242nd ACS National Meeting

August 28, 2011

Greg Malis, Ian Kennedy, Lizanne Avon, Émilie Larivière.

Environmental Assessment Directorate Ottawa, Ontario, Canada

Pest Management Regulatory Agency:

Aquatic exposure modelling for exposure

assessment in support of the regulation of pest

control products in Canada

Introduction to Canadian water modelling

• General modelling aspects

• Model features

• Modelling scenarios

• General approach to modelling

• Monitoring data

• Comparison

Modelling in Canada

• Use of Canadian scenarios to model:

• Estimated environmental

concentrations (EECs) in surface

waters for ecological risk assessment

• Pesticide Root Zone Model

(PRZM)/EXposure Analysis Modeling

System (EXAMS)

• EECs in drinking water supplies for

the human health risk assessment

• Surface Water: PRZM/EXAMS

• Groundwater: LEACHM (Leaching

Estimation and Chemistry Model)

Factors affecting pesticide levels in water

Pesticide levels

in water

Registrantstudies Fate

Assessment

Pesticideproperties

LabelsPesticide

use

Crops

soilproperties

WeatherHydrology

Varies with site

Modelling Environmental Fate of Pesticides

Foliar washoff

LeachingVolatilization

Spray Drift

PRZM EXAMSPrecipitation

(and irrigation)

Diffusion

LEACHM

Leaching

Foliardeposition

Foliarwashoff

Transformation

PRZM Model Processes

Phototransformation

Spray drift

Wa

ter

de

pth

2.7

4m

, 5

.33

m o

r 0

.8m

Volatilization

Completely mixed

Se

dim

en

t

De

pth

5c

m

Water

Transformation

TransformationSediment

Ou

tflo

w

EXAMS Model Processes

•Transport (Advection & Diffusion)

•Biotransformation

•Hydrolysis

•Partitioning (gas – solution – soil)

Evaporation of water

Volatilization of chemical

water table

Addition of rainfall and/or irrigation water

Addition of chemical:

Surface application or soil incorporated

Segment 1

Segment 2

Segment 3

Segment n

Calculated groundwater concentration

LEACHM Model Processes

Model Scenarios

• The data describing the soil, crop and weather

characteristics of a site.

• Represent areas across Canada of major agricultural

activity.

• Incorporate representative soil, weather and plant

growth information.

• Surface water bodies

• drinking water: reservoir and dugout

• environmental exposure: small pond

Model Scenarios – receiving water bodies

• Drinking water

• US EPA Index reservoir (based on Shipman reservoir in

Illinois – 5.3 hectare surface area with 2.74 m depth,

drainage area is 172.8 hectares)

• a prairie dugout (0.05 hectare surface area with 5.4 m depth,

drainage area is 4 hectares)

• Environmental exposure

• the ecoscenario represents a permanent water body (1

hectare surface area with 0.8 m depth, drainage area is 10

hectares)

• 0.15 m depth used for amphibian assessment

Scenario Locations

General Approach

• Use tiered approach (Level 1 and Level 2)

• Level 1 drinking water:

• to screen out pesticides that do not pose any drinking water

concern, and allow for simplified use expansions

• For surface water, uses PRZM/EXAMS with most vulnerable

scenarios

• For groundwater, uses highest EECs out of all 11 scenarios

• If Level 1 concentration is unacceptable, move to a Level 2

assessment

• Ecoscenario – Level 1 is a direct overspray

assessment. PRZM/EXAMS starts with level 2, using

region specific rate/use information and scenarios

Level 1 - General Approach

• Fate inputs to models – modified from PMRA Science

Policy Note (SPN) 2004-01, Estimating the Water

Component of a Dietary Exposure Assessment

• 80th percentile – half-lives in environmental media

(e.g., soil, water)

• Terrestrial field dissipation DT50s are not used in modelling

• 20th percentile – KOC or Kd coefficients where multiple

values exist

• Note – modeling inputs are being discussed as part

of a NAFTA groundwater modelling project

Level 1 - General Approach (continued)

• Assumptions about use:

