A Comparison of Models Describing Residual NAPL Formation in the Vadose Zone

Abstract

A major shortcoming of multifluid flow simulators is the inability to predict the retention of nonaqueous phase liquid (NAPL) in the vadose zone after long drainage periods. Recently, three theoretical models—the Wipfler and van der Zee (WVDZ) model; the Van Geel and Roy (VGR) model; and the Lenhard, Oostrom, and Dane (LOD) model—have been proposed for describing residual NAPL formation. The WVDZ model assumes a critical total liquid saturation below which all NAPL becomes residual. The VGR and LOD models are extensions of an existing hysteretic relative permeability–saturation–capillary pressure model and assume formation of residual NAPL during NAPL drainage and imbibition, respectively. In this study, we compared model predictions against results of a series of static pressure cell experiments. We found no experimental evidence supporting the WVDZ concept of a critical total liquid saturation. The other two models yielded reasonable predictions. The VGR and LOD models were then incorporated into a multifluid flow simulator, and simulations of two transient column experiments were conducted. Both models performed considerably better than simulations without considering the formation of residual NAPL, underwriting the importance of incorporating this process in simulators. Although the VGR and LOD models are based on different conceptual models, no clear performance differences could be observed when simulation results were compared against the transient experimental data.

Scalable Modeling of Carbon Tetrachloride Migration at the Hanford Site Using the STOMP Simulator

“…4 for the DNAPL mass. Because the results are relatively close, it can be concluded that the 78K grid block simulations reported by Oostrom et al (2004, 2005, 2007) were sufficient for the purposes intended.…”

“…Simulation results support a conceptual model for CCl 4 distribution where CCl 4 in the DNAPL phase migrated primarily in a vertical direction below the disposal site and where some DNAPL probably migrated across the water table into the regional aquifer. Results from Oostrom et al (2004, 2005, 2007) also show that the lower permeability Cold Creek Unit retained more DNAPL within the vadose zone than other hydrologic units during the infiltration and redistribution process. Due to the relatively high vapor pressure of the CCl 4 , the simulated vapor plumes were extensive and influenced by density‐driven advection.…”

“…Numerical simulation capabilities for predicting the complex migration behavior (including residual NAPL formation in the vadose zone) of volatile dense organics, such as CCl 4 , have become available to the scientific community (Falta et al, 1992; Oostrom et al, 2005; Piepho, 1996; White et al, 2004). While these computational capabilities have allowed subsurface scientists to investigate laboratory‐ and field‐scale problems, the complexity of the numerical solution limits the computational domain size and thus grid resolution for a particular physical domain.…”

Estimating the Impact of Vadose Zone Sources on Groundwater to Support Performance Assessment of Soil Vapor Extraction

“…The fully implicit, integrated finite difference wateroil-air operational mode of the Subsurface Transport Over Multiple Phases code (White and Oostrom 2006) was used to conduct the 3D flow and transport simulations in domains configured based on the generalized conceptual model shown in Figure 1. The simulator has been extensively used to simulate a variety of multi-fluid systems (e.g., White et al 2004;Oostrom et al 2005). The applicable governing equations are the component mass-conservation equations for water, organic compounds, and air.…”