Dissolved substances are called solutes, which may be natural constituents, artificial tracers, or contaminant constituents. Quantification of solute transport for aquifers or ground water systems is based on the convective-dispersive (or advective-dispersive) solute transport theory, which is also applicable in other transport media, such as surface water. Basically, the convective-dispersive solute transport theory is based on Fick’s first law (see Eq. [2-20]), which was established by the mid-19th century. The original Fick’s first law was established for molecular diffusion in surface water. Later, by the mid-20th century, Fick’s first law was extended to solute transport in ground water by including the dispersion effects. There is no doubt that the migration mechanism of solutes in ground water systems is more complex than the one in surface water flow systems. Different solute constituents have different migration rates, depending on their physical and chemical characteristics as well as the characteristics of the ground or aquifer materials. Also, some solute constituents may be subjected to decay or degradation. These phenomena make the migration of solutes in ground water relatively complex as compared with the solutes for surface water.