Geoderma, 43 (1988) 143-161 143 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Water M o v e m e n t in a G l os saq u a l f as M e a s u r e d by T w o T r a c e r s

M. DIAB 1, PH. MEROT 2 and P. CURMI 2

l Ecole Nationale Supdrieure Agronomique de Rennes; et 2Institut National de la Recherche Agronomique, Laboratoire de Science du Sol, 65 route de Saint Brieuc, 35042 Rennes Cedex (France)

(Received April 2, 1987; accepted after revision June 3, 1988)

ABSTRACT

Diab, M., Merot, Ph. and Curmi, P., 1988. Water movement in a Glossaqualf as measured by two tracers. Geoderma, 43: 143-161.

The relationships between soil organization and water movement were investigated in a Glos- saqualf. This soil is characterized by a clear discontinuity between the tongues and the matrix in both material (texture and mineralogy), and structure (porosity and pedality). Two tracer ap- proaches have been used to evaluate water movement: (1) methylene blue for studying the transfer in saturated conditions in the field and on undisturbed cores; and (2) an environmental isotope (oxygen-18) for studying the actual behaviour under natural conditions. The different approaches show that tongues are preferential paths of water transfer; the matrix

is nearly short-circuited under saturated conditions. Analogous results were obtained in the lab- oratory and under natural conditions. These results are attributed to the shapes, sizes, distribution and overall continuity of the system of pores.

INTRODUCTION

Internal differences in morphology and composition of a soil affect patterns of water movement. To find a soil with large differences, we chose the profile of a Glossaqualf (Soil Survey Staff, 1975 ), a degraded and leached soil, because of its marked internal contrast in both morphology and composition (De Con- inck et al., 1976). Patterns of water movement in the soil were determined with the use of two tracers, methylene blue and 1sO. Water movement was examined in both saturated and unsaturated states under laboratory and field conditions. Field data were then compared with a successive mixing tank model. Our pur- pose was to get a picture of the pathways followed by water in a soil differing greatly in its organization within and among horizons.

0016-7061/88/$03.50 © 1988 Elsevier Science Publishers B.V.

144

MATERIALS AND M E T H O D S

Location

The studied soil profile is located in the Noe Luce wood, 20 km west of Rennes (Brittany, France, 48°05'N 1 °41'W). It is a Glossaqualf developed in a silt loam overlying clayey and stony alluvium.

Morphological and analytical techniques

A detailed soil profile description was made following the methods of Ja- magne (1967) and C.P.C.S. (1967). Importance and morphology of macropo- rosity were studied by means of large thin sections (9 X 16 cm). Samples were air-dried and impregnated with polyester resin prior to fabrication of the thin sections (Delaye, 1984). Visualization of macroporosity was obtained by ad- dition of a fluorescent dye to the resin (UVITEX-OB from Ciba-Geigy), with 1 g/1 concentration (Murphy et al., 1977; Bullock and Thomasson, 1979; Fi~s, 1982) and high-contrast photography (Ringrose-Voase, 1984).

Bulk samples were collected from the main horizons or parts of horizons for physicochemical and mineralogical analysis. A Siemens diffractometer was used to determine the mineralogy of the clay fraction (prepared according to Robert and Tessier, 1974).

gsa t measurements and staining methods

Water movement under saturated conditions was followed by Ksat measu re - m e n t s and staining the efficient porosity with methylene blue dye (Bouma et al., 1977, 1979) in situ by means of a ring infiltrometer and on undisturbed cores in laboratory.

The in situ Ksa t of the lower part of the profile, i.e. A22g+Btg horizon and Btgd horizon, have been measured after removal of the overlying horizons to a depth of 35 cm on two sites with 35 cm diameter rings. After saturation of the profile by gravity, a constant height of 1 g/l methylene blue solution was applied on one site. The experiment was stopped after 15 days; during that period 65 1 of the due solution percolated downward. The stained porosity was studied on horizontal sections every 5 cm and also on vertical sections.

In the laboratory, seven cores from the A22g + Btg and from the Btgd hori- zons were studied. The cores, 9 cm in diameter and 15-25 cm high, were water- proofed and steadied laterally by paraffin reinforced with medical gauze straps. A 2 cm high ring fitted with an overflow was placed on top of the core and a filter was fitted at its base to prevent collapsing. A 0.1 g/1 methylene blue

145

solution was added on the top of the core after saturation by capillarity. After the test, we made sure no preferential flow phenomenon had occurred at the sample/paraffin interface.

