numerical analysis of water and solute transport in variably-saturated fractured clayey till

$: Numerical analysis of water and solute transport in variably-saturated fractured clayey till$
Journal of Contaminant Hydrology 104 (2009) 137–152

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Journal of Contaminant Hydrology

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Numerical analysis of water and solute transport in variably-saturatedfractured clayey till

Annette E. Rosenbom a,⁎, Rene Therrien b, Jens Christian Refsgaard a, Karsten H. Jensen c,Vibeke Ernstsen a, Knud Erik S. Klint a

a The Geological Survey of Denmark and Greenland (GEUS), Copenhagen K, Denmarkb Département de Géologie et de Génie Géologique, Université Laval, Québec, Canadac Department of Geography & Geology, University of Copenhagen, Copenhagen K, Denmark

a r t i c l e i n f o

⁎ Corresponding author. GEUS, Øster Voldgade 10Denmark. Tel.: +45 38142934; fax: +45 38142050.

E-mail address: [email protected] (A.E. Rosenbom).

0169-7722/$ – see front matter © 2008 Elsevier B.V.doi:10.1016/j.jconhyd.2008.09.001

a b s t r a c t

Article history:Received 22 January 2008Revised 8 June 2008Accepted 6 September 2008Available online 19 September 2008

This study numerically investigates the influence of initial water content and rain intensities onthe preferential migration of two fluorescent tracers, Acid Yellow 7 (AY7) and Sulforhodamine B(SB), through variably-saturated fractured clayey till. The simulations are based on thenumerical model HydroGeoSphere, which solves 3D variably-saturated flow and solutetransport in discretely-fractured porous media. Using detailed knowledge of the matrix,fracture, and biopore properties, the numerical model is calibrated and validated againstexperimental high-resolution tracer images/data collected under dry and wet soil conditionsand for three different rain events. The model could reproduce reasonably well the observedpreferential migration of AY7 and SB through the fractured till, although it did not capture theexact depth of migration and the negligible impact of the dead-end biopores in a near-saturatedmatrix. A sensitivity analysis suggests fast flowmechanisms and dynamic surface coating in thebiopores, and the presence of a plough pan in the till.

© 2008 Elsevier B.V. All rights reserved.

Keywords:HydroGeoSpherePreferential flowFluorescence tracerFractureBioporeClayey till

1. Introduction

In several regions of the northern hemisphere, extensiveareas of the subsurface consist of low-permeability clayey tillsoverlying aquifers that are used for water supply. Disconti-nuities, such as fractures, macropores and channel-likeopenings (biopores), have been observed in these clayey tills(for example, Fredericia,1990). These discontinuities can formpotential pathways for rapid downward transport of con-taminants from the surface to underlying aquifers, and anyassessment of potential contaminant migration throughclayey tills must therefore account for discontinuities.

Water and solute migration in variably-saturated clayeytills that contain discontinuities is still not completelyunderstood, in part because it is difficult to measure waterand solute exchange between the clay matrix and the

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discontinuities. It has often been assumed that waterinfiltration in these clayey tills is controlled by the high-capillary suction of the fine-grained clay matrix. As waterinfiltrates from the ground surface into the discontinuities,the high-capillary suction forces water imbibition into theclayey matrix, which reduces gravity-dominated downwardflow in the discontinuities. Some studies have shown,however, that a low-permeability mineralized layer, or coat-ing, often exists at the interface between the matrix and thediscontinuities. Such a layer reduces matrix imbibition andthus promotes discontinuities-dominated flow as opposed tomatrix-dominated flow (Thoma et al., 1992). Other studies offluid flow in variably-saturated fractured media have docu-mented film flow, fingering, and intermittent flow in thediscontinuities, indicating that fluid flow in a combinedmatrix and discontinuity porous medium is more complexthan previously assumed (Phillips et al., 1989; Tokunaga et al.,2000; Tofteng et al., 2002; Gjettermann et al., 2004). Visualobservation of discontinuities in a clayey till is not sufficient toassess their hydraulic behaviour, since discontinuities might

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138 A.E. Rosenbom et al. / Journal of Contaminant Hydrology 104 (2009) 137–152

not all be interconnected and hydraulically active. It istherefore essential to also quantify their hydraulic behavioureither from hydraulic or tracer tests (Rosenbom and Jakobsen,2005).

Soil water content and rain intensity have been shown toinfluence water movement and solute transport in disconti-nuities. Shipitalo and Edwards (1996) reported that therelative contribution of macropores to chemical transportand water movement appears to be greatest when the soil isdry, and that it decreases as the soil becomes wetter. Thehydrophobicity of surface litter and soils combined with theopening of cracks might explain the greater contribution ofmacropores under dry conditions. In contrast, Flury et al.(1994, 1995) found that, for well-structured soils, water andsolutes tend to move deeper under wetter than dryerconditions, indicating that macropores play a greater rolefor wet conditions. It has been widely documented also thathigh intensity rainfalls generally yield greater downwardwater flow and solute transport in macropores compared tolow intensity rainfalls (e.g., Trojan and Linden, 1992).Retardation in the soil matrix can, on the other hand, slowdown solute migration in macropores, especially if there issufficient time for retardation to be effective.

Risk assessment analyses for flow and transport infractured clayey till require a defensible conceptual modelbased on detailed field-scale characterization of three dif-ferent porous medium domains: the matrix and two types ofdiscontinuities, the fractures and the biopores (Simunek et al.,2003). The conceptualmodel should account for all significantflow and transport processes in the three domains. Thefollowing processes are potentially the most important tosimulate: (a) advection in all three domains; (b) diffusion intothe clayey matrix adjacent to the fractures and the biopores,and the water and solute interactions between all domains;and (c) retardation processes (sorption, desorption, precipita-tion and biodegradation) in the three domains (McKay et al.,1993). Current conceptual flow and transport models forfractured clayey tills include the equivalent porous medium,the dual continuum, the discrete fracture and the continuoustime random walk approaches (see for example, Sidle et al.,1998; Villholth and Jensen, 1998; Kosakowski et al., 2000;Jørgensen et al., 2002, 2004a,b,c; Mortensen et al., 2004). Theconceptual model is usually selected based on the end resultsthat are required, the geometry and scale of the fracturedporous media, the field data available and some practicallimitations such as availability of computational resources(Samardzioska and Popov, 2005). For a well-defined geome-try-based conceptual model in three dimensions, and when alarge amount of data exist on the different porous mediumdomains, which is the case in this study, a discrete fracturemodel appears to be the most appropriate (Jørgensen et al.,2004a,b,c; Samardzioska and Popov, 2005).

