Plume persistence caused by back diffusion from thin clay layers in a sand aquifer following TCE source-zone hydraulic isolation

Quantification of Degradation of Chlorinated Hydrocarbons in Saturated Low Permeability Sediments Using Compound-Specific Isotope Analysis

“…With time the accumulated DNAPL phase is dissolved and transported by advection in the aquifer and by diffusion into the underlying low permeability sediments. 1,2 After reduction of the strength of the chlorinated hydrocarbon plume in the aquifer, back-diffusion occurs from low permeability sediments toward the aquifer due to the reversal of the concentration gradient, forming a long-term contamination source. [1][2][3]5 Most previous studies assumed that chlorinated hydrocarbons are not affected by degradation in low permeability sediments.…”

“…1,2 After reduction of the strength of the chlorinated hydrocarbon plume in the aquifer, back-diffusion occurs from low permeability sediments toward the aquifer due to the reversal of the concentration gradient, forming a long-term contamination source. [1][2][3]5 Most previous studies assumed that chlorinated hydrocarbons are not affected by degradation in low permeability sediments. [2][3][4]6 It was commonly hypothesized that microbial growth is very limited in the small pores of low permeability sediments and that degradation of chlorinated hydrocarbons is thus unlikely to occur.…”

A Bayesian belief network approach for assessing uncertainty in conceptual site models at contaminated sites

“…At this site, two specific conceptual issues, both key to risk assessment, are considered. The first is the presence of a DNAPL (Dense Non Aqueous Phase Liquid) [ITRC, 2013] and consequently the long term persistence of a secondary source [Parker et al, 2008]. The second element concerns the presence or absence of fractures in the clay till.…”

“…The long term persistence of a DNAPL source is exacerbated in low permeability geological features [Hadley and Newell, 2012] or dual porosity geological media [Chambon et al, 2011]. Clay tills are common in northern Europe and parts of North America [Christiansen et al, 2008;Parker et al, 2008] and can lead to a complex system of preferential flow paths (e.g. fractures or sand lenses) in a low permeability matrix [Damgaard et al, 2013a;Damgaard et al, 2013b;Gerber et al, 2001;Hendry et al, 2004;Kessler et al, 2012;Kessler et al, 2013].…”

Release of contaminants from a heterogeneously fractured low permeability unit underlying a DNAPL source zone

“…Conversely where fractures are present, inward DNAPL migration, controlled by the fracture aperture and associated capillary entry pressures (Kueper and McWhorter, 1991), result in the potential for DNAPL mass to enter and reside within the fractures. The dissolution of DNAPL constituents and subsequent diffusion of the dissolved phase from the fractures results in significant aqueous-and sorbed-phase mass within the matrix (Espositio and Thomson, 1999;Parker et al, 2008;Parker et al, 1994;Reynolds and Kueper, 2004;Sale et al, 2008). Through successive fracture replenishments by DNAPL, mass stored in the matrix can increase until the pore water concentration asymptotically reaches the contaminant solubility throughout the matrix (Parker et al, 1997).…”