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

Abstract

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 NAPLwet 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. IntroductionThe improper storage and disposal of hazardous nonaqueous phase liquids (NAPLs) has resulted in widespread contamination of the subsurface environment [Dragun et al., 1984]. As a NAPL migrates within a formation, a portion is retained within the pore structure owing to capillary forces. This entrapped residual NAPL serves as a persistent source of pollution to flowing groundwater. Our ability to predict the performance of a particular remediation technology in a NAPL contaminated formation will be determined, to a large extent, by our understanding of the processes that control interphase mass transfer. Indeed, the success of conventional "pump-andtreat" groundwater remediation technologies [Kavanaugh, 1996] is now widely recognized to be controlled by the slow dissolution of residual NAPL to the aqueous phase.Subsurface systems containing NAPL and water are generally assumed to be preferentially wetted by water. In water-wet systems, water occupies the smaller pores and the pore space immediately adjacent to the soil grains in the larger pores; residual NAPL is entrapped in the center of the larger pores as discontinuous spherical singlets or ganglia [Chatzis et al., 1983]. Entrapped NAPL ganglia may be quite complex in shape, occupying multiple pore bodies. These shapes are controlled by variations in pore structure and size, as well as by the initial NAPL release rate and saturation history [Land, 1968] ], it is...

Interphase mass transfer between fluids in subsurface formations: A review

“…Furthermore, the curvature and surface roughness of the grain particles may even further impede the flow within the film layer. In accordance with these observations, Bradford et al [89] experimentally showed that the interphase mass transfer in NAPL-wet media is much greater than the water-wet media providing further evidence that the flow configurations depicted in Fig. 3 can be useful in identifying which components of the interfacial areas contribute the most to interphase mass transfer ''MNtoIW'' can be visualized from Fig.…”

“…Bradford et al [89] confirmed this phenomenon on fractional wettable media which is a mixture of water-wet and NAPL-wet media. The authors showed that in water-wet media residual NAPL exists as single or multi-pore blobs.…”

“…Different correlations between residual saturation and porous medium properties (i.e., median grain size, wettability, pore size distribution) have been proposed by different authors. For example, the residual saturations measured by Powers et al [9] do not have the same relationship with medium properties as Bradford et al [89] or Schnaar and Brusseau [37] reported. One reason for these variations is that the meso-scale experiments present the impact of system parameters on entrapment jointly.…”

“…The authors showed that the mass transfer formulations developed by [32,43,110] cannot simulate interphase mass transfer for different wettable media. Consequently, the authors developed a revised formulation for interphase mass transfer coefficient that considers the wettability of the media using data obtained from column experiments by Bradford et al [89]. The developed model does not consider interfacial area separately but lumps it into the interphase mass transfer rate coefficient.…”

Experimental investigations of the entrapment and persistence of organic liquid contaminants in the subsurface environment.

“…Calculated values of y&j and fofol* for organic components of the synthetic DNAPL mixture ( 16) (14). The initial organic saturation for the 0, 25, pore structure.…”