Applying the Joint Probability
Distribution Analysis for
Pacific Northwest Salmonid
Risk Assessment
Qingli Ma, Rick Reiss and Cliff Habig
Exponent, Inc.
Paul Whatling
Cheminova, Inc.
OUTLINE OF THE
PRESENTATION
Overview of EPA and NMFS approaches for endangered species
risk assessments
The joint probability distribution analysis
Assessing the potential risks of dimethoate to Pacific Northwest
salmonids using the joint probability distribution analysis
Comparison of estimated potential risks of dimethoate to Pacific
Northwest salmonids
Conclusions
EPA APPROACH FOR PESTICIDE
RISK ASSESSMENTS
– RISK QUOTIENT (RQ) METHOD
Exposure profile – estimate environmental concentration (EEC)
Tier-1 GENEEC model estimate
Tier-2 PRZM/EXAMS model estimate: 90th percentile values
Effect profile – effect level (e.g., LC50)
Risk characterization – RQ calculation
RQ= Exposure/Toxicity
For most risk assessments, the RQ is calculated by dividing a point
estimate of exposure by a point estimate of effect
Level of concern (LOC): Various risk presumptions for comparison
with RQ to determine potential concern.
EPA RISK PRESUMPTIONS FOR
AQUATIC ANIMALS
Risk presumptions
RQ
LOC
Acute high risk
EEC/LC50 or EC50
0.5
Acute restricted use
EEC/LC50 or EC50
0.1
Acute endangered
species
EEC/LC50 or EC50
0.05
Chronic risk
EEC/NOEC
1.0
NFMS APPROACH FOR
DEVELOPING BIOLOGICAL
OPINIONS (BIOPS) ON PACIFIC
NORTHWEST SALMONIDS
A similar assessment process to the EPA approach, with additional
species population modeling for developing the “jeopardy” opinions.
It integrates the status of the species, the environmental baseline, the
exposure and the effects in developing BiOps.
In the most recent BiOps (the third batch of six chemicals), NFMS
used EPA’s Tier-1 GENEEC model to estimate potential cumulative
exposure (EECs), which tends to significantly over-estimate exposure
because of the conservative nature of the model.
JOINT PROBABILITY
DISTRIBUTION ANALYSIS
In this approach, both the exposure and effect are treated as
probabilistic distributions instead of point values
Probabilistic distribution of exposure
Probabilistic distribution of effect
Joint probability distribution of exposure and effect, which
describes the probability that an effect exceeding any given
magnitude will occur under the range of exposure scenarios
SAP and ECOFRAM recommend for probabilistic assessment of
risks
PROBABILISTIC
DISTRIBUTION OF EXPOSURE
Exposure assessment for dimethoate was undertaken with the
linked PRZM/EXAMS model
Major model input parameters that EPA used for the California
red-legged frog assessment were used (for consistency with EPA
RQ method)
Parameter
Value
Parameter
Value
Water solubility (mg/L)
3200
Aerobic soil (d)
6.2
Kd (L/kg)
0.3
Aerobic aquatic (d)
16.4
Photolysis half-life (d)
353.0
Application rate (kg/ha)
0.56
Neutral hydrolysis (d)
6.8
Number of applications
3
Foliar half-life (d)
2.9
Application method
aerial
PROBABILISTIC
DISTRIBUTION OF EXPOSURE
Seven EPA standard PRZM/EXAMS scenarios were simulated that
cover the major dimethoate use patterns, with considerations of rate
and method of applications, use regions, and crops:
CA-alfalfa
CA-citrus
MS-cotton
OH-corn
CA-lettuce
OR-pears
CA-cotton
These scenarios represent the high-end exposure for the selected
crops and are expected to produce runoff > 90% of the sites where the
crop is grown.
PROBABILISTIC
DISTRIBUTION OF EXPOSURE
The model was run for 30 years to generate
statistically meaningful distributions
The model-generated probability distributions of daily
peak concentrations were used to construct the joint
probability distributions for acute risk assessment.
Probablity of Exceedance
Probability of exceedance of dimethoate
concentrations for CA-lettuce scenario
1
0.8
0.6
y = 1.086e-98.8x
R² = 0.9749
0.4
0.2
0
0
0.005
0.01
0.015
0.02
0.025
Concentration (mg/L)
0.03
0.035
0.04
PROBABILISTIC
DISTRIBUTION OF
EFFECT
For toxicity assessment, the concentration-effect
relationship was derived from two effect endpoints:
Percent of species affected, expressed as species
sensitivity distribution
The dose-response relationship
SPECIES SENSITIVITY
DISTRIBUTION (SSD)
Invertebrate species were used as they serve as
salmonid prey. Besides, fish is less sensitive than
prey
9 freshwater and marine invertebrate species
19 of 48-hr toxicity data were selected for acute
risk assessment
The EPA Species Sensitivity Distribution
Generator, which assumes a log-normal
distribution, was used to generate SSD
Species sensitivity distribution
(SSD)
DOSE-RESPONSE
RELATIONSHIP
The concentration-effect relationships between dimethoate and
daphnia from three studies were used to generate the doseresponse curve:
Anderson et al. (2006)
Hertl et al. (2002)
Song et al. (2007)
The concentrations ranged from 0.8 to 9.95 mg/L and the mortality
ranged from 0 to 100%
The exposure time was 48 hrs and the temp. was 20-21 oC
All three datasets were combined and fitted to a logarithm function
to derive the effect distribution
Dose-response relationship
between dimethoate and daphnia
Daphnia mortality (%)
100
80
y = 30.671ln(x) + 33.057
R² = 0.4952
60
40
20
0
0
2
4
6
8
10
Dimethoate concentration (mg/L)
12
GENERATION OF JOINT PROBABILITY
DISTRIBUTION FOR RISK
DETERMINATION
The exposure distribution was integrated with the effect
distribution to develop the joint probability distribution (JPD)
A risk product (RP) was calculated from the JPD and used for risk
determinations
RP = Exceedance probability X Magnitude of effect
The following four levels of 90th percentile RPs were used to
categorize the risks after Giesy et al. (1999) and Giddings et al.
