Estimating LNAPL Volumes in Unimodal and Multimodal Subsurface Pore Systems

In this chapter, we compare modeling approaches for determining the specific volume of light nonaqueous phase liquids (LNAPLs) in the subsurface on top of the water-saturated zone following spills or leaks at or near the soil surface. We employ both unimodal and multimodal pore-size capillary pressure–saturation functions in our predictions. Hydrologic properties, both fluid properties and pore-size distribution, are important for accurate predictions. Before presenting our results, we discuss fluid interfacial tensions, fluid wettability, and pore structures. Data from the literature are used in our calculations to show that the use of multimodal unsaturated soil hydraulic properties can lead to significantly different subsurface LNAPL-specific volume predictions compared to when unimodal formulations are used. The differences can be significant even when the capillary pressure–saturation curves appear similar over the capillary pressure range relevant to the hypothetical LNAPL contamination scenarios. Consequently, the multimodal pore structure of porous media needs to be addressed to avoid potentially erroneous estimates of the resources and time needed to remediate LNAPLs from contaminated areas.