Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 247

The literature suggests that nitrate leaching from the crop field may vary from
30 to 170 kg-N/ha/year depending on the crop, irrigation, fertilizer use (type &
amount), crop rotation, and management factors.

7.3.4 Mode of Pollution by Nitrate and Pesticides

Nitrogen is essential to plant growth and increases yield. Under ideal conditions,
only the fertilizer that can be used by the plant would be applied, leaving no residual
to move below the root zone. However, in most cases, not all the applied nitrogen is
assimilated by the plants, allowing some to move below the root zone. Nitrogen in
the soil that is not returned to the atmosphere in the form of nitrogen gas or ammonia
is generally converted to the nitrate form by bacteria. Nitrate is very mobile, and if
there is sufficient water in the soil, it can move readily through the soil profile.
Among the nutrients leached or allowed to run off, N is the most abundant and is of
major concern as the source of ground and surface water pollution.
The pesticides that are applied for preventing, destroying, repelling, mitigating,
or controlling any insect, fungus, bacterial organism or other plant or animal pest;
deposit in or on a site of plant, soil, or water. Then through different processes
move and enter into other environmental components. Based on the characteristics
of the component and characteristics of the pesticides such movement, their fate and
impact on the environmental components become evident. The persistent pesticides
persist in the environmental component for a long time and move intact from one
component to another even at long distances ( Fig. 7.2 ) .
In the past, most pesticides were regarded as involatile, but now volatilization is
increasingly recognized as being an important process affecting the fate of pesticides
in field soils. Another process affecting pesticide fate and transport is the relative
reactivity of solutes in the sorbed and solution phases. Several processes such as
gaseous and liquid phase molecular diffusion, and convective-dispersive transport,

Long route

Initial site of
pesticide
deposition

Water

Soil

Air

Hydrolysis, photolysis

Degradation

Fig. 7.2 Schematic of fate of pesticide residues in environment