Waterborne Environmental, Inc
. EMPM March 24, 2014 1 Waterborne Environmental, Inc
. EMPM March 24, 2014 1 

# Comparing Groundwater models – Why are there differences?

Nathan Snyder Waterborne Environmental, Inc.

Mark Cheplick (Waterborne) Ishadeep Khanijo (Waterborne) Daniel Perkins, Ph.D. (FMC)Waterborne Environmental, Inc
. EMPM March 24, 2014 2 

## Overview

 Look back of PRZM

- GW rollout

 Evaluation of Tier 2 refinement strategiesWaterborne Environmental, Inc
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## PRZM - GW “One year later”

 December 11, 2012 Memorandum from Don Brady “officially” made PRZM

- GW the standard tool for calculating estimated drinking water concentrations (EDWC) from groundwater sources
 Announced one

- year evaluation period (immediate inclusion in assessments)

 Tier 1: higher of SCI

- GROW or PRZM

- GW

 Tier 2: PRZM

- GW with refinements
 Version Control

 Official: http://www.epa.gov/oppefed1/models/water/#przm_gw

 Interim : https://sites.google.com/site/przmsurfacewater/surface

water

- calculator

- archiveWaterborne Environmental, Inc
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## Conceptual Model and Scenarios

 Six vulnerable standard scenarios, conceptual model of application over potable well.

 Rapidly declining degradation in 1m soil profileWaterborne Environmental, Inc
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## Tiered Assessment Approach

 Tier 1

• Tier 2Waterborne Environmental, Inc
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### Comparing Groundwater Model Simulations – What are the differences ? ( Tier 2 Strategies – Impact on EDWCs)
 Refinements are to the Complete Risk Assessment

 EFED and Registrants working with EPA Risk Managers to evaluate Tier 2 refinements

 HED evaluates risk based on refined approach

 Incorporation of BEAD data in assessment noted in flowchart, but not in refinement strategy details


###  USEPA Provides 5 Specific Refinement Strategies

– 3 Categories

 Modeling refinements

– EDWC changes based on changes in environmental fate, scenarios, application representation

 Mitigation refinements

– EDWC changes based on use guidance adjustments (well setbacks or reduced annual schedules)

 Analysis refinement

– utilize daily time series or other relevant duration summary of results in refined HED assessmentWaterborne Environmental, Inc
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### Modeling Refinement: “Refinement Strategy 2: Identify Pesticide Fate Parameters not Considered in the Tier 1 Simulations”

 Product Specific Consideration

 Primary inputs Sorption ( K oc 

### ), Aerobic Soil Degradation, and Hydrolysis (subsurface aqueous phase degradation)

 Additional Considerations (some require outside of GUI inputs or post processing)

 Plant Processes (interception, wash off, foliar degradation, uptake)

 Anaerobic Degradation

 Modification to Total Toxic Residue (modeling parent and metabolites independently)

 Refined Sorption Processes (discussed earlier)

 Non Linear Sorption

 Time Dependent SorptionWaterborne Environmental, Inc
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## PRZM - GW InputsWaterborne Environmental, Inc
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### Model and Parameter Sensitivity: Soil Degradation RateWaterborne Environmental, Inc
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### Model and Parameter Sensitivity: Sorption and HydrolysisWaterborne Environmental, Inc
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### Total Toxic Residue (Single Run or Addition of Components)

 Total Toxic Residue (TTR) assumptions are used when uncertainties in environmental fate and metabolite toxicity are present
 Methods to conservatively use available data provide more realistic EDWCs while maintaining TTR concepts

Efate Soil DT50 (days) Koc Hydrolysis DT50 (days) Application Rate* (Kg/ha) TTR 100 20 300 1 Parent 2 300 15 1 Metabolite 1 10 150 250 0.483 Metabolite 2 90 50 300 0.373 Metabolite 3 50 20 300 0.102 Metabolite 4 100 20 300 0.080 *Set based on maximum formation and Molecular weight (sum > TTR)Waterborne Environmental, Inc
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## Degradation Changes with Depth

 When anaerobic and aerobic degradation are similar, the assumption of decreasing degradation with depth does not seem appropriate

 If dissipation studies available, inverse modeling techniques can be used to validate (and verify conservatism)

Example: DT50=50 , K oc =20 , Hydrolysis=300Waterborne Environmental, Inc
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### Modeling Refinement: “Refinement Strategy 1: Development of Representative Scenario”
 PRZM

