rust
• Crystalline & amorphous mineral formation
– Iron oxides: magnetite (Fe3O4), ferrous hydroxide (Fe(OH)2), ferric hydroxide (Fe(OH)3)
– Carbonates: aragonite (CaCO3), magnesite (MgCO3), siderite (FeCO3), dolomite (CaMg(CO3)2)
– Others: ferrous sulfide (FeS), brucite (Mg(OH)2), green rust
• Crystalline & amorphous mineral formation
After 15 mos. operation, as reported Philips et al. (2000)
Mineral Precipitates in PRB at Oak Ridge, TN
Porosity reduction of 0.02-0.20 per year (Sarr 2001)
Mineral Precipitates in PRB at Oak Ridge, TN
After 15 mos. operation, as reported Philips et al. (2000)
PorePoreFeFe00
filingsfilings
FeOOHFeOOHCaCOCaCO33FeSFeS
PorePoreFeFe00
filingsfilings
Porosity reduction of 0.02-0.20 per year (Sarr 2001)
Mineral Precipitates in Iron Media: After 4 years of operation - PRB at Elizabeth City, NC,
Wilkin et al. (2002)
Mineral Precipitates in Iron Media: After 4 years of operation - PRB at Elizabeth City, NC,
Wilkin et al. (2002)
• MODFLOW – groundwater flow
Approach
• MODFLOW – groundwater flow
Updating Hydraulic Conductivity of the PRB
Modeling SchemeModeling Scheme
Heterogeneous Aquifer GenerationHeterogeneous Aquifer Generation
Max Simulation Max Simulation Period ?Period ? PostPost--ProcessingProcessing
YesYes
(MODFLOW)(MODFLOW)
Updating Porosity andUpdating Porosity and Reactivity of the PRBReactivity of the PRB
NoNo
Reactive Transport Simulation Reactive Transport Simulation in the Vicinity of PRBin the Vicinity of PRB
(RT3D)(RT3D)
Iron Reactions
DODO
NitrateNitrate
SulfateSulfate
WaterWater
Mineral Precipitates
Fe2+ + 2OH- Fe(OH)2 (s)
Fe2+ + 2OH- Fe(OH)2 (s)
Reaction Kinetics • Iron Corrosion Rate
– Pseudo first-order rate law for iron corrosion by DO and NO3 -
(Mayer et al. 2001)
– Zero-order rate law for iron corrosion by water under anaerobic conditions (Reardon 1995)
• Mineral Precipitation Rate – Reversible rate law based on transition state theory (Lasaga 1998)
• Microbial Sulfate Reduction Rate – Monod equation (Gu et al. 2002)
Reaction Kinetics
(Mayer et al. 2001)
– Zero-order rate law for iron corrosion by water under anaerobic conditions (Reardon 1995)
• Mineral Precipitation Rate – Reversible rate law based on transition state theory (Lasaga 1998)
• Microbial Sulfate Reduction Rate – Monod equation (Gu et al. 2002)
Heterogeneous Hydraulic Conductivity FieldHeterogeneous Hydraulic Conductivity Field ((mmlnKlnK = = --10 m/s, 10 m/s, ss lnK lnK = 1, = 1, llxx = 3 m, = 3 m, ll yy = 1 m= 1 m))
Groundwater Groundwater flowflow
PRB PRB (1 m x 25 m x 10 m)(1 m x 25 m x 10 m)
Model Validation: Moffett Field PRB
• Data from Yabusaki et al. (2001)
• PRB 3 m x 3 m x 5.5 m used to remove chlorinated solvents from groundwater
• 1D Model
Model Validation: Moffett Field PRB
• Data from Yabusaki et al. (2001)
• PRB 3 m x 3 m x 5.5 m used to remove chlorinated solvents from groundwater
• 1D Model
Flow
Pea Gravel
3.0 m
Predicted and Measured pH - Moffett Field PRBPredicted and Measured pH - Moffett Field PRB
12
pH - field data
0.005
0.004
Distance into PRB (m)
0.005
Distance into PRB (m)
Fe(OH) 2
Sensitivity Analysis Sensitivity Analysis -- Key MineralsKey Minerals
CaCO 3
MgCO 3
FeCO 3
Fe(OH) 2
Total Reduction
