Certain soils develop water repellent characteristics over time and become hydrophobic. This reduces their water holding capacity and can cause issues like poor plant growth and increased erosion. Hydrophobicity is caused by organic compounds like plant waxes coating soil particles. It is more common in coarse-textured sandy soils and soils affected by events like wildfires. Remediation methods include adding clay or other amendments to mask the hydrophobic layers, applying wetting agents, or biodegrading the hydrophobic compounds with microbes.

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2. Presented By:
Hanumanthappa S.R.
II Year M.Tech (SWE)
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3. Content
Introduction
HBS around the globe
Soil Textural relationship
Causes and Factors
Water - Repellency
Preferential Flow
Consequences
Remediation's
Methods/Measurement/Characterization
Case studies
Conclusion
References
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4. Hydrophobic:
•“fear of water”
•Refers to the degree that water “beads up” on a surface
Hydrophilic:
•“love of water”
•Refers to the degree that water “wets” or adheres to a surface
Water
Hydrophobic Surface
Hydrophilic Surface
Water
INTRODUCTION

5. Certain dry soils develop water repellent characteristics with time and do
not permit moisture infiltration. These soils are called hydrophobic or
water repellent soils.
5
Cohesion holds water molecules together to form bubble on top
of the soil. (Quizlet.com)
HYDROPHOBIC
SOILS

6. Practical & Real Life Applications
Lotus Flower
Rose
Water Sports
Car Windshields
Skiing

7. Hydrophobic soils are generally erodible (wind) and infertile due
to their negligible water holding capacity.
The affinity of water is reduced in hydrophobic soil to such an
extent that soil resists water infiltration for hours, days or weeks. Some
of these soils exhibit extreme water repellent characteristic when dry.
Soil Water Repellent has been observed in almost every part of
the world in various climates, with various types of vegetation and in soils
with various textures.
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9. Authors Places
De Bano, 1981; Reeder and Jungerius, 1979 United States
Roberts and Carbon, 1971 Australia
Nakaya, 1982 Japan
Prusinkiewics and Kosakowski, 1986 Poland
Shiels,1982 Great Britain
Barrett and Slaymaker, 1989; Roy et al., 1999 Canada
Bisdom et al., 1993; Dekker and Ritsema, 1994 The Netherlands
Giovannini and Lucchesi, 1983 Italy
Doerr et al., 1998 Portugal
Wallis et al., 1989 New Zealand
Source: Quyum, 2000 9

10. Soil Textural relationship with Hydrophobicity
• Related to coarse-grained soil (Small surface area per unit volume as
compared to the fine- grained soil)
• Relatively small amount of hydrophobic organic matter is needed to coat
coarse soil particles as compared to the fine soil particles
• Increase in soil hydrophobicity with increasing grain size
• Soil hydrophobicity is most likely to develop in soil with < 10% clay
content
Note: Increase in clay content in soil, an increased amount of hydrophobic
organic matter is required for developing hydrophobicity (De Bano, 1991)
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11. • Soil hydrophobicity is not only restricted to sandy soils (Ex: Magic
sand) but also common in soils with clay contents
Giovannini et al. (1983) water repellency was observed in soil
containing 40% clay in Italy.
• Hydrophobicity found in heavy basin clay soils. Ex: Netherlands
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12. Causes and Factors affecting Soil Hydrophobicity
• Organic matter: Roots, decomposing plant tissues, plant derived waxes,
Industrial pollution (Oil, toxic spills and old waste dumps)
• Forest Fire
• Amphiphilic compounds (fatty, fulvic and humic acids)
• Soil fungi (Pencillium nigrican, Aspergillus sydowi, and Actinomycetes)
and microorganisms contribute to give organic matter in soil
• Trees with amount of resins, waxes or aromatic oils (Eucalyptus and
Pines) are major contributor of organic matter
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13. Organic matter coating
• Long chain fatty acids (Primary cause of water repellency)
• Transfer of Lipids causes hydrophobicity from particulate organic matter
• Hydrophobic microbial by-products
• Mixing of non-coated mineral soil particle with organic coated particles
may partially coat the non-coated soil particle (induces hydrophobicity)
• Heating, causes the organic matter to coat the adjacent mineral soil
surfaces
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14. Forest Fire
• Common in North America
• Organic matter accumulates in the litter layer during the intervals
between fires
• Heating, improves the bonding of hydrophobic substance to soil particles
• Fire- induced soil hydrophobicity is temporary in nature
• Heating of hydrophilic soil containing more than 2-3% organic matter
induces hydrophobicity
• A fire can cause hydrophobicity when heating temperature is between
175 to 200° C.

