Soil Pollution

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The major components of the environment

are:

1. The lithosphere i.e. the worlds of rock

2. The atmosphere i.e. the air

3. The hydrosphere, i.e. region occupied by

water

4. Biosphere i.e. the living things

5. Pedosphere i.e. the soil 2

Diagrammatic representation of a hypothetical soil profile 3

O-Zone or A0 Zone:

It is the top layer of the Profile, partially decomposed organic

Debris. It is the region of principal soil formation known as

Humification. It contains 20-30% of organic matter, dark in

colour and acidic in nature

O - Zone

A1 - Zone

A2 - Zone

A3 - Zone

B - Zone

A-Zone: It is divided in to three main zones (A1, A2, A3)

A dark upper layer containing humus with mineral grains (A1)

Underlying light coloured horizon with little organic matter, is

due to leaching processes known as eluviation

Eluviation, enhanced by carbonic acid,

resulting from the decay of humus,

displaces bases (calcium, sodium,

magnesium, potassium) from the

exchange sites of clay minerals.

These bases move down the soil profile

as colloidal particles, dissolved ions or

as free ions complexed with hydroxyl.

Humification can be defined

as the transformation of

numerous groups of

substances (proteins,

carbohydrates, lipids etc.) and

individual molecules present

in living organic matter into

humic substances like humic

acid, fluvic acid and humin.

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Material dissolved in the A-horizon finds in some cases its way to the saturated zone

of groundwater, yet the greatest part of it is normally re-deposited in the underlying

layers forming the B-horizon.

In this process, known as illuviation, colloidal material and metal oxides are

deposited or precipitated in the B-horizon, resulting in an enrichment of its layers in

clay and Aluminium oxide. Iron oxides, if present, give the horizon its red or yellow

brown colour.

Lowest one, C-horizon, represents the stage nearest to the parent material. It is

made up of partially or poorly weathered bedrock having minimum content of

organic matter and clay.

Bed Rock

Sand Dunes Glacial Sedimentary Fluvial

(Alluvials)

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Grain sizes of clastic sediments and related rock types

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Composition by Volume of an Average Soil

Silicate

olivine, (Mg,Fe,Mn)2SiO4

Zircon (ZrSiO4),

Topaz Al2(F, OH)2SiO4

made of 85% dead matter, 8.5% living roots

and rootlets and about 6.5% soil organisms.

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Soil Water

Soil Air

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Soil Degradation and Soil Quality

Soil degradation is defined as the decline in soil quality

caused through its misuse by human activity (Barrow

1991).

The term soil quality itself may be further defined as,

The capacity or capability of a soil to produce safe

and nutritious crops in a sustained manner over a long

term, and to enhance human and animal health,

without impairing the natural resource base or

adversely affecting the environment

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Degradation or decline of soil quality may occur due to

Physical or chemical processes triggered off by natural phenomena,

or

induced by humans through misuse of land resources.

Processes such as

Physical degradation processes

• soil erosion

• nutrient run-off

• water logging

• desertification

Chemical degradation processes

• acidification

• organic matter loss

• salinization

• nutrient depletion by leaching

• Toxicants accumulation

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How do we Know/Measure Soil Degradation ?

Through Indicators !!!

They may be

Biological Indicators: de sity of icroorga is s’ populatio s, or through measuring some of the basic biological activities, such as

respiration or intensity of biogeochemical reactions.

Physical Indicator: The fundamental physical characteristics of soil,

such as bulk density, water infiltration or field water holding capacity.

Chemical Indicators : pH-values or the concentration of certain ions

such as nitrates, phosphates etc.

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Biological Indicators:

The main indicator of biological activities in soil is the soil respiration rate.

It is assessed through measuring the carbon dioxide evolution resulting from the

decomposition of organic matter.

In other words, it is closely related to the efficiency of biological processes taking

place within the soil.

Biological processes in soils depend on several factors, among which moisture,

temperature, oxygen, partial pressure, and availability of organic matter are of

central importance.

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Physical Indicators of Soil Quality

• Soil Bulk Density

Soil bulk density is defined as the mass of soil per unit volume in its natural field state,

including air space and mineral matter, plus organic substance.

High values of bulk density may restrict the movement of surface waters through the soil,

leading to a loss of nutrients by leaching.

It may also increase erosion rates.

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• Water Infiltration

Infiltration is defined as the process by which water enters the soil.

Its rate depends on soil type, soil structure and soil water content.

Infiltration is important for reducing run-off and consequent erosion.

Steady infiltration rates for different soil texture groups in very deeply wetted soils

(Hillel 1982)

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Field Water Holding Capacity

Field water holding capacity is the amount of water a soil can hold after having

been saturated and then allowed to drain for a period of one to two days.

Field water holding capacity depends also on the soil texture.

Loamy soils provide the most plant-available water, whereas clayey soils hold

their water content so tightly that it might not be available for plants.

