Exposures of Aquatic Organisms to the Organophosphorus Insecticide, Chlorpyrifos Resulting from Use in the United States - Reviews of Environmental Contamination and Toxicology - page 88

and chemical properties discussed in Sect. 1.1 , above, potential transport pathways
to aquatic systems include water and soil erosion from rainfall and irrigation and
spray drift during application.
The Pesticide Root Zone Model, PRZM, was selected to evaluate pesticide run-
off potential because of its ability to account for pertinent environmental processes
at an appropriate spatial scale and time step for chemical dissipation. PRZM is a
dynamic, compartmental model developed for simulating movement of water and
chemical in unsaturated soil systems within and below the plant root zone (Carousel
et al. 2005 ). The model simulates time-varying hydrologic behavior on a daily time
step, and includes physical processes of runoff, infiltration, erosion, and evapotrans-
piration. The chemical transport component of PRZM calculates pesticide uptake
by plants, volatilization, surface runoff, erosion, decay, vertical movement, foliar
loss, dispersion and retardation. PRZM includes the ability to simulate transport of
metabolites, irrigation, and hydraulic transport below the root zone. PRZM is the
standard model used for ecological and drinking water pesticide risk assessments by
the U.S. Environmental Protection Agency's Office of Pesticide Programs (USEPA
2009 ). The model has undergone extensive validation, in which the results were
compared with measured concentrations from numerous studies of field-scale run-
off and leaching, conducted for pesticides in the United States (Carousel et al. 2005 ;
Jones and Russell 2001 ). Moreover, PRZM has been integrated into several water-
shed assessment models in the U.S. (Parker et al. 2007 ; Snyder et al. 2011 ).
Two versions of PRZM were used here. The majority of simulations were con-
ducted using PRZM version 3.12.2; the version that is incorporated into the pesti-
cide registration review process of the USA (USEPA 2009 ). Simulations for
California were conducted using a version of WinPRZM (FOCUS 2012 ) that was
modified (Hoogeweg et al. 2012 ) to simulate pesticide losses in furrow and flood
irrigation tail-water, which is a significant source of pesticide loadings in many
areas of California and elsewhere that utilize these irrigation practices. WinPRZM
contains additional enhancements that are not available in PRZM 3.12.2 that have
been added for pesticide registration evaluation in Europe, including the ability to
simulate soil adsorption using the Freundlich isotherm, temperature and soil-
moisture-dependent degradation, and non-equilibrium sorption to soil (FOCUS
2012 ). WinPRZM produces identical exposure concentrations to PRZM version
3.12.2 when those options are not used.
The Exposure Analysis Modeling System, version 2.98.04 (EXAMS), was used
to evaluate the relative effects of use practices (i.e., CPY labels) on exposure con-
centrations. Simulations utilized standard scenarios developed by USEPA's Office
of Pesticide Programs for pesticide registration review, which are configured to run
with EXAMS (USEPA 2009 ). The scenarios represent a 10-ha field draining into a
1-ha × 2 m deep pond. EXAMS combines a chemical fate and transport model with
a steady-state hydraulic model to simulate the following processes: advection, dis-
persion, dilution, partitioning between water, biota, and sediment, and degradation
in water, biota, and sediment (Burns 2004 ).
The regulatory version 2.05 of the AgDRIFT ® model (Teske et al. 2002 ) was
selected to estimate spray drift deposition onto aquatic water bodies. This version of