Transport of Materials Through the Soil Profile

7/27/2019 Transport of Materials Through the Soil Profile

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TRANSPORT OF

MATERIALS THROUGHTHE SOIL PROFILE


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FINE-TEXTURED SOILS WITH HIGH CLAY CONTENT

-Do not drain well- Retain large amounts of water for long periods

- Aeration in these soils is limited

- Processes such as organic decomposition,,

ammonia, volatilization, and nitrification are

retarded.

Although clay particles are active sites for ion

exchange and adsorption, the effective adsorption

of dissolved materials is reduced since the water

molecules are polar and compete for adsorption.


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COARSE-TEXTURED SOILS

-Conduct large quantities of air and water,

and oxidative processes are encouraged.

- At the same time the rates of evaporation,

lateral transmissibility, and percolation are

higher.


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As a rule, passage through the soil profile resultsin purification of water because of

-Adsorption

- Volatilization

- Decomposition or degredation

- Nitrification

- Denitrification

- Plant uptake


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Adsorption-desorption

The interaction between the chemicals in water

and the soil through which the water passes largely

determines water quality changes within the soil profile.

The adsorption process entails the removal of

chemicals at solution and retention on the surface ofsoil particles by chemical or physical bonding. If the

bonds formed between the adsorbate and soil are

chemical, the process is almost always irreversible. If

the bonds are physical, by weak Van der Waals forces,the chemicals are easily removed or desorbed a change

in solution concentration of the adsorbate.


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Absorption is the process in which a fluid is

dissolved by a liquid or a solid

(absorbent). Adsorption is the process in whichatoms, ions or molecules from a substance (it could

be gas, liquid or dissolved solid) adhere to a surface of

the adsorbent. Adsorption is a surface-based process

where a film of adsorbate is created on the surfacewhile absorption involves the entire volume of the

absorbing substance.

Adsorption, the binding of molecules or

particles to a surface, must be distinguished

from absorption, the filling of pores in a solid. The

binding to the surface is usually weak and reversible.


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The quantity of a chemical that can be

adsorbed by soil depends on concentration of

adsorbate and soil temperature . In most research

on the adsorption-desorption process, the

amount of a chemical asorbed is determined as a

reaction of concentration at a constanttemperature, and the resulting function is called

an adsorption isotherm.


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The most common representation of the

adsorption-desorption process in soil is the

Freundlich isotherm:= kc1/n

Where

X is the amount of chemical adsorbed

M is the amount of the soil adsorbent

c is the equilibrium concentration of adsorbate in

solution after adsorption

k and n are empirical constants


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Organic wastes, pesticides, ammonia,

and phosphorous are adsorbed to an extent

dependent on the clay content, organicmatter content, and cation-exchange

capacity of the soil. Adsorption usually

assures that chemicals remain in the soil longenough for processes such as decomposition

and plant uptake to occur.


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Volatilization

The loss of a chemical from the soil-water system

by vaporization into the atmosphere is termed

volatilization. Certain chemicals move to the soil surface by

diffusion or mass flow.

Volatilization is the process whereby a dissolved

sample is vaporised. In atomic spectroscopy this is usually

a two step process. The analyte is turned into small

droplets in a nebuliser which are entrained in a gas flowwhich is in turn volatilised in a high temperature flame in

the case of AAS or volatilised in a gas plasma torch in the

case of ICP spectroscopy.


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At the surface the rate of volatilization q of a

chemical may be approximated by theequation

Q=-kv ym-1/2

Where

kv

is a wind and temperature-dependent

vaporization constant for the chemical

y is the saturation vapor concentration

m is the molecular weight of the chemical


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A high volatilization rate requires that

vaporized chemicals move away from the soil

surface so that additional chemicals can move

into the vapor phase. Consequently, wind

speed is a critical determinant of

volatilization rate. Volatilization can removelarge quantities of chemicals such as

ammonia and pesticides from the soil,

particularly during the initial period afterapplication.


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Decomposition or degredatrion. Organic

materials in the soil break down to form carbondioxide, water, inorganic elements such as nitrogen

and chloride. Degredation rates depend on soil

temperature, moisture, strength of binding by soils,

soil type, and soil microorganisms. Oxidation,hydrolysis, and microbial enzyme action are the

most common methods of degradation. In many

soils the combined processes of adsorption and

degredation can remove 99 percent or more of the

organic content of heavily polluted water.


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Nitrification.

The two-step process in which ammonia NH2 isoxidized to Nitrite NO2 and then to nitrate NO3 is

termed nitrification. This is an important reaction

in the soil-water system because a largely

immobile form of nitrogen (ammonia) isconverted to a highly mobile form (nitrate) which

may be absorbed by plants or lost by leaching and

denitrification.


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Nitrification is a two-step process. The first stage is

the oxidation of ammonium (NH4+) to nitrite (NO2

-), a

function carried out by bacteria in thegenus Nitrosomonas. The nitrite formed is rapidly oxidized

to nitrate (NO3-) by bacteria in the genus Nitrobacter.

Because nitrate and nitrite are much more mobile in soils

than ammonium, nitrification can be viewed as a process

that mobilizes nitrogen, making it more available

for plant uptake but potentially allowing it to leach from

the ecosystem. The latter is an undesirable attribute of

nitrification because fixed nitrogen is an important

component of the nutrient capital of ecosystems. Inaddition, large concentrations of nitrate or nitrite can

pollute groundwater and surface waters.


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Denitrification

The denitrification process involves the conversion

of nitrate to gaseous nitrogen species such aselementl nitrogen gas, nitrous oxide, or nitric acis.

A relatively large group of bacteria accomplish

denitrifiaction by using niutrate as an oxygen

source in their respiration. Biological

denitrification can cause a 5 to 10 percent

reduction in total nitrogen in percolating water.


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Denitrification also becomes the key pathway

for dissimilative nitrate reduction, which is the

process in which nitrates are reduced from the soil,the former being highly toxic for living organisms.

Denitrification tends to produce large amounts

of by-products.

The process of denitrification lowers the

fertility of the soil and thus is less common in areas

where the land is rather well-cultivated. But this

loss of nitrogen to the atmosphere can eventually

be regained via introduced food and water, as part

of the nitrogen cycle.


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Plant uptake

In soils with heavy vegetal cover, the major

mechanism for removal of inorganic nitrogen

and phosphorous is uptake by plants. Flow of

water toward roots in response to transpiration

results in the transport of non adsorbednutrients with high solubilities, such as nitrate.

Diffusion is the most active mechanisms for

transporting adsorbed species (e.g.phosphorous, potassium, iron) to plant roots.


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Table 15-4 demonstrates the overall effectiveness of chemical

removal during percolation. The data indicate a high degree of

purification for water carrying an extreme chemical burden, i.e.domestic wastewater. Data are provided for the effectiveness of

two land application processes for wastewater treatment i.e.

slow rate application and rapid infiltration. Slow rate

application is the application of wastewater to vegetated lands

with moderate permeability. Rapid infiltration refers toapplication to highly permeable soils with minimal vegetation.

Since natural waters carry a much lighter chemical, burden than

wastewater, a high degree of purification can be expected for

most waters passing through the soil during the runoff cycle.


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Transport of Materials Through the Soil Profile