- GW may be expanded to capture the variability in anticipated leaching to groundwater based on differences in soil and weather (National Implementation)

 Used to place standard scenarios into context and link to monitoring locations

 May be used to focus analysis to anticipated use area with adjustments to allow for crop/product specific analysisWaterborne Environmental, Inc
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### National Implementation of PRZM - GW

 PRZM

- GW methodology applied to national soils database (STATSGO) and distributed weather stations

 STATSGO Soils with layer thickness and bulk density used directly and FC and WP calculated from sand, clay, organic carbon using Rawls and Brakensiek method
 Weather from 237 SAMSON Stations assigned from10,498 STASGO MUIDs based on largest area of the MUID polygon created by intersecting MUID centroid with MLRA SAMSON weather region.


###  Cropping assumed as row crops with regional planting dates and cropping parameters based on USEPA standard scenarios and the PRZM manual (can be modified based on label)
 CN for Row Crops based on Soil Hydrologic Soil Group

 HSG A:74/70/74 B:83/80/83 C: 89/87/89 D: 92/90/92


###  Groundwater table depth for all scenarios assumed as 9 m with 1 m screen length.
 Post Processing results include maximum daily concentration, simulation average, post breakthrough average and breakthrough time.Waterborne Environmental, Inc
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## National PRZM - GWWaterborne Environmental, Inc
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### National PRZM - GW : Placing Standard Scenarios into Context
 Example DT50=50 days, K oc 

### =20, Hydrolysis=0 days, CN=10Waterborne Environmental, Inc
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## National PRZM - GW: Impact of CN

 The CN in PRZM influences runoff processes

 Setting CN artificially low, highlights areas of runoff vulnerability as having leaching vulnerability

 Example DT50=50 days, K oc 

### =20 , Hydrolysis=0 daysWaterborne Environmental, Inc
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## National PRZM - GW: Environmental Fate Sensitivity

Example: DT50=300 , K oc =20 , Hydrolysis=300Waterborne Environmental, Inc
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## Spatial Representation

 Presentation of results mapped by soil polygon (STATSGO MUID)

 Options to scale up to political boundaries (counties) or down to specific SSURGO layers based on soil properties and location

 Filter Results based on cropping patternsWaterborne Environmental, Inc
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## Mapping Options

 Maximum in a Soil Polygon versus Area Weighted AverageWaterborne Environmental, Inc
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## Spatial Comparison of Inputs

 Evaluate differences in Environmental Fate

 Hydrolysis=300, K oc 

### =20, Variable Soil Degradation Soil Aerobic Degradation Rate 50 Days 300 daysWaterborne Environmental, Inc
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## Review Areas Exceeding Level of ConcernWaterborne Environmental, Inc
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### Mitigation Refinement: “Refinement Strategy 3: Consideration of Annual Application Retreatment”
 Resistance management plans control use frequency, while registration focused on the product label

 BEAD data can help Risk Manager and HED bridge the label and actual useWaterborne Environmental, Inc
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### Mitigation Refinement: “Refinement Strategy 4: Considerations of Well Setbacks”

 C/C 0 

### = exp ((

- L/v)k )

 where :

 C = concentration at well [mass/volume ]

 C 0 = concentration at point of application [mass/ vol ]

 L = well setback distance [length ]

 v = lateral groundwater velocity [ length/time ] (e.g., 0.15 m/day )

 k = degradation rate in aquifer [time

- 1 ]

 Hydrolysis as surrogate for degradation rate

 Other Study Designs

– non - guideline and generally not availableWaterborne Environmental, Inc
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### Analysis Refinement: “Refinement Strategy 5: Exploring Exposure Durations That Are Representative of the Exposure Duration of Concern”

 Compound specific determination

 Environmental fate and use pattern must result in a significant annual fluctuation

 Limited utility to most compounds identified as having a groundwater concernWaterborne Environmental, Inc
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## Discussion

 Change in estimation approach (SCI

- GROW to PRZM

- GW) has resulted in a significant increase in the perception of risk

 Actual exposure has not changed....just our way of viewing it

 New modeling tools are causing registration delays across industry with the impact felt immediately (during evaluation period)

 PRZM

- GW continues to evolve as bugs are fixed and capabilities added

 Pragmatic approaches to shifting perception of risk and use of refinements are needed to support registrations

 Using best available data and reasonable surrogates for incomplete data should be included in discussion


###  Interaction between EFED, Registrant, HED, and Risk Manager on refinement and analysis approaches and management of risk is a critical element of the process