Spatial Variations of Residual NAPL Zone Concentration in Subsurface

Improved models of residual nonaqueous phase liquid (NAPL) dissolution in the subsurface are presented to incorporate spatial variation of the residual zone source concentration. The new models are proposed based on the previously developed simple screening models under the assumption of proportionality between the residual NAPL source zone concentration and the remaining residual NAPL mass. The objective of the models is to predict the solute concentration in the zone of the residual NAPL as a result of dissolution. The proposed residual NAPL dissolution models enable the pseudo-equilibrium formulation to be used and therefore the numerical simulations for field application problems can be simplified compared to the non-equilibrium counterpart. With proper choice of empirical parameters, the proposed simple screening models can work as well as more complex dissolution rate correlation models. The models are especially useful for situations of small residual NAPL saturation, which are typical for many field applications. The previous screening models lumped the entire NAPL source zone together without considering the spatial distribution of NAPL source zone concentrations. The new models will incorporate the fact that the NAPL mass in the upstream will dissolve first. It will create a dissolution zone and this zone will travel downstream at certain velocity. Based on this consideration, the NAPL zone concentration is dependent on not only the time but also the distance into the NAPL zone.