Environmental Modeling Public Meeting

April 28, 2015

Gabe Rothman, Meridith Fry, Dirk Young, Chuck Peck, Jim 

Lin, Faruque Khan, James Hetrick, Mah Shamim
Background

 Volatilization algorithm evaluated: last of remaining PRZM 

routines needing formal evaluation 

 Calculates daily loss of pesticide due to volatilization flux 

from treated field to the atmosphere 

 Scope of review: only evaluated soil component of 

algorithm at this time (bare soil applications only, foliar 

applications not evaluated)

 Provides potential for refinement of aquatic EECs for soil 

fumigants and semi-volatiles

 Intended use to address quantity of pesticide available for 

runoff to water bodies

 Not intended for use with terrestrial air exposure assessments

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Evaluation and Final Products

 QA/QC Tasks

 Phase I: Evaluation of PRZM Code

 Phase II: Case Study (Ground Truth) Comparison

 Final Products

 QA/QC Report: Documentation of Phase I and Phase II 

Findings

 Input parameter guidance document specific for 

volatilization algorithm and soil application methods

 Updated version of SWCC containing critical inputs for 

the volatilization algorithm

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Phase I Evaluation of PRZM Code, 

 First Step: Reviewed critical routines for volatilization production from literature and Step 1 Overview

inspected calculation process for accurate soil concentration and volatilization flux

Routine Key Parameters Process

Dynamic Soil 

Temperature Routine

(Hanks et al., 1971)

• Air temperature, solar radiation, & 

albedo

• Soil lower boundary temperature

• Initial HLC input (at 25 degrees 

Celsius)

• Heat of Henry

(e.g., Enthalpy of Vaporization)

• %Sand, %Clay

Adjusted HLC and resulting 

vapor-phase concentration 

in soil

Diffusion in Soil 

Adjustment

(Millington and Quirk,

1961)

• Diffusion in Air Coefficient

• Field Capacity

• Wilting Point

Adjusted diffusion

coefficient in soil accounting 

for soil porosity and 

tortuosity

Volatilization Flux 

Boundary Condition

(Jury et al., 1983)

• HLC

• Diffusion in Air Coefficient

• Stagnant BL Height

• Vapor phase soil concentration in top 

soil compartment

Soil volatilization flux to the 

atmosphere from top soil 

layer through stagnant 

boundary layer

Citations:

Dynamic Soil Temperature Routine: Hanks, R. J., D. D. Austin, and W. T. Ondrechen. 1971. Soil temperature estimation by a numerical method. Soil Science Society of 

America Proceedings 35:665-667.

Millington - Quirk Expression: Millington and Quirk, 1961. Permeability of porous solids. Trans. Faraday Soc., 57, 1200-1207, doi: 10.1039/TF9615701200.

Jury Equation: Jury, W. A., W. F. Spencer, and W. J. Farmer, 1983. Use of models for assessing relative volatility, mobility, and persistence of pesticides and other trace 

organics in soil systems. Pages 1-43 in J. Saxena, editor. Hazard Assessment of Chemicals. Academic Press, New York, New York, USA. 4
Phase I Evaluation of PRZM Code, 

Step 1 Results

 Bug Found and Corrected: Index to incorrect soil compartment with HLC in solution for “C term” of advection-dispersion equation for top soil 

compartment

 Only impacted volatilization algorithm function

C(1)= -(DISP(K,2)*THETN(2) + new_henry(K,2)*DGAIR(2) )*DELT/(DELX(1)* 

0.5*(DELX(1)+DELX(2)))

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Phase I Evaluation of PRZM Code, 

Step 2 Overview

 Second Step: Functionality tests for volatilization 

algorithm - mass balance and sensible volatilization 

calculation criteria for success:

Mass balance completely accounted for in soil

Flux the same for applied non-partitioning gas under both dry and saturated moisture conditions

Flux higher for highly volatile chemical versus semi- volatile chemical

Flux higher for same chemical under high soil 

heating (low albedo, high temperature) than low 

soil heating (high albedo, low temperature)

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Additions to SWCC Output Options 

for Mass Balance in Soil

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Phase I Evaluation of PRZM Code,

Step 2 Results

 Dry vs. Moist Conditions

• Soil Moisture (Dry = 0.00 cm water input, Moist = 20.o0 cm water input)

• Field capacity and wilting point the same in dry and moist scenarios

• Gas, Non-Partitioning Chemical (HLC = 104)

• No degradation or sorption

• Chemical Incorporated Uniformly to 46 cm depth

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Phase I Evaluation of PRZM Code,

Step 2 Results

 Highly Volatile vs. Semi-Volatile

• Chemical State Inputs (Highly Volatile HLC = 104, Semi-Volatile HLC = 0.1) 

• Dry Conditions(Water Input= 0.00 cm)