The objectives of this study are to perform a detailedinterpretation of the fluorescent tracer experiments in clayeytills presented in Rosenbom et al. (2008), using the three-dimensional discrete fracture model HydroGeoSphere (Ther-rien et al., 2004). The tracer experiments were performedunder both wet and dry conditions, to investigate differentrain events and initial water contents. High-resolution imagesof the fluorescent tracers were taken in the field to obtainquantitative distributions of tracer concentrations in the clay

matrix, the fractures and the biopores. In connectionwith thetracer experiments, detailed data were collected on thegeometry and hydraulic properties of the till matrix, thefractures and the biopores, as well as the tracer character-istics. Based on that data, a three-dimensional conceptualmodel was built into the HydroGeoSphere model to simulatevariably-saturated flow and solute transport in the clayey till.In the numerical analysis of the tracer experiments, emphasisis on understanding the different tracer migration pathwaysfor various initial water content, rain intensities, tracercharacteristics, geometry of the discontinuities, and hydraulicproperties. This study contrasts with those listed previously,which have considered either fully-saturated conditions orexcluded detailed information on the hydraulic behaviour ofdiscontinuities, especially the interaction between bioporesand fractures.

2. Site description

The field experiments were performed at Gjorslev, Den-mark, where the geology consists of a regional weaklyundulating till plain, with 7–15 m of clayey till overlyingfractured whitish Danien bryozoan limestone, which is theprimary groundwater reservoir in the region. The field site islocated in an agricultural area and contains parallel tile drains.The drains are installed at a depth of 1.2 m below surface(hereafter referred to as BS) and their spacing is uniform andequal to 20 m. The piezometric head in the limestone aquiferis approximately 4 m BS and the water table elevation in thetill fluctuates between approximately 4 m BS in the summerto the tile drain level located at 1.2 m BS in the fall and winter.A 5 meter deep pit was excavated at the site to observe andclassify macropores in the till.

2.1. Geological description

The till can be divided into three zones according to thedegree of weathering. Zone 1 is the weathered and oxidizedportion of the till located in the upper 1.4 m below groundsurface, zone 2 is the unweathered and oxidized portionlocated between 1.4 m and 4.5 m below ground surface, andzone 3 is the unweathered and reduced zone at depths greaterthan 4.5 m. Zone 1 contains, from the surface downward,about 30 cm of dark grayish-brown organic rich (TOC: 0.97%C), non-calcareous (pHCaCl2: 6.6–6.9), oxidized clayey andhighly-porous top soil (USDA texture: loam), followed by abrownish, massive, strongly-sandy, non-calcareous (pHCaCl2:6.6–6.9) clayey till (USDA texture: loam; 16–20% clay). Zone 2is an olive-brown, calcareous (pHCaCl2: 7.4 to 7.5) clayey till(19–20% clay) containingmultiple lenses of sheared chalk andsome stones. Zone 3 is an olive-gray reduced, silty, clayey till(USDA texture: 16–20% clay) that is strongly calcareous (18%CaCO3). The till composition in zone 3 indicates reducingconditions.

At a depth of 1 m, the most dominant clay mineral issmectite, followed by a significant amount of illite–smectiteand smaller amounts of vermiculite, illite and koalinite. Atdepths of 3 and 7 m, the smectite content decreases by about50% and is approximately equal to the amount of illite–smectite and illite (Ernstsen and Rosenberg, 2004). Thetransport of fine particles and a dynamic clay-coating of

139A.E. Rosenbom et al. / Journal of Contaminant Hydrology 104 (2009) 137–152

smectite, which has swelling properties, is expected on themacropore walls.

A fabric analysis of the till, performed at depths of 2 m and3.5m BS (Krüger, 1994), shows a preferred SE–NWorientationof elongated clasts (Fig. 1). Glacial strikes on boulders andstones have a similar preferred SE–NWorientation. Combinedwith kinetostratigraphical investigations of folded andsheared chalk lenses (Berthelsen, 1978), the fabric analysisindicates that the till can be classified as a basal till of type B(Benn and Evans, 1996; Klint, 2001) implying a systematicallyfractured or faulted massive matrix. The till has been de-posited when a glacier transgressed the area from the SE,which corresponds to the Young-Baltic Ice-advance that

Fig. 1. Lithological log and fracture distribution at the Gjorslev site

occurred approximately 16,000 years ago (Houmark-Nielsen,1987). The orientation of the fractures as well as thestreamlining of the till plain indicate that the direction ofice movement changed from SE–NW to E–W during the laststage of the ice-advance. Additional information on thelithology and till characteristics can be found in Rosenbomet al. (2008).

2.2. Classification of the macropores and fractures

Macropores observed in the till include root holes andwormholes, or burrows. Because of their origin, macroporesare therefore also referred to as biopores in the text. The

. Facies description is according to Krüger and Kjær (1999).

Table 1Measured and simulated water content initially and after 100 min irrigationat 45, 70, 95 and 120 cm BS (Rosenbom, in press) for the fall/summer tests

Season, depth Average water content [%]

Measured Simulated

Initial After 100 minirrigation

Initial After 100 minirrigation

Fall, 45 and 70 cm BS 0.30 0.36 0.31 0.35Summer, 45 and 70 cm BS 0.25 0.27 0.26 0.27Fall, 95 and 120 cm BS 0.36 0.36 0.36 0.36Summer, 95 and 120 cm BS 0.31 0.31 with

pulses of ∼5%0.30 0.30

For the fall and summer tests, the water content is represented by averagevalues given the similarity between the water content measurements of the100 min rain event with return periods of 1 year, 5 years and 20 years.


upper 0.3 m of organic rich topsoil is strongly bioturbated(100–1000 biopores per m2). At depths between 0.3 m and1.4 m, biopores with a diameter of 3 to 6 mm are abundantand their observed number ranges between 100 and4000 biopores/m2. Among these biopores, wormholes arecoated with clay and organic matter.

The till contains desiccation fractures and fractures oftectonic origin. Desiccation fractures are located in the upper2 m of the till. They are vertical, they have a randomorientation and their number decreases with depth. Thetectonic fractures are of glacial origin and are observed atdepths greater than 1.4 m (Fig. 1). They are divided in twogroups. Group 1 consists of subvertical fractures with aNorth–South strike. This group contains two conjugatedfracture sets, with one set dipping 70–80° towards the Eastand the other set being approximately vertical. Their averagespacing is 10 cm at a depth of 2 m and it increases with depth,from 50 cm at a depth of 4 m to 1.5 m at depth of 5 m. Thesefractures are distributed throughout the area and theirorientation indicates that they were developed as conjugatingshear fractures during the retreat of the Young-Baltic ice-advance, when the ice movement direction rotated from SE–NW to E–W as indicated by fracture orientation andgeomorphological evidence. Because the group 1 fractureswere formed by glaciotectonic loading and shearing, they canbe open for fluid flow. Group 2 consists of sub-horizontalfractures whose spacing ranges between 1 cm and 4 cm atdepths between 1.8 m and 3.5 m, increases to 4–10 cm atdepths between 3.5 m and 4 m, and is non-existing at depthsgreater than 4 m. The group 2 fractures are classified as shearfractures formed as a result of shear movement in the de-forming bed under the sole of a glacier. Because of their modeof formation, they are less likely to be open than fractures ofgroup 1.

