Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 81

Spray-drift

Degradation

Deposition

Volatilization

Photodegradation

Biological/chemical
degradation

Absorcion

Leaching

FIGURE 4.1
Main pathways of pesticides to degrade and to enter into aquatic compartment.

pathways for pesticides to degrade and to enter into aquatic compartments once they are
applied. Once the transformation products are formed in the environment, they are sub-
jected to the same processes as their precursor.
Pesticides can degrade by three common processes that take place in the surface envi-
ronment and include the following:

1. Biological degradation, which can lead, to either transformation products or com-
plete mineralization to form inorganic constituents.

2. Chemical degradation (hydrolysis, thermolysis).

3. Photochemical reactions, which require solar energy.

Surface-water concentrations of pesticides are highest for those associated with direct
applications to water bodies such as for aquatic weed control. For example, initial herbicide
concentrations of 0.2-3 mg/L are generally targeted for aquatic weed. Indirect or uninten-
tional entries of pesticides into water generally result in much lower concentrations, in the
range of 0.001-0.01 mg/L or lower (Racke 2003). Such entries can occur due to the follow-
ing physical processes:

1. Runoff movement in either a dissolved or sorbed state

2. Atmospheric deposition after volatilization

3. Leaching movement as soluble constituents of the aqueous phase

4. Spray drift.

Understanding these dissipation routes is the key to ensuring the accurate assessment of
environmental fate and the behavior of pesticides.
Surface runoff occurs whenever the rate of water application to the ground surface
exceeds the rate of infiltration and the surface storage capacity is exceeded. Losses of
chemicals in runoff depends primarily on the duration and intensity of rainfall and water-
shed characteristics such as soil properties, land use, vegetation cover, soil moisture, and
topography (Wolfe 2001). Surface runoff may become more significant under very heavy