in situ remediation (isr mt3dms local domain approach · in‐situ remediation (isr‐mt3dmstm)...
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In‐Situ Remediation(ISR‐MT3DMSTM)
Local Domain Approach
1ISR‐MT3DMS Local Domain Approach
1 Local Domain
5 m
Global Domain
Global Domain
Sand
DNAPL
Well
Diffusion Into Clay
Silt or Clay
Forward Diffusion
2ISR‐MT3DMS Local Domain Approach
Sand
Well
Back-Diffusion out of Clay
Back‐Diffusion
3
Silt or Clay
ISR‐MT3DMS Local Domain Approach
Factors Influencing Remediation Timeframe
4
THICK silt/clay:
‐ Sale et al., 2008‐Matrix Diffusion ToolKit(ESTCP, www.gsi‐net.com)
Influencing factors: ‐ Velocity‐ Thickness‐ Retardation‐ Diffusion rate‐ Transverse dispersion
‐ Length of clay lens‐ Biodegradation‐ Contact time
ISR‐MT3DMS Local Domain Approach
Introduction• Modeling diffusion‐dominated transport may require the addition of dozens to hundreds of layers to a model, depending on the thickness of silt/clay layers. This can have prohibitive costs, particularly for 3‐D models which already incorporate a large number of rows and columns.
• While there are analytical solutions for simulating diffusion in thicker layers of silt/clay, a numerical model is often needed for thinner silt/clay layers, or when complex degradation reactions occur.
ISR‐MT3DMS Local Domain Approach 5
Introduction• ISR‐MT3DMSTM offers the option to use local 1‐D domains to represent diffusion‐dominated transport. These 1‐D domains are outside the global model grid, and thus may result in significant cost and time savings for some sites.
• The local 1‐D domains incorporate the same reaction options that are available for the global model, so the effects of in‐situ bioremediation or ISCO, for example, may be simulated in silt/clay layers.
• Users also have the option to specify different horizontal and temporal discretization for the local 1‐D domains, relative to the global model.
ISR‐MT3DMS Local Domain Approach 6
Local domain(clay with limited extent, 50 layers)
Global domain
1 2 3
Each clay lens:10 to 100+ layers
100+ layers
Each clay lens:20 to 100+ layers
Water Table
Example Applications
7ISR‐MT3DMS Local Domain Approach
0 1 2 3
Scale, in meters
0 1 2 3
Scale, in meters
Global domain
Local domain
8
Example Applications
ISR‐MT3DMS Local Domain Approach
0 1 2 3
Scale, in meters
Global domain
Local domain
9
Example Applications
ISR‐MT3DMS Local Domain Approach
Example #1: Ontario Site
ISR‐MT3DMS Local Domain Approach 10
Case Study #1 – Ontario Site
Treatment zone Waterloo Emitter
Groundwater elevation contour Other injection well
0 5 10
Scale, in meters
Sequenced Injections:1. Surfactant (NAPL)2. Hydrogen peroxide3. CaO24. Waterloo emitters
11ISR‐MT3DMS Local Domain Approach
0 5 10
Scale, in meters
0 5 10 15 20 25 30 35 4090
92
94
96
98
100
Distance (m)
Section K-K'
Silty sand
Sand
Silt / Clay
Screened interval
Legend
WEST EAST
Sand
12ISR‐MT3DMS Local Domain Approach
Model Grid
0 1 2 3
Scale, in meters
Minimum spacing = 4 inches(Waterloo Emitter diameter)
2‐D: 450 columns, 280 rows
Time step = 0.05 d
Phase I – 5 solutes(4‐hour run‐time)
0 5 10
Scale, in meters
Phase I – Waterloo Emitters horizontal influence
13ISR‐MT3DMS Local Domain Approach
Phase I: Waterloo Emitters (t=3y)Case 1: PHC Koc = 5,000 mL/g
Case 2: PHC Koc = 50,000 mL/g
Electron Donors:
• GRO, DRO, Fe(II)
Electron Acceptors:
• DO, Fe(III)s
Reactions:
• Instantaneous or first‐order
• Reductive dissolution
Phase II model:
• Hydrogen peroxide• CaPO• GRO/DRO Conc.• Diffusion into silt
14ISR‐MT3DMS Local Domain Approach
Case Study #1• Waterloo emitters used for passive oxygen injection, with approx. 1 meter spacing between wells
• Same wells also used for periodic active injections of oxygen‐releasing compound, chemical oxidant, or surfactant (depending on the event)
• Tight spacing of passive injection wells required high resolution grid discretization to evaluate zone of influence from passive injections
• Geology is interbedded sands with tight fine‐grained layers.
