NAPL POOL DISSOLUTION IN STRATIFIED AND ANISOTROPIC POROUS FORMATIONS

A two-dimensional numerical model is developed to study contaminant tr ansport resulting from the dissolution of single- and multicomponent d ense nonaqueous-phase liquid (DNAPL) pools in heterogeneous porous med ia. The aqueous-phase concentration of each dissolved component is ass umed to undergo first-order decay as well as sorb under local equilibr ium conditions. Pool shrinkage is accounted for by modeling the progre ssive reduction of the DNAPL pool surface area as a time-dependent bou ndary. Multicomponent pool dissolution is modeled using an effective s olubility (or equilibrium aqueous solubility) relationship, where the nonaqueous-phase activity coefficient for each constituent is evaluate d at each and every time step. Subsurface heterogeneities are depicted by an ideally stratified porous formation and by a statistically anis otropic aquifer. In the stratified formation, a multicomponent DNAPL p ool is assumed to be formed at the interface between a sand layer and a clay layer, where DNAPL dissolution occurs simultaneously in both st rata. The ground-water velocity inside the sand stratum is uniform in the longitudinal direction whereas the interstitial liquid in the aqui tard is stagnant. In the statistically anisotropic aquifer, a single-c omponent DNAPL pool is assumed to be formed on top of an impermeable b edrock, where DNAPL dissolution occurs in the aquifer only. Results fr om several model simulations indicate that dissolved contaminant conce ntrations in aquifers are reduced significantly in the presence of aqu itards, and most importantly, the transfer of dissolved contaminants a long the pool-water interface is slower within statistically anisotrop ic than within homogeneous aquifers.