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 (Nash–Sutcliffe 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.
Highlights
It 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)
A close fit was achieved between the dual porosity model output and the original data, with a Nash– Sutcliffe model efficiency coefficient (NSMEC) of 0.97, 0.99 and 0.97 and a normalised root mean square error (NRMSE) of 5, 3, and 5 % recorded for D2O breakthrough data from core samples taken from 5.03, 9.52, and 21.75 m b.g.l., respectively
The D2O breakthrough curves for all core samples exhibited a relatively elongated shape, with 100 % breakthrough not recorded for any of the timescales tested. This was expected given that a “long tailing” is a common feature of dualporosity materials, i.e. systems where the mobile domain is coupled to a less mobile, or immobile, domain. In such instances the dominant solute transport mechanism during imposed flow in the mobile domain(s) is typically advection; solute exchange occurs in parallel with the immobile domain(s), typically via molecular diffusion
Summary
It 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). Central to this hypothesis is that the magnitude of the change in nonreactive tracer concentration in effluent samples taken immediately after a no-flow period is indicative of such nonequilibrium Subsequent work within this field has included determination of physical (e.g. diffusive mass transfer between advective and nonadvective water) and chemical (e.g. nonlinear sorption) nonequilibrium processes in soil (Brusseau et al, 1997), determination of nonreactive solute exchange between the matrix porosity and preferential flow paths in fractured shale (Reedy et al, 1996), quantifying the effect of aggregate radius on diffusive timescales in dual porosity media (Cote et al, 1999), numerical modelling of aqueous contaminant release in nonequilibrium flow conditions (Wehrer and Totsche, 2003), empirical modelling of the release of dissolved organic species (Guimont et al, 2005; Ma and Selim, 1996; Totsche et al, 2006; Wehrer and Totsche, 2005, 2009) and heavy metals (Buczko et al, 2004), increasing the efficiency of solute leaching (Cote et al, 2000), empirical modelling of conservative tracer transport in a laminated sandstone core sample (Bashar and Tellam, 2006), and characterising in situ aquifer heterogeneity (Gong et al, 2010). A novel dual domain model is described which has been used to guide physical interpretation of the experimental tracer breakthrough curves
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