Fossil rootlet biopores as conduits for contaminant transport through clay horizons: a case study of DNAPL behaviour in Severn alluvium, UK

Abstract

This paper presents a case study of several DNAPL contaminated sites around the Severn Estuary (UK) where a combination of detailed observations, core dissections and physicochemical characterisation of alluvial clay-silt horizons have revealed the presence of fossil rootlet biopores which act and have the potential to act as conduits for contaminant migration through up to 13 m of clay-silt. The biopores are shown to penetrate the lowpermeability (K * 10 -10 m/s) clay-silt matrix throughout its entire depth (up to 13 m) and provide a preferential transport pathway for DNAPLs from near surface to the underlying aquifer, with particularly high concentrations measured in the biopores themselves. Capillary rise experiments with coal tar distillate demonstrate that DNAPLs are drawn into biopores, with values of surface interfacial tension for the system calculated as 8 9 10 -2 J/ m 2 . Wicking by residual plant fibres was demonstrated qualitatively and is thought to be an important additional transport mechanism. The DNAPL contamination below and throughout the Severn alluvial clay-silt horizons demonstrates that the assumption that these horizons act as an effective seal protecting underlying aquifers from severe pollution from the legacy sites around the Severn is flawed and highlights the failure of current protocols for sampling of clay horizons for hydraulic conductivity assessments, since current methods can destroy delicate in situ biopore structures. The study demonstrates that the interplay between ecological and the geological depositional environment can, at a regional scale, result in a network of biopores that can potentially act as conduits for contaminant transport.

The Influence of Syndepositional Macropores on the Hydraulic Integrity of Thick Alluvial Clay Aquitards

“…Root remains were not found during preparation of these samples but were noted during the microscope examination of other samples from the cores at depths of 1.0, 1.5, 2.0, 3.2, 14.0, and 15 m. Shell material, characteristic of moluscs living in fresh muddy water, was also recovered throughout the column. To the authors' best knowledge, the only other published work on this topic are those by White et al () and Emanuel and Sapsford () where such syndepositional macropores were recorded to a depth of 2.0 m and 13 m, respectively.…”

“…In such instances, preferential flow pathways typically propagate from the surface (e.g., fracturing due to stress relief on unloading, dessication cracks, rootholes, biogenic channels) and are therefore mostly restricted to the top few meters of the aquitard (Klint & Gravesen, ; O'Hara et al, ). In contrast, there have been few investigations to date on the influence of syndepositional preferential flow pathways (i.e., paleorootholes, paleosols, tectonic fractures) on aquitard hydraulics (Emanuel & Sapsford, ; Jorgensen et al, ; Neuzil, ; White et al, ). In such scenarios, preferential flow pathways are formed during aquitard deposition (e.g., vegetation growth and associated root holes that are pervasive or provide channels) and are buried as the aquitard deposits accumulate.…”

Assessing Human Health Risk to DNAPLs Exposure in Bayesian Uncertainty Analysis

“…Modeling uncertainty has been well recognized in groundwater/hydrology community, the outputs of groundwater model could be unreliable due to biased model parameters and model structure. Especially, simulating DNAPLs transport in groundwater is even tougher because DNAPLs could accumulate and form a pool when they are hindered by an aquitard or a lense with low permeability (Emanuel and Sapsford, 2016). Moreover, the DNAPL pools would become the new sources of the contaminant, which makes the simulation of DNAPL transport more complicated (Kueper and Mason, 1996).…”