the breakdown of water repellency and solute transport through a hydrophobic soil

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The breakdown of water repellency and solute transport through a hydrophobic soil B.E. Clothier a, * , I. Vogeler a , G.N. Magesan b a Environment & Risk Management Group, HortResearch, PB 11-030, Palmerston North, New Zealand b Manaaki Whenua—Landcare Research, PB 3127, Hamilton, New Zealand Received 14 October 1998; accepted 6 April 1999 Abstract Unsaturated infiltration into the Ramiha silt loam, an Andic Dystrochrept, follows the classic pattern. A rapid drop-off from a high flow rate, seemingly induced by capillary attraction, appears followed by an apparent steady-flow maintained by gravity at around 0.5 mms 21 . Beyond 100 min, however, the infiltration rate climbs nearly linearly to exceed 4 mms 21 as the soil’s water repellency breaks down. This is only evident after a period that might exceed the observer’s attention span. The hydrophobicity in this case could be due to one, or a combination, of the many unusual characteristics of this soil—its low bulk density (0.8 Mg m 23 ), its strongly aggregated nature, the presence of mycorrhizal fungi, its high organic matter content (16.5%), or the presence of allophanic clay (4%). Our measurements of infiltration into undisturbed cores of Ramiha silt loam were made with disc permeameters set at the unsaturated pressure head of h 0 240 mm: The permeameters contained a solution of electrolytic tracer (KBr) so that we could observe solute transport in this soil. Vertical three-wire rods for Time Domain Reflectometry (TDR) measurement were inserted directly through the base plate of the permeameter so that we could continuously monitor the soil’s changing water content and resident concentration of electrolyte. The TDR measurements revealed the transient behaviour of fingered prefer- ential flow into this soil during the breakdown of hydrophobicity. At the conclusion of the experiment, the soil cores were sectioned to permit measurement of the profiles in the resident concentration of the invading chemical. Near the surface, at the conclusion of the experiment, the resident concentration of bromide was found to be exactly that of the invading solution. So, despite the initial water repellency of the soil, the infiltrating bromide solution was subsequently able to invade the entire pore space—once the hydrophobicity had dissipated. Classic theory would then seem capable of describing solute transport after the effects of water repellency had faded. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Sorptivity; Time domain reflectometry; Infiltration; Fingering; Andisol 1. Introduction The Water Drop Penetration Test (WDPT) relies on the gradual breakdown of hydrophobicity to register a measure of the soil’s water repellency (Letey, 1969). However, little attention has been directed towards understanding the mechanisms and consequences that result from the loss of ephemeral hydrophobicity which can follow the wetting of the soil surface (DeBano, 1969; John, 1978). Thus there has probably been a diminution in the reliance of the WDPT as a criterion of repellency. Other more dynamic measures Journal of Hydrology 231–232 (2000) 255–264 0022-1694/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(00)00199-2 www.elsevier.com/locate/jhydrol * Corresponding author. Tel.: 164-6-356-8080; fax: 164-6-354- 6731. E-mail address: [email protected] (B.E. Clothier).

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Page 1: The breakdown of water repellency and solute transport through a hydrophobic soil

The breakdown of water repellency and solute transportthrough a hydrophobic soil

B.E. Clothiera,* , I. Vogelera, G.N. Magesanb

aEnvironment & Risk Management Group, HortResearch, PB 11-030, Palmerston North, New ZealandbManaaki Whenua—Landcare Research, PB 3127, Hamilton, New Zealand

Received 14 October 1998; accepted 6 April 1999

Abstract

Unsaturated infiltration into the Ramiha silt loam, an Andic Dystrochrept, follows the classic pattern. A rapid drop-off from ahigh flow rate, seemingly induced by capillary attraction, appears followed by an apparent steady-flow maintained by gravity ataround 0.5mm s21. Beyond 100 min, however, the infiltration rate climbs nearly linearly to exceed 4mm s21 as the soil’s waterrepellency breaks down. This is only evident after a period that might exceed the observer’s attention span. The hydrophobicityin this case could be due to one, or a combination, of the many unusual characteristics of this soil—its low bulk density(0.8 Mg m23), its strongly aggregated nature, the presence of mycorrhizal fungi, its high organic matter content (16.5%), or thepresence of allophanic clay (4%).

