Pollution of Water Resources from Agricultural Fields and Its Control - Practices of Irrigation and On-farm Water Management - page 258

7.5 Measurement of Solute Transport Parameters

7.5.1 Different Parameters

Study of mobility of contaminants in an aquifer is an important issue for the proper
remediation of contaminated groundwater. Transport of contaminants in aquifer
depends largely upon the aquifer and their chemical properties. The aquifer property
can be defined as the medium or material property which consequently is related to
the velocity and dispersion of contaminants. These intrinsic factors arising from the
physical aspects of the aquifer materials finally construct two important solute trans-
portation parameters, i.e., advection and dispersion coefficients. Analysis of solute
transport parameters therefore is essential for prediction of the fate of contaminants.
The main parameters of the solute transport equations are as follows: advec-
tion coefficient ( V ), hydrodynamic dispersion coefficient ( D ), retardation factor ( R ),
first-order rate constant, zero-order rate constant, partition coefficient, pore water
velocity ( ν ), dispersivity, and length ( α ).

Dispersivity : It indicates the spreading of a solute across the “front” of average
bulk flow. It is a geometrical proportionality constant of a media. It is deter-
mined by the pore-size distribution of the channels involved in transmission
of a bulk solution.
Retardation factor (R) : It is a unique medium property. It is a dimensionless
parameter reflecting the retarding effect of adsorption on solute transport. It
is expressed as

R = 1+ ρ K d / θ ,

where ρ is the bulk density, K d is partition coefficient, θ is volumetric water

content.
First-order rate constant : In first-order kinetics (a form of reaction process),
the reaction rate is proportional to the concentrations of the component reac-
tants. The proportional constant is termed as first-order rate constant (or
degradation constant, in degradation reaction).
Zero-order rate constant : In zero-order kinetics, the constant reaction rate is
termed as zero-order rate constant. Zero-order kinetics refers to the reaction
process in which the reaction rate is independent of the concentrations of the
reactants.
Partition coefficient : It is a proportionality constant in an equilibrium adsorp-
tion isotherm. It is determined as the ratio of adsorbed content to the
concentration in an aqueous solution for the linear equilibrium isotherm.
Hydrodynamic dispersion : It refers to the spreading of a solute beyond the
region expected due to the flow alone. It is the sum of hydraulic dispersion
and molecular diffusion.
Pore water velocity (v) : It is the average velocity through soil pores. It is deter-
mined as: v

= q / θ , where v is the pore water velocity, q is the Darcy velocity,

θ is the water content.