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Soil Erosion

Soil Compaction

Soil Degradation

Processes

Physical Soil Degradation

Chemical Soil Degradation

Acidification

Salinization and

Sodification

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Soil Erosion

Soil erosion occurs when the rate of soil removal by water and/or wind exceeds the

rate of soil formation.

Soil erodibility. This is a measure of the soil resistance to detachment and transport.

It depends particularly on soil texture, organic content, structure, and

permeability.

Generally, soils with low contents of clay and organic matter are more

readily eroded than soils with a higher content of the same.

Erosivity. This is a measure of the potential of the eroding agent to erode and is

commonly expressed in kinetic energy (Morgan 1995).

With regards to rainfall, this will be related to the intensity of the rainfall

as well as to the size of raindrops. 17

Vegetation cover. Plant cover on soils may function as a buffer between the soil surface

and the rest of the environment, including all natural forces such as

wind, rain and running water.

It dissipates the energy of raindrops, or run-off, allowing water to

enter the soil.

Topography. Increases in slope steepness and slope length are factors that make

soil more susceptible to erosion due to the respective increases in

velocity and run-off volume.

The relation between erosion and slope can be mathematically

expressed in the following equation:

E ∝ tanmθ L n

where E is soil loss per u it area, θ is the slope a gle a d L is the slope

length.

According to Zingg (1940), this relation may be written as:

E ∝ tan1.4θ L0.6

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Soil Conservation Strategies

Agronomic practices. These aim to minimise the period of exposure to erosion when

the soil is left bare, by encouraging the cultivation of dense vegetation cover and plant

root network.

Soil management techniques. These aim to increase the resistance of the soil to

erosion, by following techniques that improve and maintain the soil structure.

Such methods mainly apply processes such as mulching, reduced or zero tillage and

addition of synthetic soil conditioners, e.g. PVA (polyvinyl alcohol), PAM (polyacryl

amide) and PEG (polyethylene glycol).

Mechanical techniques. The main strategy here is to reduce the energy of the eroding

agent through modifying the surface topography.

This is attained by geotechnical methods such as bunding, terracing or constructing

diversionary spillways to direct water away from areas that are highly susceptible to

erosion.

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Soil Compaction

Soil compaction is usually a combination

of both engineering compaction and

consolidation, so may occur due to a lack

of water in the soil, the applied stress

being internal suction due to water

evaporation as well as due to passage of

animal feet.

Affected soils become less able to absorb

rainfall, thus increasing runoff and

erosion. Plants have difficulty in

compacted soil because the mineral

grains are pressed together, leaving little

space for air and water, which are

essential for root growth. 20

Chemical Soil Degradation

Acidification Like all other problems of soil, acidification has an anthropogenic

dimension as well as a natural one, arising from background

factors.

Natural processes that lead to acidification range from

long-term base leaching and microbial respiration to nitrification

and plant growth.

Nitrifying bacteria also helps in lowering the pH of the soil by

oxidation of ammonium, according to the equation:

Anthropogenic processes leading to acidification

Excessive use of inorganic nitrogen fertilizers.

Acid rain

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The Impact of Acidity on Soil Quality

• Aluminium and/or manganese toxicity

• Deficiency of phosphorus, due to its tying up by iron and aluminium

• Calcium and magnesium deficiency :

Nutrients such as phosphorus, potassium, magnesium and calcium decrease through

acidity.

Liming Materials

Liming materials most commonly used include limestone, chalk, marl and basic slag.

They may contain minor amounts of quick lime (CaO), slaked lime (Ca(OH)2), and

magnesium carbonate (MgCO3).

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Salinization and Sodification:

In regions where evapotranspiration is higher than precipitation, soil water flow will

be driven by capillary action in an upward direction and eventually, due to evaporation,

saline precipitates will be formed in the interstitial pores of the soil, leading to

salt accumulation between the soil grains. This phenomenon is known collectively as

salinization.

In cases where the parent material of the soil is rich in sodium, salinization will also lead to

the increase of sodium in the soil water, leading to sodification, as this is termed by soil

scientists.

Amendment of Soils Affected by Salinity and Sodicity

Control of salinity. The main factors for salinity occurrence in soils are a relatively

high water table and a high degree of evapotranspiration; this is why the controlling

of salinity must be carried out using technical methods that can mitigate the effect of

these two factors. Subsurface drainage may provide an effective means of lowering

the water table, while surface mulches may effectively help reduce evapotranspiration

and stop salt accumulation.

Amendment of sodic soils. In addition to drainage management techniques, in the

case of saline soils, two main techniques are essential for the amendment of these

soils, namely the disruption of clay pans and the use of soil chemical conditioners

to replace the adsorbed sodium with calcium. A popular chemical conditioner used

for this purpose is gypsum (CaSO4· H2O). 23

The quality of soil is adversely influenced by contamination (pollution) of the

system.

The co cepts of co ta i atio a d pollutio of soil are used i a comparable way as they reflect only a difference in degree of drainage to the

soil system.

Any addition to soil, i.e. contaminants- meaning the compounds that may

exert adverse effect on soil functioning. It can be defined as soil

contamination.