Our measurements of infiltration into undisturbed cores of Ramiha silt loam were made with disc permeameters set at theunsaturated pressure head ofh0 � 240 mm: The permeameters contained a solution of electrolytic tracer (KBr) so that wecould observe solute transport in this soil. Vertical three-wire rods for Time Domain Reflectometry (TDR) measurement wereinserted directly through the base plate of the permeameter so that we could continuously monitor the soil’s changing watercontent and resident concentration of electrolyte. The TDR measurements revealed the transient behaviour of fingered prefer-ential flow into this soil during the breakdown of hydrophobicity. At the conclusion of the experiment, the soil cores weresectioned to permit measurement of the profiles in the resident concentration of the invading chemical. Near the surface, at theconclusion of the experiment, the resident concentration of bromide was found to be exactly that of the invading solution. So,despite the initial water repellency of the soil, the infiltrating bromide solution was subsequently able to invade the entire porespace—once the hydrophobicity had dissipated. Classic theory would then seem capable of describing solute transport after theeffects of water repellency had faded.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Sorptivity; Time domain reflectometry; Infiltration; Fingering; Andisol

1. Introduction

The Water Drop Penetration Test (WDPT) relies onthe gradual breakdown of hydrophobicity to register a

measure of the soil’s water repellency (Letey, 1969).However, little attention has been directed towardsunderstanding the mechanisms and consequencesthat result from the loss of ephemeral hydrophobicitywhich can follow the wetting of the soil surface(DeBano, 1969; John, 1978). Thus there has probablybeen a diminution in the reliance of the WDPT as acriterion of repellency. Other more dynamic measures

Journal of Hydrology 231–232 (2000) 255–264

0022-1694/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0022-1694(00)00199-2

www.elsevier.com/locate/jhydrol

* Corresponding author. Tel.:164-6-356-8080; fax:164-6-354-6731.

E-mail address:[email protected] (B.E. Clothier).

Page 2: The breakdown of water repellency and solute transport through a hydrophobic soil

of hydrophobicity such as the Molarity of an EthanolDroplet (MED) (King, 1981), or the Repellency Index(RI) that relies on measuring the intrinsic sorptivity(Tillman et al., 1989), would seem better able todescribe the degree of hydrophobicity.

Our interest here is to describe the transient behaviourof infiltration into an ephemerally hydrophobic soil, andto describe its consequences upon solute transport.

We did not set out initially to study water repel-lency. Rather we were interested, at first, in determin-ing the relative roles of interpedal convection andintra-aggregate diffusion on solute transport andexchange in a strongly aggregated soil. Thus, anexperimental requirement was to have sufficientwaterborne tracer enter the soil so that we could betterdiscriminate between the various transport andexchange process. So our measurements of infiltrationwere maintained until a cumulative infiltration ofI <80 mm had entered the soil. This often required obser-vations up to 10 h, well exceeding that which mightoften be considered the attention span of an operatorintent on measuring the soil’s hydraulic properties. Itwas through these extended observations that wecame to record the breakdown of hydrophobicity.Our experimental technique relied on a disc permea-meter, set at an unsaturated head and containing acocktail of inert and reactive tracers, to record infil-tration into the soil. A three-wire TDR probe (Vogeleret al., 1996) inserted through the permeameterallowed us to observe water infiltration and electro-lytic tracer penetration into the soil during the break-down of hydrophobicity. Interpretations of certain ofthe results of these experiments in terms of determi-nation of the exchange isotherm of reactive chemicalhave already been published (Clothier et al., 1996).Here we present new results relating to the temporalchanges in the measured infiltration rate as the repellencybreaks down, and we interpret the TDR traces in relationto the temporalpatternof water entry into,and inert solutetransport through this ephemerally hydrophobic soil.

