Identification, characterization, and hydrological implications of water repellency in mountain soils, southern British Columbia

Identification, characterization, and hydrological implications of water repellency in mountain soils, southern British Columbia

CATENA vol. 16, p. 477-489 Cremlingen 1989 ] I D E N T I F I C A T I O N , CHARACTERIZATION, A N D H Y D R O L O G I C A L I M P L I C A T I O N S ...

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vol. 16, p. 477-489

Cremlingen 1989 ]

I D E N T I F I C A T I O N , CHARACTERIZATION, A N D H Y D R O L O G I C A L I M P L I C A T I O N S OF WATER R E P E L L E N C Y IN M O U N T A I N SOILS, S O U T H E R N BRITISH C O L U M B I A G. Barrett, Mississauga O. Slaymaker, Vancouver Summary The physicochemical properties of soils, which determine how readily the soils wet, were shown to vary significantly in mountain soils collected at six sites in southern British Columbia, even within individual profiles. The results of water drop penetration time tests were used to classify samples using a very simple scheme which is based upon our current understanding of the possible physicochemical interactions between solid surfaces, water, and soil air. In all cases where the samples were collected at sites in the subalpine-alpine ecotone, a layer which either wets reluctantly or is water repellent exists at or near the surface of the profile. These layers occur only where there is evidence for accumulation of organic matter, and are usually no more than a few centimetres thick. At the one site which was below the alpine-subalpine ecotone, the soils wet readily throughout the profile. These results suggest that the type of organic matter which accumulates in soils of the alpine-subalpine ecotone of southern

British Columbia either limits the affinity of soils for water or renders the soil water repellent. The relation between infiltration rate and ponding depth was explored experimentally for a set of soil samples from a site in which a repellent layer was developed to depths greater than thirty centimetres. It was found that the infiltration rate, which was less than 2.0 millimetres per day for all samples, was insensitive to changes in the ponding depth to a maximum applied depth of 400 millimetres, and that it remained approximately constant over time. These results suggest that water was transported primarily as a vapour rather than as a liquid. Given that water repellent soils are not uncommon in the alpine-subalpine ecotone of southern British Columbia, and that ponding depths on hillslopes would be several orders of magnitude less than those applied experimentally, it is inferred that transfer of water as a vapour within may be an important mechanism in such environments.

ISSN 0341 8162 @1989 by CATENA VERLAG, D-3302 Cremlingen-Destedt, W. Germany 0341-8162/89/5011851/US$ 2.00 + 0.25

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Molecule

hydroF $o1~ surf~

hydrophilic

hydrophobic

Fig. 1: Coating of a surface by amphophilic molecules. In their interpretation of variable solute sources and hydrological pathways in a small subalpine basin in British Columbia, G A L L I E & SLAYMAKER (1984) suggested that water repellency could account, in part, for the observed tendency of water to bypass the soil matrix. They noted that water repellency seemed to be preferentially associated with three of the six soil-vegetation complexes found in their field area. In explaining the relatively low solute yields from these same soils, G A L L I E & S L A Y M A K E R (1985) inferred the importance of hydrophobicity in years when segregated soil ice did not form. Reasonable though these assumptions were, they were based on inference from scattered observations and BARRETT's Masters thesis (1981). No systematic analysis of the phenomenon of water repellency was undertaken. Water repeUency appears to be associated with accumulation of certain types of organic compounds on mineral grains. BOZER, B R A N D T & H E M W A L L (1969) suggested that the molecules involved are probably amphophilic; that is, they have a polar

end which is attracted to mineral surfaces, and a nonpolar end which is directed outwards to form a hydrophobic surface (fig.l). It has been documented that forest fires and slash burning may result in transformation, volatilization, and distribution of organic compounds to produce a repellent layer, which may revert to a non-repellent condition over a number of years (DEBANO, M A N N & H A M I L T O N 1970, SAVAGE, OSBORNE, LETEY & H E A T O N 1972, R E E D E R & J U R G E N S O N 1979, and GIOVANNINI & L U C C H E S I 1983). Fungi and algae have been implicated in the production of water repellent soil (BOND & HARRIS 1964, SAVAGE, M A R T I N & L E T E Y 1969, and M I L L E R & W I L K I N S O N 1977), as have a variety of plant species, especially those native to semiarid environments. The vegetation implicated includes: Chaparral in California ( K R A M M E S & D E B A N O 1965), Juniper in Utah (SCHOLL 1971) and Mallet trees in Australia ( M C G H I E & POSN E R 1980). As far as the authors are aware, there has been no previous documentation of water repellency as char-

