An analytic method is described for estimating the volume of mobile Light Nonaqueous Phase Liquids (LNAPL) in porous media from observed LNAPL thicknesses in monitoring wells. Static (mechanical) equilibrium of fluids in a homogeneous porous medium is the key condition on which the method is based. Both the Brooks-Corey and van Genuchten equations, with parameters derived from laboratory column experiments reported in the literature, are used to relate fluid contents to capillary pressures. The calculations show that LNAPL in the vadose zone does not distribute itself as a distinct layer floating on the top of a capillary fringe. Rather, the traditional concept of a capillary fringe is not applicable when LNAPL is present. Further, neither the LNAPL level nor the water level in monitoring wells is equal to the water-table elevation. The water table, being the surface on which the water pressure is zero gage, is located above the LNAPL-water interface in the well, and LNAPL in the porous media will reside below the water table.
It is shown that finite volumes of LNAPL theoretically can exist in materials with positive entry pressures (e.g., Brooks-Corey porous media) without revealing their presence in the form of an LNAPL layer in monitoring wells. However, LNAPL in porous media with zero entry pressure will always appear in monitoring wells, regardless of the volume of LNAPL in the porous medium. In addition, the theory shows that the ratio of the volume of LNAPL per unit area in the vadose zone to the thickness of LNAPL in monitoring wells is strongly dependent upon the capillary properties of the porous medium. Only in porous media with very uniform pore sizes is the volume of LNAPL in the vadose zone approximately proportional to the thickness of LNAPL in monitoring wells.