2. Materials and methods

The soil on which we carried out our field tests, andwith which we conducted laboratory experiments onundisturbed cores, is the Ramiha silt loam—an AndicDystrochrept. The pedology of this soil has already

been detailed by Parfitt et al. (1984). Clothier et al.(1996) described the set-up of the laboratory experi-ments we will use here, although they analysed themin terms of the transport and exchange of reactive35Stransport. Here we only provide details that are salientto hydrophobicity and the transport of solute duringthe breakdown of repellency.

The soil of loessial origin has a bulk density0.82 Mg m23, and a strong nut structure in the A hori-zon. The smeary consistency suggests that allophaneis present, and indicates that the soil is an admixture ofloess and tephra. Allophane comprises 4% by weightof the soil in the top 700 mm. A 20,000 yr BP ashband occurs in the soil at a depth 1 m to record therate of loessial accretion. The organic carbon contentof the top soil is 16.5%. When a clod is broken, thesurface soil breaks into 5–10 mm diameter sphericalaggregates, many of which are found to be coated withthe white hyphae of a mycorrhizal fungus.

Infiltration experiments were carried out both in thefield, and on undisturbed cores taken to the laboratory,using a disc permeameter set at the unsaturated headh0 of 240 mm. The permeameters used in the fieldwere of disc diameter 200 mm, whereas those atop theundisturbed cores were 97.5 mm in diameter. In bothcases, a 2–10 mm depth of fine sand ensured goodhydraulic contact between the permeameter and thesurface of the soil that had been gently shaved flat at adepth of about 50 mm below the soil surface, after thepasture thatch and roots had been removed. Thedescription of the procedure for the collectionof the four undisturbed cores, and their set-upin the laboratory is described elsewhere (Clothieret al., 1996). The reservoir of the permeameterscontained a solution of 0.1 M KBr to permit tran-sient measurement of electrolyte invasion byTDR, and also so that a terminal sampling in thesoil would reveal the final pattern of solute transport.The vertical arrangement of the three-wire TDR probeof length L � 125 mm, directly through the base ofthe permeameter, was described by Vogeler et al.(1996). They also outlined the procedures we usedhere to interpret the TDR traces in terms of soilwater contentu , and the soil solution concentrationof bromide ions. In their experiments with repackedRamiha silt loam, they used this set-up to examine theexchange of invading K1 ions with resident Ca1,however here we seek to observe the consequences

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264256

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of the breakdown of hydrophobicity in undisturbedcores on the transport through them of inert bromidetracer. At the end of the experiment all four columnswere sectioned, and five small cores of about 1 g wereextracted in a row across the diameter of the column atdepths of 5, 15, 25, 45 and 75 mm. These sampleswere individually analysed for the Br2 concentrationusing a bromide-specific electrode.

The field soil, both at the time of sampling andcolumn exhumation, had a gravimetrically measuredvolumetric water contentun of around 0.5 m3 m23.From gravimetric sampling under all the permeameterexperiments, ath0 � 240 mm on the undisturbedcores, Clothier et al. (1996) found the soil to wet tou0 � u�h0� � 0:62^ 0:04 m3 m23

: From 10 samplestaken from the column immediately under the disc

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264 257

Fig. 1. (a) The time course of unsaturated infiltration ratei(t) from a disc permeameter�ho � 240 mm� into two undisturbed cores of Ramiha siltloam, and a field site. The inset provides an expansion of the initial 100 min and reveals an apparent steady-state flow rate. (b) The infiltrationdata of Fig. 1(a) are here plotted to show the instantaneous rate of infiltrationi against the cumulative infiltrationI up to that time.

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with the TDR rods through the permeameter (core 4),it was found thatu0 � 0:643^ 0:03 m3 m23

:

The Repellency Index of the soil, i.e. the ratio of thewater sorptivitySw at h0 � 240 mm to the ethanolsorptivitySe at the sameh0, was also determined (Till-man et al., 1989; Wallis and Horne, 1992). TheSw wastaken from the early-time infiltration of water into twoof the undisturbed cores described above. Two smal-ler, undisturbed cores of radius 70 mm were used forSe. Glass permeameters of radius 68 mm were used tosupply an 85% solution of ethanol to the cores so thatthe time-course of the cumulative infiltrationI(t)could be monitored.

