Laboratory and model simulations of a LNAPL spill in a variably-saturated sand, 1. Laboratory experiment and image analysis techniques

A two-dimensional, multiphase flow experiment was conducted in the laboratory. The saturation distribution of a lighter than water non-aqueous-phase liquid (LNAPL) as it migrated through a variably-saturated sand medium was determined using image analysis techniques. The pressures in the water and LNAPL phases were measured using hydrophilic and hydrophobic porous cups connected to a series of pressure transducers and a data acquisition system. The LNAPL inflow and water outflow were also recorded. A series of capillary pressure-saturation experiments were conducted for each two-phase system. The capillary pressures recorded by the pressure transducers were used to calculate the pphase saturations based on a fully hysteretic capillary pressure-saturation algorithm. The static equilibrium capillary pressure-saturation relationships proved to be invalid immediately ahead of the LNAPL front. The capillary pressure-saturation relationship appeared to be dynamic for short periods of time as the LNAPL front arrived at each transducer location. The presence of an entrapped air phase as the LNAPL migrated through the unsaturated zone clearly affected the migration of the LNAPL and the maximum LNAPL saturation reached in the sand medium.