Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 82

storms (Bronstert et al. 2002). During runoff, pesticide residues can be both transported in
solution and sorbed to soil particles. The magnitude of the latter loss path is generally small
compared to that transported in the water phase because of the relative amounts of water
moved compared to eroded soil. Losses of molecules with high solubility via surface runoff
are more important than losses of molecules with K oc greater than ca. 1000 L/Kg (e.g., syn-
thetic pyrethroids), where erosion is considered to be the main loss pathway (Dabrowski
et al. 2002; Holvoet et al. 2007; Wu et al. 2004).
The amount of organic chemicals that move off soil into surface waters is normally less
than 1% or 2% of that applied (Bloomfield et al. 2006; Wauchope 1996). Studies of large agri-
cultural watersheds have shown that the atrazine flux in rivers varied between 0.25% and
1.5% of the amount spread on the land (Richardson 2007; Schottler and Eisenreich 1994).
This suggests that, under most circumstances, the runoff process plays a minor role in the
transport and fate of organic compounds. However, exceptional fields with steep slopes
and heavy precipitation can make the runoff a dominant transport process and, in some
cases, up to 5% has been measured (Burgoa and Wauchope 1995).
The fate of a pesticide from its point of application through surface soil and into sub-
soil is governed by the interactive processes of adsorption, transformation, and transport
(Weber and Miller 1989). Leaching is the vertical movement of the dissolved pesticide resi-
dues and transformation products down soil profiles as a result of a downward poten-
tial gradient caused by the infiltration of rain at the surface. Xenobiotics dissolved in
water tend to move more slowly than the water, as they are subject to sorption to the soil
as well as degradation processes. Moreover, slow groundwater movement and pesticide
residue attenuation in the groundwater due to sorption and degradation further dimin-
ish the residue concentrations (Röpke et al. 2004). The rate of movement of water through
the soil profile is controlled by the hydraulic conductivity of the soil, which depends on
the type and water content of the soil. Wetter soils have higher hydraulic conductivities
and therefore, the organic compounds can move more rapidly from the surface of the
fields through the soil. The chemical properties of the soil particles, their distribution
and size, and the amount of organic matter will influence the capacity of the soil to retain
more hydrophobic compounds. The mobility of the organic compounds through the soil
is largely dominated by sorption processes. These processes are controlled by the lipo-
philicity of the molecules, the chemical properties, distribution, and size of the soil par-
ticles, organic matter, and soil humidity. In general, it has been stated that an increase in
water content reduces adsorption and increases the mobility, whereas an increase in clay
and organic matter content increases adsorption and reduces mobility. Mobilities of some
compounds are directly related to pH, showing higher mobility in soils with higher pH
(Somasundaram et al. 1991).
Likewise, irrigation practices and rainfall frequency and intensity also influence the
leachability of pesticides and transformation products; a higher water input favors the
transport of compounds to groundwater. Furthermore, physicochemical properties of pes-
ticides and transformation products play an important role in their transport to the sub-
soil. Hydrophobic compounds with a high organic carbon partition coefficient (K oc ) have
a high affinity to be retained in the soil and therefore, their lixiviating only takes place
under conditions when residues have a very high half-life.
According to the literature, an organic compound is able to contaminate the ground-
water if its solubility in water is higher than 30 mg/L, its adsorptivity K oc (organic carbon
partition coefficient) is lower than 300-500, its K d (distribution adsorption constant) is
lower than 5 mL/g, and its soil half-life is longer than 3 weeks (Aharonson 1987; Demoliner
et al. 2010).