The purpose of this work is to identify the mechanisms that govern the removal of carbon tetrachloride (CT) during soil vapor extraction (SVE) by comparing multiphase flow simulations with a detailed data set from a well-defined two-dimensional flow cell experiment. The flow cell was packed with two sandy soils including an embedded fine-grained sand layer. Gas concentrations at the outlet of the flow cell and 15 sampling ports inside the flow cell were measured during SVE. A dual-energy gamma radiation system was used to measure an initial NAPL saturation profile in a fine-grained sand layer. Imaging result from a dual-energy gamma radiation system with dyed CT mark along CT migration was used to construct the distribution of initial NAPL saturation in the flow cell for input to numerical simulations. Gas concentration results and photographs during SVE were compared to simulation results using a continuum-based multiphase flow simulator, STOMP (Subsurface Transport Over Multiple Phases). The measured effluent gas concentration decreased quickly at first, and then started to decrease gradually, resulting in long-term tailing. CT mass was removed quickly in coarse sand, followed by a slow removal from the fine-grained sand layer. An analytical solution for a one-dimensional advection and first-order volatilization model matched the tailing well with two fitting parameters. However, given detailed knowledge of the permeability field and initial NAPL distribution, we can predict the tailing and gas concentration profiles at sampling ports using equilibrium NAPL volatilization. NAPL flow occurs in the presence of free NAPL, and must be accounted for to accurately predict NAPL removal during the SVE experiment. The model prediction was accurate within the uncertainty of the measured or literature derived parameters (i.e., dispersivity and soil parameters). This study provides insights into the physical mechanisms of NAPL removal from a low permeability zone, and use of the local equilibrium assumption for NAPL volatilization during SVE. In addition, this study demonstrates that lack of detailed information regarding NAPL distribution and heterogeneity pattern lead overall NAPL removal to a kinetically controlled system at a 2-D flow cell scale.
Revised: April 7, 2011 |
Published: October 13, 2009
Yoon H., M. Oostrom, T.W. Wietsma, C.J. Werth, and A.J. Valocchi. 2009.Numerical and experimental investigation of DNAPL removal mechanisms in a layered porous medium by means of soil vapor extraction.Journal of Contaminant Hydrology 109, no. 1-4:1-13.PNNL-SA-64091.doi:10.1016/j.jconhyd.2009.07.001