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Water Repellent Mountains, British Columbia

acteristic of soils in the alpine-subalpine ecotone of southern British Columbia. This paper begins with a discussion of the conceptual basis for characterization of the physicochemical properties of soils in general, and of water repellent soils in particular. The results of field studies of water repellent mountain soils at the site studied previously by G A L L I E & SLAYM A K E R (1984, 1985) and at other sites in southern British Columbia are presented. The paper closes with a summary of the results and conclusions. A subsequent paper will explore the implications of this phenomenon for overland flow and runoff generation ( B A R R E T T & S L A Y M A K E R , in preparation).

Characterization of the physieoehemieal properties of soils The contact angle, ~, is often used to characterize the physicochemical properties of a surface: if it is less than ninety degrees, the surface is hydrophilic, and if it is greater than ninety degrees, th surface is hydrophobic. The contact angle for a solid-air-water system is defined by the Young equation: (~'~o -

~w)/'~w,,

= cosM

(1)

where as~, Osw, and ~rw~ are, respectively, the solid-air, solid-water, and water-air interfacial tensions. This equation applies only for: (2) since the cosine of the contact angle is defined only within these limits. If the ratio is greater than one; that is, if:

(os.--a~w)/aw. > 1 ('AI ENA

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then a film of water will spread over the surface, eliminating the solid-air interface entirely. Under such conditions, the angle of contact is effectively zero: it no longer varies as a function of the difference between the solid-air and solidwater interfacial tensions. For hydrophobic surfaces, the absolute value of the difference between the solid-air and solidwater interfacial tensions may exceed the water-air interfacial tension; that is:

(o~a-asw)/Owa < - 1

(4)

If equation (4) is satisfied, the Young equation is not satisfied; although a drop of water will rest upon a film of air such that the contact angle is effectively 180 ° . These limitations of the contact angle as a measure of the affinity of a surface for water are often overlooked, even though they are associated with significant thresholds in the way water interacts with solid surfaces. If the contact angle is zero, water spreads spontaneously over the entire solid surface, thus guaranteeing continuity of the liquid phase. This is, in fact, one of the central assumptions underlying RICHARDS'(1931) development of the theory of capillary conduction in porous media: water can flow from any point within the medium to any other point through this film of water. Although the issue is not pursued further in this paper, discontinuity of the liquid phase is expected to have a significant effect upon the mechanism of transfer of water in porous media, especially if it is water repellent. For soils in which the contact angle is 180 °, a film of air will remain on solid surfaces, thus ensuring continuity of the gaseous phase. For contact angles between zero and 180°, there is no guarantee of continuity of either phase. GEOMORPHOLOGY

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In situations where a soil is dry initially, transient water repellency is sometimes observed (see, for example WESSEL 1986, and J U N G E R I U S & VAN DER M E U L E N 1988). A number of changes in the physicochemical properties of soils may be involved. It has been observed that the solid-air interfacial tension of some solids decreases as the air approaches saturation (OSIPOW 1977). It is also possible that the solid-water interfacial tension may decrease as the surface interacts with liquid water; it seems plausible that relatively slow chemical changes may take place, or that amphophilic compounds may be displaced slowly from mineral surfaces in the presence of water, perhaps forming micelles. One or both of these changes will affect the angle of contact. TOPP (1966) suggested that surface active agents derived from soil surfaces may produce changes in both the waterair and solid-water interfacial tensions, thus causing the contact angle to change. It is important to understand that water repellent soils can be saturated if a pressure is applied which is sufficiently high to force water into the pores. The pressure required to force water into a pore depends upon the physicochemical properties and the geometry of the pore: the higher the contact angle or the smaller the "neck" of the pore, the higher the pressure required to fill the pore. If the pressure applied to a sample is reduced, air will enter the soil. The narrowest portions of the pores fill at the highest pressures, hence these "necks" of the pores are expected to drain first, leaving the wider bodies of many pores filled. Thus, during the draining of a water repellent soil, the liquid phase may become discontinuous: filled pores will empty only through transport of water CATENA

vapour. The water content of a repellent soil may, therefore, be higher than would otherwise be expected. It is important to differentiate between this apparent loss of repellency and the true loss of repellency due to alteration of the physicochemical properties of soils through interaction with water.

