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Dissolution of Trapped Nonaqueous Phase Liquids in Sand Columns

Département des sols et de génie agro-alimentaire, Université Laval, Québec, Canada;

W. A. Jury and M. A. Anderson

Department of Soil and Environmental Sciences, University of California, Riverside.

^* Corresponding author (Josee.Fortin{at}sls.ulaval.ca).

ABSTRACT

Soil column experiments were designed to evaluate the dissolution and leaching under continuous water flow of two immobilized nonaqueous phase liquids (NAPL): o-xylene, which is lighter than water (LNAPL) and o-dichlorobenzene, which is denser than water (DNAPL). Major differences were observed in the mobility and dissolution of the LNAPL and the DNAPL under continuous leaching. Under water-saturated conditions and downward flow, the trapped LNAPL was removed from sand columns by dissolution and by mobilization of small droplets. In contrast, the DNAPL remained trapped in the soil matrix during the leaching phase and was removed by dissolution only. The model used to describe the data assumes that the NAPL is trapped in the soil volume as ideal spheres of initial diameter d₀. The transport parameters of the model were determined by a separate tracer experiment, so that the only parameter fitted to the resident residual NAPL and outflow observations was the initial diameter of the spheres. The sphere model described the data fairly well, with optimum initial sphere diameter of 0.12 cm for o-dichlorobenzene and varying between 0.17 to 0.21 cm for o-xylene. The optimum diameter of the spheres was used to calculate the length of the mass transfer zone (L_m) over which solution passing through the NAPL region changes from zero NAPL concentration to NAPL water solubility. L_m varied between 7.9 to 11.1 cm for o-xylene, and from 3.7 cm in the finer sand to 5.2 cm in the coarser material for o-dichlorobenzene.