Mass loss from LNAPL pools under fluctuating water table conditions

Nonaqueous-phase liquids less dense than water (LNAPLs) form pools at the ground-water table, when leaked in sufficient quantities into the subsurface. Subsequent fluctuations of the ground-water table create a smear zone in the vicinity of the water table with the LNAPLs trapped in the pore spaces in the form of blobs. In addition to the pool, these blobs participate in dissolution and biodegradation processes in the saturated zone. A quantitative method is developed to evaluate mass loss of LNAPLs in the saturated zone under fluctuating water table conditions. A mathematical scheme is used to transform the moving domain to a stationary domain and the transformed advective-dispersive-reaction equation is solved numerically. The mass loss is computed in terms of two components: (1) Biodegradation within the domain; and (2) advective flux leaving the domain. The results indicate that the fluctuating water table conditions enhance the mass loss inasmuch as the entrapped blobs contribute to dissolution and biodegradation processes. Analysis of mass loss as a function of the Damkohler number (Da) indicates that biodegradation increases the mass removal from the system only after Da exceeds 0.1. Furthermore, for Da > 0.1, the biodegradation of solute dissolved from the entrapped blobs enhances the mass loss from the pool to a much greater extent than when no entrapped blobs exist under static water table conditions.