DISSOLUTION FINGERING DURING THE SOLUBILIZATION OF NONAQUEOUS PHASE LIQUIDS IN SATURATED POROUS-MEDIA .2. EXPERIMENTAL-OBSERVATIONS

Nonaqueous phase liquids (NAPLs) are a common source of contamination at polluted groundwater sites, where they frequently remain trapped wi thin the pore space at residual saturation and reduce the permeability of the medium to aqueous phase flow. The model presented in a compani on paper [Imhoff and Miller, this issue] suggested that when fluid flo w is imposed on such a system, the aqueous phase may interact with dis solution-induced permeability changes, and lead to fingered patterns. In this investigation, a two-dimensional flow cell was used to study t he effects of porous medium structure, Darcy flux, initial residual NA PL saturation, median particle diameter, gravity, and NAPL composition on dissolution fingering. Fingering occurred when two conditions were met: (1) 11 to 80 e-fold times had elapsed, where e-fold time is the time required for the instability to grow by a factor e and was predic ted from the linear stability analysis in the companion paper; and (2) the length of the dissolution front before finger development was sma ller than the zone of NAPL residual. Where fingers formed, finger stru cture was similar and showed no systematic variation within the parame ters investigated. Observed finger wavelengths compared well with mode l predictions. A single experiment in a three-dimensional cell, 1 m lo ng, demonstrated that fingers can grow to at least 30 cm in length. Wh en experimental observations in this cell were compared with predictio ns of NAPL dissolution based on models that did not include fingering, the measurements of changing NAPL saturation differed significantly f rom model predictions.