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