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Wel-come

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“HYDROPHOBICITY OF SOILS”

Presented By:

Hanumanthappa S.R.

II Year M.Tech (SWE)

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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

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

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Certain dry soils develop water repellent characteristics with time and do

not permit moisture infiltration. These soils are called hydrophobic or

water repellent soils.

Cohesion holds water molecules together to form bubble on top

of the soil. (Quizlet.com)

HYDROPHOBIC SOILS

Practical & Real Life Applications

Lotus Flower

Rose

Water Sports

Car WindshieldsSkiing

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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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

Reviews in Hydrophobic Soils around the Globe

Source: Quyum, 2000 9

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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)

• 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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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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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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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.

djdjsjfdksdskjkjdslkljdskjfl Post-Fire Hydrophobic Layer and Erosion

Unburned Vegetated Landscape, High Infiltration 15

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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.• Dry soils at high relative humidity resulted in an increase of

repellency

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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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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.

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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

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).

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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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• 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

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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

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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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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Remediation of Hydrophobic Soil

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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• 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

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).

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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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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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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

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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.

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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

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• 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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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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THANK YOU…