• Costs were prohibitive to develop a 3‐D model for evaluating vertical diffusion‐dominated transport in thick fine‐grained layers, given tight horizontal spacing and number of species in reactive transport model.
ISR‐MT3DMS Local Domain Approach 15
Local Domain Approach
16
Area of interest for modeling diffusion
Global Model Domain
ISR‐MT3DMS Local Domain Approach
Cross‐Section in Global Model (3 layers)
17
Sand Seam #1
SILT
Sand Seam #2
Source Area
Global model domain
ISR‐MT3DMS Local Domain Approach
Local Model Domains for Silt(1‐D Diffusion)
18
Sand Seam #1
SILT
Sand Seam #2
Area of InterestMultiple 1‐D vertical (Local) models
are linked to sand seamconcentrations in global model.
Silt layer is inactive to transport in global model.
ISR‐MT3DMS Local Domain Approach
Next Steps• Using the local domain approach substantially reduced the size of the global model domain.
• We are currently simulating the influence of active remediation on mass in the finer‐grained layers using the local domain approach.
ISR‐MT3DMS Local Domain Approach 19
Case Study #2: Florida Site
ISR‐MT3DMS Local Domain Approach 20
Approx. source zone extent
Site Characteristics
• Beach sand aquifer
• Continuous, thin clay layer across site
• Other discontinuous, thin silt/clay layers
• Multiple, thin suspended DNAPL layers in source zone
Extraction WellTransect
Injection WellTransect
Case Study – Florida Site
21ISR‐MT3DMS Local Domain Approach
1
10
100
1,000
10,000
100,000
2002 2003 2004 2005 2006 2007 2008
TVOC Co
ncentration (ug/L)
Year
TCE MCL = 5 ug/L
Hydraulic isolation system started August 2002
Expected trendwithout back‐diffusion
Observed Trend
Modified from Parker et al., 2008
TVOC Trend After Source Containment
22ISR‐MT3DMS Local Domain Approach
v = 130 ft/yαtv = 1.5 mm
v = 65 ft/yαtv = 1.5 mm
Distance (m)
Elevation (ft)
200 columns, 158 rows (layers)Minimum grid spacing: z = 1.25 cm, x = 0.5 m
Run‐time = 45 minutes for 85‐y simulation (t = 0.24 d)
Clay layer thickness = 0.2 m, foc = 0.5%
2‐inch thick TCE pool
2‐D Model Grid
23
Carey et al. (2015)
ISR‐MT3DMS Local Domain Approach
Distance (m)
Elevation (ft)
Clay
TCE pool: S=1100 mg/L, 5 m x 0.05 m
16 layersin clay
C=1,100 mg/L
t=35 y t=85 y0
TCESourceModel
Source Characteristics
24
DNAPL source removed at t=35 y.
Carey et al. (2015)
ISR‐MT3DMS Local Domain Approach
t = 0
Mclay = 136 kg
0 10 20 30 40 50 60 70 80 90 1000
5
10
0
0.005
0.1
1
10
100
0 10 20 30 40 50 60 70 80 90 1000
5
10
0 10 20 30 40 50 60 70 80 90 1000
5
10
Elevation (m
)Elevation (m
)Elevation (m
)
Distance (m)
TCEConcentration
(mg/L)
Mclay = TCE mass in clay assuming 20 m width.t = time since source removal.
t = 20 y
Mclay = 1.1 kg
t = 30 y
Mclay = 0.06 kg
30 years after source removal: 99.96% mass depletion in clay, avg. Cwell = 12 to 126 ug/L
Simulated TCE After Source Removal
25
Carey et al. (2015)
ISR‐MT3DMS Local Domain Approach
10987654321
5 m
1 Local Domain
5 m
(a) Local domain models with xLD = 0.5 m (b) Local domain models with xLD = 5 m
Conceptual illustration of local domains for two cases: (a) global and local domains have the same horizontal spacing; and (b) local domain has a larger horizontal spacing than the global domain grid.
