Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 259

Dispersivity length ( ) : Generally it increases with the travel distance of solute.

The discrepancy between laboratory and field test value of

α

α is termed as

scale effect of dispersion.

Solute transport parameters can be obtained by column studies and breakthrough
curve (BTC) experiments in soils and well-tracer tests in aquifers.

7.5.2 Breakthrough Curve and Breakthrough Experiment

7.5.2.1 Breakthrough Curve

Breakthrough curves are the graphical presentation of relative solute concentration
(outflow/inflow) of water passing through a soil column against the corresponding
time of measurement or the pore volume. The curve is used to derive solute transport
parameters.
Instead of relative solute concentration, the normalized concentrations are also
used in constructing breakthrough curves. The pore volume, also called the dimen-
sionless time, is the total volume of water in the column of soil at a particular water
content.
The shape of the breakthrough curve (See Fig.7.3) varies with water flux and
solute concentration (input rate).

1

0.8

0.6

0.4

0.2

0

0 10 20 30 40

Time, hr

Fig. 7.3 A typical breakthrough curve

7.5.2.2 Breakthrough Experiment

The breakthrough experiments may be conducted by applying a solution of salt
to the column of soil as a pulse or in a step. For the pulse-type experiment, a small
volume of salt solution is spread uniformly over the surface of the soils in the column