3. Field-scale tracer experiment

Two tracers, Sulforhodamine B (SB) and Acid Yellow 7(AY7), were added to irrigationwater applied over three plotshaving each an area equal to 2 m×2 m and located betweentwo parallel drains, with a distance of 10 m to each drain. Theupper 20 cm of organic rich topsoil was removed at each plotto focus on the properties of the underlying clayey till.Irrigationwater was therefore applied at a depth of 0.2 cm BS.The irrigation rate varied for each plot and corresponded torain intensities of 14 mm/100 min, 27 mm/100 min, and36 mm/100 min, respectively. Concentrations of the twotracers in the irrigation water were CAY7=10 g L−1 andCSB=0.12 g L−1. Two series of tests were conducted. The firsttests were during a relatively wet season, in the fall, when thetemperature was around 10 °C and the water table was high.The second series of tests were during a dry season, insummer, when the average temperature was around 20 °Cand the water table was low. TDR probes were used to mea-sure water contents at depths of 25 cm, 50 cm, 75 cm and100 cm below the irrigated surface (Table 1). Water contentswere measured at two different times, prior to irrigation and100 min after beginning irrigation. Approximately 12 h afterstopping irrigation, each plot was excavated to take images ofthe fluorescent tracer distribution and to map discontinuities(biopores and fractures) on a vertical face of the excavation.

The vertical face was then carefully scraped and tracerimaging and fracture mapping were repeated on the newlyexposed vertical face. The procedure was repeated severaltimes over 7 days to produce tracer images and fracture mapsfor a series of parallel vertical soil profiles. More details on thetracer experiments can be found in Rosenbom et al. (2008).

3.1. Tracer characteristics

Using the method originally developed by Hayduk andLaudie (Tucker and Nelken, 1990), the aqueous moleculardiffusion coefficient for the fluorescent tracers SB and AY7 havebeen determined to be 2.2·10−6 cm2 s−1 and 2.9·10−6 cm2 s−1,respectively. Neither tracer is expected to be significantlyaffected by sorption, precipitation and biodegradation. On theotherhand, the ionic charge of the tracers seems tohave amajorimpact on their migration pattern (Rosenbom et al., 2008), asdiscussed below.

3.2. Experimental results

Apparent tracer concentrations for AY7 and SB wereobtained by digitally processing the tracer images taken in theexcavation, assumingafluorescence detectiondepth of 1mminthe third dimension, perpendicular to the vertical soil profile,and a spatial resolution of 1 mm2. From the apparent con-centrations along vertical faces, the vertical migration patternsfor the two tracers have been schematized in Fig. 2, whichshows that: (a) in the upper 1.4 m, vertical migration of bothtracers occurs primarily in the biopores, (b) below1.4m, tracersare observed only in fractures connected to biopores that arehydraulically active and contain tracers, (c) as opposed tosummer, no tracerhasmigrated indead-endbiopores in the fall,(d) in contrast to SB, AY7 undergoes piston-type migration inthe till matrix in summer, down to a depth of 20 cm below theirrigation level, (e) the maximum migration depths observedfor SB and AY7 are equal to 1.7m BS and 3mBS, respectively. Inaddition to the observations shown in Fig. 2, tracer imagesreveal that the highest apparent tracer concentrations are oftenfound in macropores, particularly in summer.

4. The HydroGeoSphere model

The numerical model HydroGeoSphere is used for thesimulations presented here (Therrien et al., 2004). It is a

Fig. 2. Conceptual model for the migration pattern of AY7 and SB in the clayey till at the field site after 100 min of irrigation, for both summer and fall experiments.


three-dimensional control-volume finite element model thatsimulates variably-saturated subsurface flow and advective–dispersive mass transport in discretely-fractured or non-fractured porous media. The model simulates flow andtransport in three dimensions in the porous medium and intwo dimensions in fractures. The model can also simulatetwo-dimensional overland flowcoupledwith subsurface flow,but overland flow has not been simulated here.

Variably-saturated flow is described by a modified form ofRichards' equation, where the storage term is expanded toconsider water and soil compressibility (Therrien andSudicky, 1996). Fractures are idealized as two-dimensionalparallel plates, implying uniform total head and concentra-tion across the fracture width, and flow velocities aredetermined by the cubic law (Witherspoon et al., 1980).Retention and relative permeability curves for both thefractures and the matrix are given either by van Genuchten's(1980) functions or they are specified in a tabular form, buthysteresis is not considered. In themodel, the porousmediumis discretized with 3D finite elements and fractures are

discretizedwith 2D finite elements. The nodes forming the 2Dfracture elements are common with nodes forming the 3Dporous medium. It is assumed that there is continuity ofhydraulic head and concentration in the fracture and matrixat these common nodes, which corresponds to instantaneousfluid and solute exchange between the domains. For solutetransport, the model assumes linear equilibrium sorption thatis independent of the sorption capacity of the medium, theflow velocity, or the solute residence time. Sorption isdescribed by a retardation factor for the fracture (Rf) and forthe matrix (R), respectively. The effective diffusion coefficientfor solutes in the matrix is given by the product of the freewater diffusion coefficient and the tortuosity, and it varieswith the water saturation according to the Millington–Quirkrelationship (Millington and Quirk, 1961). Mechanical disper-sion in the fractures and the matrix is described by long-itudinal and transverse dispersivities. For transversedispersivity in the 3D porous medium, HydroGeoSphereaccounts for a horizontal and a vertical component. The twocomponents are identical if one assumes that the clay matrix


is isotropic and homogeneous and that the primary flowdirection is vertical.

5. Conceptual model

The domain considered for the simulations of the tracerexperiments extends vertically from the top of the irrigationzone, located at a depth of 0.2 m, to a depth of 3.0 m. Theupper 0.2 m of the soil profile that was removed prior to thefield experiments is not part of the conceptual model andsimulation domain. Variably-saturated flow and transport areexamined for three different boundary conditions, corre-sponding to the three different rain intensities applied in thefield, and for two different initial distributions of watercontent representing the dry and wet seasons, respectively. Inaddition to the till matrix, discontinuities identified andmapped in the field (Rosenbom et al., 2008) have beenincorporated in the numerical model. From the near surfacedown to a depth of 1.4 m, discontinuities are classified asbiopores and consist mostly of wormholes. Discontinuitiesbelow 1.4 m are classified here as fractures. Three differentsubsurface domains are therefore represented in the simula-tions: the till matrix, the biopores, and the fractures. Thesethree domains are further described below.

