Modeling DNAPL Migration and Persistence: Twenty Years of Progress and Challenges for the Future

It is now widely recognized that conventional pump-and-treat technologies are often an inefficient and ineffective approach for the remediation of aquifers contaminated by nonaqueous phase organic liquids (NAPLs). Of particular concern are dense NAPLs (DNAPLs) of low solubility, such as chlorinated solvents, that can migrate along tortuous flow paths deep into a formation and pool in relatively inaccessible locations. Capillary entrapped residuals and pools of DNAPL may then serve as persistent sources of contamination to flowing groundwater. The mechanisms of DNAPL migration and persistence were first introduced in the hydrology literature in the early 1980's, and the last fifteen years have seen extensive development of multiphase flow and transport models for applications to DNAPL contamination and remediation scenarios. This presentation provides an overview of research focused on the development and application of these models. An historical perspective is provided for the parallel evolution of conceptual models and numerical simulators. The capabilities of state-of-the-art simulators are then illustrated through a series of comparisons of model predictions with laboratory and field observations of tetrachloroethylene (PCE) infiltration and recovery. Model/experimental comparisons highlight the importance of accounting for rate-limited solubilization, buoyancy forces, and chemical/physical heterogeneity in predictions of PCE migration and remediation. Example simulations reveal the extreme sensitivity of model predictions to grid resolution and subsurface heterogeneity, and point to the need for further conceptual and computational advances.