The role of geological heterogeneity and variability in water infiltration on non-aqueous phase liquid migration

Abstract

This study investigates the influence of two factors—geological heterogeneity and variability in water infiltration—on non-aqueous phase liquid (NAPL) migration in the unsaturated zone. NAPL migration under three-phase flow conditions resulting from a ground surface spill is modeled for multiple heterogeneous realizations of a porous medium with various water infiltration scenarios. Increased water infiltration before the spill has two counteracting effects: NAPL relative permeability (k rn) increases with increasing water saturation (S w) for a given NAPL saturation, while higher S w in the soil near the NAPL source zone leads to less NAPL mass infiltration. It is found that the former effect is overwhelmed by the latter effect, the net effect being that with longer infiltration durations before the spill, both the infiltrated NAPL mass and the depth of the front migration decrease. Simulation results also show strong effect of the medium heterogeneity. Results suggest that total infiltrated mass, front depth and plume spread increase with an increasing standard deviation of log-permeability. Also variability in modeling results among realizations is largely impacted by the log-permeability standard deviation. Spatial correlation in permeability also strongly influences NAPL infiltration. An increase in the isotropic correlation length from 0.75 to 1.5 m leads to a decrease in total infiltrated mass, plume migration depth as well as vertical spread. Lateral spread in this case is not shown to be affected by the correlation length.