A Mass Balance Approach to Resolving LNAPL Stability

Abstract

This article explores the hypothesis that natural losses of light nonaqueous phase liquids (LNAPLs) through dissolution and evaporation can control the overall extent of LNAPL bodies and LNAPL fluxes observed within LNAPL bodies. First, a proof-of-concept sand tank experiment is presented. An LNAPL (methyl tert-butyl ether) was injected into a sand tank at five constant injection rates that were increased stepwise. Initially, for each injection rate the LNAPL bodies expanded quickly. With time the rate of expansion of the LNAPL bodies slowed and at extended times the extent of the LNAPL became constant. Attainment of a stable LNAPL extent is attributed to rates of LNAPL addition being equal to rates of LNAPL losses through dissolution and evaporation. Secondly, analytical solutions are developed to extrapolate the processes observed in the proof-of-concept experiment to dimensions and time frames that are consistent with field-scale LNAPL bodies. Three LNAPL body geometries that are representative of common field conditions are considered including one-dimensional, circular, and oblong shapes. Using idealized conditions, the solutions describe volumetric LNAPL fluxes as a function of position in LNAPL bodies and the overall extent of LNAPL bodies as a function of time. Results from both the proof-of-concept experiment and the mathematical developments illustrate that natural losses of LNAPL can play an important role in governing LNAPL fluxes within LNAPL bodies and the overall extent of LNAPL bodies.