Often the sorptivity is found by plotting the initialI(t1/2) infiltration data, for at early times, the two-termPhilip (1957) infiltration equation

I � St1=2 1 Kt �1�

is dominated by the first term, and so the sorptivitySisfound as the slope of the plot. Where soils are extre-mely repellent, sinceSw ! 0; use of this equationbecomes problematic. Nonetheless, if it were found

thatSw < 0 then the Repellency Index would, throughdivision by zero, indicate a very hydrophobic soil! Soit is critical that good measures of the sorptivity beobtained. As Smiles and Knight (1976) showed, morerobust estimates of the terms in the Philip two-term infil-tration equation can be obtained whenIt21/2 is plottedagainstt1/2. The Philip (1957) expression is therefore

It21=2 � S1 Kt1=2 �2�and a better estimate ofS is found from the intercept ofIt21/2(t1/2). We used Eq. (2) to determineSw andSeath0 �240 mm:

3. Results and discussion

Water repellency is known to determine the patternof water entry into soil (Dekker, 1998), and therebycontrol the processes of flow and transport (Ritsema,1998). Our interest here is to examine the breakdownof hydrophobicity in an ephemerally repellent soil,and to determine the consequences of this on flowand transport mechanisms.

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264258

Fig. 1. (continued)

Page 5: The breakdown of water repellency and solute transport through a hydrophobic soil

3.1. Infiltration

If the attention span of the operator of a discpermeameter were only 100 min or so, then theywould observe a classic pattern of infiltration intothe Ramiha silt loam (Fig. 1(a), inset). Both in thefield, and with the two cores in the laboratory, atthis unsaturated head ofh0 of 240 mm, the infiltrationrate i drops from what seems to be a capillarity-induced high of around 3–5mm s21, to an apparentlysteady-state flow of about 0.5–0.7mm s21. Care wastaken to exclude transient effects due to the initialwetting of the thin layer of contact sand. So, after2 h of work, such an operator would feel confidentwith the values ofS and K that they might havederived from theseI(t) observations. An operatorwith a keen eye might have been perplexed as towhy such an open, nut-structured silt loam hadapparently returned a hydraulic conductivity measure-ment in the range of just 1–2 mm h21.

Anyway, our primary interest was to decipher therelative transport roles of convection and diffusion inmoving solute through this aggregated soil. Thus werequired that more than just thisI of 5 mm of solution

enter the soil. So we left the permeameters on the soilfor much longer. Soon after 100 min, a dramatic risein the infiltration rate commenced. (Fig. 1(a)). Aftersome 9–10 h, the infiltration rate is around 4–5mm s21, indicating that the apparent conductivitythat might have been inferred, from the inset after100 min, isanorderof magnitude too low.Appropriately,now the Ramiha silt loam can be found to have a conduc-tivity at h0 � 240 mm of around 20–30 mm h21.

These infiltration rate data clearly record the break-down of ephemeral hydrophobicity in this soil. Suchdata would seem rare, maybe because few operators hadthe incentive to extend their period of observation.

In Fig. 1(b), thei(t) of Fig. 1(a) are replotted asi(I)so that it can be seen that the breakdown of hydro-phobicity occurred after just 5 mm of solution hadinfiltrated. For future reference, it is worth notingthat the presence of TDR rods through the base ofthe permeameter on core 4 made no difference tothe recorded pattern of infiltration. This is not surpris-ing, for all the experiments were conducted at theunsaturated head of240 mm, such that preferentialflow, or enhanced wetting created by the rods, wouldbe unlikely.

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264 259

Fig. 2. The short-time, unsaturated infiltration data from the inset of Fig. 1(a) are here plotted asI =t1=2 against the square root of elapsed timet1=2

so that the intercept provides the sorptivity of water,Sw (Eq. (2)). Results from unsaturated infiltration experiments using ethanol in a glasssorptivity tube are used to provide the sorptivity of ethanol,Se.

