Determining actual LNAPL plume thickness: review and case study in a fractured aquifer

Determining the thickness of a LNAPL (light non-aqueous phase liquid) hydrocarbon plume in the fractured rock surrounding a borehole is a very important aspect in determining the quantity and degree of contamination of the groundwater and soil, as well as deciding on applicable remediation measures. This review aims to compare some of the field and empirical methods to eventually propose a method of confirming the plausibility of the determined thickness. General insight is supplied relating to the occurrence of groundwater in the Earth’s crust, the basic principles of multiphase flow and the properties of the three fluids of importance, being water, Jet A-1 fuel and air. From the methods applied, the field bail-down method of Hughes et al. and the mathematical approach according to CONCAWE supplied reasonable results, with the Zillox and Muntzer equation providing no rational outcome due to changing water levels from regular bailing of the LNAPL. Furthermore, practically none of these methods have been developed for deep, fractured aquifers. By plotting regular depths to water and fuel on the abscissa axis with free product thickness on the ordinate, the depth to water and depth to fuel linear plots will intersect where the free product thickness equals zero. This indicates the static water level (i.e., the ground water level prior to hydrocarbon contamination) and by subtracting this from the free product depth, a maximum product thickness is obtained.