Preferential flow and aging of NAPL in the unsaturated soil zone of a hazardous waste site: implications for contaminant transport

Abstract

Flow of non‐aqueous phase liquids (NAPL) in the unsaturated zone is thought to be driven by gravity with a dominant vertical flow direction, and lateral spreading to be limited to the gradient of the relative permeabilities. The effect of soil profile build‐up, preferential flow, aging, and groundwater level fluctuations is mostly neglected. The objective of our study was to check the effects of such processes on the fate of NAPL in the unsaturated soil zone. At a hazardous waste site, we conducted a field survey of the unsaturated soil zone and monitored the groundwater for a two year period. We conducted spatially resolved and depth dependent soil sampling and analysis and the evaluation of former ram and core drilling protocols. The samples were analyzed for the 16 EPA PAH and alkanes with GC‐MS and GC‐FID. 13C‐NMR spectroscopy was used to assess structural changes of the NAPL phase. Flow of bulk NAPL along macropores and along preferential permeability structures, like sedimentation discontinuities, are the dominant transport pathways which cause large lateral spreading beyond those expected by the relative permeability gradient. Accumulation of NAPL was found at locations with abrupt textural changes and within the zone of capillary rise. Aging of NAPL results in the depletion in soluble and volatile compounds but also in oxidation and polymerization. It increases the chemical diversity and decreases the mobility of the NAPL. Thus, NAPL flow ceases much earlier than expected from the capillary forces. As chemical transformation is restricted to the NAPL water/air interface, a skin‐like thin film is formed which encapsulates and preserves the bulk NAPL from further hardening, limiting contaminant mass transfer from the NAPL to the aqueous phase.

Single Event–Driven Export of Polycyclic Aromatic Hydrocarbons and Suspended Matter from Coal Tar–Contaminated Soil

“…Nonequilibrium release may arise from the formation of high‐viscous boundary layers as a result of aging, that is, the depletion in soluble and volatile compounds, the biochemical transformation and polymerization of NAPLs at the NAPL–air or NAPL–water interface. Rate limitations to the mass transfer of NAPL‐borne compounds are the consequence (Luthy et al, 1993; Nelson et al, 1996; Totsche et al, 2003; Ghoshal et al, 2004). Thus, Raoult's law may not be applicable for predicting aqueous concentrations of total PAHs (Mahjoub et al, 2000).…”

DOE-EMSP Final Report: Characterization of Changes in Colloid and DNAPL Affecting Surface Chemistry and Remediation

“…NMR spectroscopy is an analytical technique used to characterize and determine the structure of functional groups using common elements as hydrogen [Totsche, 2003]. In this study, NMR was used to determine the presence of acid functional group in the SRS DNAPL that would be detected by looking at the presence of hydrogen atom.…”

Combining steam injection with hydraulic fracturing for the in situ remediation of the unsaturated zone of a fractured soil polluted by jet fuel

“…Typically, the movement of NAPL is governed mainly by natural fractures that form hydraulic avenues through an otherwise low permeable porous matrix (Klint and Gravesen, 1999;D'Astous et al, 1989;McKay and Fredericia, 1995;Sidle et al, 1998;Broholm et al, 2000). After a long period of contamination, the NAPL may have accumulated inside the low permeability porous matrix, which has limited accessibility to the high permeability pathways (Totsche et al, 2003;Harrar et al, 2007). This situation becomes more complicated when dealing with highly heterogeneous soils (Rosqvist and Destouni, 2000;Reynolds and Kueper, 2001;Papafotiou et al, 2008;Tzovolou et al, 2009).…”