Absorption involves the assimilation of one substance into another, such as liquids into solids or gases into liquids. Adsorption is the adhesion of gas molecules, or of ions or molecules in solution, to the surface of solid bodies with which they are in contact.

A colleague made these analogies. Absorption is like pouring water over bread—the bread soaks up the water and changes in character. Adsorption is like spreading butter on bread—it sticks to the surface and cannot be entirely removed, but neither the bread nor the butter changes in character except at the surface.

These terms commonly occur in environmental site assessment reports, especially those involving the petroleum industry. If gasoline or fuel oil leaks underground, does the soil absorb it or adsorb it? Perhaps both.What if the fuel reaches groundwater? Is it absorbed? No, but some of it may be dissolved.

As the goal of such an assessment is to enable managers and regulatory agencies to decide if the site requires remediation, the precise terms may not matter. If tons of soil must be removed, who cares whether the fuel was absorbed or adsorbed? Some writers use the term sorption to indicate a lack of differentiation. Of importance in selecting remedial technologies is whether the hydrocarbons are in the liquid state, t he vapor (or gaseous) state, or the solid state. Since gasoline generally doesn't solidify, a service station that is excavating solids is probably cleaning up soil containing adsorbed-phase hydrocarbons.

Gasoline dissolved in groundwater is referred to as dissolved-phase or aqueous-phase hydrocarbons. Gaseous molecules of fuel trapped between tiny openings in the soil are called vapor-phase hydrocarbons. Recoverable gasoline floating on top of the groundwater is called free product or, variously, separate-phase, free-phase, or nonaqueous-phase hydrocarbons.

DNAPLs are dense, nonaqueous-phase liquids (such as trichloroethylene), which are heavier than water and sink to the bottom of waterbodies. LNAPLs are light, nonaqueous-phase liquids (such as gasoline and oils), which are lighter than water and float on the water table.

How utterly absorbing!

—Retta Whinnery