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

Abstract

Clay‐rich deposits are commonly assumed to be aquitards which act as natural hydraulic barriers due to their low hydraulic connectivity. Postdepositional weathering processes are known to increase the permeability of aquitards in the near surface but not impact on deeper parts of relatively thick formations. However, syndepositional processes affecting the hydraulic properties of aquitards have previously received little attention in the literature. Here, we analyze a 31 m deep sediment core recovered from an inland clay‐rich sedimentary sequence using a combination of techniques including particle size distribution and microscopy, centrifuge dye tracer testing and micro X‐ray CT imaging. Subaerial deposition of soils within these fine grained alluvial deposits has led to the preservation of considerable macropores (root channels or animal burrows). Connected pores and macropores thus account for vertical hydraulic conductivity (K) of 4.2×10−9 m/s (geometric mean of 13 samples) throughout the thick aquitard, compared to a matrix K that is likely < 10−10 m/s, the minimum K value that was measured. Our testing demonstrates that such syndepositional features may compromise the hydraulic integrity of what otherwise appears to have the characteristics of a much lower permeability aquitard. Heterogeneity within a clay‐rich matrix could also enhance vertical connectivity, as indicated by digital analysis of pore morphology in CT images. We highlight that the paleo‐environment under which the sediment was deposited must be considered when aquitards are investigated as potential natural hydraulic barriers and illustrate the value of combining multiple investigation techniques for characterizing clay‐rich deposits.

In-situ estimation of subsurface hydro-geomechanical properties using the groundwater response to Earth and atmospheric tides

“…Further studies at this site include Acworth et al (2016) and Acworth et al (2017), which were precursors to Rau et al (2018) in the investigation of pore pressure response to atmospheric tides, and Timms et al (2018) on a core scale analysis of the site's laterally extensive and thick aquitard. Due to the sufficient M 2 response, time-series data of groundwater pressure heads measured between the 21/01/2016 and 14/04/2018 were used from the bore BH30061 which is located at latitude −31.518340 • , longitude 150.468332 • and an elevation of 313 MASL (WGS84).…”

Immiscible fluid displacement in porous media with spatially correlated particle sizes

“…For example, it has been shown that structural heterogeneity can impact water infiltration in soils and the emergence of preferential flow [59], the development of unsaturated zones at the river-aquifer interface [58], and the distribution of saline and fresh water within the continental shelf [40]. The occurrence of structural heterogeneity, in the form of high permeability zones within aquitards, can also compromise their ability to act as barriers for water and contaminant migration [60]. In addition, the contrast in permeability between different geological layers may control the mechanisms for attenuation of CO 2 , when considering leakage from an underground reservoir used for carbon capture and storage [43,51,32].…”

Learning Groundwater Contaminant Diffusion‐Sorption Processes With a Finite Volume Neural Network

“…For the sake of demonstration, we focus here on diffusion and sorption of groundwater contaminant in fully saturated clay. This process is relevant, for example, in clay liners of landfills (Hendry et al., 2003; Timms et al., 2018) or in long‐term tailing of groundwater pollution (Huang & Goltz, 2015; Johnson et al., 2003).…”