Because most soils do have a certain buffering capacity, it usually takes some

time before the negative effects become apparent. Once this situation occur,

the soil can be considered as polluted, which for all practical purposes means

that malfunctioning of the soil is apparent due to abundant pressure or

availability of the compounds.

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A schematic overview of the processes representing soil-

pollutant interactions

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Pollution is often broadly categorized according to its

source:

Point-source pollution: As the name implies, is pollution with a

clearly identifiable point of

Discharge, e.g.

waste water treatment plant etc

Nonpoint-source pollution: Is pollution without an obvious single

point of discharge, e.g.

surface runoff of a commonly used lawn herbicide.

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Soil Pollution

Soil pollution is caused by the presence of chemicals or other

alteration in the natural soil environment.

Resulting in a change of the soil quality

likely to affect the normal use of the soil or endangering public

health and the living environment.

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CAUSES OF SOIL DEGRADATION

• Soil erosion/degradation is the loss of top soil.

Fertility of soil & its water-holding capacity is

reduced.

• Excessive farming, construction, overgrazing,

burning of grass cover and deforestation

• Excess salts and water (Salinization)

• Excessive use of fertilizers & pesticides

• Solid waste

:

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First effect of pollutants

• Washed away: might accumulate somewhere

• Evaporate: can be a source of air pollution

• Infiltrate through the unsaturated soil to the groundwater

• DDT: fat soluble, stored in fatty tissues – Interferes with calcium metabolism

– Results in thin egg shells in birds

• Agent orange: code name for one of the herbicides and defoliants (results in leaf fall) used by the U.S. military as part of its herbicidal warfare program, During the Vietnam War, between 1962 and 1971, the United States military sprayed 20,000,000 US gallons (80,000,000 L) of chemical herbicides and

defoliants in Vietnam – anti fertility, skin problems, cancer

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Mechanism of Pollution Transport through soil

The environment plays a key role in the ultimate fate and

transport of contaminants.

fate of contaminant released into the environment, depends on

(i) Chemical structure of contaminant which is highly variable

abiotic factors within the receiving environment (e.g. organic

carbon ,ph ,surface water)

(ii) Interaction with the biotic environment which can result in

degradation, transformation or bioconcentration of the

contaminants

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A summary of the mechanisms involved in soil pollution

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Once contaminants also reach the soil, they move

in one of these directions:

1) Vaporized into the atmosphere without chemical change

2) Absorbed by soils, move downward through the soil in

liquid or solution form and can be lost from the soil by

leaching.

3) Undergo chemical reactions within or on the surface of the

soil

4) Broken down by soil micro-organisms

5) Washed into streams and rivers in surface run-off

6) Taken up by plants or soil animal and move up the food

chain 35

Transport Mechanisms

1. Mass flow of dissolved chemicals within

moving solution

2. Liquid diffusion within soil solution

3. Gaseous diffusion within soil air voids.

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Determination of affected areas and risk assessment of pesticide spills

Not every spill of pesticide implies health risk:

Some important factor determines the risk of the spill:

The characteristics of the stored pesticides

Toxicity

Life time of the pesticide

Quantity and time span of spillage

Problem addressed?

Whether it is likely that the soil or groundwater in the surroundings of the storage facility is

contaminated

Weather such a possible contamination is causing risk

What measures can be taken to reduce these risks.

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Assessing Contamination

• Distribution of pesticide into the environment

• Pesticides may infiltrate the soil

• They may be carried away by wind

• They may be spread by runoff

• They may leach out into the groundwater and

subsequently spread underground , eventually entering

rivers or lakes

Pesticide

Storage

Infiltration

In top soil

Spreading by

groundwater

Spreading

by wind Top Soil

Water Body Groundwater

table

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Different Steps for Assessing

Contamination

• Step 1: Determine which of pesticides that have been spilled are relevant i.e. may have caused contamination

• Step 2: Determine Whether these relevant pesticides have infiltrated the soil and, if so, to what depth

• Step 3: Determine whether the relevant pesticides have leached into the groundwater and, if so, which area around the store will have contaminated ground water

• Step 4: Determine whether the relevant pesticides have been distributed by wind and, if so, which area around the store will be contaminated

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Identifying possible human health

risks through contamination

• Step 5: Identifying exposure points

• Step 6: Predicting concentrations at the exposure points

• Step 7: Identifying exposure routes

• Step 8: Determining when permissible exposure levels have been exceeded

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Control of soil pollution

• Use of pesticides and fertilizers should be minimized.

• Cropping techniques should be improved to prevent growth of weeds.

• Special pits should be selected for dumping wastes.

• Controlled grazing and forest management.

• Wind breaks and wind shield in areas exposed to wind erosion

• Afforestation and reforestation.

• 3 Rs: reduce, reuse, recycle

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Bioremediation

• The use of naturally occurring microorganisms such as bacteria, fungi & plants to

break down or degrade toxic chemical compounds that have accumulated in the

environment

• It is a method that treats the soils and renders them non-hazardous, thus

eliminating any future liability that may result from landfill problems or violations.

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