It is analogous to the retardation factor of chromatography.

The retardation factor depends on the chemical nature of both the contaminant and the aquifer.

The retardation factor in planar chromatography, R can be mathematically described by the following ratio:

The specific Retardation factor (Rf) of each chemical can be used to aid in the identification of an unknown substance.

The retardation factor is another very important feature that make the motion of the contaminant to deviate from the average groundwater motion.

In terms of retention factor (k), retardation factor (R) is defined as follows:

By assuming linear, equilibrium sorption, for example, the advection-dispersion equation can be modified by a simple retardation factor and solved analytically.

In chromatography, the retardation factor (R) is defined as the fraction of an analyte in the mobile phase of a chromatographic system.

Unlike diffusion and dispersion, which simply spread the contaminant, the retardation factor changes its global average velocity, so that it can be much slower than that of water.

In planar chromatography in particular, the retardation factor (R) is defined as the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front.

It is important to note that, although the term retention factor is sometimes used synonymously with retardation factor (R), in regards to planar chromatography, the term is not defined in this context.

Once visible, the R value, or retardation factor, of each spot can be determined by dividing the distance the product traveled by the distance the solvent front traveled using the initial spotting site as reference.

Where equilibrium sorption is described by a linear distribution coefficient, for example, the sorbing solute's velocity is retarded relative to that of a nonreactive tracer; the relative velocities can be described with a retardation factor.