Impacts of transport mechanisms and plume history on tailing of sorbing plumes in heterogeneous porous formations Mahdi Maghrebi a,⇑ , Igor Jankovic a , Richelle M. Allen-King b , Alan J. Rabideau a , Indra Kalinovich c , Gary S. Weissmann d a Department of Civil, Structural, and Environmental Engineering, The State University of New York at Buffalo, USA b Department of Geology, The State University of New York at Buffalo, USA c Dillon Consulting Limited, Manitoba, Canada d Department of Earth and Planetary Sciences, The University of New Mexico, USA article info Article history: Received 28 January 2014 Received in revised form 10 July 2014 Accepted 14 July 2014 Available online 23 July 2014 Keywords: Plume tailing Heterogeneity Transport mechanisms Plume history abstract This work investigated the impacts of permeability and sorption heterogeneity on contaminant transport in groundwater using simulation experiments designed to elucidate the causes of tailing. The effects of advection, diffusion and sorption mechanisms and plume history were explored. A simple conceptual model consisting of a single inclusion (heterogeneity) of uniform hydraulic conductivity K and sorption distribution coefficient K d was adopted. The 3D inclusion, shaped as a horizontal oblate ellipsoid of var- iable thickness, was placed in a homogeneous anisotropic background of different hydraulic conductivity and sorption distribution coefficient. The background represents average K and K d of a heterogeneous porous formation. A closed-form analytic flow solution for uniform flow past the inclusion was coupled with a numerical transport solution to simulate contaminant migration for a wide range of transport parameters and two distinct source conditions. Over 2600 numerical simulations were performed in par- allel. Transport results were presented in terms of travel time distributions at a control plane down- stream of the inclusion and used to quantify tailing for a wide range of transport parameters, in order to separate advection-dominated from diffusion-dominated transport regime and to investigate effects of inclusion shape, diffusion, sorption and plume history on tailing. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction and background The success of conventional subsurface cleanup technologies is strongly influenced by contaminant tailing – the presence of con- taminants that remain in the subsurface for long periods of time (decade or longer) [6,8,11,18,40,42,43,46,50,57]. Tailing often results in groundwater concentrations that are above the maxi- mum contaminant level (MCL) limits for drinking water consump- tion. For example, reviews of 77 active sites in the USA where pump-and-treat was used have shown that cleanup goals based on drinking water MCLs have been reached for only 8 sites within a decade [42]. The lack of adequate knowledge about time scales that characterize contaminant tailing presents a serious challenge for the application of pump-and-treat and similar cleanup technologies. In addition to its practical significance, plume tailing is highly relevant in modeling groundwater transport. Because of presence of plume tailing, the macroscopic (Gaussian) advection–dispersion equation cannot properly predict skewed plume distributions in moderately to highly heterogeneous formations [5,19,25,26,29,37]. Plume tailing is directly influenced by spatial variability in hydraulic and sorbing properties of the subsurface materials that form the overall physical and chemical heterogeneity of a porous formation. Several field and numerical studies have attributed plume tail- ing to specific geologic units in the subsurface. For example, Parker et al. [45] found that one or a few thin clayey beds in a sand aquifer at an industrial site in Florida caused TCE tailing long after the source zone was isolated. Similarly, Rasa et al. [47] developed a 2D numerical model for MTBE (methyl tertiary-butyl ether) and TBA (tertiary-butyl alcohol) plume transport in a shallow aquifer at Vandenberg Air Force Base, California, and found that two silty layers were responsible for plume tailing long after the original source was depleted. The conceptual model of subsurface adopted for the present study is partially motivated by these findings. http://dx.doi.org/10.1016/j.advwatres.2014.07.007 0309-1708/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Address: 204 Jarvis Hall, The State University of New York at Buffalo, Buffalo, NY 14260, USA. Tel.: +1 716 645 4004; fax: +1 716 645 3667. E-mail address: maghrebi@buffalo.edu (M. Maghrebi). Advances in Water Resources 73 (2014) 123–133 Contents lists available at ScienceDirect Advances in Water Resources journal homepage: www.elsevier.com/locate/advwatres