2

Classification of the physicochemical properties of soils

There is no reason to expect, a priori, that the physicochemical properties of a soil matrix will be the same everywhere. Within soils, surface properties will vary as a result of heterogeneity of the inorganic and organic components exposed at the surface of grains: hydrophobic and hydrophilic sites may exist side by side. Soils may be repellent or nonrepellent, or they may have mixed properties. There is no test procedure which adequately reflects the entire range of possibilities, but a simple procedure such as the water drop penetration time test (see LETEY 1969) may be used to place a soil within broad categories. Very short penetration times indicate that hydrophilic surfaces dominate, while long penetration times suggest that hydrophobic surfaces dominate. For soils with intermediate penetration times, several interpretations are possible. The soil may have a very low affinity for water; that is, the contact angle is close to ninety degrees. Alternatively, the soil may exhibit transient water repellency: after exposure to water the contact angle falls gradually to a value below ninety degrees. These interpretations of the water drop penetration time test results are summarized in tab. 1.

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> 600s water repellent 10 - 600s limitedaffinity or transient repellency < 10s non-repellent Tab. 1: Interpretation of water drop penetration time tests.

3

Field and laboratory investigations

There were three phases to the field and laboratory investigations: the first extends the observations of G A L L I E & S L A Y M A K E R (1984, 1985) regarding repellency at G o a t Meadows; the second involves a limited survey of the physicochemical properties of soils in the subalpine of southern British Columbia; and the third is concerned with estimation of infiltration rates in water repellent soils from Ash Lake, a site near Goat Meadows. The design and results of these investigations are described in the sections following.

3.1

Goat Meadows

Goat Meadows Basin is located on a ridge top near Pemberton, British Columbia (fig.2). Bedrock is primarily late Cretaceous metasediments of the Gambier Group, which forms a roof pendant on quartz diorite of the Coast Plutonic Complex ( W O O D S W O R T H 1977, R O D D I C K 1976). Surficial deposits are primarily loess, colluvium, and talus. Orthic and Dystric Brunisols are developed in the loess, which blankets bedrock to a depth of about thirty centimetres on average. G A L L I E (1983) estimated that the annual precipitation averages about 1800 millimetres, 1500 millimetres of which falls as snow. The samples were collected from each of the three soil-vegetation assemblages identified by G A L L I E & S L A Y M A K E R

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(1984) as possibly being associated with water repellency (see fig.3). The samples were air-dried, and then characterized by application of the water drop penetration time test at one centimetre intervals. The results of these tests are shown graphically in fig.4. Ten of the thirty samples had layers which exhibited permanent water repellency in a layer which was no more than three centimetres thick, and which was never encountered below a depth of three centimetres. In some cases the repellent layer was coincident with the root mat, but more often it extended below it. The repellent layer was always associated with zones which show evidence of enrichment in organic matter, through variations in colour. Water repellent layers were found in samples from all three of the soil-vegetation assemblages sampled. Twenty of the samples lacked a repellent layer, but all had a layer characterized by a long penetration time. In addition, such layers were found to grade into the water repellent layer in most of the remaining ten samples. These layers were up to six centimetres thick, and extended to six centimetres below the surface. As for the repellent layers, they were never associated with layers which did not show some visible evidence of accumulation of organic matter.

3.2

Field survey of sites in southern British Columbia

In order to determine if water repellent and water resistant layers are present

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__~X\%E=°E.,O."

E ( ~ " LAKEL~MEADOWS

"---

o

PEMBERTON VALLEY

ILK LAKEPARK PEAK lEE GLACIERPARK

Fig. 2: Sample sites.

k n ...,=., ~ Sod°. D =.==J

Fig.

3:

Goat Meadows sampling

sites.