Percolation was stopped when the optical density of the effluent became stable, which occurred in 4 to 9 weeks. Bouma et al. (1977) had proposed that percolation be stopped at the moment that the optical densities of the perco- lation solution and the effluent became equal, but we could not use that crite- rion. In our trials percolation was continued long enough so that a film due to methylene blue aggregation (Duff and Giles, 1975 ) appeared on the tops of the cores. Because of the film the concentration of the percolation solution was reduced at the core surface. Consequently, the actual concentration of the so- lution entering the core could not be evaluated. To overcome the effects of the film in the field trials, the methylene blue solution was made 10 times as strong as that used in the laboratory.

Observations in situ and in the laboratory provide information about water movement in differing volumes of soil. In the laboratory, the hydraulic con- ductivity can be tested on one tongue at a time emphasizing the differences between tongues. With the larger volume covered by in-situ tracing, however, a network of tongues can be covered. Possible lateral redistribution of the tracer may therefore be spotted within that network.

Gravimetric water contents and amounts of 1so

Oxygen-18 (180, a n environmental isotope) was used to follow water move- ment in both saturated and unsaturated states under natural conditions (Merot, 1981). This approach permits observations of water movement in different horizons and in parts of horizons.

Two variables were studied: the gravimetric water content and the amount of oxygen-18 in the soil water. The first accounts for the state of moisture in the given material. The second is an indicator of the water movement in nat- ural conditions (Fontes, 1983 ). As a mat ter of fact, for precipitation of a given amount and isotopic content, the vertical distribution of water isotopes along a soil profile (so-called isotopic profile) depends on the transfer and mixing conditions in the soil. The influence of evaporation on isotopic values, which is slight under conditions of this study, will not be considered.

The following protocol was chosen: at the end of the wet season (April 1984) we dug a ditch at the Noe Luce site at about 10 m of the described profile. Sampling was immediately done on the faces of the pit, according to soil struc- ture along four vertical axes (referred to as profiles I to 4), each centred on a tongue. For each profile we sampled at different depths, on the one hand inside a tongue and on the other in its peripheral matrix.

We put our samples directly into flasks with calibrated apertures and kept them in a freezer. The water was extracted in the laboratory with an extraction device under vacuum. The isotopic values obtained after analysis were cor-

146

rected by a coefficient taking into account the specific yield of each distillation t Gascuel-Odoux and Merot, 1986). The results are expressed by the relative isotopic abundance with respect to a standard value. They are determined by a well known method (I.A.E.A., 1983), using a double collecting mass spec- trometer. The relative difference is called the 6%~ value and is defined as:

~%( = (Rsample/Rstandard - - 1 )" 10 :~

with, for oxygen-18, R = [1sO ] / [1~O ]. The standard is the SMOW: Standard Mean of Ocean Water.

Simulation of isotopic profiles

A model with successive mixing tanks (Mtinnich, 1983) simulating a verti- cal dispersive transfer of a tracer along a homogeneous soil profile was used to test the assumptions made for the isotopic tracer. The aim was to compare the field data with computed data in the case of a soil without tongues (or with non-functioning tongues). The model describes the unsaturated zone as a se- ries of soil layers with internally well mixed soil water.

The layer thickness controls the longitudinal dispersion in the course of simulated water movement. The characteristics of the model are based upon physical measurements: field capacity and total porosity. As infiltration pro- ceeds, a portion of P mm water with a 6p%~ amount of oxygen-18 enters step- wise in the top box that formerly contained H mm of water with a 6H%O amount of oxygen-18. The P water is mixed with this stationary water and yields a new tracer concentration:

(~c(i:t = (~p~;i,(: "P+t~Hqle "H)/ (P+ H)

6c%o is the new concentration in the box and of the surplus above the field capacity. The surplus enters the next lower box, where the same mixing process is repeated until the last box is reached. The whole process continues for the next infiltration step.

The initial conditions of the soil moisture and oxygen-18 content of soil water in September, 1983 were estimated. Because of these approximations, the simulated profiles show only general trends. We will compare the last com- puted profiles obtained in mid-April, date of the sampling campaign, with the field profiles measured at the same date.

