Well,

Si = Vi /Vv

Where Si is phase saturation, Vi is the volume of phase (i) and Vv is the volume of voids in the material. The sum of the saturations of all phases present equals one.

Sair + Swater + SNAPLS =1.

In the unsaturated zone with air, water, and NAPL, the corresponding saturations total one: Sair + Swater + SNAPLS =1.

Accordingly, and for example; Sair =0.19 , Swater= 0.80, SNAPLS = 0.01, and when Sair becomes 0.39, the Swater is already reduced from 0.80 to 0.60 and so, SNAPLS = 0.01.

Regards