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3.2. Repellency index

From the short-time data in the inset of Fig. 1(a), aplot of It 21/2(t1/2) provides anSw of 24mm s21/2 (Fig.2, Eq. (2)), a value that is very low for such a well-structured, medium-textured soil withu0 2 un <0:15: The short-time, infiltration data obtained withthe ethanol-filled, glass permeameters are alsoshown in Fig. 2. Use of Eq. (2) provides anSe of0.48 mm s21/2. As Tillman et al. (1989) noted, for awettable soil the ratio ofSw/Se should be 1.95, dueprimarily to greater surface tension of water. Shouldthe measured ratio be lower than this, then there isdynamic evidence of some water repellency. Wallisand Horne (1992) thus defined the Repellency Index(RI) as being 1.95 divided by the measuredSw/Se, sothat values of RI greater than unity indicate hydropho-bicity. Moderately repellent soils tend to have RIsbetween 20–40, and severely repellent soils havebeen found to have RIs of up to 80. Here our soilhas an RI of 40, placing it at the bottom end of theclass of severely repellent soils. However, here wehave observed that after about 100 min, or some5 mm of infiltration, this severe repellency breaks

down, and the soil eventually becomes hydrophilic.Now we will explore the pattern of soil wetting duringthis breakdown, and its consequences on the solutetransport of the bromide tracer.

3.3. Transient wetting

With the arrangement of the TDR rods through thepermeameter, we are able to measure continuously theaverage water contentu�t� over the length of the rods�L � 125 mm� during infiltration I(t). Since at anytime t we know how much water has entered thesoil from the permeameter, viz.I, our contempora-neous TDR measure ofu can be used to infer some-thing about the shape of the profile-of-wettingu (x),with the depth beingx. This permits answers to keyquestions. Is the shape of the profile fully rectangular,as might happen with a Green-Ampt soil thatpossesses a Dirac-d diffusivity function that is highlynon-linear (Philip, 1969)? Or, is the profile of wettingmuch flatter, with there being just a gradual wettingbehind the wet-frontxf as might characterise a soilwhose diffusivity is only weakly dependent uponu(Philip, 1969)?

Here, for simplicity, we consider two syntheticprofiles of wettingu(x) so that we can easily interpretouru�I � data to infer which pattern of wetting we mustbe observing with our combined permeameter-TDRinfiltration device. We choose to do this simply byconsidering two analytical profiles that would delimitthe bounds of likely behaviour for the wetting of soilduring infiltration—a profile of rectangular wetting,and one where the profile is always a triangle (Fig. 3).Wetting of most soils would lie somewhere betweenthese extremes (Clothier and White, 1982).

3.4. Rectangular wetting

For infiltration into the highly non-linear soil ofFig. 3(a), the profile of wetting can be written as

u�x� �u0 x # xf

un x . xf

����� �3�

so that the cumulative infiltration is given by

I �Zxf

0�u�x�2 un� dx� xf �u0 2 un� �4�

In this case, TDR rods of lengthL will measure an

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264260

Fig. 3. Synthetic profiles of water content in soil, in which (a) theprofile of wetting corresponds to a rectangle as would happen for asoil with a Dirac d-function diffusivity relationship, and (b) thewetting provides a triangular profile. The simple analytical expres-sions for these synthetic profiles provide a straightforward relation-ship between the cumulative infiltrationI and the mean watercontentu measured by TDR over rod lengthsL (Eqs. (6) and (10)).

Page 7: The breakdown of water repellency and solute transport through a hydrophobic soil

average water content ofu given by

u �u0xf 1 un�L 2 xf �

Lxf # L

u0 xf . L

������� �5�

By elimination ofxf from Eqs. (4) and (5), we obtainthe expression foru�I �t�� that would be observed for asoil that wets according to Fig. 3(a)

u�t� � un 1I �t�L

I # L�u0 2 un�u0 I . L�u0 2 un�

������� �6�

3.5. Triangular wetting.

For infiltration into the weakly non-linear soil ofFig. 3(b), the profile of wetting can be written as

u � un 1 �u0 2 un� 1 2xxf

� �x # xf

un x . xf

������� �7�

so that the cumulative infiltration is given by

I � 12

xf �u0 2 un� �8�

In this triangular case, the TDR rods of lengthL willmeasure an average water content ofu given by

