Post on 20-May-2018
Emerging Contaminant Soil Fate Model Subroutine Development for
SWAT
Louis J. Thibodeaux and Eileen M. Canfield Louisiana State University
Cain Department of Chemical Engineering Jesse Coates Hall, South Stadium Drive
Baton Rouge, LA 70803 thibod@lsu.edu
2013 International SWAT Conference
Toulouse, France
Acknowledgement
• Funding: US Department of Agriculture-Agri. Res. Service. Grassland, Soil and Water Res. Lab., Temple, TX USA.
• Funding: Louisiana State Univ., Dept. Chemical Engineering, Baton Rouge, LA USA.
• Undergraduate Research Student, Miss. Kalpanee Gunasingha, BS ChE 2013.
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Project Objectives
• Develop a comprehensive, process-based mass balance theory, advection-diffusion transport model with reaction for projecting the FATE of EmCons on surface soil of watersheds.
• In doing so update and expand the present soil pesticide module in SWAT.
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Existing Pesticide Soil Module-Characteristics, construction and assumptions. Conventional Box-Model Approach. Soil layers as boxes with uniform concentration within. Number of layers typically four (4). Processes (5): a) particle erosion, b) solute run-off, c) reaction/evaporation, d) lateral flow, and e) infiltration.
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Continued: Equilibrium pesticide partitioning solid/water.
Transient mass balance, daily, on pesticide in soil layers. Analytical solution using “operator splitting” approach [aka, sequential]. Concentration in water is the state variable. Pesticide mass rate [aka, flux] out by the four processes captures its FATE on soil.
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Outline. Introduction.
The G-Box model [“G” denotes gradient as in concentration gradient] character, construct and assumptions. Simple diffusion, reaction and advection G-Box vs. exact analytical solution comparisons. A more realistic G-Box application of Mirex behavior patterns on soil. Summary.
H DT
DB
vT
vB
RX
Da
Do vo
CAi
CA
CBi
CB (EmCon Sink)
Ca (EmCon Source)
Soil Horizon A
Soil Horizon B
Atmosphere
Air/Soil Interface
A/B Interface
HRU Area S
G-Box Structure & Mass Balance
Di = diffusive type EmCon flux (kg/m2s) E = machine application flux (kg/m2s) vi = advective type EmCon flux (kg/m2s) RX = reaction EmCon degradation (kg/m3s) CA = EmCon concentration (kg/m3) H = A horizon depth (m) S = surface area (m2)
Mass Balance Horizon A: d[CAHS]/dt = DTS + vTS – DoS – voS - RXHS Flux Continuity a/A: DaS + E= DTS + vTS [solve for A/S interface conc.] Flux Continuity A/B: DBS + vBS = DoS + voS [solve for A/B interface conc.]
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SOIL G-BOX MODEL: EMCON FLUX PROCESSES [18]
• Atmospheric boundary layer – Dry deposition of vapor – Dry deposition of particles with sorbed fraction – Wet deposition scavenging, both vapor and
particles
• On soil surface – Chemical Evaporation – Water run-off
• Water erosion of soil particles
– Wind erosion of soil particles • Plant wash-off • Machine application
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SOIL G-BOX MODEL: EMCON FLUX PROCESSES [18]
• In soil horizons – Molecular diffusion
• Vapor phase • Aqueous phase
– Brownian diffusion • Aqua sols (aka colloids)
– Bioturbation • Soil solids • Soil pore water
– Infiltration • Vertical, down column (wicking upward?) • Lateral, horizontal in horizon
– Plant uptake, at specified depth in horizon – Reaction, degradation or transformation
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Other Processes and Assumptions
–Equilibrium • 3 bulk soil phases: gas, aqueous and solid • Sub-phases: particles in the atmosphere, organic
fraction on soil solids and colloids in the pore water • Linear equilibrium assumption (LEA) used throughout;
may be problematic for some metals and organics.
–Assumption
• The existing SWAT modules available from which to import data such as: soil water in horizon, precipitation, infiltration, soil organic matter, runoff, sediment erosion, etc., as needed by Soil G-Box.
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0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-0.01 7E-17 0.01 0.02 0.03 0.04 0.05
Soil Depth (cm)
Concentration (µg/L)
k1
k2
k3
k1
k2
k3
n Exact Solution n Gradient Box
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Steady State: Fixed Concentration (Diffusion & Reaction) Exact Solution vs. Gradient Box
Reaction Constants
1.5 Boxes
0
1
2
3
4
5
6
0 0.01 0.02 0.03 0.04 0.05
Soil Depth (cm)
Concentration (µg/L)
k1
k2
k3
k1
k2
k3
n Exact Solution n Gradient Box
Reaction Constants
2 Boxes
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Steady State: Zero Flux Bottom (Diffusion & Reaction) Exact Solution vs. Gradient Box
0
1
2
3
4
5
6
7
8
9
10
0 0.01 0.02 0.03 0.04 0.05
Soil Depth (cm)
Concentration (µg/L)
1 Day
3 Months
1 Year
1 Day
3 Months
1 Year
n Exact Solution n Gradient Box
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Un-Steady State: Semi-Infinite Slab (Diffusion) Exact Solution vs. Gradient Box
5 Boxes Ln Size Distribution
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Parameters Needed for Model
• Height of box layers (cm) • Number of box layers • Rain washout ratio • Groundcover fraction • Plant wash off fraction • HRU Area (cm2) • Mass Transfer Coefficient
(cm/s) • Particle velocity (cm/s) • Rainfall rate (cm/s) • Concentration of Particles in
air (µg/L)
• Partition Coefficients • Pesticide Concentration
(µg/L) • Erosion rates • Reaction Rate Constant
(1/s) • Pesticide Application rate
(µg/cm2s) • Lateral Flow rate (cm/s) • Percolation rate (cm/s) • Bulk Density of Soil (µg/L) • Diffusion Coefficients
(cm2/s)
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Mirex
• C10Cl12, white crystal, MW = 546 g/mol
• Insecticide for fire ant control; EPA prohibited use in 1976.
