Application of gravity drainage to the recovery of residual LNAPL in homogeneous and lensed sand packs

Abstract

Residual LNAPL (light non-aqueous-phase liquid) trapped as immobile, isolated blobs under the water table often constitutes a large fraction of the total amount of LNAPL in the subsurface. When the water table is lowered using pumping wells, these blobs are remobilized on contract with the layer of free LNAPL that lies on top of the water table. In the dewatered zone above the LNAPL layer, the LNAPL phase is kept continuous by LNAPL films which spread between water coating the pore walls and air occupying the centre of the pores. The LNAPL films can flow under the action of gravity forces, resulting in the accumulation of LNAPL on top of the water table where it can be recovered. Extremely small residual LNAPL saturations in the dewatered zone can be eventually achieved by gravity drainage. However, gravity drainage is a slow process due to the low conductivity of LNAPL films. Two experiments have been carried out in a two-dimensional laboratory model packed with sand. In the first experiment, the time scale involved in the recovery of residual Soltrol oil 100® by gravity drainage in a homogeneous sand pack was investigated. In the second experiment, a heterogeneous (lensed) sand pack has been used to study: (1) the influence of different-permeability lenses on the spatial distribution of LNAPL leaked at the top of the sand pack; (2) the effects of water table fluctuations; and (3) the recovery of residual LNAPL by gravity drainage in a lensed porous medium. Special attention has been given to the case where the lenses have smaller permeability than the surrounding medium.