5.1. Matrix properties

The upper section of the variably-saturated till unit isdivided in zones 1 and 2 defined previously. Zone 1 is locatedat depths between 0.2 m and 1.4 m BS and zone 2 extendsfrom 1.4 m to 3 m BS. Zone 3 corresponds to the deeper

Table 2Model input parameters used in HydroGeoSphere simulations

Media Parameter Value

Matrix Zone 1 Saturated conductivity, Ks [cm s−1] 5.4·10−4

Porosity, θs[cm3 cm−3] 0.36Residual saturation, Sr [cm3 cm−3] 0.08van Genuchten parameter, α [1/cm] 0.00698van Genuchten parameter, n[–] 2.0Longitudinal dispersivity, αl [cm] 5.0Transverse dispersivity, αt [cm] 1.0Tortuosity, τ [–] 0.71Bulk density, ρ [kg cm−3] 0.002

Matrix Zone 2 Saturated conductivity, Ks [cm s−1] 1.8·10−6

Porosity, θs[cm3 cm−3] 0.31Residual saturation, Sr [cm3 cm−3] 0.07van Genuchten parameter, α [1/cm] 0.00293van Genuchten parameter, n[–] 1.07442Longitudinal dispersivity, αl [cm] 10.0Transverse dispersivity, αt [cm] 1.0Tortuosity, τ [–] 0.68Bulk density, ρ [kg cm−3] 0.002

Tectonic fractures Aperture, rc [cm] 0.01Residual saturation, Sr [cm3 cm−3] 0.01van Genuchten parameter, α [cm−1] 0.04687van Genuchten parameter, n[–] 2.29719Longitudinal dispersivity, αl [cm] 10.0Transverse dispersivity, αt [cm] 0.1

Biopores Aperture, rc [cm] 0.3Residual saturation, Sr [cm3 cm−3] 0.01van Genuchten parameter, α [cm−1] 0.1van Genuchten parameter, n[–] 2.0Longitudinal dispersivity, αl [cm] 10.0Transverse dispersivity, αt [cm] 0.1

portion of the till and wasn't considered in the simulations. Tomeasure the hydraulic properties of the till, undisturbed coresamples were collected at depths of approximately 90 cm forzone 1 and 215 cm BS for zone 2 (Jacobsen and Iversen, 2004).For each zone, drainage curves were measured on five100 cm3 cores (height 3.4 cm, inner diameter 6.1 cm), usinga procedure described by Schjønning (1985), and bulksaturated hydraulic conductivities were measured on threelarger vertically-oriented cores, of height equal to 20 cm andwith an inner diameter of 20 cm, using the proceduredescribed by Klute and Dirksen (1986). The bulk saturatedhydraulic conductivity of the till matrix in zone 1 wasestimated from additional measurements on three largerhorizontal cores that did not consider the macropores.

From the experimental drainage curve data, van Genuch-ten's parameters α and n were estimated using van Genuch-ten's m=1−1/n retention curve model in the software RETC(van Genuchten et al., 1991), assuming that the saturatedwater content (θs) is equal to porosity and the residual watercontent (θr) is equal to 0.08 (Jacobsen and Iversen, 2004). Thevan Genuchten–Mualem model was then used to predict therelative permeability of the till matrix. All hydraulic andtransport properties for the till matrix are listed in Table 2.

5.2. Biopore properties

The biopores in the upper 1.4 m of the till unit areprimarily vertical wormholes, where most of the tracermigrated in the field. The biopores are represented in themodel by vertical 2D fracture planes with an aperture of0.3 cm, which is their typical diameter. Although representing

Source

Jacobsen and Iversen (2004).Jacobsen and Iversen (2004).Jacobsen and Iversen (2004).Estimated from drainage curves presented in Jacobsen and Iversen (2004).Estimated from drainage curves presented in Jacobsen and Iversen (2004).Estimated.Jørgensen et al. (1998).Estimated from θsJacobsen and Iversen (2004).Jacobsen and Iversen (2004).Jacobsen and Iversen (2004).Jacobsen and Iversen (2004).Estimated from drainage curves presented in Jacobsen and Iversen (2004).Estimated from drainage curves presented in Jacobsen and Iversen (2004).Jørgensen et al. (1998).Jørgensen et al. (1998).Estimated from θsJacobsen and Iversen (2004).Estimated from SEM-image.Assumed.Estimated from SEM-image.Estimated from SEM-image.Jørgensen et al. (1998).Assumed.Estimated from SEM-image.Assumed.Gerke and van Genuchten (1993).Gerke and van Genuchten (1993).Assumed.Assumed.

Fig. 4. SEM-image of a 1st order fracture zone in a clayey till unit. White togrey areas represent matrix aggregates. Fractures appear as elongated zonesof multiple channels/tubes (pores) (Rosenbom et al., 2001).


biopores by fracture planes approximates the cylindricalgeometry of the biopores, it is assumed here that thisapproximation does not introduce significant errors in theconceptual model. Since the biopores are represented byfracture planes, their saturated hydraulic conductivity iscalculated as Ks,biopore= rc2ρg / (12μ)=654 cm/s, using a uniformaperture rc=0.3 cm. This relationship for the saturatedhydraulic conductivity applies to parallel plates of aperturerc and it underestimates by a factor of 1.5 the saturatedhydraulic conductivity of a cylinder of diameter equal to rc(Bear, 1972).

The retention curve and relative permeability of thebiopores are not known but it is assumed that they can bedescribed by van Genuchten–Mualem's relationships. Thevalues of van Genuchten's parameters α and n suggested byGerke and van Genuchten (1993) for macropores were used asinitial estimates for the biopores (Table 2). The retentioncurve computed with these values shows a sharp decrease insaturationwhen the pressure head decreases from −0.5 cm to−1.0 cm, which is similar to the capillary pressure character-istics of a capillary tube with a diameter of 3–6 mm and alsoconsistent with the fact that biopores also consist of acombination of micropores on the periphery of a wormhole(Fig. 3).

5.3. Fracture properties

Because they cannot be isolated from the till matrix, thehydraulic properties of the desiccation fractures located in theupper 1.4 m are embedded in the till matrix measurementsfor zone 1. The desiccation fractures are therefore notexplicitly represented in the model but instead lumped withthe matrix for this zone by treating the till and fractures as acomposite equivalent continuum.

Fig. 3. SEM-image of a wormhole in a clayey till unit. Dark grey to black areasrepresent pore space, whereas white to grey areas mostly represent matrixaggregates, as well as impregnation material in the centre of the large pore.