u �un 1

xf �u0 2 un�2L

xf # L

12

u0 1 un 1 �u0 2 un� 1 2Lxf

� �� �xf . L

����������9�

By elimination ofxf from Eqs. (8) and (9), we obtainthe expression foru�I �t�� that would be observed for asoil that wets according to Fig. 3(b)

u�t� �un 1

I �t�L

I # L�u0 2 un�=2

u0 2�u0 2 un�2L

4I �t� I . L�u0 2 un�=2

��������� �10�

Eqs. (6) and (10) bracket the likely profiles of wettingthat are possible. So via inverse interpretation of ourmeasuredu�I �; or u�t�; we can infer the profile in thechanging pattern of wetting within the soil during thebreakdown of hydrophobicity. The measuredu�t� datafor core 4 are presented in Fig. 4, along with thepredictions of Eqs. (6) and (10).

The measured rise inu�t� over the first 200 min is

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264 261

Fig. 4. The time course in the TDR measured water content in the soil (X) in relation to that predicted by the expression that assumes arectangular profile (…, Eq. (6)), and by that which considers wetting to be in the form of a triangle (- - -, Eq. (10)).

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far greater than that predicted by either Eqs. (6) or(10)! It should not be physically possible, given ourreliance on mass balance (Eq. (4)), to observe morewater in the soil, than that which could have infil-trated. Instead, the TDR probes must be measuring apocket of preferential wetting that is due to the localpresence of a wetted finger around one, or more, of thethree TDR rods. Given that infiltration is at an unsa-turated head of240 mm, it is unlikely that this fingerhad been created by disturbance of the soil. Rather it islikely that these TDR rods are recording the localpassage of a wetted finger, a phenomenon found typicalof infiltration into hydrophobic soils (Ritsema, 1998).

There is initially a rapid rise in the TDR-measuredwater content as the rods�L � 125 mm� measure thepassage of a ‘finger’. After about 100 min themeasured rate of wetting slows. At around 165 min,water was noted to be freely dripping out of the baseof the core atx� 250 mm: The wetted finger musthave reached the bottom of the core, and the pressurepotential there eventually risen to zero. Now flow isbeing driven by a steady hydraulic-head gradient of0.84 (� 210/250). At this time (Fig. 1(a)), the flowrate was just over 1mm s21, whereas by the end of theexperiment, the flow was around 4.5mm s21. So over

the period 165–600 min, the breakdown of hydropho-bicity has seen the hydraulic conductivityK0 (i.e.K�h0 � 240 mm�� rise nearly five-fold. The patternof wetting within the soil during this period can beseen to possess a slope that is better predicted by Eq.(10), suggesting that the breakdown of hydrophobicityis achieved with a flat, triangular like profile ofwetting. However, between 165–500 min, theincrease inu is quite small (<0.04), relative tomassive increase inK0, so to account for this itwould seem that there must also have been an accel-eration of water flow inside the wetted fingers, as well.Maybe this is in response to a widening of the wettedfingers as the hydrophobicity breaks down (Ritsema etal., 1998). In addition, this observation could beexplained by the later arrival of slower-movingfingers.

The rise in the soil’s volumetric water contentduring the breakdown of hydrophobicity appearsgradual, and characterised by a triangular profile ofwetting away from the proximal surface. However,during this period, the soil’s hydraulic conductivitychanged dramatically. Now we examine theconsequences of this hydraulic behaviour on the trans-port of the bromide tracer.

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264262

Fig. 5. The time course in the TDR-inferred concentration of bromide in the soil solution.

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3.6. Solute transport

Following the procedure developed by Vogeler etal. (1996) for the Ramiha silt loam, our contempora-neous TDR measurements of the bulk electricalconductivity and the soil’s volumetric water content,can be used to infer the soil solution concentration ofbromide averaged over the length of the rods. Theresults of this analysis are shown in Fig. 5 and therecan be seen a nearly linear rise in the measured resi-dent concentration of bromide throughout the experi-ment with core 4.

