DISSOLUTION FINGERING DURING THE SOLUBILIZATION OF NONAQUEOUS PHASE LIQUIDS IN SATURATED POROUS-MEDIA .1. MODEL PREDICTIONS

The dissolution of nonaqueous phase liquids (NAPLs) trapped at residua l saturation is an important problem at many contaminated groundwater sites. It is well known that NAPL ganglia trapped within the pore spac e reduce the permeability of the medium to aqueous phase flow. When fl uid flow is imposed on such a system, the aqueous phase may interact w ith the dissolution-induced permeability changes, leading to fingered patterns. This mechanism is very similar to that of mineral dissolutio n instabilities, which are a particular example of reactive infiltrati on instabilities. Extending that literature, we present a nonlinear mo del describing the dissolution of NAPL ganglia and perform a linear st ability analysis of the resultant moving free boundary problem, demons trating that instabilities may develop from a planar dissolution front . Predicted finger wavelengths are a function of both residual NAPL sa turation and the imposed aqueous phase flow rate; they range from cent imeters to meters. Experimental observations of dissolution fingering are presented in a companion paper [Imhoff et al., this issue] and are compared with predictions from this model. Dissolution fingering may affect the solubilization of NAPL ganglia in natural. environments and in experimental studies of NAPL dissolution intended to quantify mass transfer rates.