Fractures that are explicitly considered in the model aretectonic fractures located below 1.4 m BS. Their spacing isapproximately equal to 10 cm and they dip at 90°. Theaperture, retention characteristics, and relative permeabilitycurve for the tectonic fractures were not directly measuredbut were estimated from high-resolution images takenwith ascanning electron microscope (SEM) of thin slices of resinimpregnated with intact samples of the fractured clayey till(Fig. 4). A 0.5 cm long fracture is shown in Fig. 4. Its geometryand aperture are irregular, which suggests that the parallelplate assumption for that fracture is approximate. To estimateits retention curve, the irregular geometry of the fractureshown in Fig. 4 is filled by a series of circles of variablediameter, thus representing the fracture as a series ofcylindrical pores, or capillary tubes. A variable distributionof circle diameters is obtained and is assumed to representthe pore size variation of the fracture plane. The capillarypressure for each class of circles, expressed as the height ofwater, was then estimated with Purcell's equation (Purcell,1949) and a retention curve was calculated for the fracturegiven the distribution of cylindrical pores. Based on theretention curve data, van Genuchten's parameter α and nwere estimated using van Genuchten's m=1−1/n retentioncurve model in the program RETC, with θsaturated and θresidualassumed equal to 0.99 and 0.01, respectively (Gerke and vanGenuchten, 1993). The average aperture of tectonic fractures,also estimated from SEM-images, is approximately equal to100 μm, and their saturated hydraulic conductivity is given byKs,biopore=rc2ρg / (12μ)=0.73 cm/s, using the average aperturevalue of 0.01 cm is used for rc.

5.4. Numerical modelling

The simulation domain is a 2D soil column oriented in thexz-plane. The domain length is equal to 280 cm in the verticalz-direction, with the top of the column located at 20 cm BS(z=280 cm) and the bottom of the column located at 3 m BS


(z=0 m). The width of the column along the x-direction is70 cm, which is the fluorescence image width. In the z-direction, the nodal spacing is 5 cm except at the terminationof biopores, where the grid is refined over a total height of5 cm below and above the termination of a biopore, withnodal spacing ranging from 0.1 to 5 cmwith a multiplicationfactor of 2. In the x-direction, the grid is refined over a widthof 0.3 cm, representing the maximum radius of the biopores,on both sides of the vertical biopore planes. As the distance tothe biopore plane increases, the nodal spacing graduallyincreases from 0.01 cm to 0.1 cm, with a multiplication factorof 1.5. A nodal spacing of 0.2 cm is used between these refinedbiopore grid zones.

The numerical model uses three-dimensional elementsand a length equal to 0.3 m is specified along the y-axis,which corresponds to the average diameter of the biopores. Asingle column of elements is used in the y-direction, whichmakes the simulation two-dimensional. Because of the shorttimeframe of the experiment, it is assumed that representingthe column in two dimensions and neglecting flow andtransport in the third dimension still captures the main flowand transport processes.

The initial and boundary conditions for the flow simula-tions were defined from water contents measured by TDRprobes and from thewater table locations (Table 1). The initialconditions were assumed to represent steady-state condi-tions with a constant vertical flux prevailing throughout thedomain. The location of the water table was used to specifythe lower boundary condition. In summer, the typical watertable depth is about 300–400 cm, which corresponds to apressure head of approximately −1 cm of water at the lowermodel boundary. The typical water table depth in the fall was100 cm, corresponding to a pressure head of 200 cm at thelower model boundary. At the upper boundary, a first-typeboundary condition was used, with specified pressure headssuch that the resulting steady-state water content profilesclosely matched the measurements (Table 1).

For each season, the field experiments were simulated byusing the computed steady-state hydraulic heads relevant tothe season as initial hydraulic heads, and by imposing a fluidflux condition at the upper boundary that corresponds to theapplied rainfall rate. The flux values used for the different rainintensities are q1 year=2.4·10−4 cm/s, q5 year=4.5·10−4 cm/s,and q20 year=6.0·10−4 cm/s.

The total simulation period was 7 days. Adaptive timestepping was used with an initial time step size of 0.01 s, amaximum time step multiplier of 1.1 and concentrationcontrol of 0.1 (Therrien et al., 2004).

6. Field-scale simulations of tracer migration

6.1. Calibration

The model was calibrated by simulating the summerexperiment with the one-year rain event, using trial-and-error adjustment of a few parameters. For that experiment,both tracers migrated into the biopore network, even thoughrelatively high suctions prevailed in the matrix and the rainintensity was rather low. This experiment therefore portrayedmost explicitly the complexity of flowand transport at the siteand was thus considered well-suited to calibrate the model.

The model input includes the parameter values listed inTable 2, themapped geometrical structure of the biopores andthe tectonic fractures, and the free solute diffusion coefficient,Dfree, for the tracers. For the two tracers, the retardationfactors in the matrix, R, and in the fracture/biopores, Rf, wereselected as calibration parameters. We also intended duringcalibration to modify the van Genuchten parameters for thebiopores, to match the measured saturation distributionof the summer study both initially and after 100 min ofirrigation. However, the simulated water content resultsagreed with measurements (Table 1) using the original valuesfor the initial van Genuchten parameters, and these para-meters were left unchanged. The simulated water saturationand pressure head, before and after 100 min of irrigation(Fig. 5a–b), indicate that, even for high suction values in thematrix, water entered the biopores. Because of its highersaturated hydraulic conductivity, the velocity in a biopore willbe much larger than the velocity in a matrix pore of similarsize.

The target for calibration was to capture the overall tracermigration pattern observed in the field (Fig. 2), in particularthe migration depth of the tracers after 7 days, which wasapproximately 3 m BS and 1.7 m BS for AY7 and SB,respectively. The overall observed tracer migration patternof the two tracers were reproduced for the simulationwithoutretardation in the biopores/fractures (Rf =1) and for matrixretardation with specified distribution coefficients KAY7′ =0.01 cm3/g and KSB′ =1.00 cm3/g, which corresponds toretardation factors Rm=1.06 and Rm=6.56 for AY7 and SB,respectively. These retardation factors agree with the findingsof Rosenbom et al. (2008), given that dynamic coating and theexistence of a fast flow mechanism such as film flow in thebiopores are not accounted for in the simulation. Comparisonof the measured apparent and simulated concentrations(Fig. 5c–d) indicates that the model reproduced the overalltracer migration pattern in the biopores. The model alsocorrectly simulated tracer AY7 entering the upper 20 cm ofthe matrix, and reproduced the limited migration of SB in thematrix. These simulation results are coherent with highermatrix retardation for SB compared to AY7, which leads to agreater mass of AY7 migrating into the active biopores/fractures system compared to SB. Themodel did not, however,simulate the deeper migration of tracer into the tectonicfractures, which is shown in the plots of apparent concentra-tion, especially for AY7 (Fig. 5c–d).

