Environmental Fate and Transport of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 38

sorption affinity would lead to more dissolved pesticide molecules in water and thus faster
leaching rate. Conversely, pesticides with low aqueous solubility and higher affinity for
soils are less likely to leach through the soil profile.
Although still poorly understood, the phenomenon of “preferential flow” has been often
noticed in field studies or when real soils are used. In these cases, pesticides move rapidly
and preferentially along macropores or decayed root and earthworm channels in soils,
and their retention times are comparable with conservative tracer solutes (Andreini and
Steenhuis 1990; Flury 1996).
The rainfall events also greatly influence the leaching processes. It was estimated by
Flury (1996) that if there is no heavy rainfall shortly after pesticide application, the annual
losses of the pesticide are between <0.1% and 1%, whereas up to 5% of applied pesticide
can leach out of the soil at the application site in the worst rainfall case.
2.4 Transport Models
Due to the wide application of pesticides in agricultural and industrial practices, their
presence has been detected and reported in many aquifers and surface waters. Transport
of pesticides encompasses infiltration, runoff, and preferential flow (or macropore flow).
Modeling of the transport behavior has been described in a number of reviews and
studies. At the end of this chapter, a brief overview of these studied transport models is
provided.
2.4.1 HERBSIM and SIMULAT Models
HERBSIM is a computer model that primarily predicts the fate of pesticides and their
metabolites in the soil considering processes such as sorption, degradation, and evaporation
and their dependencies on environmental variables, whereas SIMULAT Model can also be
used for one-dimensional transport (Diekkrüger et al. 1995; Aden and Diekkrüger 2000).
Generally, in these two models, water flow, solute transport, potential evapotranspiration,
sorption, and degradation can be calculated by Richards' equation, convection-dispersion
equation, Penman-Monteith equation, linear/Freundlich/Langmuir adsorption isotherm
with up to three different binding sites considered, and Walker equation, respectively (Aden
and Diekkrüger 2000). In addition, plant growth, heat flux, and tile drainage can also be
simulated in the SIMULAT model.
2.4.2 HYDRUS Model
HYDRUS model applies to water flow and solute transport in vadose zone. It incor-
porates many physical and chemical nonequilibrium models, such as the mobile-
immobile water content model, Dual-Permeability Model, One Kinetic Site Model,
and Two-Site Sorption model, to simulate the transport of solutes (Simunek and van
Genuchten 2008). Advantageously, HYDRUS has a large capacity and is suitable for
one-, two-, and three-dimensional applications. With respect to pesticides, Malone
et al. (2004) analyzed the effects of having equilibrium or kinetic sorption models on
the simulated degradation in using HYDRUS and summarized the transport of pesti-
cide in soil-water system.