### 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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# ( ) d b sed sed w ai w 

## K d d CF m C ρ θ + + = '

where: C w = water concentration [μg /L] m ai ’ = mass applied per unit area [kg/ha] CF = conversion factor (100 μg

- m - ha/L

- kg) d sed = sediment depth [0.01 m] d w = water column depth [0.1 m] θ sed = porosity of sediment [0.509] ρ b = bulk density of sediment [1.3 kg/L] K d = water

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

d ai w 

## K m C 00013

. 0 00105

. 0 ' + =

# 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, pre

emergence 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)

10 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

11 Index Reservoir (large body, relatively little flow) Event

- Based Runoff from Non

Rice 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, 1 st 

### application to a dry field, 2 nd 

### 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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