Direct comparison of the apparent concentration imagesand simulated concentrations remains approximate because(a) apparent concentrations are estimated on a two-dimen-sional vertical plane (Fig. 5c–d) but the tracer concentration inthe third dimension is unknown because the depth ofpenetration of the fluorescence detection is unknown,(b) the 3D-distribution of hydraulic-active discontinuities inthe irrigated till unit is unknown, which in turn implies thatspreading of tracer on the irrigated surface is unknown, and(c) the resolution of the measured apparent and simulatedimages is different. For further comparison of measured andsimulated tracer migration, vertical mass plots were extractedfor the tracers (Fig. 5c–d), using the apparent tracerconcentration and assuming that these concentrations extend1 mm in the third dimension, as well as assuming a uniformdistribution of tracer on the irrigated surface. The simulated

Fig. 5. Calibration results for AY7 and SB tracers for the “1 year rain event— Summer season”. Simulated water saturation (a) and pressure head (b) distribution areshown before and after 100min of irrigation. Measured and simulated AY7 (c) and SB (d) concentration images andmass plots are presented at a simulation time of7 days after irrigation began. Themeasured and simulated concentration images are however not directly comparable given an unknown representation/resolutionin the third dimension.


mass per 1 mm depth perpendicular to the vertical sectionwas also computed. The simulated mass profile for bothtracers underestimates the measured apparent mass profile(Fig. 5c–d). Non-uniform tracer spreading at the surface and

the experimental focus on exposing the visual tracer paths inthe profiles could explain why the migration depth and massof the two tracers as measured in the field was not fullycaptured.


6.2. Validation

To test the predictive capability of the calibrated model,the remaining five field experiments were also simulated.Input parameters for the calibrated model were keptunchanged except for the structural characteristics of thefractures and biopores. The initial saturation distribution andthe upper and lower boundary conditions applicable to eachexperiment were also used.

The simulated saturations of the five experiments com-pare reasonably well to saturations measured initially andafter 100 min of irrigation (Table 1). As also documented byothers (Trojan and Linden, 1992), increased rainfall intensitiesare expected to result in greater water flow and tracertransport in the fractures and biopores. The simulationsindicated that the 20-year rain event represents a watervolume larger than the unsaturated volume of the till in thefall scenario, leading to a higher water migration rate throughthe till since no ponding at the surface is registered, neitherwhile performing the tracer experiment nor in the simulation.

The preferential migration of the two tracers is alsoreproduced to some degree in the simulation, as shown forexample by Fig. 6a. The model tends, however, to simulate amore pronounced interaction between the discontinuitiesand the surrounding matrix. It is the case for the summerscenarios compared with the fall scenarios and for AY7compared to SB. The observed migration depth is also notcaptured for the “Validated” simulations shown in Fig. 6a. Oneimportant result of the tracer experiment, also not repro-duced in the simulations, was that the tracer entered thedead-end biopores in the fall. Given this discrepancy betweenthe measured apparent and simulated concentration distri-bution, a sensitivity analysis was initiated with focus on themost uncertain parameters.

6.3. Sensitivity analysis

The sensitivity of the calibrated and validated model wastested against the use of imbibition as opposed to drainagecurve, the longitudinal dispersivity, the lower boundarycondition, the geometry of the discontinuities, and thebiopores coating. The effect of each of these parameters wasevaluated for simulations of the “5 year — Fall and Summer”scenario and results at a time of 0.5 day after irrigation areshown in Fig. 6. Additional simulations tested the sensitivityof the reference model to the choice of retention curveparameters and saturated hydraulic conductivity for thebiopores and for matrix zone 1, as well as the impact ofadding a plough pan at 30–35 cm BS. Results for theseadditional simulations for the “1 year — Summer” scenarioand 7 days after irrigation, are shown in Fig. 7.

6.3.1. Imbibition vs drainage curveIn the calibrated model, measured drainage curves for the

twomatrix zones were used, even though an imbibition curvefor the upper unsaturated matrix zone 1 would be morerelevant during the rainfall application. An imbibition curvefor this matrix zone was not measured, but imbibition/drainage investigations performed on tills (Jensen andRefsgaard, 1991; Rosenbom et al., 2000) have shown anaverage reduction in the pore volume of about 13% at full

saturation when comparing drainage to imbibition. Theimbibition and drainage retention curves reported in thesestudies have the same slope in the range of saturationsobserved in our field experiment, and therefore they have thesame van Genuchten parameters in that range.

The use of an imbibition curve can therefore be simulatedby reducing the pore volume of the upper till zone by 13%,which reduces its porosity from 0.36 to 0.31, and keeping thesame van Genuchten parameters. Results shown in Fig. 6bindicate that using an imbibition curve instead of a drainagecurve has very little effect. The simulations shown, however,that for less pore volume in the matrix, more tracer migratesin the biopores as the interaction between the matrix andbiopores will be reduced with the increased saturation. Thissuggests that, to simulate the worst case scenario for the spillof contaminants in a variably-saturated clayey till, imbibitioncurves should be used instead of drainage curves, unless themodel accounts for hysteresis.

6.3.2. Longitudinal and transverse dispersivitiesThe longitudinal and transverse dispersivities of the clay

matrix influence the migration depth of AY7 into the clay,between the macropores. To reproduce the observed migra-tion depth in the clay for the “5 year — Summer” scenario, alongitudinal dispersivity of 5 cm for matrix zone 1 wasspecified. Increasing the dispersivity to 10 cm, which is thevalue estimated by Jørgensen et al. (1998) at a depth of 1m fora saturated clayey till, produced a simulated migration depththat was too large (Fig. 6c), For both seasons, the migrationdepth in biopores was decreased by an increased migration inthe matrix. An increase in saturation of the clayey till seemedto reduce the influence of longitudinal dispersivity on down-ward tracer migration.

The transverse dispersivity for matrix zone 1 was reducedfrom 1.0 cm to 0.5 cm and it increased the migration of AY7into the biopores andminimized its dispersion into thematrixfrom the biopores. The impact of transverse dispersivity onmigration and dispersion of AY4 was enhanced for increasingsaturation.

6.3.3. Influence of structure geometryTo simulate the “5 year — Summer” scenario with the

structure geometry of the “5 year — Fall” scenario, threeextra biopores and one extra tectonic fracturewere introduced.The degree of connectivity between biopores and tectonicfractures were kept unchanged. By comparing the simulatedrelative concentration distributions for the two differentbiopore and fracture geometries (Fig. 6e), it is clear that thenumber of biopores is a controlling factor for the tracermigration depth, because tracer storage is proportional to thenumber of biopores. Given the incomplete numerical descrip-tion of the biopores, their storage can be overestimated.