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0

£¢D a

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2

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4 5 6 Sedge samples

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[ ] reduceditltniiy

[ ] IoeoI

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[ ] non-feint

Fig. 4: Characteristics of the Goat Meadows samples. at other subalpine locations in southern British Columbia, a limited survey was undertaken. As indicated in fig.l, four additional sampling sites were selected:

3. Kokanee Lake, where the samples were collected in a subalpine meadow at an elevation of approximately 1940 metres, below tree limit; and

1. Manning Park, where the samples were collected in a subalpine meadow at an elevation of approximately 2150 metres, just below tree limit;

4. Elk Lake Park, where the samples were collected in a meadow at an elevation of approximately 1800 metres.

2. Idaho Peak, where the samples were collected in a subalpine meadow at an elevation of approximately 1800 metres, below tree-limit; CA'II!N a

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All of the sites are clearly subalpine meadow, except for the Elk Lake Park site, which is well below local tree-limit. Samples were collected by driving PVC (polyvinylchloride) tubes with an inside diameter of 76 millimetres, into the GEOMORPHOLOGY

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E

0

i J=

G] 20 1

2

1

2

3 4 Manning Park samples

5

6

5

6

? a

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Idaho Peak samples

U"I non.repellenI i

bottom o~

sample a

20

1

2

3

4

Fig. 5: Characteristics of

Elk Lake Park samples

the survey samples.

earth. The tubes were sharpened to limit disruption of the samples. Six samples were collected from each site; however, two of the samples from Elk Lake Park and two from Idaho Peak were disrupted during transport, and could not be analyzed. The tubes were opened by cutting a window in the tubing parallel to its axis with a table saw. The samples were air dried, and tested at one centimetre intervals using the water drop penetration time test. The results of these tests are summarized in fig.5.

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All samples from the sites at Manning Park, Kokanee Lake, and Idaho Peak had layers which appeared to be either water repellent or water resistant, but not even one of the four samples from Elk Lake Park appeared to have either a repellent or a resistant layer. The layers were typically on the order of a few centimetres thick, and were confined to the top few centimetres of the profile. The repellent and resistant layers were never associated with portions of the profile which did not exhibit the usual signs of accumulation of organic matter.

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These results suggest strongly that water resistant and water repellent layers are common in the subalpine of southern British Columbia, and that they are associated with the accumulation of organic matter in the profile. No attempt was made to stratify samples according to soil-vegetation complex, but it is significant, perhaps, that the samples from Elk Lake Park, which were collected below the alpine-subalpine ecotone, did not exhibit water repellency. However, the thct that the material was rather coarse, and was in an area which may have been subject to surface erosion confounds interpretation. Further sampling and analysis is necessary to resolve the issue. It is clear that the presence of a water repellent or of a water resistant layer will have a profound impact upon the hydrological behavior of a basin, as the observations of G A L L I E & S L A Y M A K E R (1984, 1985) demonstrate. Field observations in other basins would, of course, add to our understanding of the role of these layers in regulation of hydrological processes in the subalpine, but in order to achieve a more clear understanding of these processes, laboratory determination of the relevant hydrological characteristics of these repellent or resistant layers is necessary. Thus, the next phase of our research focused upon determination of these properties. 3.3

Infiltration into water repellent soil - - the Ash Lake samples

The fact that the properties of soil samples varied rapidly in the vertical makes analysis difficult. Conventional methods of preparation of soil samples, which may involve grinding of samples and destruction of organic matter using an oxidizing agent, such as hydrogen per(AI ENA

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oxide, are unacceptable for analysis of these soils. An intact sample with homogeneous characteristics is desirable for analysis of infiltration; thus, an attempt was made to locate a site where a repellent layer was particularly deep. A suitable site, which will be referred to as the Ash Lake site (fig.l), was identified. It is located less than one kilometre downstream from the site studied by G A L L I E & S L A Y M A K E R (1984, 1985). Ash Lake is located within the alpinesubalpine ecotone at an elevation of about 1680 metres. The parent material of the soil appears to be primarily a clay-silt lacustrine deposit, which is rich in what appears to be peat. The depth to till may exceed one metre, which is far greater than the ten to thirty centimetres observed at the Goat Meadows site. Vegetation is comprised primarily of grasses and moss. Preliminary analyses suggested that water repellency was developed to a depth in excess of thirty centimetres at this site, which is far greater than that observed at any of the other sites. Eight soil cores, each thirty centimetres in length were collected for analysis (fig.6). The samples were sealed immediately to prevent drying. It was expected that the infiltration rate would be very low until the ponding depth exceeds some critical value. The critical depth, hp, required to fill an individual pore is expected to be governed by the equation:

hp = -2¢rwacos [~]/pgr

(5)

where p is the density of water, g is the acceleration due to gravity, and r is the minimum radius of the pore (it is assumed that the pore is radially symmetrical). Equation (5) differs from the equation for capillary rise only in sign. GEOMORPHOLIIGY