R E S U L T S A N D D I S C U S S I O N

Profile description (Fig. 1; colors are for moist soil)

O1,Of, Oh A1

7-0 cm; organic matter of the "mor" type. 0-2 cm; very dark grayish brown (10YR 3/2) silt loam; weakly

147

A21g

A22g + Btg

Btgd

developed fine granular structure; few fine roots; clear wavy boundary. 2-22 cm; light grey (10YR 7/2.5 ) silt loam with common, me- dium, reddish yellow (7.5YR 7/6) mottles; weakly developed coarse platy structure; common, very fine and fine, tubular pores; many medium and fine roots with prevailing horizontal orienta- tion; clear smooth boundary. 22-54 cm; this horizon has three distinct parts of equal impor- tance and abrupt irregular boundaries between each other.

( 1 ) Matrix: light yellowish brown (10YR 6/4 ) silt loam; mas- sive structure; few, very fine and fine tubular pores; very dark grey (5YR 3/1) nodules and few strong brown (7.5YR 5/8) coatings on tubular pores; few fine and very fine roots.

(2) "Relict peds of the B horizon": this part is larger at the base of the horizon and located in the continuation of Btgd prisms. Yellowish brown (10YR 5/6) silty clay loam with common, me- dium, brighter (10YR 5/8) mottles and coarse lighter (2.5YR 6/ 4) mottles surrounded by a strong brown (7.5YR 5/8) lining; weakly developed coarse angular blocky structure; few, fine tu- bular and vesicular pores; black coatings on ped faces and in the tubular pores; few black nodules and red (2.5YR 4/6) coatings; no roots.

(3) "Tongues": this part is located above the tongues of the Btgd horizon and has the same characteristics. 54-80/90 cm; this horizon is composed of: (1) a prismatic brown matrix, and (2) subvertical grey tongues surrounding the prisms and constituting an interconnected network in the horizontal plane.

(1) Matrix: strong brown (7.5YR 5.5/6) silty clay loam with common, fine, faint, brownish yellow (10YR 6/6) mottles; mod- erate very coarse prisms parting to moderate medium blocks; common, fine, tubular and vesicular pores; brown (7.5YR 5/4) clay coatings on ped faces and in tubular pores, locally covered in the upper part of the horizon by black coatings; prisms are sur- rounded by a yellowish red (5YR 5/8) lining 5 mm wide, more consistent than the matrix and with a sharp limit to the tongue; no roots; gradual, wavy boundary with the sandy clay alluvium.

(2) Tongues: these tongues are 10 cm wide at the top of the horizon and 2-4 cm wide at the bottom; light grey (10YR 7/1.5) silt loam with brownish yellow (10YR 6.5/6) mottles, scarce in the finer tongues and common in the wider tongues; massive structure; many, very fine to coarse, tubular and vesicular pores,

148

coarse tubular pores are vertical and continuous; ~bw, medium to

fine roots flattened somewhat and with a vertical orientation.

Macroporosi~,

The spatial distribution of the macroporosity of the different horizons was also examined. The tubular porosity of the A21g horizon has an homogeneous spatial distribution. The A22g + Btg horizon is characterized by an important spatial heterogeneity of the macroporosity related to the distinctive parts re- corded in the field description. Macroporosity of the Btgd horizon was ob- served on both vertical and horizontal sections (Figs. 2 and 3 ). Macroporosity of the Btgd matrix is formed mainly by millimetric vughs and channels weakly connected and by fine (<0.2 mm width), unconnected planes. This has an homogeneous spatial distribution with the same aspect in vertical and hori- zontal sections. Even so, a decrease in porosity along a border of 5-10 mm can be noticed at the contacts with the tongues ( Fig. 3 ), corresponding to the strong brown lining observed in the field.

Macroporosity is more important in the tongues than it is in the matrix. It is mainly formed by a highly connected network of millimetric planar voids with non-fitting edges and a prevailing vertical orientation and fine to coarse channels (0.5-3 mm diameter). A vertical crack is quite often observed at the limit between matrix and tongue.

