DISSOLUTION OF TRAPPED NONAQUEOUS PHASE LIQUIDS IN SAND COLUMNS

Soil column experiments were designed to evaluate the dissolution and leaching under continuous water flow of tno immobilized nonaqueous pha se liquids (NAPL): o-xylene, which is lighter than water (LNAPL) and o -dichlorobenzene, which is denser than water (DNAPL). Major difference s were observed in the mobility and dissolution of the LNAPL and the D NAPL under continuous leaching, Under water-saturated conditions and d ownward flow, the trapped LNAPL was removed from sand columns by disso lution and by mobilization of small droplets, In contrast, the DNAPL r emained trapped in the soil matrix during the leaching phase and,vas r emoved by dissolution only, The model used to describe the data assume s that the NAPL is trapped in the soil volume as ideal spheres of init ial diameter d(0). The transport parameters of the model were determin ed by a separate tracer experiment, so that the only parameter fitted to the resident residual NAPL and outflow observations was the initial diameter of tile spheres, The sphere model described the data fairly well, with optimum initial sphere diameter of 0.12 cm for o-dichlorobe nzene and varying between 0.17 to 0.21 cm for o-xylene, The optimum di ameter of the spheres was used to calculate the length of the mass tra nsfer zone (L-m) over which solution passing through the NAPL region c hanges from zero NAPL concentration to NAPL water solubility. L-m vari ed between 7.9 to 11.1 cm for o-xylene, and from 3.7 cm in the finer s and to 5.2 cm in the coarser material for o-dichlorobenzene.