15. djdjsjfdksdskjkjdslkljdskjfl
Post-Fire Hydrophobic Layer
and Erosion
Unburned Vegetated
Landscape, High Infiltration 15

16. Water content
• Repellency is generally considered to increase with increasing
dryness of soil.
• Dekker & Ritsema (1994) measured actual SWR in field moist
state and potential SWR after air drying or oven drying and
defined a critical water content (WCcrit) above which a soil
sample is wettable and below which it becomes repellent.
• Studies found that repellency is maximum at intermediate to
small water content between air-dried states and wilting points.
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• Dry soils at high relative humidity resulted in an increase of
repellency

17. Soil pH
• Alkaline soils to be less prone to SWR compared to acidic soils
• SWR can been successfully reduced in acidic soils by increasing
soil pH via liming.
• Increasing pH will improve the wettability and decreasing pH will
intensify water repellency.
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18. Temperature
• Changes in temperature are linked with changes in surface tension
and viscosity of liquids, in solubility of salts and gases, in
evaporation rates and rates of chemical reactions.
3 possible mechanisms:
• Temperature-induced changes in contact angle
• Changes in liquid-gas in interfacial tension because of solute effects
• Changes of the enthalpy of immersion with temperature or capillary
pressure.
The influence of higher temperatures, which under field
conditions may be expected only during wild-land fires, are
intensively investigated since burnt soils are especially prone to
SWR. 18

19. Theoretical Background (Water repellency)
• Surface property of a solid which impedes complete wetting, i.e. it
prevents water from spreading on its surface and forming a
continuous water layer.
• Ball up as droplets with a finite contact angle
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Initially, it was believed that the occurrence of
hydrophobicity is associated with burns, in
which the heat of the fire vaporizes the
hydrophobic compounds of soil organic matter.
Since the compounds could move into the
atmosphere, they condense on the soil mineral
particles and form hydrophobic coatings on such
particles (Savage, 1974).
!

20. Water repellency (Physical background)
• The surface tension of a substance is based on the difference in
energetic state between molecules in the bulk phase and the
molecules at the surface
• The molecules at the surface are attracted by a reduced number of
neighbors and therefore in an energetically un-favourable state. The
creation of new surfaces is thus energetically costly, and a fluid
system will act to minimize surface areas.
• Principally, the same is valid for solid surfaces, although solid
surfaces cannot react in minimizing surface areas like fluids.
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21. • For the liquid, which forms a droplet on the solid, is in mechanical
equilibrium and for an ideal smooth surface, the
contact angle Q is defined by the Young equation (Young, 1855):
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The higher the surface tension of a solid (for
solid also called surface free energy) the better
it is wettable by water. The condition for
complete wetting shows that only solids with
surface free energy, significantly higher than
the surface tension of water with 72.75 mNm-1
at 21.5 °C are completely wettable and are
therefore called Hydrophilic.
Partially wettable substances have a surface free energy lower
than 72.75 mN m-1 and are therefore called hydrophobic.
Each water molecule may interact
with up to four other water molecules

23. Preferential Flow
• When a small area of subsurface carry large portion of flow is Preferential
flow
• It is a characteristic of hydrophobic soils
• Due to spatial distribution of hydrophobicity with in the soil profile
• Decreases, where degree of hydrophobicity decreases with depth due to
increase in soil moisture, hence preferential flow disappears.
• Ritsema and Dekker (1994 and 1998) conducted field studies on the
moisture movement through hydrophobic soils. The moisture migrated
only through certain sections of the soil. The soil between these sections
was relatively dry
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24. Preferential Flow Path
Wallis et al. (1991) concluded that water infiltration in initially dry
hydrophobic soi1 is retarded and water is retained in a top layer at first.
With prolonged infiltration, Minor perturbations in an originally uniform
wetting front may go to form channels or fingers.
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25. Consequences of Hydrophobicity of Soils
• Dry patches and poor soil wetting causing increased erosion
by wind and water
• Poor seed germination
• Drainage, leaching of nutrients , accelerated solute migration
through preferential pathways
• Runoff
• Pesticide leaching and nutrient loss
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26. Remediation of Hydrophobic Soil
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Reduced crop growth is associated with hydrophobic soil. It is estimated
that hydrophobicity and its associated phenomena caused 40% reduction
in crop production in Australia (Blackwell et al., 1994).
• Top Layer (Claying) Source: Franco et al., 2000
• Spraying of wetting agents
• Masking
• Furrow Sowing
• Wax degradation by microorganisms (Biodegradation)
• Deep cultivation

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28. • Dilution of hydrophobic soil (done through cultivation) with
hydrophilic soil would allow water infiltration into the soil profile.
• Cultivation involves the abrasion of soil particle (removes or
decreases soil hydrophobicity)
• Soil claying, spreading of large amount of clay on the top of the
hydrophobic soil layer is very common in Australia
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Amending of hydrophobic soils with fine textured soils (clay, fly ash
and silica) could overcome the effect of hydrophobicity.