Laboratory analysis

Separate analyses were made of the different parts of A22g+Btg horizon and Btgd horizon. Physicochemical data are displayed in Table I, whereas the mineralogy of the clay fraction is given in Table II. The matrix of the Btgd and the relict peds of the argillic horizon have 21-25% clay as contrasted to 11- 12% in the A21g horizon, the A22g+Btg matrix and the tongues extending down from it into the Btgd horizon. Moreover, different clay mineralogical compositions are observed in these eluvial and argillic volumes. Quartz pre- vails over kaolinite and hydroxy-aluminous vermiculite in the eluvial volumes (A22g, matrix of A22g + Btg, tongues), whereas kaolinite and micaceous clays prevail over smectites in the argillic horizon matrix. Moreover, the cation- exchange capacities of the clay fractions are 26 and 24 meq./100 g of clay for the A22g + Btg matrix and the tongues, respectively, and 36 meq./100 g of clay for the Btgd matrix.

The interpenetrating masses in the A22g+ Btg and Btgd horizons differ in their composition as well as in macroporosity. Distinctions in macroporosity parallel those in proportions and nature of clay. The matrix of the Btgd horizon and the relict peds above it have low to medium macroporosity, mainly con- sisting of poorly connected vughs and channels. In contrast, the A22g+Btg

149

20

40

60

80

"MOOr" Micro- podzol

A21 g

A22 g

8~

B tgd

Fig. 1. Interpretative diagram of the No~-Luce profile.

,(

F

/ '

,'%

Matrix

[ ] Tongue

i i

2 cm

Fig. 2. Vertical macroporosity in the Btgd horizon.

matrix and the tongues have a moderate to high macroporosity that includes large, connected fissural and tubular voids.

The distinctions between these horizons were the reason for their further

150

~-3 ctn '

~ Matrix

Tongue

Fig. 3. Horizontal macroporosity in the Btgd horizon.

investigation. Because of the differences between and within the horizons, they provide natural models for the study of the functions of different structures in the movement of water in soils.

Saturated hydraulic conductivity

The K~a t data for in-situ observations and for cores are presented in Fig. 4 using a logarithmic scale. This scale was used because the Ksat frequency dis- tribution is generally lognormal (Nielsen et al., 1973; Freeze, 1975). Permea- bility measured in situ is low: 0.6.10 -6 m s-1. It fits class L1 as defined by McKeague et al. (1982) which, according to those authors, corresponds to a silty or clay material with few biopores and massive structure or a more or less expressed coarse structure in which structural elements are strictly matching. Such conditions correspond well to those of the soil under study.

As the set of data is too small, the mean values of in-situ Ksa t and column Ksat (A22g + Btg, Btgd) are not statistically different. We did, however, notice the following: (i) A22g + Btg columns have a little higher Ksa t values and the

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152

TABLE II

Clay fraction mineralogy by horizons of the Nod-Luce profile

Horizons SM CL VR1 VR2 VM MI KK QZ FD

A21g 1 1 3 - 2 3 4 2

A22g + Btg: A22g matrix 1 3 2 3 4 2 Relictual B clods 1 2 1 1 4 3 2 1

Btgd: Matrix 3 2 2 - 2 4 4 2 Tongues - 2 2 1 2 3 4

Mineral code: SM = smectite; CL = chlorite; VR1 = hydroxyaluminous vermiculite; VR2-- s.s. ver- miculite; VM=interstratified vermiculite/mica; MI=mica; KK=kaolinite; QZ=quartz; FD = feldspar. Approximate weight fractions (x-ray): 4 = one-half to one-third; 3 = one-third to one-fifth; 2 = one- fifth to one-twentieth; 1 = less than one twentieth.

<a)-

<b)-

(c)_

• • • l+, m + e

• "l • • •

I I

8 -7 -'s -'5 log Ksa t

Fig. 4. Saturated hydraulic conductivity: vertical bar is mean value; (a) =A22g+Btg columns; (b) =Btgd columns; (c) =in-situ measurements.

scat ter of data is smaller t h a n for B tgd columns; (ii) m e a n value of Ksat for B tgd cores is very similar to in-si tu Ksa t measurements . The larger sca t ter of Ksat da ta for the B tgd hor izon (magni tude 100) under l ines its more heteroge- neous nature . As a tongue was sampled in each Btgd column, this means t h a t p robab ly non - func t i on ing tongues exist.

