Emerging Contaminant Soil Fate Model Subroutine Development for SWAT

Transcription

1 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 International SWAT Conference Toulouse, France

2 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

3 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.

4 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. 4

5 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. 5

6 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. 6

7 G-Box Structure & Mass Balance C a (EmCon Source) Atmosphere Soil Horizon A H D a D T D B HRU Area S v T R X v B C Ai CA C Bi C B (EmCon Sink) Air/Soil Interface A/B Interface Soil Horizon B D o v o D i = diffusive type EmCon flux (kg/m 2 s) E = machine application flux (kg/m 2 s) v i = advective type EmCon flux (kg/m 2 s) R X = reaction EmCon degradation (kg/m 3 s) C A = EmCon concentration (kg/m 3 ) H = A horizon depth (m) S = surface area (m 2 ) Mass Balance Horizon A: Flux Continuity a/a: Flux Continuity A/B: d[c A HS]/dt = D T S + v T S D o S v o S - R X HS D a S + E= D T S + v T S [solve for A/S interface conc.] D B S + v B S = D o S + v o S [solve for A/B interface conc.] 7

8 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 8

9 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 9

10 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. 10

11 Steady State: Fixed Concentration (Diffusion & Reaction) Exact Solution vs. Gradient Box Concentration (µg/l) E Boxes Soil Depth (cm) Exact Solution Gradient Box Reaction Constants k1 k2 k3 k1 k2 k

12 0 Steady State: Zero Flux Bottom (Diffusion & Reaction) Exact Solution vs. Gradient Box Concentration (µg/l) Boxes 2 Soil Depth (cm) Exact Solution Gradient Box Reaction Constants k1 k2 k3 k1 k2 k3 6 12

13 0 Un-Steady State: Semi-Infinite Slab (Diffusion) Exact Solution vs. Gradient Box Concentration (µg/l) Soil Depth (cm) Boxes Ln Size Distribution Exact Solution Gradient Box 1 Day 3 Months 1 Year 1 Day 3 Months 1 Year 10 13

14 14

15 Parameters Needed for Model Height of box layers (cm) Number of box layers Rain washout ratio Groundcover fraction Plant wash off fraction HRU Area (cm 2 ) 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/cm 2 s) Lateral Flow rate (cm/s) Percolation rate (cm/s) Bulk Density of Soil (µg/l) Diffusion Coefficients (cm 2 /s) 15

16 Mirex C 10 Cl 12, white crystal, MW = 546 g/mol Insecticide for fire ant control; EPA prohibited use in vp = 8*10-7 mm Hg at 25 C log K ow = Water solubility = mg/l at 25 C Henry s Constant = log K oa =

17 Soil Horizons Horizon A Horizon B Horizon C Porosity 0-20 cm cm cm Water ϵ Air ϵ Soil 1-ϵ T

18 18

19 19

20 20

21 21

22 22

23 23

24 24

25 25

26 26

27 27

28 28

29 29

30 30

31 31

32 32

33 33

34 34

35 Horizon A: 0-20 cm Horizon B: cm Horizon C: cm 35

36 36

37 37

38 38

39 39

40 40

41 Final Model: Wet Deposition, Gas & Vapor Deposition, Diffusion (Wind & Water Erosion, Bioturbation, Infiltration) 0 Concentration (µg/l) Soil Depth (cm) With Bioturbation

42 42

43 43

44 44

45 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). 45

46 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

47 47

48 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. 48

49 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. 49

50 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. 50

51 *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. 51

52 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. 52

53 G-Box Structure & Mass Balance C a (EmCon Source) Atmosphere Soil Horizon A H D a D T D B HRU Area S v T R X v B C Ai CA C Bi C B (EmCon Sink) Air/Soil Interface A/B Interface Soil Horizon B D o v o D i = diffusive type EmCon flux (kg/m 2 s) E = machine application flux (kg/m 2 s) v i = advective type EmCon flux (kg/m 2 s) R X = reaction EmCon degradation (kg/m 3 s) C A = EmCon concentration (kg/m 3 ) H = A horizon depth (m) S = surface area (m 2 ) Mass Balance Horizon A: Flux Continuity a/a: Flux Continuity A/B: d[c A HS]/dt = D T S + v T S D o S v o S - R X HS D a S + E= D T S + v T S [solve for A/S interface conc.] D B S + v B S = D o S + v o S [solve for A/B interface conc.] 53