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O f course, since the sizes of pores are distributed over a range of values, saturation of a water repellent soil is expected to occur gradually as the pressure is increased. Water was ponded above the samples using the apparatus illustrated in fig.7. The bottoms of the samples were vented, so that flow from the base was possible. The observation tube had only one sixtyfourth of the cross-sectional area of the sample, thus providing a sixty-four fold increase in the precision of determination of the amount of water infiltrating into sample. Changes in moisture within samples were determined by following changes in the weight of the sample. In preliminary experiments, it was found that at ponding depths greater than 500 millimetres, sealing of the samples became a problem: water began to flow between the sample and the container walls. Thus, the ponding depth was not increased beyond 400 millimetres in the experiments. It was expected that as the ponding depth was increased in steps, the infiltration rate would increase due, in part, to the increase in the pressure head at the surface, but more importantly to the increase in hydraulic conductivity which should accompany wetting of an ever greater fraction of the pores up to the saturation point for the soil. Cumulative infiltration is plotted against time in fig.8 for all eight samples. The rate of infiltration is given by the slope of the line: the greater the slope, the greater the infiltration rate. As can be seen from fig.8, the infiltration rate is essentially constant for all of the samples, even though the ponding depth is increased in steps to a maximum of 400 millimetres. These results were contrary to our expectations. The most straightforward interpreta( A I ENA

tion of these results is simply that the ponding depth was insufficient to cause most pores within the sample to fill. If the contact angle of the soil were 180 °, then application of equation 5 suggests that a ponding depth of 400 millimetres will fill only pores with "necks" of radius 0.037 millimetres or larger. Given the fine texture of the soil, it is not unreasonable to expect that only a small proportion of the pores would be filled at this pressure.

4

Summary and conclusions

The results presented in this paper suggest that water repellent layers are not uncommon in the alpine-subalpine ecotone of southern British Columbia: they were identified at four of the five sites studied. The repellency of these layers appears to be "permanent"; that is, there is no obvious diminution of the effect over time as a result of contact with water. The layers appear to grade into layers which are penetrated only slowly by water, which suggests either that they have a very limited affinity for water or that they exhibit transient water repellency. Both types of layer tend to extend from the surface to a depth of a few centimetres, although at one site, water repellency was developed to a depth of more than thirty centimetres. These layers appear to be associated with the accumulation of organic matter in the profile. The research results presented in this paper, while important for understanding of the alpine-subalpine ecotone in southern British Columbia, have implications for other environments. The contact angle is not determined routinely; it is usually determined only if water repellency is expected. Indeed, it was the

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Fig. 6: Ash Lake sampling sites.

Water

head Stopper

Soil

,.- Screen

_

('Aq E N A

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Fig. 7' Apparatus for measurement of infiltration.

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150

porld~ depth (nYn) I 250 ] 350 [

400

2O



samote 1

• 8

SanW,e2 sample3 san'ere4



sample 5

---o---

samples Saml~ 7

*

lO~ ~e E

sample 8

i

0

10

20

Time (days)

Fig. 8: Cumulative infiltration for Ash Lake samples. search for water repellent layers which led to our discovery of layers which have a limited affinity for water. It seems probable that significant variations in the contact angle occur in many soils, associated perhaps with variations in the type and amount of organic compounds in the profile. The contact angle of a soil is not directly observable in the way that soil texture, the other major determinant of infiltrability is. Furthermore, standard methods of preparation of soil for determination of hydraulic properties may involve crushing, sieving, and destruction of organic matter with oxidizing agents such as hydrogen peroxide. Thus, the impact of variations of the contact angle upon hydrological processes may routinely be underestimated. The importance of the physicochemical properties of soils is illustrated by the results of experiments with the extemely repellent samples collected from the Ash Lake site, which indicate clearly that the physicochemical properties of soils can have a profound effect upon both the

