LNAPL THICKNESS IN MONITORING WELLS CONSIDERING HYSTERESIS AND ENTRAPMENT

Case histories of LNAPL contaminated sites indicate that LNAPL thickne sses in monitoring wells exhibit certain characteristic behaviors when related to water-table fluctuations. This paper investigates these be haviors using established theories for fluid saturations in three-phas e (air-LNAPL-water) porous media systems. A semianalytical model is de veloped to predict the LNAPL thickness in a monitoring well considerin g the effects of saturation hysteresis and air/LNAPL entrapment. Given the total LNAPL volume in the soil profile and the historical sequenc e of water-table elevations, the model predicts LNAPL thicknesses, bas ed on the assumption of a succession of hydrostatic pressure distribut ions. For example simulations, the model indicates that the LNAPL thic kness in a monitoring well can exhibit complex behaviors that are stro ngly dependent on the previous saturation history. While many of these predicted behaviors are consistent with field observations, some are not. This suggests that certain aspects of current theories pertaining to three-phase fluid saturations warrant further technical evaluation .