INFLUENCE OF ANTECEDENT MOISTURE-CONTENT ON RESIDUAL LNAPL SATURATION

In a water wet porous medium, the antecedent moisture content controls the entrapment of LNAPL at low capillary numbers. A two-phase retenti on cell study of entrapment was conducted for LNAPL water systems in a carbonate sand. For two different LNAPL water systems, if was found t hat a linear expression related the residual LNAPL saturation to the a ntecedent water saturation. The prediction of trapped LNAPL saturation compares favorably to long-column test data obtained for the same san d. A trapping model was developed, and its predictions were compared t o field measurements of trapped LNAPL saturation. Deviations between p redicted and measured LNAPL saturations arose when there was a change in the median pore size distribution. In addition, a series of three-p hase (air-LNAPL-water) retention cell tests measured the amount of LNA PL that became trapped when the water phase was maintained at a consta nt saturation during LNAPL withdrawal under a negative pressure head. Under these conditions, the amount of trapped LNAPL was higher than th at measured by two-phase tests at the same water saturation.