• vp = 8*10-7 mm Hg at 25°C
• log Kow = 7.0105
• Water solubility = 0.085 mg/L at 25°C
• Henry’s Constant = 0.0524
• log Koa = 8.369
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Soil Horizons
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Horizon A Horizon B Horizon C Porosity 0-20 cm 20-100 cm 100-250 cm
Water ϵ2 0.1 0.05 0.005 Air ϵ1 0.4 0.1 0.05 Soil 1-ϵT 0.5 0.85 0.945
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19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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Horizon A: 0-20 cm Horizon B: 20-100 cm Horizon C: 100-250 cm
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37
38
39
40
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0
50
100
150
200
250
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Soil Depth (cm)
Concentration (µg/L)
Final Model: Wet Deposition, Gas & Vapor Deposition, Diffusion (Wind & Water Erosion, Bioturbation, Infiltration)
n With Bioturbation
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Summary
• Develop soil EmCon fate model for SWAT.
• Conventional Box to G-Box.
• Numerical solution for simultaneous n-boxes vs. exact solution (nice!)
• Mirex behavior, conceptual validation (nice?)
• Experimental verification, soil profile comparisons (needed).
• Theoretical mathematical stability of G-Box (I need help).
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Acknowledgement
• Funding: US Department of Agriculture-Agri. Res. Service. Grassland, Soil and Water Res. Lab., Temple, TX USA.
• Funding: Louisiana State Univ., Dept. Chemical Engineering, Baton Rouge, LA USA.
• Undergraduate Research Student, Miss. Kalpanee Gunasingha, BS ChE 2013.
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THE G-BOX MODEL : AN INTRODUCTION
This is a new environmental modeling approach for
addressing chemical fate in natural media is based on the
Lavoisier mass balance (aka, law of conservation of mass). It
is an advancement of the conventional approach which
compartmentalizes the air, water, plant, soil phases, etc. into
boxes of uniform concentration. However, it retains the
transparency and simplicity of the former. The “G” is from
the word gradient, like in concentration gradient specifically.
In the following description it will be contrasted to the
conventional box model so as to highlight and it’s similar and
special features.
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The Conventional Box Model.
1) Multi boxes used for a multimedia system. For example a
four compartment (n=4) system consists of air, water, plant and
soil, one box or compartment for each media.
2) Material moves between the adjoining boxes by diffusive and
advective processes.
3) Reaction and equilibrium occurs within the each box,
4) Chemical concentration and material density within the box
is a single uniform value.
5) A species mass balance, steady state or transient, is
performed on each box.
6) A mathematical procedure is used to solve the set of n=4
equations; typically concentration is the state variable.
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Gradient Box (G-Box) Enhanced Features.
1. A concentration gradient is formed within by defining single
concentration at the center of each box,
2. The interface planes that separate the boxes have defined
individual concentrations based on any, one, selected,
convenient mobile fluid phase of the multi-media system
[*Typically for the environmental media two interface planes,
one above and one below, are used.]
3. Chemical flux continuity exist across each interface plane.
4. Chemical phase equilibrium exist at adjoining gas/liquid,
liquid/solid, etc. interface planes.
5. A steady-state species mass balance is performed across each
the interface plane to ensure flux continuity. In the example
three additional mass balances are performed.
6. A mathematical procedure is used to solve the set of seven
[n=4+3] equations.
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*Common features of the Earth’s bulk media structure.
1. Made of horizontal layers as is soil, aquatic bodies (lakes,
oceans, etc.) and the atmosphere,
2. Layers are stratified in the vertical dimension,
3. Composition and/or density of material within the media
layers is typically highly variable in the vertical dimension,
4. The √area-to-depth is large numerically, and
5. Vertical transport typically controls chemical mobility in
comparison to lateral.
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Summary:
Based upon Earth’s unique physical structure of the key
environmental compartments and the special characteristics of
the G-Box modeling approach, a first principle mass balance
procedure can yield a convenient and realistic mathematical
description of connected and complex processes for quantifying
chemical fate. Its application deriving the EmCon module for
SWAT is a good example.
H DT
DB
vT
vB
RX
Da
Do vo
CAi
CA
CBi
CB (EmCon Sink)
Ca (EmCon Source)
Soil Horizon A
Soil Horizon B
Atmosphere
Air/Soil Interface
A/B Interface
HRU Area S
G-Box Structure & Mass Balance
Di = diffusive type EmCon flux (kg/m2s) E = machine application flux (kg/m2s) vi = advective type EmCon flux (kg/m2s) RX = reaction EmCon degradation (kg/m3s) CA = EmCon concentration (kg/m3) H = A horizon depth (m) S = surface area (m2)
Mass Balance Horizon A: d[CAHS]/dt = DTS + vTS – DoS – voS - RXHS Flux Continuity a/A: DaS + E= DTS + vTS [solve for A/S interface conc.] Flux Continuity A/B: DBS + vBS = DoS + voS [solve for A/B interface conc.]
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