ISR‐MT3DMS Local Domain Approach 26
vl
vu Hydrodynamic Dispersion (Dz = Dm + De)
Comparison of vertical mechanical dispersion (Dm) and effective diffusion coefficient (De) magnitudes in each grid cell of a 1‐D local domain. Vertical mechanical dispersion is shown to be significant at the top and bottom clay‐sand interfaces due to the use of a three‐dimensional dispersion tensor and horizontal velocity components at each clay‐sand interface. Application of a 1‐D diffusion model will result in underestimation of the mass flux between the transmissive zone and clay layer.
Elevation (m
)
De = Do
Dm = αtv v
Dz = 2.5x10‐5 m2/d
Dz = 1.7x10‐5 m2/d
Dz = 0.9x10‐5 m2/d
ISR‐MT3DMS Local Domain Approach 27
0.001
0.01
0.1
1
10
100
-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50
Sim
ulat
ed W
ell C
once
ntra
tion
(mg/
L)
Time (y)
Simulated monitoring well concentrations at x=5, 25, and 100 m. Solid lines represent the global domain model, dashed lines represent the local domain model with local grid x=0.5 m, and dotted lines represent the local domain model with local grid x=5.0 m.
x=5 m
x=100 m
x=25 m
ISR‐MT3DMS Local Domain Approach 28
0
5
10
15
20
25
30
35
1 2 3 4 5 6
Remed
iatio
n Timeframe (y)
x = 5m x = 10m x = 25m x = 50m x = 75m x = 100m
Simulated remediation timeframe for three model cases: (a) no local domains are used; (b) 200 local domains are used with horizontal spacing of 0.5 m; and (c) 20 local domains are used with horizontal spacing of 5.0 m. Based on monitoring well with Lscreen=3 m.
200 Local domains 20 Local domainsNo Local domainsxLD = 0.5 m xLD = 5.0 m
ISR‐MT3DMS Local Domain Approach 29
0
5
10
15
20
25
30
35
0 20 40 60 80 100
Remed
iatio
n Timeframe (y)
Clay Layer Length (m)
(a) Remediation timeframe versus clay layer length (Lscreen=3 m)
(b) Remediation timeframe versus well screen length (x = 50 m)
0
5
10
15
20
25
30
35
40
0.1 1 10
Remed
iatio
n Timeframe (y)
Well Screen Length, Lscreen (m)
ISR‐MT3DMS Local Domain Approach 30
05
101520253035
0 5 10 15 20Remed
iatio
n Timeframe
(y)
Transverse Dispersivity, αtv (mm)
050
100150200250300350400
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Remed
iatio
n Timeframe
(y)
Clay Layer Thickness, Hclay (m)
foc = 1.5%R = 8.7
foc = 0.5%R = 3.5
(a) Remediation timeframe versus αtv.
(b) Remediation timeframe versus clay layer thickness (Hclay).
(c) Remediation timeframe versus v.
0
5
10
15
20
25
30
0 5 10 15 20Remed
iatio
n Timeframe (y)
Contact Time, tc (y)
(d) Remediation timeframe versus contact timebetween DNAPL and clay aquitard.
0
10
20
30
40
50
60
0.00 0.10 0.20 0.30 0.40Remed
iatio
n Timeframe
(y)
Groundwater Velocity, v (m/d)
ISR‐MT3DMS Local Domain Approach 31
1
10
100
1 2 3 4 5 6 7 8 9 10
Ratio
of M
axim
um to
Minim
umRe
med
ation Timeframe
Exhibit 14 – Comparison of relative sensitivity of remediation timeframe to various input parameters, based on the ratio of maximum to minimum timeframe for each set of modeled parameter adjustments. Hclay is the clay layer thickness, R is the retardation coefficient, v is groundwater velocity, Lclay is the length of the clay layer, Csol is solubility, is the tortuosity coefficient, and Lscreen is the monitoring well screen length. Based on clay layer length of 50 m and well screen length of 3 m unless except for Lclay and Lscreen parameter adjustments.
Hclay
R
v
Lclay αtv
Csol Lscreen
ClayHalf‐life Contact
Time
ISR‐MT3DMS Local Domain Approach 32