Porosity Reductions: Averages and MaximaPorosity Reductions: Averages and Maxima
0.16
0.12
Distance into PRB (m)
Groundwater Flow
Groundwater Flow
P o
ro si
ty R
ed u
ct io
Porosity Reduction Related to Balance Between Porosity Reduction Related to Balance Between Seepage Velocity and Reaction RateSeepage Velocity and Reaction Rate
Average and Maximum Porosity Reduction at 10, 30, 50 yr
Average and Maximum Porosity Reduction at 10, 30, 50 yr
0.64 Initial Porosity in PRB = 0.60
Max
Avg
Distance in PRB (m)
Average and Maximum Conductivity Reduction at 10, 30, 50 yr
Average and Maximum Conductivity Reduction at 10, 30, 50 yr
R at
io o
f H
yd ra
u lic
C o
n d
u ct
iv it
y K
/K o
Distance in PRB (m)
Initial Condition
30 Years
10 Years
50 Years
Initial Condition
Inflow Darcy Velocity Over TimeInflow Darcy Velocity Over Time
lightlight--low velocity, dark high velocitylow velocity, dark high velocity
Darcy Velocities in PRB Over TimeDarcy Velocities in PRB Over Time D
ar cy
V el
oc iti
es in
P R
0.12
0.10
0.08
0.06
0.04
0.02
Reduction in high velocities Reduction in high velocities due to filling of preferential due to filling of preferential flow pathsflow paths
Initial 10 yr 30 yr 50 yr
Seepage Velocities and Residence Times
Darcy velocity (q) largely controlled by facies in aquifer, Elder et al. 2002
Seepage velocity (vs) increases because porosity decreases (vs = q/n)
Residence decreases as porosity fills because seepage velocity is increasing
Seepage Velocities and Residence Times R
es id
en ce
T im
e (d
Existing Existing InstallationsInstallations Future?Future?
Darcy velocity (q) largely controlled by facies in aquifer, Elder et al. 2002
Seepage velocity (vs) increases because porosity decreases (vs = q/n)
Residence decreases as porosity fills because seepage velocity is increasing
Initial 10 yr 30 yr 50 yr
1 0.3
2
4
6
8
10
Hydraulic Head Distribution h (m) at y=25 m, Initial
P R
2
4
6
8
10
Hydraulic Head Distribution h (m) at y=25 m, After 50 years
P R
50 yrs50 yrs
Head DistributionsHead Distributions Initially and at 50 YrsInitially and at 50 Yrs
Gradient becomes Gradient becomes steeper, but dramatic steeper, but dramatic
head changes not head changes not apparentapparent
Effect on Flow PathsEffect on Flow Paths
50 yrs50 yrs
Increasing thickness ofIncreasing thickness of sacrificial zonesacrificial zone
6.0
6.5
7.0
7.5
8.0
8.5
9.0
0 5 10 15 20 25 30
No auger mixing Mixing every 15 yrs Mixing every 10 yrs Mixing every 5 yrs
M ed
ia n
R es
id en
ce T
im e
in P
R B
(d ay
Operational Time (year)
Effect of Auger Mixing on Residence TimeEffect of Auger Mixing on Residence Time
Summary
• Reductions are spatially variable due to flow heterogeneities (larger reductions where flow rates are higher)
• Impacts on hydraulic performance: – Re-distribution of flow paths – Reductions in residence time, largely after 10 yrs – Flow bypassing, largely after 10 yrs – Hydraulic gradient build-up, but subtle
• Auger mixing and sacrificial iron/gravel zones have modest effect on porosity reductions and changes in hydraulics
Summary
• Reductions are spatially variable due to flow heterogeneities (larger reductions where flow rates are higher)
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