An experimental investigation of NAPL pool dissolution enhancement by flushing

Experiments coupled with mathematical modeling are used to further elucidate the quantification 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, Xtol≈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 quantify the impact of flushing on the NAPL pool dissolution flux, J, for wide ranges of two of the key controlling parameters—equilibrium concentration, CS, and average pore water velocity, vx. 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 demonstrate the expected trend of increasing J as vx increased from approximately 2 to 30 m/day. The LE model and the NE model with an average mass transfer coefficient, kl=4.76 m/day, were able to describe the neat toluene pool data for vx<18 m/day reasonably well. However, for vx>18 m/day, the NE model with kl=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 vx. The LE model describes the binary pool data reasonably well for the entire range of vx studied (approximately 0.1 to 10 m/day), and the NE model predictions with the average kl=4.76 m/day determined in the neat-pool studies do not deviate significantly from the LE model in this vx range, although small deviations occur for >10 m/day.