Chuck Peck, EFED

Dirk Young, EFED

3/19/2013

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Outline

 Background on rice modeling and issues

 Case study

 Background

 Refinements

 Future rice modeling refinements

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Typical Drinking Water Modeling

 For drinking water assessments (DWAs), EFED uses FIRST 

(Tier I) or PRZM and EXAMS using the index reservoir 

(Tier II) to model exposure.

 Models account for

 Amount of pesticide applied

 Abiotic and biotic degradation

 Sorption to soil and sediment

 Runoff events

 Spray drift

 Models provide acute and chronic estimated drinking 

water concentrations (EDWCs).

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Drinking Water Modeling - Rice

 Modeling pesticide applications to rice paddies and 

their discharges is somewhat unique

 Pesticide can be applied to a field when it’s wet or dry

 Fields are flooded for an extended period of time

 In some cases, multiple flooding events and discharges 

occur

 Large discharge events not dictated by precipitation

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Drinking Water Modeling - Rice

 In 2002, EFED developed the Interim Tier I Rice Model 

for use in estimating pesticide surface water exposure 

in rice paddies.

 Estimated screening-level concentrations to represent 

short and long term surface water exposure, for use in 

aquatic ERAs and DWAs.

 Evaluated from 2002-2007 using aquatic field 

dissipation (AFD) studies and monitoring data. 

Approved for use in 2007.

 Tier I Rice model only provides a peak value (no 

degradation) for the water in a rice paddy.

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Tier I Rice Model

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( ) w sed sed b d

ai

w d d K

m CF C

+ θ + ρ

=

'

where: 

Cw = water concentration [μg /L]

mai’ = mass applied per unit area [kg/ha]

CF = conversion factor (100 μg-m-ha/L-kg)

dsed = sediment depth [0.01 m]

dw = water column depth [0.1 m]

θsed = porosity of sediment [0.509]

ρb = bulk density of sediment [1.3 kg/L]

Kd = water-sediment partitioning coefficient [L/kg], or 0.01 x Koc

d

ai

w K

m C

0.00105 0.00013

'

+

=
Case Study - Background

 Conducted a DWA for a non-selective, foliar 

herbicide.

 Herbicide is registered for a number of non-ag and 

ag uses, including rice.

 Could be applied to dry or flooded fields, preemergence or post-emergence.

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Case Study – Pesticide Use and Properties

 Parent (only) – very soluble, stable to hydrolysis, but broke 

down in biotic studies (soil – 30 days, water – 90 days).

 Three degradates of concern plus parent made total toxic 

residue stable in all environments.

 Aquatic field dissipation study done under dry and flooded 

conditions using 12-inch bare soil paddies in CA and LA 

 Fairly rapid dissipation of parent in water (half-lives ~ 7 days, 

water concentrations at or below 2 ppb in 14 days). 

 Concentrations for 2 of 3 degradates of concern captured.

 Done at lowest rate on label.

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Case Study - Refinements

 No monitoring data available for runoff from rice paddies. 

 Limited data available from BEAD on use patterns. 

 Looked at different options to estimate a chronic EDWC

 Use a simplified dilution method – line of evidence.

 Develop a dilution factor based on measured concentrations 

for other pesticides used in rice production – line of evidence.

 Use Pesticide Flooded Application model (PFAM) and 

postprocess results to account for aerobic aquatic 

degradation.

 Update PFAM to account for discharge to receiving waterbody

- characterization.

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Pesticide Flooded Application Model (PFAM)

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Potential Overflow

Water Input

Pesticide Application

Volatilization

degradation

degradation

mass transfer

Depth

Evaporation/Precipitation

Adjustable

Weir

Tolerance

Use mathematical processes in EXAMS and VVM to estimate 

paddy water concentration. 
PFAM with Index Reservoir

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Index Reservoir (large 

body, relatively little 

flow)

Event-Based Runoff from NonRice Areas

Runoff Generated by Rice Areas

Rice Field

Acute and chronic EDWCs estimated by model. Allows for 

incorporation of runoff from precipitation in estimating EDWCs 

over time in index reservoir.
Case Study – Additional Refinements

 Worked with HED to re-evaluate residues of concern 

and potentially eliminate degradates that were 

resulting in persistence of herbicide.

 Worked with registrant to refine label rates and 

application conditions.

 In the end, were able to make a safety finding for rice 

use based on chronic drinking water concentrations 

based on PFAM estimates.

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Case Study – Refinement Results

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• Two applications at 0.73 lbs ai/acre, 1st application to a 

dry field, 2nd application to a flooded field

• Retreatment interval of 10 days, 

• Holding period of 55 days (4” paddy), 30 days (8” paddy)

Refinement

Acute EDWC 

(μg/L)

Annual Average 

EDWC (μg/L)

4” 8” 4” 8”

Based on effluent paddy water concentration

Tier I w/ degradation 555 431 136 105

PFAM w/ dilution factor 390 320 195 160

PFAM w/ degradation 390 320 95 78

Based on index reservoir concentration

PFAM w/ index reservoir 90 158 22 39
Next Steps for EFED with Rice

 Currently EFED has approved the use of PFAM on a 

case-by-case basis in ERAs and DWAs. 

 Develop scenarios that include flowing water bodies 

and index reservoir to incorporate dilution into the 

acute and chronic concentrations.

 Analyze rice ag practices, including continuous flow.

 Evaluate model against measured concentrations. 

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Questions?

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