• No degradation or sorption

• Chemical Incorporated Uniformly to 46 cm depth 9
Phase I Evaluation of PRZM Code,

Step 2 Results

 High vs. Low Soil Heating: Dynamic Soil Temperature Routine Functionality

• High Soil Heating Parameters (Air Temperature = 35°C, Albedo =0.14)

• Low Soil Heating Parameters ( Air Temperature = 5°C, Albedo =0.60)

• Dry Conditions(Water Input= 0.00 cm)

• No degradation or sorption

• Chemical Incorporated Uniformly to 46 cm depth

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Phase 2 Case Study Comparison,

Overview

Compared modeled volatile flux to 

measured volatile flux in the field

Considered bare soil broadcast applications 

only (no tarps)

 Evaluated case studies with a mix of shallow 

and deep incorporation applications

Case studies encompassed three field 

volatility studies done with three soil 

fumigants 

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Phase 2 Case Study Comparison,

SWCC Application Method Inputs

Case Study

Modeled 

Application Method SWCC Demo

Chemical A

Washington State

Multi-Nozzle

Shank Injection

Uniform 

Incorporation to 14 

inches

Chemical B

Florida

Shank Injection Incorporate Entirely 

to 5 inches

Chemical C

Arizona

Shank Injection Incorporate Entirely 

to 13 inches

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Phase 2 Case Study Comparison,

Preliminary Results

Magnitude

Underpredictions: 2.9 1.6 1.8 1.3 963.4 136.1

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Addressing Incorporation Depth 

Issue

 Underprediction in volatilization with preliminary analysis 

appears to occur with deep incorporation depths

 Preliminary analysis modeled entire chemical incorporated

“@ depth” for shank injection applications

 Literature on actual chemical distributions with shank 

injection applications with fumigants 

Figure citation: Guo, M. et al.,2004. Distribution and Leaching of Methyl Iodide in Soil Following Emulated Shank and Drip Application. 

J. Enviro. Qual. 33: p. 2151

Also referenced: Ajwa. et al.,2002. Application of Alternative Fumigants Through Drip Irrigation Systems. 

American Phytopathological Society 92: 1349-1355

Injected Chemical

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Examining Soil Chemical Distribution,

SWCC Inputs

Case Study

Modeled 

Application Method SWCC Demo

Chemical B

Florida

Shank 

Injection

Linearly Increasing

to 5 inch Injection 

Depth

Chemical C

Arizona

Shank 

Injection

Linearly Increasing

to 14 inch Injection 

Depth

Atmosphere

depth

Δ Option for App. Method

Injection Depth

Soil Chemical 

Distribution 15
Examining Soil Chemical Distribution,

Results

Before:

@ Depth

After:

Δ

1.8 1.3 963.4 136.1

1.4 1.5 4.8 4.7

Magnitude

Underpredictions:

Magnitude

Underpredictions:

Trace

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Impact on Aquatic EECs

 For shank injection cases ran with Δ, EECs without volatilization greater than EECs with volatilization for study sites ran with historical weather

 Reduction in EECs associated with underpredicted volatile loss from soil for these cases 

produce reasonably conservative EECs

Chemical Case Study

SWCC One-in-Ten Year Surface Water Peak Concentrations 

(ppb)

No Volatilization Volatilization

Chemical B

with Tampa, FL Historical Weather 27.2 14.6

Chemical C

with Phoenix, AZ Historical Weather

3.4 x 10-3 1.1 x 10-3

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QA Findings and Conclusions

 Volatilization algorithm functions sensibly after bug fix

 Underpredictions in flux occur within 3X of measured 

values with shallow shank injections and uniformally

incorporated applications

 Large underprediction of flux rates with deep 

injections of soil fumigant applications when modeled 

with “@ depth” option

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Guidance for Soil Applications with 

Volatilization Algorithm in SWCC

 Shank injection applications: Recommend “Δ” option 

− Consistent with empirical soil chemical distributions from research 

with deeply-applied soil fumigants

− Improves agreement in measured flux rates with deeper incorporation depths more common with soil fumigants

− Impact of initial chemical distribution SWCC input minimal with shallower incorporation depths (< 5 inches), occurring with both soil 

fumigants and soil-applied conventional chemicals

 Surface applications: Recommend “Below Crop” default option 

 Recommend use of volatilization algorithm for both soil fumigants and soil-applied conventional chemicals

− Expected model sensitivity of flux with HLC verified during Phase I of 

evaluation

− Modeled flux calculations within acceptable range of measured values 

in case study comparisons following SWCC guidance above

− Not recommended for chemicals with HLC< 10-6 atm•m3/mol

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Future Work

 Corrected version of volatilization algorithm to be 

released in next version of SWCC

 Also will be incorporated into PRZM-GW

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Questions ?

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