An experimental investigation of NAPL pool dissolution enhancement by flushing

摘要

Experiments coupled with mathematical modeling are used to further elucidate the quantifica- tion of NAPL pool dissolution and its enhancement by water flushing. The experiments were performed using glass bead-packed column reactors with small pools of neat toluene or a toluene in dodecane mixture (toluene mole fraction, X_tol ≈ 0.02 or 0.09). Experimental quasi--steady-state toluene dissolution fluxes were determined by a mass--balance approach. The results of these experiments are used to quantity the impact of flushing on the NAPL pool dissolution flux, J, for wide ranges of two of the key controlling parameters--equilibrium concentration, C_s, and average pore water velocity, υ_x. In addition, the experimental dissolution flux data are used to evaluate predictions made using independently obtained model parameters and analytical mathematical models incorporating the two basic approaches used for mathematically describing the interphase distribution of NAPLs: the local equilibrium (LE) approach; and the mass transfer limited, or nonequilibrium (NE) approach. The data from the experiments with a neat toluene pool demon- strate the expected trend of increasing J as υ_x. increased from approximately 2 to 30 m/day. The LE model and the NE model with an average mass transfer coefficient, k_1 = 4.76 m/day, were able to describe the neat toluene pool data for υ_x < 18 m/day reasonably well. However, for υ_x > 18 m/day, the NE model with k_1 = 4.76 m/day provides a better description of the data, suggesting that the equilibrium boundary condition may become invalid for very high velocities. The experiments with the binary toluene in dodecane pools also show the expected trend of increasing J with increasing υ_x. The LE model describes the binary pool data reasonably well for the entire range of υ_x studied (approximately 0.1 to 10 m/day), and the NE model predictions with the average k_1 = 4.76 m/day determined in the neat-pool studies do not deviate significantly from the LE model in this υ_x range, although small deviations occur for > 10 m/day.