Methylene-blue tracing

In-s i tu tracing. Fig. 5 shows the color features observed in a vert ical section. In the A 2 2 g + B t g hor izon (35-50 cm) relict peds of the B hor izon are free of color; color features are main ly in the tongue areas. In the first cen t imet res the

153

[ ] Hethylene blue stained feafures

] Tongues

] Relictual B clods

Fig. 5. In situ methylene-blue stained features displayed on a vertical section.

] A22g and Bfg, matrix and bleached zones

] Bfgd , mafmx

35cm

45

55

65

.75

whole tongue is coloured but then the colour decreases rapidly with depth. In this horizon, saturated flow may be related mainly to the tongues but the ty- pology of the relevant porosity cannot be defined.

In the Btgd horizon a few coloured areas can be noticed in the matrix. These are pores communicating with the tongues: wide planes related to the prismatic structure and cracks, channels and vughs inside the matrix. But the main col- oured features are located in the tongues. Some tongues are not coloured and others are coloured only at their bases. Coloured features relate to coarse pores ( > 1 mm), interconnecting large planes, channels and vughs, i.e. a porosity characterized by its strong continuity. The lack of coloration of some tongues may be related either to a shading effect, i.e. a lack of top feeding or to the existence of non-functional tongues with a poorly connecting porosity. If the tongues form a three-dimensional interconnected network, the presence of tongues with only base coloration can be explained by lateral flow.

The in situ tracing therefore confirms that in a Glossaqualfpreferential flow in a saturated state occurs in the tongues and more particularly in their coarse interconnected macroporosity.

Tracing in cores. In the first 3-7 cm, a decrease of the coloured surface is

154

generally observed. This is probably due to methylene-blue diffusion caused by the duration of the trials. Furthermore, there is no relationship between depth and surface colouring. Only the sides of the voids are then coloured to a thickness of 0.5-1.5 mm, without any extra diffusion.

In the A22g+ Btg cores, additional information is obtained as compared to the in-situ tracing. Here, tongue areas are not completely coloured: bleached and very dense areas with no coloration whatsoever are observed. We can al- ready spot tongues which are more or less functional due to the morphology of the pore system. Relict peds of B horizon remain free of coloration apart from a few channels communicating with bleached areas and rare channels not ap- parently connected with a coloured area, at least on any observed surface. The behaviour of these relict peds is therefore similar to that of the Btgd matrix.

Lastly, the A22g+ Btg matrix shows an intermediate behaviour. Macropores larger than 1 mm diameter free of colour can often be found whereas others are coloured.

Color features observed in Btgd cores support the observations in situ. Most of the coloured areas are found either in the coarse connected porosity of the tongues or in channels and cracks inside the matrix connected with the col- oured area of a tongue {Fig. 6 ). Nevertheless, at the top of the column, coloured areas occur inside the matrix, showing the presence of potentially conducting pores. This seems to be due to the sampling of the cores where the prism tops are truncated whereas they normally are covered by impervious relict peds of the B horizon in situ. One of the cores shows very little color in the tongue corresponding to a Ksat of 3" 10-7 m s - ~. The existence of low-functional tongues is thus indicated.

The hydraulic behaviour in a saturated condition of this Glossaqualf with

, ~ I c m

F~FT~ Matrix

Tongues

S~ained macropores

Fig. 6. Stained macropore features displayed at the bo t tom of a core of the Btgd horizon.

155

methylene-blue tracing can be summarized as follows: (1) tongues have their specific behaviours and constitute preferential flow patterns; (2) this specific- ity can be observed at the base of the A22g + Btg horizon where tongue volumes behave differently from relict peds of the B horizon; (3) this behaviour is di- rectly related to the high macroporosity as well as coarse and highly connecting porosity of the tongues; (4) non- or low-functional tongues are observed which reflects high variability of hydraulic conductivity measurements made on cores and is related to the variability in the morphology of the macroporosity of the tongues; (5) the low conductivity on the Btgd matrix is shown as well as that of relict peds of the B horizon in the A22g+ Btg horizon.

Soil water contents and oxygen-18 amounts

The soil data are presented by water content profiles (Fig. 7) and by isotopic profiles (Fig. 8). At the same depths different values may appear: on the one hand, a value from a sample in the tongue and, on the other, one or two values from samples in the matrix from each side of the tongue.

