Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 179

LiquidPARAM calculates the probability of maximum average TDI across all birds
exceeding the chronic NOEL and the corresponding probability for exceeding the
LOEL. Section 5 describes the assessment of chronic effects in detail.

4 Exposure Assessment for Granular Chlorpyrifos

The Granular Pesticide Avian Risk Assessment Model (GranPARAM) was used to
estimate exposure and fate of birds as a result of consuming pesticide granules in
CPY-treated agricultural fi elds. The model as originally described (Moore et al.
2010b , c ) has been updated for this assessment.
GranPARAM simulates the grit ingestion behavior of individual birds and deter-
mines how many pesticide granules and the associated dose each bird ingests during
the 24-h period immediately following CPY application. Each bird in a GranPARAM
simulation is assumed to be actively foraging for grit in and around the agricultural
fi eld to which CPY has been applied. The scheme that GranPARAM follows to
model granule ingestion behavior is depicted in Fig. 5 .
In GranPARAM, each bird is randomly assigned a daily grit intake rate from a
large database of grit counts for the species being considered and estimated grit
retention rate. This step defi nes the number of medium- and coarse-sized particles
(i.e., particles in the same size range as Lorsban 15G granules) that the individual
ingests during the peak day of the simulation. For CPY, the peak day was assumed
to be the 24 h immediately following application. The work of Stafford et al. ( 1996 )
and Stafford and Best ( 1997 ) showed that most pesticide granules are incorporated
into soil, and thus unavailable to birds, within 1-d of application. Rainfall accelerates
this process (Stafford and Best 1997 ). GranPARAM relies on estimates of granule
counts on the soil surface immediately after application, which clearly represents
the maximum possible exposure for birds (Solomon et al. 2001 ).
Each site of application of the granular formulation is randomly assigned a soil
texture (e.g., Silt-Loam) with a probability equal to the occurrence of that texture
fraction in the crop-capable acreage in the region of interest. The database in the
model was originally for corn, but has been expanded to include other crops and
areas to which CPY is applied. Once the soil texture category is assigned, the
application site is then randomly assigned a specifi c soil particle size profi le (% of
soil mass represented by various particle size categories) from a large soils database
of measurements. This step defi nes the levels of medium- and coarse-sized sand
particles available as natural grit.
For each exposure scenario (Table 3 ), the method of application, rate of application,
incorporation effi ciency, bird species, region of interest, and other aspects of the
analysis included in the simulation were defi ned (Fig. 5 ). The rate of application of
CPY defi nes the relative numbers of medium- and coarse-sized granules applied.
The method of application (e.g., in-furrow, band, broadcast) determines the spatial
placement of these granules and the number available as a source of particles to
birds. The choice of bird species determines the number of particles ingested.