The biopores directly connected to tectonic fractures seemto control the deep tracer penetration in the “Validated”simulation (Fig. 6a) with less biopores. This controlling effectwas not as pronounced for the “Fall geometry”, where thedistance to the neighbour biopores and the number of tec-tonic fractures that are close to the biopores seem to be thecontrolling factors.

Comparing the fall season's “Validated” simulation(Fig. 6a) with the summer season's “Fall geometry” simulation

Fig. 6. Validation-results (a) and results of sensitivity analysis of AY7-scenario (at 1/2 days simulation): “5 years rain event— Fall season” (b–e). Simulations results are presented for (b) the use of imbibition/drainage curve forthe upper matrix unit, (c) the longitudinal and transverse dispersivity, (d) the lower boundary condition, and (e) the geometry of discontinuities. 147

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Fig. 7. Simulated AY7 concentrations for “1 year rain event— Summer season” after 7 days for (a) the calibrated model, (b) modified retention parameters α and n for the biopores, (c) modified retention parameters α and nfor the biopores andmodified saturated hydraulic conductivity of the biopores, (d) imbibition curve for the uppermatrix unit, (e) reduced saturated hydraulic conductivity of matrix zone 1 and (f) introduction of a plough pan30–35 cm BS.

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(Fig. 6e) suggests that biopores directly connected to tectonicfractures also control the deep tracer penetration. Thus, thetracers were penetrating deeper into the system of disconti-nuities in summer, suggesting that the effect of the disconti-nuities on the tracer migration becomes more pronounced asthe till desaturates. Given that the hydraulic inactivity of thedead-end biopores in fall is not accounted for, these resultsremain uncertain.

6.3.4. Lower boundary conditionTo further examine the causes for the discrepancy be-

tween observations and simulations for the fall, where themodel predicted that the dead-end biopores were activatedcontrary to the observations, the lower boundary conditionwas changed from 200 cm H2O to 250 cm H2O. A justificationfor this change is that the water table in the field couldpotentially be higher than assumed here, because the drainsmay not have lowered the water table as much as anticipatedsince they are 10 m away from the test plot. As shown inFig. 6d, a higher saturation in the profile tended to reduce thepenetration depth of the tracer.

6.3.5. Influence of biopore coatingAs mentioned above, the model incorrectly predicted that

tracer entered the dead-end biopores (primarywormholes) inthe fall season. A plausible reason for this discrepancy is thatthe model does not consider the influence of biopore coating.In summer, the till becomes more and more unsaturated inthe upper part, where micro-fissures are formed on the sur-face of the wormholes. The areas between the micro-fissuresare stabilized with organic material and mucus (Schachtscha-bel et al., 1989) deposited when worms are moving up anddown the wormholes. A mucus layer of 10–15 μm is depositedeach time a worm is passing (Kretzschmar, 2004). In the fall/winter, the worms hibernate and the wormholes destabilizesince the mucus is highly bioavailable and easily soluble. Atthe same time, given the increase in saturation of the media,the clay minerals start to swell and close the micro-fissuresand thereby minimize the flow and transport interactionbetween the wormhole and the matrix. For this reason, watercan build up in the dead-end biopores in the fall/winter,which makes them inactive in relation to the overall flow andtransport of the media. To incorporate the processes takingplace in these dead-end wormholes in models such asHydroGeoSphere, more thorough investigations are needed.

To test this hypothesis, an attempt was made to introducecoating of low permeability and different realistic thicknessesaround the biopore faces in the model. However, due to thelarge contrast in hydraulic conductivity of the biopores andthe coating, the numerical simulations could not converge.

6.3.6. The influence of matrix and biopore properties on tracermigration depth

Using the calibrated model (Fig. 7a), the retention curveparameters and the saturated hydraulic conductivity for thebiopores andmatrix zone 1 were modified (Fig. 7b to e), and aplough pan was added at 30–35 cm BS (Fig. 7f), to investigateif the tracer migration depth could be more accuratelysimulated.

Fig. 7(a) shows the migration depth simulated for thecalibrated model, which uses a realistic drainage curve for

biopores that contain a range of micropores, and a saturatedhydraulic conductivity for the biopores that is based on thecubic law and incorporates the aperture of the largest singlepore/tube representing the biopore regime. It could be arguedthat this combination of the drainage curve parameters andthe saturated hydraulic conductivity represents the samebiopore setting, and different combinations could be moreappropriate. To test other combinations, results from a secondsimulation are presented, with the same saturated hydraulicconductivity for the biopores, equal to 654 cm s−1, but wherethe drainage curve parameters are α=1 cm−1 and n=4. Theseparameters are obtained by neglecting the range of micro-pores present in the biopores and they are assumed to be amore adequate representation of the tension conditions in acylinder/wormhole with a diameter of 3–6 mm. Thiscombination of parameters resulted in a decreased migrationdepth compared to the calibrated model (Fig. 7b), which iscaused by a change in entrance pressure to the biopore fromjust below −1000 cm to larger than −5 cm. In the thirdsimulation, the first set of drainage curve parameters for thebiopores is used, but their hydraulic conductivity is decreasedto 18 cm s−1 to account for the aperture distribution of all themicropores represented in the biopore regime (Fig. 3). Also inthis simulation, less vertical migrationwas predicted (Fig. 7c).A fourth simulation uses the same parameters as the third onebut it assumes that the imbibition curve is used instead of adrainage curve for matrix zone 1, by lowering its porosityfrom 0.36 to 0.31. This simulation produced a minor increasein the migration depth (Fig. 7d) compared to the depthsimulated with the drainage curve and shown in Fig. 7c,which is a direct result of reducing the porosity of the matrixwith 14%. For the fifth simulation discussed here, thesaturated hydraulic conductivity of matrix zone 1 wasdecreased by a factor of 10. As shown in Fig. 7e, the simulatedmigration depth slightly increased compared to the simula-tion using the imbibition curve (Fig. 7d). By reducing thesaturated hydraulic conductivity of the matrix, pressurebuilds up more quickly in the soil resulting in an increaseddrainage from the soil matrix to the biopores.

None of the simulations shown in Fig. 7a–e captured theobserved migration depth and a last simulation was con-ducted, based on the simulation shown in Fig. 7d but with thehydraulic conductivity of matrix zone 1 decreased by a factorof 100 at elevations between 30 cm and 35 cm BS, to representa plough pan. The depth of the plough pan was estimatedfrom on the depth of the AY-migration in the till matrix insummer, as well as drastic changes in the organic content andagricultural practice in the field area. There are however nodirect observations to document the presence of plough pan.The hydraulic conductivity of the plough pan was based onestimates from Christiansen et al. (2004). The simulationwiththe plough pan shown in Fig. 7f produced a maximummigration depth nearly similar to the calibrated model(Fig. 7a) and producedminor tracer migration into thematrix,consistent with the field observations.