The precipitation was sampled near the Noe Luce site. The amounts of pre- cipitation and their isotopic values are presented in Table III. To show clearly the significant variation of oxygen-18 amounts, we have presented {Fig. 9) the

Profile 1 Prof i le 2 Profile 3 Profile 4 0,20 0,25 0,20 0,25 0,20 0,25 0,20 0,25

10 . . . . . . .

20 ~ . . . .

T! 3 0 - - - ::

l

I" ~o |

50 i] I i

60-

70- ]

8O

, o ] ! 1oo

! I "

[1' . . . .

l]

- - ] - -

!

J

(cm.) Depth

Fig. 7. Gravimetric soil moisture profiles l i n e s = tongues.

| [ 1 :

I : !

] ' I

] !

l

i 7 !

]' ! !

I I

7 i

I l

(mid-April 1984). Plain l ines=matrix; dashed

156

90

100 "

(c m ) O e p t h

--8 - 7 - 8 - 7 - 8 7 -8 ,~ [ % ° )

4

: i }" p ,

" - ! . . . . . . . - t -

' ' 1 ± " ' , , 1 " ¢ 0 0 . . . . . . . . . . .

, ! 1 !

,~o- i I i ~ L ,

Fig. 8. P r o f i l e s o f o x y g e n - 1 8 v a l u e s o f soil water ( m i d - A p r i l 1 9 8 4 ) . Plain lines=matrix; dashed lines = tongues.

T A B L E I I I

Raw data of precipitation, for each sample date: oxygen-18 v a l u e s ( i n 6%,; ) and amounts of p r e -

c i p i t a t i o n (in m m )

Date H ( m m ) 5%c D a t e H ( m m ) 6%c

1983: 1984:

12.09 2.9 - 4 .50 3.01 19.0 - 5 .73

12.09 16.9 - 5 .66 3.01 2.5 - 7.59

18.09 17.0 - 2 .84 6.01 5.0 - 6 .22

10.10 7.0 - 2 .70 12.01 12.0 - 5 .67

17.10 12.0 - 6 .04 25.01 39.0 - 6.81

7.11 1.7 - 4 .96 31.01 20.0 - 8 .55

21.11 1.0 - 4.32 17.02 10.0 - 6 .29

28.11 23.0 - 4 .10 14.03 16.0 - 9 .65

5.12 3.5 - 3 .39 23.03 2.6 - 8 .88

12.12 28.0 - 11.37 24.03 10.0 - 11.84

19.12 15.0 - 9 .61 25 .03 34.0 - 8 .10

21 .12 21.0 - 6 .80 17.04 40.0 - 6 .52

isotopic values weighted for a given amount of precipitation rather than the monthly weighted values. The essential cause of water transfer in a soil is not the time but the quantity of water provided.

Soil water contents. For profiles 2, 3 and 4 the water contents are at the same level, clearly higher in the tongues than in the matrix (Fig. 7). This is some-

157

Cummulofed prectpitofions 100 200 300 400 [mm.)

I I I i

- 4. ~5_10 ~ P

\ - 5 "

~5_11 - 6 2 1o_oi

f"~20_01 47 04

-,- k,o_, l \ / °:

-10"~ (%o)

Fig. 9. Weighted isotopic values for a given amount of precipitation, versus the cumulative precip- itation: between each date (15-10/25-11; 25-11/10-12 etc...), 38 mm rain fell with the corre- sponding oxygen - 18 values.

times observed by pedologists describing this type of soil (Gascuel-Odoux, 1984) and is generally presented as a mark of specific hydric behaviour of the tongue, zone of preferential transfer. On the other hand, we noticed that for the first profile the tongue did not exhibit a water content different from the matrix: we inferred that this tongue was not functional.

Oxygen-18 amounts. As depicted by Fig. 8, the oxygen-18 contents in the tongues of profiles 2, 3 and 4 are, at each level, greater than in the surrounding matrix. The contents are also less variable in each tongue than in the matrix; the water in the tongues is isotopically more homogeneous. For these three tongues, the 5%o values range from -6.15 to -7 .38 (mean-- -6 .91 , standard deviation--0.34). For the matrix, the 5%o value ranges from -6.25 to -7.97 (mean= -7.33, standard deviation--0.39). Moreover, this difference in- creases with depth. Therefore, for these three profiles the water movement is greater in the tongues than in the surrounding matrix. In contrast no differ- ence between tongues and matrix was evident in profile 1.