If there were a piston-displacement invasion ofsolute tracer, then we would expect the bromidefront xs to be atI/u0. Over the length of this experi-ment, I < 80 mm (Fig. 1(b)), andu0 � 0:643; so wewould expect for non-preferential flow thatxs be125 mm—i.e. the length of the TDR rods. Thus forsuch a uniform case, dispersion aside, we wouldexpect the TDR-measured resident concentration, atthe end of the experiment, to be the influent concen-tration of 0.1 M. However, as can be seen in Fig. 5, atthe end of the experiment, we only record a resident

concentration of 0.06 M, such that through highlydispersive processes of solute transport, we are ‘miss-ing’ some 40% of the applied solute.

Such data might, at first glance, suggest that theRamiha is a mobile–immobile water soil (Clothieret al., 1998), with there being a small mobile domain,so that the linear rise in the resident concentration ofbromide in Fig. 5 reflects interdomain diffusion.

The resident Br2 data measured at the end of theexperiment (Clothier et al., 1998) are presented herein Fig. 6, for they reveal that at the proximal surfacethe resident concentration of bromide had finally risento exactly the influent concentration. Therefore, at theend of the experiment, after hydrophobicity hadabated, the invading solute was travelling throughthe entire pore space. At this stage, at this depth,there was no preferential flow.

Clothier et al. (1998) considered that solute trans-port in this geometrically complicated and hydrauli-cally complex soil, could be simply described using afully mobile version of the convection–dispersionequation, as long as a very large dispersivity wereused to smear the profile of solute invasion. A

B.E. Clothier et al. / Journal of Hydrology 231–232 (2000) 255–264 263

Fig. 6. The measured depthwise profile in the resident concentration of bromide in the soil solution obtained by sampling at the end of theexperiments. If the profile of bromide invasion were rectangular in shape, a mass-conservative calculation would place the tracer front at a depthof aroundz� 125 mm (i.e.I/uo). Data from four cores have been combined here, and for each core a row of five samples were taken at eachdepth. Thus every data point comprises 20 samples, and the shaded bands provide the standard deviation.

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dispersivityl of the order of 20–30 mm, would gener-ate a flat profile shape such as that in Fig. 6, and wouldcorroborate the linear rise in the temporal pattern ofsolute invasion Fig. 5. In Fig. 6, a piston invasion-front of bromide would be at 125 mm, however thedispersed nose we have observed in the solute concen-tration profile has lead to some 40% of the bromidepreferentially moving beyond thisxs. This supportsour observation in Fig. 5. It is this smearing Fig. 6,and the linear rise in the resident concentration Fig. 5that al < 20 mm would mimic. The dispersion processwould describe the sum effect of many mechanisms; thenature of the aggregates, the breakdown of the hydro-phobicity, the gradual wetting of the soil, and the accel-eration of flow within the widening preferential fingersof wetting, and the slower penetration of some fractionof the fingers. Whereas the hydraulic functioning of thissoil during the breakdown of ephemeral hydrophobicityappears to defy simple description in terms of classicalsoil physics (Fig. 1), solute transport would appearcapable of prediction using the standard convection–dispersion equation with the wetted domain beingfully mobile. However, an unusually large dispersivityis needed to account for the effect of soil structureand patterns of preferential flow (Magesan et al.,1995).

4. Conclusion

Observation of unsaturated infiltration in the Ramihasilt loam beyond 100 min revealed a breakdown inhydrophobicity that saw the effective hydraulic conduc-tivity of the soil rise nearly fivefold. Possible reasons forthis soil being ephemerally repellent are its stronglyaggregated structure, the presence of mycorrhizalfungi, a high allophane content, or its elevated level oforganic matter. The breakdown began some 100 minafter the soil was first wetted, and was still not completeafter 10 h. So whether or not this soil would exhibitrepellency in the field would depend on the intensityand duration of rainstorms.

Despite being unable to provide a rational physicalprediction of the process of ephemeral hydrophobicity,it was found possible to describe its impact on solutetransport using the classical convective–dispersiveapproach. The entire wetted pore space could be consid-ered mobile, although a large dispersivity was needed to

predict the smeared profile of solute invasion caused bythe multitude of preferential flow processes.

References

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