[1] The utility of existing models for describing upscaled mass transfer from nonaqueous phase liquid (NAPL) were examined when preferential dissolution pathways form in NAPL-contaminated zones that extend over the scale of decimeters. Laboratory experiments were conducted in two well-characterized, heterogeneous packings. Using data from these experiments and simulations, existing methods for upscaling the mass transfer rate coefficient for NAPL dissolution based on dissolution front length growth (LDF), aquifer heterogeneity and spatial moments of NAPL distribution, and the ganglia-to-pool ratio (GTP) were evaluated along with an equilibrium stream tube (EST) model for predicting contaminant flux. When the correlation length of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of dissolution fingers, only 4.8% of the NAPL resided in pools. Dissolution fingers formed in this experiment, and the LDF, GTP, and EST models resulted in similar predictions of effluent concentrations, with root-mean-square errors (RMSEs) between 0.035 and 0.079 and the LDF-heterogeneous model best. When the correlation scale was smaller (1.0 cm), 66.7% of the NAPL was in pools, and preferential dissolution pathways were dominated by channeling, preferential dissolution caused by spatial variations in aqueous phase permeability, and NAPL saturation. For this experiment the EST and GTP models performed well, with RMSEs of 0.055 and 0.103, respectively. Dissolution fingering was important when the permeability correlation length was sufficiently large that dissolution finger formation was not disrupted and NAPL pools were not dominant.
Despite the installation in the 1980s and 1990s of hydraulic containment systems around known source zones (four slurry walls and ten pump-and-treat systems), trichloroethene (TCE) plumes persist in the three uppermost groundwater-bearing units at the Middlefield-Ellis-Whisman (MEW) Superfund Study Area in Mountain View, California. In analyzing TCE data from 15 recovery wells, the observed TCE mass discharge decreased less than an order of magnitude over a 10-year period despite the removal of an average of 11 pore volumes of affected groundwater. Two groundwater models were applied to long-term groundwater pump-and-treat data from 15 recovery wells to determine if matrix diffusion could explain the long-term persistence of a TCE plume. The first model assumed that TCE concentrations in the plume are controlled only by advection, dispersion, and retardation (ADR model). The second model used a one-dimensional diffusion equation in contact with two low-permeability zones (i.e., upper and lower aquitard) to estimate the potential effects of matrix diffusion of TCE into and out of low-permeability media in the plume. In all 15 wells, the matrix diffusion model fit the data much better than the ADR model (normalized root mean square error of 0.17 vs. 0.29; r 2 of 0.99 vs. 0.19), indicating that matrix diffusion is a likely contributing factor to the persistence of the TCE plume in the non-source-capture zones of the MEW Study Area's groundwater-extraction wells. O Virgilio (Vic) Cocianni is the remediation manager for Schlumberger, a leading oilfield services company. Cocianni provides leadership and guidance on remediation activities in the various countries where Schlumberger operates.
Du'Bois (Joe) Ferguson is the remediation technical manager for Schlumberger Technology Corporation,where he provides the technical leadership for the company's remediation projects on a global basis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.