The mean 5%0 value of the tongues in profiles 2, 3 and 4 is roughly equal to the value of recent precipitation (mean 5%o for the 10-4 and 17-4 precipita- tion = - 6.93 ). The 5%o value for the matrix is more negative, as is the winter precipitation (mean 5%o from the 25-11 to 20-3 precipitation = -7 .4) . Thus at the same depth, the water in the tongues is younger than the water in the peripheral matrix. The oxygen-18 data thus indicate that tongues with higher water contents are also more favorable pathways for water movement.

Comparison with a computed "isotopic profile". A model simulating vertical transfer of water in a homogeneous soil was described in the section Materials and Methods. After each rain, the model gives a simulated "isotopic profile"

158

but only the last one (mid-April) is shown in Fig. 10. For comparison, parallel data are included for profile No. 1 in the graph. Our evidence indicates that water movement is uniform or nearly so in profile No. l, the tongue apparently being non-functional.

The isotopic values for the "computer profile", the tongues and matrix of profile No. 1, and the matrixes of profiles No. 2, 3 and 4 have negative values (6%~ = - 7 . 6 to - 8 . 0 5 ) at depth. For the "computer profile", the values are stable throughout the winter months. The "computer profile" also has a neg- ative value of 6%(, = - 7.8 between depths of 10 and 20 cm and a smaller neg- ative value of 6%~ = - 6.65 at a depth near 45 cm. On the basis of the oxygen- 18 values of the rainfall, the negative value at the shallower depth seems due to precipitation in March, especially on the 14th and 25th, whereas the second and lower value at greater depth is at t r ibuted to the negative values of precip- itation in January. From these data, the modal effective velocity of water trans- fer in the profiles can be calculated (Merot, 1985 ) as about 6.4-10 -8 to 7.5-10 -s m s -]"

The computed "profile" values fit most of the measured values for profile 1 fairly well. We therefore concluded that there is no preferential transfer in the latter profile. On the contrary, for profiles No. 2, 3 and 4, the values of the matrix water are slightly more negative than for the computed "profile". More

z,O.

50-

60-

70-

80-

90-

I00-

-8 -7 0

10-

20-

30. [

%o

' c318~

(cm) Depth

Fig. 10. Computed profile (continuous double line ) compared with profile 1 (small vertical lines ), for oxygen-18.

159

striking, however, is the fact that the oxygen-18 amounts of the tongue water (from -6 .2 to -6 .85) do not fit those of the simulated "profile". The differ- ence seems due to an important input of recent precipitation (40 mm with 5%o = -6.52) in these tongues during the last two weeks before sampling. Therefore, inside these tongues there is a rapid and preferential transfer of water to a depth of 90 cm. The velocity of the water is about 10 to 15 times greater in the tongues than in similar soils with non-functional tongues (as profile 1 ).

CONCLUSIONS

Tracing water movement by oxygen isotopes, methylene blue and morphol- ogical analysis of the porosity provide converging lines of evidence and com- plement one another. Environmental tracing by isotopes shows that under natural conditions in saturated or unsaturated states, mean water transfer is ten times faster in the tongues than in the Btgd matrix, even though some tongues are non-functional. The values obtained by in-situ Ksat measurements of the entire Btgd horizon show a similar trend for the conductivity of water. Lastly, methylene-blue tracing shows that the only functional porosity under saturated conditions comprises the coarser and more continuous macroporos- ity inside the tongues.

These different approaches exhibit the specific role of a pedologic feature - - the tongues in Glossaqualfs - - on water movement. Compared to studies show- ing preferential flow along macropores, of structural (Bouma, 1981, 1984) or biological (Smettem and Collis-George, 1985) origin, the tongues have a mi- neralogical constitution quite different from the matrix - - the clay content and the CEC of the clay are lower in a tongue than in the matrix. This may enhance the role of preferential flow in the tongues and the chemical filtering capacity of this type of soil.

ACKNOWLEDGEMENTS

The work of the Laboratoire de Biogdochimie des Isotopes Stables (INRA) where the isotope analyses were performed is gratefully acknowledged. More- over, the authors are indebted to the referees of Geoderma for their construc- tive criticism and suggestions for improvement of the presentation.

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