• Label information

– Highest rate and max. number of applications

– Shortest time interval between applications

• The pesticide is applied every year of the simulation

– Range of starting application dates

• 100% of the watershed is assumed to be cropped

• 100% of the crop is assumed to be treated with pesticide

• For drinking water assessment, we report both 90th

percentile of daily and yearly average concentrations for surface water and groundwater EECs

• For ecological assessment, we report 90th percentile of daily, 96-hour, 21-day, 60-day, 90-day and yearly average concentrations for ecoscenario EECs

Transformation products

• PRZM does not correctly handle formation and decline of

transformation products

• LEACHM handles only the case of 100% conversion

• PMRA typically simplifies by handling transformation products in

one of two ways

1. Combined residues – calculate the combined half-life, and then model all

compounds of interest together

– assume all compounds have similar phys. chem. characteristics as the

parent

2. Treat separately – assume that transformation products are applied to the

soil surface then same as the parent compound.

– assume pulse application rather than slow formation

– missing phys. chem. Data

3. For groundwater, if possible, simulate transformation using LEACHM

– Requires sufficient transformation product fate data

– When compounds have differing KOC

Level 2 – General Approach

• Case-by-case approach

• Use pattern items considered include:

• restricting the modelling to scenarios that reflect the current

or proposed uses and relevant application dates of the

pesticide

• percent cropped area

• agricultural practices (e.g. crop rotation, resistance

management)

• Refinements are intended to be more accurate, not

just less conservative

• PMRA tries to choose reasonably conservative inputs at

Level 1

• Use expansion could require additional modelling

Modelling data quality

• Modelled EECs are intended to be a realistic upper

bound

• Uncertainty and the degree of conservatism are not

easily characterized due to

• Simplification of processes in the models

• Uncertainty in fate data

• Quantitative effect of choice of generally conservative inputs

• Not all model assumptions and parameters are

conservative.

• LEACHM neglects preferential flow

• Degradation rates from lab studies conducted at 20°C or 25°C

which may not reflect a colder Canadian climate (PRZM)

Use of Monitoring Data

• When available, PMRA considers monitoring data (surface

and groundwater) for re-evaluation and special reviews

• Evaluate the quality of data received

• Experimental design

• Spatial and temporal conditions

• Available ancillary data

• Data sources

• Environment Canada, Department of Fisheries and Oceans,

Provincial Ministries

• US data: NAWQA, USDA, STORET databases

• Registrant

• Literature

Comparison to other jurisdictions

• EPA Surface water – similar to PMRA

• Tier 1 EPA – GENEEC and FIRST, PMRA – PRZM/EXAMS

• Tier 2 both use PRZM/EXAMS

• Different scenarios

• US chooses chemical parameters differently

• EPA Groundwater – different

• SCI-GROW is a regression based on field (PGW) studies

• SCI-GROW is based on single application (taken as

maximum yearly total)

• SCI-GROW is considered valid for KOC between 13 and 180

L/kg OC

• EU uses different models

NAFTA groundwater modelling project

• Intended to harmonize Canada and US groundwater

modelling

• Common conceptual model

• Common numerical model (i.e. modelling program)

• Mostly common guidance on scenario creation and

input parameter selection

• Completion within 2011

Future directions

• NAFTA projects• Groundwater, degradation kinetics, PGW guidance

• Surface water modelling• Canadian reservoir and watershed scenario development, Percent Crop

Area (PCA), probabilistic, variable volume pond, watershed model

• Study improvements• Link sorption and degradation, field – aerobic soil lab study improvements,

plant uptake and metabolism, more evenly distributed data, second

chemical spike, sterile control

• Model advancements• Model advancements will require more parameters and will increase the

complexity of choosing them. Examples include; time dependent sorption,

transformation products, nonlinear sorption isotherm

Questions

Input parameter comparison with US and EU

EU EPA PMRA

Sorption Freundlich

isotherm where

model allows

Linear isotherm

using Kf

Linear isotherm

using Kd

Half-life Geometric mean Mean or median

or 3x or 90%

confidence

80th percentile of

calculated half-

lives

t1/2 temperature

correction

Yes Yes Occasionally

t1/2 moisture

correction

Yes No No

Foliar

interception

Removed from

application

GW: none

SW: PRZM

GW: none

SW: PRZM

Run length ? 30 50