The entrapment and long‐term dissolution of tetrachloroethylene in fractional wettability porous media

Abstract

Theoretical and modeling studies for the prediction of nonaqueous phase liquid (NAPL) entrapment and dissolution have largely assumed that the soil is preferentially water‐wet. Many natural systems, however, have both water‐ and NAPL‐wet solids as a result of spatial and temporal variations in fluid and soil properties. This condition is referred to as fractional wettability. This work presents long‐term dissolution data for tetrachloroethylene (PCE) entrapped in porous media representing a range of grain sizes, gradations, and fractional wettabilities. Entrapment data suggest that for given wettability conditions, initial residual NAPL saturations tend to increase with decreasing soil mean grain size. In addition, residual NAPL saturations in finer‐textured sands were observed to reach a minimum at intermediate wetting conditions, whereas residual saturations in coarser‐textured media decreased asymptotically with increasing fraction of NAPL‐wet sand. In dissolution studies, an increase in the NAPL‐wet fraction tended to result in decreased PCE dissolution time, characterized by a longer period of high effluent concentrations, followed by a more rapid reduction in concentration. Increases in NAPL‐wet fraction, however, were also associated with higher sorptive capacities, leading to enhanced PCE concentration tailing. The influence of fractional wettability on PCE dissolution behavior was also found to depend on media grain size distribution, particularly for more water‐wet soils. Experimental observations are discussed in the context of pore‐scale conceptual models of entrapment.