(2005):
If the RP < 0.25%, the risk is characterized as minimal
If the 0.25% < RP < 2%, the risk is low
If 2% < RP < 10%, the risk is intermediate, and
If RP > 10%, the risk is characterized as high
-
1
0.1
Joint Probability
Risk Product
0.8
0.08
Max RP = 0.085%
90th percentile RP =0.071%
0.6
0.06
0.4
0.04
0.2
0.02
0
0
0
20
40
60
Species Affected (%)
80
100
Risk Product (%)
Probability of Exceedance
Joint Probability Derived from Acute Species
Sensitivity Distribution (SSD) and Exposure
Distribution for CA-Lettuce
-
Probablity of Exceedance
1
0.8
1E-08
1E-09
1E-10
1E-11
1E-12
1E-13
1E-14
1E-15
1E-16
1E-17
1E-18
1E-19
1E-20
Joint Probability
Risk Product
0.6
Max RP << 0.25%
0.4
0.2
0
0
20
40
60
Mortality (%)
80
100
Risk Product (%)
Joint Probability Derived from Daphnia Exposure
and Dose-Response Relationship for CA-Lettuce
SUMMARY OF THE RISKS
DETERMINED FROM THE JOINT
PROBABILITY DISTRIBUTION
ANALYSIS
The calculated risk product from the joint probability distribution
derived from species sensitivity distribution and exposure was
significantly less than 0.25% under one of the worst-case use
scenarios (CA-lettuce) for dimethoate.
Likewise, the calculated risk product from dose-response curve
and exposure under the same use scenario was also significantly
below 0.25%.
Thus, it can be concluded that the risks imposed by dimathoate
uses to salmonid preys are minimal.
RISKS DETERMINED USING THE
EPA RISK QUOTIENT METHOD
The 90th percentile peak daily concentration predicted by
PRZM/EXAMS from the same exposure scenario was 0.024
mg/L. This is the EEC EPA used for calculating RQ for acute risks
The lowest acute toxicity endpoint for invertebrate species
(marsh mosquito) is 0.031 mg/L
The resulting RQ is 0.77
The EPA LOCs for aquatic animals are 0.5, 0.1, and 0.05 for
acute high risk, acute restricted use, and acute endangered
species, respectively. Thus, according to the EPA RQ method,
dimethoate would trigger the acute risks for all three categories.
COMPARISON OF RISKS
DETERMINED BY JPD AND RQ
METHODS
The joint probability distribution analysis showed minimal risks
from dimethoate use:
The maximum and 90th percentile risk products were << 0.25%
based on SSD analysis
The maximum and 90th percentile risk products were << 0.25%
based on dose-response analysis
The RQ method calculated a RQ value of 0.77 and indicated
potential risks for all three risk categories for aquatic animals
The bias of the RQ method lies in its overly conservative
assumptions and single point/species estimates for RQ
calculations
CONCLUSIONS
The JPD analysis shows that the risks imposed by dimethoate
uses to salmonid preys are negligible
The JPD approach is more appropriate for risk determinations for
the Pacific Northwest salmonids because they consume more
than one invertebrate species
It is a better tool for EPA and NMFS for jeopardy and habitat
modification determinations
The assessment is still highly conservative because it uses the
stagnant PRZM/EXAMS farm pond scenario to estimate
exposures in salmonid habitat
REFERENCES
Anderson, T.H., Tjornhoj, R., Wollenberger, L., Slothuus, and T.,
Baun, A. 2006. Acute and chronic effects of pulse exposure of
Daphnia magna to dimethoate and pirimicarb. Environ. Toxicol.
Chem. 25: 1187-1195
Song, M.Y., Stark, J.D., and Brown, J.I. 1997. Comparative
toxicity of four insecticides, including imidacloprid and
tebufenozide, to four aquatic anthropods. Environ. Toxicol. Chem.
16:2494-2500.
Hertl, J. 2002. Acute toxicity of dimethoate to Daphnia magna in a
48-hour immobilization test. IBACON-Institut fur Biologische
Analytik und Consulting. IBACON GmbH Study No. 10591220,
10591220A; Unpubl. Report, CHA Doc. No. 482 DMT.