Technical Note: The use of an interrupted-flow centrifugation method to characterise preferential flow in low permeability media

Abstract

Abstract. We present an interrupted-flow centrifugation technique to characterise preferential flow in low permeability media. The method entails a minimum of three phases: centrifuge-induced flow, no flow and centrifuge-induced flow, which may be repeated several times in order to most effectively characterise multi-rate mass transfer behaviour. In addition, the method enables accurate simulation of relevant in situ total stress conditions during flow by selecting an appropriate centrifugal force. We demonstrate the utility of the technique for characterising the hydraulic properties of smectite-clay-dominated core samples. All core samples exhibited a non-Fickian tracer breakthrough (early tracer arrival), combined with a decrease in tracer concentration immediately after each period of interrupted flow. This is indicative of dual (or multi-)porosity behaviour, with solute migration predominately via advection during induced flow, and via molecular diffusion (between the preferential flow network(s) and the low hydraulic conductivity domain) during interrupted flow. Tracer breakthrough curves were simulated using a bespoke dual porosity model with excellent agreement between the data and model output (NashSutcliffe model efficiency coefficient was > 0.97 for all samples). In combination, interrupted-flow centrifuge experiments and dual porosity transport modelling are shown to be a powerful method to characterise preferential flow in low permeability media. IntroductionIt is well known that heterogeneities, including biogenic pores/channels, desiccation cracks, fissures, fractures, nonuniform particle size distributions and inter-aggregate pores, are widespread in the subsurface and lead to a range of preferential flow phenomena (Beven and Germann, 1982;Cuthbert et al., 2013;Cuthbert and Tindimugaya, 2010;Flury et al., 1994). The coexistence of a relatively high hydraulic conductivity (K) domain(s) and an impermeable one, often termed dual porosity, results in a non-Fickian breakthrough curve. Solute transport in such systems is often characterised by an early arrival of solutes originating from the more mobile domain (macropores) and a slow approach to the final concentration caused by diffusion into the immobile domain (matrix or microporous network). When fitting breakthrough curves, therefore, it is often difficult to differentiate between contributions from the micro-and macropore transport mechanisms. As a consequence, in recent years there has been much research into the development of effective empirical and modelling techniques to characterise solute transport processes for dual porosity systems. One method investigated has been the use of interruptedflow solute-breakthrough experiments. Amongst the original work on this topic Murali and Aylmore (1980) discussed the influence of nonconstant flow on solute transport in aggregated soil. Brusseau et al. (1989) developed a flowinterruption method for use in measuring rate-controlled sorption processes in soil systems, which was subsequently applied by Koc...

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

“…Visual dye tracing demonstrates that preferential flow pathways are present as submillimeter‐ to millimeter‐scale sinuous channels, and that relatively high vertical K occurs to a depth of at least 31.4 m BGL (i.e., 31.4 m was the maximum core sampling depth). These results are consistent with evidence for dual porosity flow from interrupted flow experiments and modeling (Crane et al, ). Based on the morphology and scale of these channels, they are considered to be biogenic in origin, for example, paleo‐rootholes.…”

“…The total porosity of water and gas‐filled pores identified by CT imaging at a resolution of 49 μm was 15% (Table ). By comparison, a total porosity of 43%–47% was measured, and a mobile domain (macroporosity) of 4%–8% was estimated from modeling, on cores from this site at a depth of 5.03, 9.52, and 21.75 m BGL (Crane et al, ). Sequential centrifuge permeability tests followed by CT imaging on additional cores are clearly warranted to verify the extent to which sample variability contributes to these results.…”

“…Macropores potentially compromise the integrity of aquitard because early breakthrough of contaminants could occur, relative to transport through a homogenous matrix that has a similar bulk hydraulic conductivity (Crane et al, ). Retardation due to diffusion of contaminants from the more mobile domain (macropores) to the less mobile domain (matrix or micropores) can be inferred from this work, and is commonly reported in literature (e.g., Jorgensen et al, ).…”

“…Investigations of a 31 m deep clayey silt paleovalley fill from Cattle Lane, Australia (Acworth et al, 2015), have revealed the potential for hydraulically active flow paths. Geotechnical centrifuge techniques (Crane et al, 2015;Timms et al, 2016) found that vertical K varied between 1:4310 28 and 1:1310 210 m/s with a geometric mean value of 4:2310 29 m/s (13 samples, 5-31.4 m BGL), values that were consistent with in situ vertical K values (Timms & Acworth, 2005). The vertical K values are high for an aquitard (as defined by K of 10 28 210 216 m/s; Neuzil, 1994) and have prompted more detailed investigation into the depositional properties and groundwater processes at this site.…”

“…Steady state K measurements of core subsamples taken from 5.0, 9.5, and 21.8m depth BGL (below ground level) were reported by Crane et al (2015) and Timms et al (2016). Details are provided on steps to ensure reliable K measurements of intact samples (Crane et al, 2015;Timms et al, 2016), primarily related to core storage, stress application and detection of rapid leakage. Potential errors were evaluated and minimized for example, by using relatively large diameter cores (101.6 mm) and application of stresses less than or equivalent to in situ stresses at the depth of core collection.…”