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ORIGINAL RESEARCH

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

^a Hydrology Group, Pacific Northwest National Laboratory, P.O. Box 999 MS K9-33, Richland, WA 99352
^b Subsurface Science Initiative, Idaho National Environmental and Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2025
^c Department of Civil and Environmental Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6
^d EMSL, Pacific Northwest National Laboratory, P.O. Box 999 MS K8-96, Richland, WA 99352
^* Corresponding author (mart.oostrom{at}pnl.gov)

Received 5 February 2004.

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.

Abbreviations: LOD, Lenhard, Oostrom, and Dane • NAPL, nonaqueous phase liquid • NH, nonhysteretic Brooks and Corey–Burdine constitutive model • PCE, perchloroethylene • VGR, Van Geel and Roy • WVDZ, Wipfler and van der Zee

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