MODELING QUASI-STEADY NAPL DISSOLUTION IN FRACTURED PERMEABLE MEDIA

This work explores the rate-limited dissolution of entrapped nonaqueou s phase liquids (NAPLs) in a fractured ''permeable'' formation through the use of mathematical models, developed on the basis of a simple co nceptual framework. The feasibility of pump-and-treat operations in su ch formations is assessed through characterization of dimensionless sy stem parameters governing NAPL dissolution and solute transport. These dimensionless parameters include the ratio of permeable block to frac ture flow (mobility number), the system dimensionless mass transfer co efficient, and the number of contaminated fracture sections. A closed form analytical solution is developed for the simple case of a single contaminated fracture section. For domains with a large number of cont aminated fracture sections, a numerical approach is presented that com bines a finite difference scheme for simulation of solute transport in the fracture network together with an analytical solution for solute transport in the permeable block flow. The most favorable conditions f or pump-and-treat systems are shown to be systems with large mobility numbers and large mass transfer coefficients. In systems with intermed iate and small values of these parameters, effluent solute concentrati ons may be significantly below equilibrium values, reducing the effect iveness of pump-and-treat remediation. It is also shown that an equiva lent continuum approach can be used to model rate-limited NAPL dissolu tion and transport in the simplified fractured permeable formation. Th e effective mass transfer coefficient for the representative continuum , however, is nonlinearly related to the local mass transfer coefficie nt.