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Predicting Pesticide Volatility Through Coupled Above- and Belowground Multiphysics Modeling

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Abstract

Pesticides are applied to agricultural fields to control unwanted pests but can volatilize and be transported downwind by wind currents to create the potential for non-target organism exposure. Volatilization rates change through the growing season due to pesticide application timing, meteorological differences, and the differential flux rates from soil and vegetation matrices. Field studies quantifying pesticide volatility are expensive and cannot capture the semi-infinite parameter combinations of soil, crop, management, and meteorological conditions encountered under regional agronomic practices. A numerical approach was used to simulate pesticide dissipation above- and belowground to augment field and laboratory experimental observations. Above- and belowground physics are coupled into a single numerical tool using the COMSOL Multiphysics® software package with the current emphasis on pesticide volatility into air from soil and vegetation and resulting near field neighboring air concentrations. Comparison of simulation results against experimental observations for an insecticide (chlorpyrifos) applied to potato and alfalfa fields shows good agreement (R2 0.68–0.98). Chlorpyrifos volatility from plant surfaces drives the overall volatility within the first several days post application. The maximum volatility flux rate simulated and observed were 0.79 and 0.66 μg m−2 s−1 for the alfalfa trial and 2.72 and 2.17 μg m−2 s−1 for the potato field, respectively. This coupled multiphysics tool [computational fluid dynamics (CFD), mass transfer coefficients, and variably saturated flow in soil] can be used to estimate volatility flux rates of pesticides when little or no prior knowledge is available and for extrapolating field study observations to different and diverse scenarios.

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Abbreviations

α :

coefficient in the Baker model; PEARL uses a value of 0.137

α f :

fluid thermal diffusivity (L2 τ−1)

ρ b :

soil bulk density (m L−3)

μ L :

solute first-order kinetic degradation coefficients in the dissolved phase (τ−1)

μ S :

solute first-order kinetic degradation coefficients in the adsorbed phase (τ−1)

μ G :

solute first-order kinetic degradation coefficients in the vapor phase (τ−1)

μ p :

pesticide first-order kinetic degradation coefficients on the leaf surfaces (τ−1)

u :

fluid velocity (L τ−1)

θ :

soil water content (L3 L−3)

θ s :

the saturation water content (volume fraction of “void” space (porosity)) of the soil (L3 L−3)

ν T :

turbulent kinematic viscosity of air (L2 τ−1)

a :

volumetric air content of the soil (L3 L−3)

A p :

areic concentration of pesticide on the leaf surfaces (mol L−2)

A ref :

the reference areic concentration (mol L−2)

A p, 0 :

areic concentration of pesticide on the leaf surfaces at time 0 (mol L−2)

c G, air :

the pesticide vapor concentration in the air aboveground (mol L−3)

c G|z = 0 :

the pesticide concentration at the soil surface (mol L−3)

c S :

solute concentration in the adsorbed phase (mol m−1)

c L :

solute concentration in the liquid phase (mol L−3)

c G :

solute concentration in the vapor phase (mol L−3)

c T :

total solute concentration in the soil medium (mol L−3)

c G, sat :

saturation vapor concentration in the air (mol L−3)

D L :

diffusion-dispersion coefficient for dissolved phase (L2 τ−1)

D G :

molecular diffusivity of solute in the air-filled pore space (L2 τ−1)

\( {D}_G^{Air} \) :

the molecular diffusivity of pesticide vapor in air (L2 τ−1)

D T :

turbulent diffusivity (L2 τ−1)

Du i :

difference function between two velocity profiles (m s−1)

d :

the thickness of the stationary air boundary layer (L)

d i :

initial depth of soil within which pesticide uniformly distributes (L)

f p :

portion of the pesticide applied to the field that is intercepted by the canopy (−)

h :

suction pressure head (defines saturation for h ≥ 0) (L)

h m, ss :

the mass transfer coefficient at the soil surface (L τ−1)

h m, p :

the mass transfer coefficient at the plant surface (L τ−1)

H p :

the height of the canopy (L)

J ss :

volatility flux from soil surface (m L−2 τ−1)

J p :

volatility flux from plant surface (m L−2 τ−1)

\( \widehat{\boldsymbol{k}} \) :

unit vector in the vertically upwards direction

K :

hydraulic conductivity (L τ−1)

K D :

equilibrium partition coefficient between liquid solid phases (L3 m−1)

K H :

Henrys law constant representing partitioning between liquid and vapor phases (−)

L soil :

depth of soil region under consideration (L)

L :

the characteristic length (that is, cross-section diameter of the tube or leaf) (L)

M total :

areic concentration (that is, concentration per unit area) applied to the field (mol L−2)

Nu L :

the Nusselt number defined in calculating the heat transfer coefficient (−)

n, M, C :

Parameters defined in calculating the heat or mass transfer coefficient (−)

Pr:

the Prandtl number defined in calculating the heat transfer coefficient (−)

p diff :

the pressure difference applied between the inlet and outlet of the box (Pa)

q :

ground water velocity vector (L τ−1)

R c :

pesticide sink terms (that is., degradation) = − CL(μLθ + μSρbKD + μGaKH) (mol L−3 τ−1)

r a :

aerodynamic resistance (τ L−1)

r b :

boundary layer resistance (τ L−1)

Re :

the Reynolds number defined in the Baker model (–)

Re L :

the Reynolds number defined in calculating the heat or mass transfer coefficient (–)

\( {Sh}_L=\frac{h_{m,p}\;L}{D_G^{Air}} \) :

the Sherwood number defined in calculating the mass transfer coefficient (–)

\( Sc=\frac{\upsilon }{D_G^{Air}} \) :

the Schmidt number defined in calculating the mass transfer coefficient (–)

Sc T :

turbulent Schmidt number, usually taken to be 0.71 (–)

t :

time (τ)

u :

the turbulent air flow velocity vector above the soil surface (L τ−1)

u* :

frictional velocity calculated from experiment or computational fluid dynamics (CFD)

U 0 :

wind speed at site of application (L τ−1)

υ :

kinematic viscosity of fluid (L2 τ−1)

x, y, z :

spatial coordinates (L)

z 0 m :

the roughness length for momentum, usually taken as z0m = 0.123Hp (L)

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Acknowledgements

The authors thank former colleague Dr. Dong Wang for review and feedback on this manuscript. Work was fully supported by Dow AgroSciences, LLC.

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Correspondence to Steven A. Cryer.

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Mao, M., Cryer, S.A., Altieri, A. et al. Predicting Pesticide Volatility Through Coupled Above- and Belowground Multiphysics Modeling. Environ Model Assess 23, 569–582 (2018). https://doi.org/10.1007/s10666-018-9594-6

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  • DOI: https://doi.org/10.1007/s10666-018-9594-6

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