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rates and mechanisms of water transport through soil. References BARRETI', G.E. (1981): Streamflow generation in a subalpine basin in the Coast Mountains of British Columbia. M.Sc. thesis, University of British Columbia, 89 p. BOZER, K.B., BRANDT, G.H. & HEMWALL, J.B. (1969): Chemistry of materials that make soils hydrophobic. In: Water repellent soils Proceedings of the symposium on water repellent soils. University of California, Riverside, May 6 10, 1968, 1~. BOND, K.B. & HARRIS, J.R. (1964): The influence of the microflora on physical properties of soils I. Effects associated with filamentous algae and fungi. Australian Journal of Soil Research 2, 111 122. DEBANO, L.F., MANN, L.D. & HAMILTON, D.A. (1970): Translocation of hydrophobic substances into soil by burning organic litter. Soil Science Society of America Proceedings 34, 130133. GALLIE, T.M. (1983): Chemical denudation and hydrology near tree limit, Coast Mountains, British Columbia. 262 p. GALLIE, T.M. & SLAYMAKER, O. (1984): Variable solute sources in a coastal sub-alpine environment. In: T.P. Burt and D.E. Walling

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(eds.), Catchment experiments in fluvial geomorphology. Geobooks, 347-357. GALLIE, T.M. & SLAYMAKER, O. (1985): Hydrological controls on stream chemistry. In: Water quality evolution within the hydrological cycle of watersheds. Proceedings of the 15th Hydrology Symposium of the National Research Council of Canada, Quebec City, 228-306.

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SAVAGE, S.M., OSBORNE, J., LETEY, J. & HEATON, C. (1972): Substances contributing to fire-induced water repellency in soils. Soil Science Society of America Proceedings 35, 674678. SCHOLL, D.G. (1971): Soil wettability in Utah Juniper stands. Soil Science Society of America Proceedings 35, 344 345.

GIOVANNINI, G. & LUCCHESI, S. (1983): Effect of fire on hydrophobic and cementing substances of soil aggregates. Soil Science 136, 231-236.

TOPP, G.C. (1966): Surface tension and water contaminants as related to the selection of flow system components. Soil Science Society of America Proceedings 30, 128 129.

J U N G E R I U S , P.D. & VAN DER M E U L E N , F. (1988): Erosion processes in a dune landscape along the Dutch coast. CATENA 16, ??%???

WESSEL, F. (1986): Water repellency of dune sand in relation to the volumetric moisture content and the organic matter content. Gereserveerd voor CATENA S U P P L E M E N T

KRAMMES, J.S. & DEBANO, L.F. (1965): Soil wettability: a neglected factor in watershed management. Water Resources Research 1, 283-286.

W O O D S W O R T H , G.J. (1977): Geology of the Pemberton (92J) map area. Geological Survey of Canada Open File Report 482, 1 map sheet.

LETEY, J. (1969): Measurement of contact angle, water drop penetration time, and critical surface tension. In: Water repellent soils - Proceedings of the symposium on water repellent soils, University of California, Riverside, May 6-10, 1968, 4347. MCGHIE, D.A. & POSNER, A.M. (1980): Water repellence of a heavy textured Western Australia Surfae Soil. Australian Journal of Soil Research 18, 309-323. MILLER, R.D. & WILKINSON, J.F. (1977): Nature of the organic coating on sand grains of nonwettable golf greens. Soil Science Society of America Proceedings 41, 1203-1204. OSIPOW, L.1. (1977): Surface chemistry: theory and industrial applications. Robert E. Krieger Publishing Company, New York, 473 p. REEDER, C.J. & J U R G E N S O N , M.F. (1979): Fire-induced water repellency in forest soils of upper Michigan. Canadian Journal of Forest Research 9, 369-373. RICHARDS, L.A. (1931): Capillary conduction of fluids through porous mediums. Physics 1, 318-333. RODDICK, J.A. (1976): Summary of the Coast Plutonic Complex of British Columbia. Geological Survey of America Abstracts 8, 405. SAVAGE, S.M., MARTIN, J.P. & LETEY, J. (1969): Contribution of some fungi in natural and heat induced water repellency in sand. Soil Science Society of America Proceedings 13, 149150.

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Addresses of authors: Dr. Gary Barrett Department of Geography University of Toronto - - Erindale 3359 Mississauga Road Mississauga, Ontario L5L 1C6 Dr. Olav Slaymaker Department of Geography University of British Columbia 217- 1984 West Mall Vancouver, B.C. V6T lW5

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