Free Nonaqueous Phase Liquid Research Articles

Experimental study on non-aqueous phase liquid multiphase flow characteristics and controlling factors in heterogeneous porous media

Enhanced understanding of non-aqueous phase liquid (NAPL) infiltration into porous media is important for the effective design of remediation strategies. Six model multi-flow experiments in two-dimensional polymethyl methacrylate tank were carried out to verify the influences of soil initial water content, local low permeability lens, lithologic sharp interface and leakage position on light non-aqueous phase liquid (LNAPL) and dense on non-aqueous phase liquid (DNAPL) transport velocities in soil, with diesel and perchlorethylene as LNAPL and DNAPL, respectively. The evolutions of the plume were recorded through the transparent side of the tank by CCD camera and the contaminant fronts were traced using gel pen on a plastic film coated on tank wall at appropriate intervals. The controlling factors of free NAPL migration paths and speeds were analyzed thoroughly, and capillary pressure equilibrium models of NAPL-water/NAPL-air interface and the partition movement theory were put forward. Results show that the effect of NAPL types: soil initial water content, local low permeability lens, leakage position and lithologic sharp interface on NAPL migration paths and velocities are diverse. The NAPL movement principles should be presented in accordance with partitions as related to the initial water content namely dry soil area, capillary zone and water-saturated zone. The vertical movement and lateral extension of NAPL are interdependent, and the vertical migration is dominant in initial leakage stage. The vertical migration is hindered by lateral growth when meeting with local low permeability lens, lithologic sharp interface or capillary fringe. Vertical migration rate reduction is mainly caused by the pore resistance, NAPL residual and lateral spreading and the causes of corresponding increase of horizontal expansion speed are the decrease of vertical velocity and the increase of buoyancy. The sufficiently short time allows to neglect chemical reactions, dissolution and sorption of NAPL and heterogeneous porous media.

Remediation of Nonaqueous Phase Liquid Polluted Sites Using Surfactant‐Enhanced Air Sparging and Soil Vapor Extraction

A two-dimensional laboratory sand tank was installed to study the remediation efficiency of surfactant-enhanced air sparging (-SEAS) coupled with soil vapor extraction (SVE) in nonaqueous phase liquid (NAPL) polluted sites. During initial stages of remediation, it was more reasonable to use conventional air sparging coupled with SVE. When most free NAPLs were removed and contaminant removal rate was maintained at a relatively low level, surfactant was added to the groundwater. During enhanced remediation, lower interfacial tension caused residual NAPLs in the porous media to slightly migrate, making the downstream contaminant concentration somewhat higher. The polluted area, however, was not more enlarged than before. The decrease in surface tension resulted in increased air saturation in the groundwater and the extent of the air influence zone. After 310 hours, 78.7% of the initial chlorobenzene mass had volatilized, 3.3% had migrated out of the sand profile, 17.5% was in the vadose zone, and 0.5% remained in the groundwater, thus revealing that SEAS/SVE can effectively improve the remediation of NAPL polluted sites.

Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction

An existing multiphase flow simulator was modified in order to determine the effects of four mechanisms on NAPL mass removal in a strongly layered heterogeneous vadose zone during soil vapor extraction (SVE): a) NAPL flow, b) diffusion and dispersion from low permeability zones, c) slow desorption from sediment grains, and d) rate-limited dissolution of trapped NAPL. The impacts of water and NAPL saturation distribution, NAPL-type (i.e., free, residual, or trapped) distribution, and spatial heterogeneity of the permeability field on these mechanisms were evaluated. Two different initial source zone architectures (one with and one without trapped NAPL) were considered and these architectures were used to evaluate seven different SVE scenarios. For all runs, slow diffusion from low permeability zones that gas flow bypassed was a dominant factor for diminished SVE effectiveness at later times. This effect was more significant at high water saturation due to the decrease of gas-phase relative permeability. Transverse dispersion contributed to fast NAPL mass removal from the low permeability layer in both source zone architectures, but longitudinal dispersion did not affect overall mass removal time. Both slow desorption from sediment grains and rate-limited mass transfer from trapped NAPL only marginally affected removal times. However, mass transfer from trapped NAPL did affect mass removal at later time, as well as the NAPL distribution. NAPL flow from low to high permeability zones contributed to faster mass removal from the low permeability layer, and this effect increased when water infiltration was eliminated. These simulations indicate that if trapped NAPL exists in heterogeneous porous media, mass transfer can be improved by delivering gas directly to zones with trapped NAPL and by lowering the water content, which increases the gas relative permeability and changes trapped NAPL to free NAPL.

A constitutive model for air–NAPL–water flow in the vadose zone accounting for immobile, non-occluded (residual) NAPL in strongly water-wet porous media

A hysteretic constitutive model describing relations among relative permeabilities, saturations, and pressures in fluid systems consisting of air, nonaqueous-phase liquid (NAPL), and water is modified to account for NAPL that is postulated to be immobile in small pores and pore wedges and as films or lenses on water surfaces. A direct outcome of the model is prediction of the NAPL saturation that remains in the vadose zone after long drainage periods (residual NAPL). Using the modified model, water and NAPL (free, entrapped by water, and residual) saturations can be predicted from the capillary pressures and the water and total-liquid saturation-path histories. Relations between relative permeabilities and saturations are modified to account for the residual NAPL by adjusting the limits of integration in the integral expression used for predicting the NAPL relative permeability. When all of the NAPL is either residual or entrapped (i.e., no free NAPL), then the NAPL relative permeability will be zero. We model residual NAPL using concepts similar to those used to model residual water. As an initial test of the constitutive model, we compare predictions to published measurements of residual NAPL. Furthermore, we present results using the modified constitutive theory for a scenario involving NAPL imbibition and drainage.

A model for stripping multicomponent vapor from unsaturated soil with free and trapped residual nonaqueous phase liquid

We present a model for the multicomponent vapor transport due to air venting in an unsaturated zone in the presence of free and trapped phases of residual nonaqueous phase liquid (NAPL). On the microscale the soil particles are assumed to form spherical aggregates with micropores filled with immobile water, trapped phases of NAPL and air. The interaggregate space is occupied with mobile air, and a thin film of free NAPL adheres on the aggregate surface. While the free NAPL can readily be in equilibrium with macropore vapor, the mass transfer from immobile phases in aggregates is rate‐limited by aqueous diffusion. This model enables us to predict the vapor concentrations of various chemical species and the free NAPL saturation over the macroscale, based on the detailed understanding of the aqueous concentrations of the species and the trapped NAPL saturation within the aggregates. The model is compared favorably with some experimental data of sparging multicomponent vapor out of an intact core taken from a contaminated site. The distinctive features of multicomponent transport, clearly exhibited by the data, are further examined in the simulations of a hypothetical case of three‐aromatic vapor transport under a radial flow field. It is found that while the vapor concentration of the most volatile component drops monotonically with time, those of the less volatile may rise as their mole fractions in the NAPL increase. The vapor concentration of a heavy component may have a local maximum at the evaporation front of the free NAPL. In the case of radial flow the free NAPL has two receding evaporation fronts. Condensation of the heavy component downstream of the far front causes a temporary increase of its total concentration there. With trapped NAPL and soil aggregation the macroscale transport is retarded, and the effluent concentrations end up in noticeable tailing.