7. Discussion and conclusions

Detailed fluorescent tracer experiments have been con-ducted in a variably-saturated fractured clayey till (Rosenbomet al., 2008) to investigate flow and transport processes


controlling the downward migration of tracers from groundsurface. The variably-saturated fractured clayey till containsthree different porous medium domains with contrastingproperties: matrix, biopores and tectonic fractures. A detailedmechanistic analysis of these tracer experiments, which hasnot hitherto been conducted, requires application of anumerical model that considers flow and transport processesin all domains, along with the interaction between domains.Based on a detailed conceptual model that includes thegeometry of the fracture and biopore network at a relativelysmall scale, variably-saturated flow and transport simulationswith the HydroGeoSphere model reproduced the overallobserved tracer pattern, and helped analyze the field-scaletracer experiments.

According to the simulations, flowand transport in bioporesoccur at matrix suction up to −185 cmH2O, which is supportedby tracer and soilmoisture data/observations and in agreementwith previous observations (Shipitalo and Edwards, 1996). Thatvalue formatrix suction is, however, significantly different fromthe commonly applied macropore models where macroporeflow is initiated at matrix suction values of −10 to −50 cm H2O(Beulke et al., 2002). The difference lies in the conceptualizationof the biopore structures,whether theyare assumed to bemadeof a macropore supplemented by a combination of microporesas observed in the specific soil (Fig. 3) or whether they areformed by a single macropore as assumed in the commonlyapplied macropore models.

The fact that high intensity rainfalls generally yield greaterwater flow and transport in the discontinuities than lowintensity rainfall (Trojan and Linden, 1992) was also simu-lated. By using different retardation factors for the tracers, itwas possible to reproduce the AY7 and SB migration patternsin the simulations, albeit with a more pronounced interactionbetween the discontinuities and matrix especially for thesummer scenarios. Exact agreement between the observedand simulated tracer migration depth in the discontinuitieswas not achieved for the calibrated model. It was also im-possible to avoid simulating tracer migration into the dead-end biopores in the fall. These differences indicate thatprobably not all the mechanism/processes taken place in thebiopores are accounted for. A plausible explanation may bethe presence of a fast flow mechanism (film flow, fingeringand/or intermittent flow) combined with dynamic coating onthe walls of the biopores, which cannot be directly simulatedwith the current version of HydroGeoSphere.

Given that the parameterization of HydroGeoSphere is nottrivial for the combined till, biopore and fracture system, asensitivity analysis investigated the impact of using imbibi-tion instead of drainage curves, as well as modifying thelongitudinal dispersivity, the lower boundary condition, thegeometry of the discontinuities, the biopore coating, and theinfluence of matrix and biopores properties on tracermigration depth. The following conclusions can be derivedfrom this analysis:

• Using an imbibition curve instead of a drainage curve formatrix zone 1 reduces the pore volume, increases the tracermigration into the discontinuities and reduces the interac-tion between the matrix and the discontinuities. Given themore rapid transport of solutes in the discontinuitiescompared to the matrix, use of the imbibition curve for

the matrix is conservative with respect to risk assessmentand therefore recommended.

• Increasing the longitudinal dispersivity leads to matrix-dominated flow and transport in the upper till unit, andreduces the contribution of biopores to flow and transport.By decreasing the transverse dispersivity, biopores becomemore active. The impact of increasing the longitudinal andtransverse dispersivity is not as pronounced in the fallcompared to summer.

• Changing the geometry of the discontinuities, including theamount of biopores, fractures and interconnected disconti-nuities, has a large influence on the simulated migrationdepth of the tracers. Increasing the number of biopores alsoincreases storage. Simulation shows that the activity of agiven bioporewith respect to flow seems to be controlled bythe distance to a neighbour biopore and by the number oftectonic fractures in its vicinity. The simulations alsoproduce increased flow and transport in the discontinuities,given a decrease in the saturation of the media, whichagrees with findings of Shipitalo and Edwards (1996).Uncertainty still exist, however, because (a) the bioporesare represented as fracture planes in the model and not astubes, and (b) the hydraulic inactivity of the biopores in thefall are not accounted for.

• By increasing the pressure head at the lower boundary, thetracer migration depth is reduced, and the dead-endbiopores remain active for flow.

• Using a more adequate combination of retention curve and/or the saturated hydraulic conductivity for the bioporesleads to a decrease in the tracer migration depth comparedto the field observations. An important consequence is thatthe unsaturated hydraulic properties of the biopores play akey role for their function as preferential flow mediators.Using the imbibition curve instead of the drainage curve formatrix zone 1 slightly increases the migration depth, butdoes not reproduce observations. However, by including alow-permeability plough pan at 30–35 cm BS and ade-quately choosing the hydraulic parameters for the biopores,the simulated migration depth is almost as large as for thecalibrated model, but with much less interaction betweenthe matrix and biopores.

Overall, the study shows that the vulnerability tocontamination of a groundwater reservoir overlain byfractured clayey tills is controlled by the presence ofdiscontinuities, even for high matrix suction values. A riskassessment analysis of contaminant transport in fracturedclayey tills should account for the fast flow mechanism, thetemporal variability of the coating along the discontinuities,and the presence of a plough pan. The HydroGeoSpheremodel incorporated detailed information on the fracturedclayey till but could not fully account for all the mechanism/processes taking place in the media. The model has howeverproven to be a valuable tool for evaluating migration ofcontaminant in this type of setting and has a great potential tosupport decisions in planning, regulation, and contaminantremediation.

From the detailed mechanistic analysis of the field datafrom Rosenbom et al. (2008) presented here, controllingmechanisms/processes of the fractured clayey till have beenelucidated. Decisions and assessments in management


would, however, require incorporation/upscaling of thisknowledge from plot/field-scale to regional/catchment scale.Therefore additional research is required to (a) examinewhether these controlling mechanisms/processes identifiedat the plot/field-scale play a similar role across scales, and(b) develop upscaling methods appropriate for representingdiscontinuities that control flow and transport in this type ofvariably-saturated fractured clayey till (Beven, 2000; Ver-eecken et al., 2007).

Acknowledgements

The work presented here has been mainly funded byGEUS.We also thank the DONG Corporation for partly fundingthe collaboration between Université Laval and the GeologicalSurvey of Denmark and Greenland (GEUS) and the NaturalSciences and Engineering Research Council of Canada(NSERC) for funding R. Therrien. The authors thank NicholasJarvis (Swedish University of Agricultural Sciences) and SørenHansen (Faculty of Life Science, University of Copenhagen) forcommenting on an early version of the manuscript.

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numerical analysis of water and solute transport in variably-saturated fractured clayey till

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