Solute Transport in Soil - Transport in the Atmosphere-Vegetation-Soil Continuum - page 178

1976; Van de Pol et al., 1977 ; Van der Zee and Van Riemsdijk, 1987 ). Most of this
variation is caused by variation of the soil hydraulic functions, preferential low due
to macropores in structured soils or due to unstable wetting fronts in nonstructured
soils. In general it is impossible to determine the range and correlations of all relevant
physical parameters. A practical approach is to measure for a period of time the solute
concentrations in the soil proile and drainage water and apply calibration or inverse
modelling to determine 'effective' transport parameters (Groen, 1997 ). Another
approach is the use of Monte-Carlo simulations, where the variation of the transport
parameters is derived from stochastic parameter distributions of comparable ields
(Boesten and Van der Linden, 1991 ). Jury ( 1982 ) proposed to use transfer functions,
which consider the transport processes within a soil column as a black box, and just
describe the relation between solutes that enter and leave the soil column. The main
limitations of the transfer function approach are that it requires ield experiments to
calibrate the transport parameters and that extrapolation to other circumstances is
risky because of its stochastic rather than physical basis.

Question 5.2: Mention ive methods that can be used to analyse the variability of solute
luxes within a ield.

This chapter focuses on the vadose zone transport of salts, pesticides and other sol-
utes that can be described with relatively simple kinetics. We consider the processes
convection, dispersion, adsorption, root uptake and decomposition. Processes outside
the scope of this chapter are (1) volatilization and gas transport, (2) transport of non-
mixing or immiscible luids (e.g., oil and water), (3) chemical equilibria of various
solutes (e.g., between Na + , Ca 2+ and Mg 2+ ), and (4) chemical and biological chain
reactions (e.g., mineralization, nitriication).

5.2 Solute Flux through Soil
The three main solute transport mechanisms in soil water are diffusion, convec-
tion and dispersion. Diffusion is solute transport that is caused by the solute gradi-
ent. Thermal motion of solute molecules within soil water causes a net transport of
molecules from high to low concentrations. The solute diffusion lux J dif (kg m -2 d -1 )
is generally described by Fick's irst law:

∂
∂

C
z

J =− θ D

l

(5.1)

dif dif

with θ the volumetric water content (m 3 m -3 ), D dif the diffusion coeficient (m 2 d -1 )
and C l the solute concentration in soil water (kg m -3 ). J dif is very sensitive to the actual
water content, which affects the effective cross-sectional transport area and the tortu-
osity of the solute path.