Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 171

Exposure assessment—Lorsban Advanced . The model used in the assessment of
exposure for birds was a refi nement of EPA's Terrestrial Investigation Model (TIM)
(USEPA 2005 , 2008 ) and is known as the Liquid Pesticide Avian Risk Assessment
Model (LiquidPARAM). Version 1 of TIM (TIM v1) estimates the fate of each
of 20 birds on each of 1,000 fi elds following an acute exposure (USEPA 2005 ).
TIM v1 is a species-specifi c model that estimates risks over a defi ned exposure
window of 7-d. The time-step in the model is 12 h. The spatial scale is the treated
fi eld where the fi eld and surrounding area are assumed to meet the habitat require-
ments of a defi ned cohort of individuals for each focal species. Pesticide contamina-
tion of edge or adjacent habitat from drift is assumed to be zero. Version 2 of TIM
( USEPA 2008 ) is similar to TIM v1 except that it has a 1-h time step and includes a
more refi ned puddle exposure algorithm as well as screening-level algorithms for
dermal and inhalation exposure.
Major components included in TIM are: (1) Food preferences of selected focal
species; (2) Daily ingestion rates of food and water which are randomly assigned
from species-specifi c body mass distributions; (3) Frequency of feeding and drink-
ing on the treated fi eld; (4) Water sources, including dew and puddles; (5)
Distributions of residues on food items and in on-fi eld water sources as a function
of application rate; (6) Degradation rates of food and water residues over time; and
(7) Interspecies distribution-based estimates of dose-response acute toxicity curves
for focal species when laboratory-derived toxicity estimates are not available, or the
dose-response curve derived from laboratory toxicity tests for focal species (see SI
Appendix 2 for additional details on model structure).
For each simulated bird, values are randomly selected for the input parameters in
TIM required to estimate exposure. The estimated risk of lethality for each indi-
vidual bird is calculated from the dose-response curve. Once the fate of an individ-
ual on a particular fi eld is determined (i.e., dead or alive), a new individual is carried
through the same process. This process is repeated for a total of 20 individuals on
the fi eld. The model then moves to the next fi eld. This outer loop continues for a
sample size of 1,000 fi elds, which results in a risk estimate for a total of 20,000
birds on treated fi elds.
LiquidPARAM shares some of the similar basic structure of TIM (e.g., each
model estimates the fate of each of 20 birds on each of 1,000 fi elds). However, sev-
eral important refi nements have been made and are briefl y described below. A more
detailed description is given in SI Appendix 3, Sect. 1.
In TIM, concentrations in dietary items within a fi eld are randomly selected from
distributions at each time step. Often this leads to situations where concentrations
increase several-fold, 4 or more days after application. This situation seems unlikely
in normal use given the fairly short half-life of CPY in the fi eld. LiquidPARAM
assumes that factors causing variation in concentrations of CPY on dietary items are
relatively small within a fi eld at a particular time step relative to those factors
that cause variation between fi elds. Factors affecting relationships between rate of
application and concentrations of pesticides on dietary items include: ambient tem-
perature, wind speed, fi eld slope, soil type, rainfall patterns, applicator experience,
and type of equipment used to apply the spray. These factors vary only slightly