Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 261

7.6 Estimation of Solute Load (Pollution) from
Agricultural Field

Estimates of diffusive-source pollution loads over different temporal and spa-
tial scales are required to evaluate the severity of the pollution problems and to
determine appropriate management measures.

7.6.1 Sampling from Controlled Lysimeter Box

Solute leaching from the agricultural field can be estimated by growing the crop
in the controlled lysimeter (drainage type) and collecting the free drainage out-
flow at the bottom of the lysimeter. To obtain a realistic estimate, the soil and
other conditions in the lysimeter should resemble those of the field. The outflow
tap is kept closed but only during drainage collection. Drainage collection may be
accomplished at various stages of the crop, or at predetermined dates from fertilizer
application. The solute load in a particular drainage collection is

M s = V d × C s (7.16)

where

M s =

mass of solute leached in the particular drainage event (mg)

V d = volume of drainage outflow (l)

C s = concentration of solute in the drainage sample (mg/l)

Seasonal or yearly amount of solute is obtained by summing up the individual
mass of solute for each drainage event.

7.6.2 Sampling from Crop Field

Samples of percolated water from the crop field can be collected by installing
water sampler and using vacuum extractor (vacuum hand pump). Soil water sam-
pler consists of porous ceramic cup. The samplers are installed at various depths
(30-250 cm) and intensities depending on the specific objectives. Sampling may be
done at various stages of the crop or at predetermined dates since fertilizer appli-
cation. At the predetermined /scheduled date, vacuum is created in the sampler and
one has to wait for several hours to obtain an appreciable amount of water. Then
the sample is collected through the pump. A schematic view of the ceramic cup,
vacuum suction pump, and sampling arrangement is shown in Fig. 7.4.
If water enters the cup, the pressure in the dial of the suction pump falls down.
In case of loose fitting of the arrangement, the pressure may also fall down. In such
a case, the suction is created repeatedly. If soil-water is low, the suction may need
to prolong for 24 h to get an appreciable amount of sample.