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Addition of 3% clay in hydrophobic soil decreased water drop
penetration time (WDPT) from minutes to seconds. (McGhie and
Posner, 1981)
Masking is a technique used in amelioration of hydrophobic soils. In
masking, clay is applied on hydrophobic soils. The clay particles
cover the hydrophobic soil surface, and improve water infiltration
slowly. The masking process also helps in reduce surface water
contamination by acting as an adsorption sites.
Lime has been employed to improve the wettability of hydrophobic soils.

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Wetting agents are chemicals frequently employed for combating
hydrophobicity in turf grass (increases infiltration)
Sowing plant in wide furrows was suggested to increase water infiltration
into hydrophobic soil (Blackwell, 1994). Widely spaced furrows
increased ponding and the ponded water slowly infiltrate into the soil.
Problem: Erosion of soil (wind and water) and rapid loss of moisture due
to evaporation
Extraction techniques, may facilitate the change in surface chemistry.
Limitation: Cannot be applied in field at large scale

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A new approach for removing hydrophobicity involves the
addition of wax degrading bacteria into hydrophobic soils.
The bacteria remove the hydrophobic substances from the
surface of soil and remove its water repellent character (Blackwell,
1994)
The biological activity and rate of break down was very slow
in hydrophobic soils. Slow releasing of fertilizer into hydrophobic soi1
increased the microbial population and activity and as a result, break
down of hydrophobic substances was stimulated (Franco et al., 2000).

32. Methods/Measurement/Characterization of HPBS
• WDPT and MED tests for soil classification according to degree of
hydrophobicity
• Laboratory infiltration tests and moisture content measurements with
depth (gravimetrically), i.e. mapping of wetting front movement with
depth
• Cyclic wetting and drying (oven and air) followed by infiltration tests
• Infiltration tests on soil samples having ratios of hydrophobic and
hydrophilic soil
• X-ray CAT scanning (Computer Assisted Tomography) for 3-D imaging of
moisture profiles
• Pore size distribution in hydrophobic soil by mercury porosimeter
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33. MED Test (Molarity of Ethanol Droplet )Test
• Proposed by Watsun and Letey (1970)
• MED tests takes less time than WDPT test and hence it is widely
used
• Molarity of ethanol droplet necessary for moisture infiltration in the
hydrophobic soil within 10 seconds is measured
• Ethanol lowers the surface tension of the liquid and enables
infiltration regardless of the soil contact angles
• Ethanol concentrations of 0.2 M intervals in the range 0-6.0 M were
used to determine soil water repellency.
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34. Procedure
• 100µL of aqueous ethanol solution of a known molarity to be
placed (using Eppendorf reference dropper)
• Time needed for the aqueous ethanol droplet to penetrate into the
soil to be recorded
• Test to be repeated on different soil samples
• The molarity of ethanol droplet that permeated the soil in 10
seconds was regarded as its MED value
• Maximum 7 samples had to be tested to obtain 5 consistent MED
values
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Classification of Hydrophobic soils by MED test
(King 1981)
Class MED (M)
Non-repellent 0
Low-repellency <1
Moderate repellency 1-2.2
Severe repellency >2.2

36. WDPT (Water drop penetration time) Test
• Simple method for determining the degree of hydrophobicity
• Test divides the soil into broad categories (contact angles > 90°)
i.e. non- wettable soils and those with (contact angles < 90°) i.e.
wettable soils
• Test measures the time taken by a water drop to completely
penetrate the hydrophobic soil sample
• A drop will penetrate only if the contact angle is < 90°
• As most hydrophobic soils have greater contact angles > 90°, the
drop of water does not penetrate immediately but takes some time
which can range from few seconds to hours. 36

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The longer the drop stays on the soil surface, the more stable and
persistent the water repellency (Dekker and Jungerius, 1990)
Dekker and Jungerius, 1990
Class WDPT (s)
Wettable <5
Slightly wettable 5-60
Strongly non-wettable 60-600
Severely non-wettable 600-3600
Extremely non-wettable >3600

38. • These results show that increase in clay reduces surfactant
effectiveness and contaminated soil texture (at least 30% clay
content) should be considered in surfactant-assisted remediation.
• Also, these results show that sandy soils are more suitable for
surfactant remediation than clay soils because clay sorption reduces
surfactant effectiveness,
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39. CONCLUSION (overall)
• Hydrophobicity when encountered in fine-grained soil (more severe)
• Whereas coarse-grained sandy soils are (more prone) to develop
hydrophobicity due to their small surface area per unit volume.
• Amending hydrophobic soil with conditioners increases the
moisture infiltration capacity of hydrophobic soil.
• The amount of soil conditioners required to improve the water
intake property of soil depended on soil type and the moisture
retaining capacity of the soil conditioners.
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