12

CHAPTER 2

Review of Literature

13

2.1. Pesticide residues in soil, water and plants

Contamination of the soil, water and air with pesticides is a social and

economical matter of concern to all. Use of metal-based inorganic and

hard pesticides like organochlorines attracted widespread opposition due

to their persistent and polluting nature. Strong public movements

opposing the indiscriminate use of pesticides started in the early sixties.

The widespread use of DDT was found to be harmful to the ecosystem.

The famous book ‘Silent Spring’ deals entirely with the environmental

impact caused by DDT. However the use of pesticides in modern

agricultural technology has become a unavoidable evil. The danger of

using such poisons in agriculture is still not known fully to many farmers.

Many reports are available on the presence of pesticide residues in soil,

water and their spread and transportation to plants and surroundings.

Reports are also emerging on the level of pesticide residues present in

agricultural products. The pesticide residues are also reported to be

present in non-agricultural products for public consumption like meat,

fish and milk. The surface water samples from Nilgiris in India were

analysed for the residues of carbofuran, quinalphos and phorate by K

Bhuvaneswari et. al. [1]. The percent occurrence of insecticide residues

observed in the samples was 60.6 for carbofuran. The insecticide residues

were detected in the range of 50 to 1200 ng/lit for carbofuran. This

happens because of the persistent nature of the pesticides. DDT, BHC,

aldrin, heptachlor and endosulfan are some of the long persisting

organochlorine group of pesticides. These find their way into cereals,

vegetables, fruits, and oil seeds from soil and water in the environment.

14

Data on pesticide residues from selected fields are being generated by

various agricultural universities and Indian Council of Agricultural

Research Institutions in the country (ICAR’s). The All India Coordinated

Research Project on Pesticide Residues sponsored by ICAR has further

accelerated the pace of research in this area [2, 3, 4].

Many chlorinated pesticides like DDT, BHC, aldrin and endrin, which are

non-biodegradable and soil, plant, water and air, the four co-ordinated

components of the environment are the first victims of pesticide

contamination. Human beings and animals are affected directly during the

handling of the pesticide products and also indirectly through the food

chain.

The chemical affects the air through its volatilization followed by wind

action. They further migrate from the site of application and reach ground

water by percolation and leaching through soil. DDT, BHC, Aldrin, and

Entrin, which are non-biodegradable and highly persistent, undergo

bioaccumulation and grow to fatal levels [5,6,7].

Analysis of agricultural products in various parts of India showed the

widespread contamination of materials of human consumption with

pesticide residues. It was found that out of 313 vegetable samples analyzed in

Mumbai, more than 50 % contained residues of HCH, lindane, DDT, aldrin

and endrin. It was also observed that leafy vegetables and potatoes contained

higher amount of residues. In Delhi, 80 % of vegetable samples tested had

pesticide residues in them. Those were samples analyzed from places like

Haryana, UP and Maharashtra.

15

Ground water, which is expected to be free from contamination, also

reported the presence of pesticide residue. Recent reports have indicated

that in Delhi area, ground water is contaminated by pesticides like Atrazin

and chloropyrifos [8]. The National Symposium on ‘Pesticides’ ‘Myths

Realities and Remedies’ held in New Delhi in 2004 had highlighted the

issue of pesticide residues in soil, water and agricultural crops as well as

the role of microorganisms in their degradation. The presence of

organochlorine pesticide residues in milk and milk products was reported

by Anoop and coworkers [3] and J. Rao [9]. The presence of residues of

chloropyriphos and mono chrotophos in paddy and non-target organisms

was reported. Several studies were carried out on pesticides residues in crops

like cotton, brinjal, onion, apple, mango, chilli, coconut and tea [10].

Pesticide, on application to agricultural fields leaches into small streams

and reaches rivers and ultimately ocean. This endangers aquatic life.

These are several reports on the contamination of river water by pesticide

residues. Reports on pesticide contamination of water sources like rivers,

ponds and lakes show that most of them are related to contamination by

organochlorines like HCH, heptachlor, aldrin and DDT. Some reports are

also related to endosulfan, malathion, parathion, dimethoate and ethion,

the last four belonging to organophosphorous group.

2.2 Classification of Pesticides

2.2.1. Classification based on usage.

Depending on the purpose for which they are used, pesticides are divided

into following basic groups.

16

1. Acaricides - for the control of mites and ticks.

2. Algicides - for the destruction of algae and

other aquatic vegetation.

3. Arboricides - for the destruction of undesirable

arbored and bushy vegetation.

4. Bactericides - for the control bacteria and

bacterial diseases of plants.

5. Fungicides - for the control of plant diseases

and various fungi.

6. Herbicides (Weedicide) - for the control of weeds.

7. Insecticides - for the control of harmful insects.

Individual groups of insecticides also have more specific names such as

aphicides, preparations for the control of aphids.

Limacides or molluskicides - for the control of various

mollusks, including gastropods.

Nematicides - for the control of nematodes

Zoocides - for the control of rodents. Materials

of this group are often called

rodenticides

17

2.2.2. Classification based on chemistry

Pesticides are also classified based on the chemical group under which

they fall. A few examples are as follows:

1. Organo chlorines

2. Organo phosphates

3. Carbamates

4. Pyrethroids

5. Nicotinoids

6. Rotenoids

Organochlorines

As the name indicates, the organo chlorines are insecticides that contain

carbon, chlorine and hydrogen. They are also referred to by other names

like chlorinated hydrocarbons, chlorinated organics, chlorinated

insecticides, and chlorinated synthetics.

One of the most important organo chlorine pesticide introduced in the

early forties is DDT which was first synthesized by Zeidler in 1874.The

chemical was used in the initial years by British army to control body lice.

Later the chemical found wide application in the health programme to

control mosquitoes causing malaria. It is chemically known as 1, 1, 1-

trichloro-2, 2-bis (p-chloro phenyl) ethane. DDT belongs to the chemical

class of diphenyl aliphatic, and as the name indicates, it consists of an

aliphatic or straight carbon chain, with two (di-) phenyl rings attached.

18

DDT was first known chemically as p,p’ dichloro diphenyl tri chloro

ethane, hence DDT Structure 2.1.

Chemical Structure 2.1. Chemical Structure of DDT

It is nearly insoluble in water but has a good solubility in

most organic solvents, fats, and oils. Trade names that DDT has been

marketed under include Anofex, Cezarex, Chlorophenothane,

Clofenotane, Dicophane, Dinocide, Gesarol, Guesapon, Guesarol, Gyron,

Ixodex, Neocid, Neocidol, and Zerdane [11]. From 1950 to 1980, DDT was

extensively used in agriculture—more than 40,000 tonnes were used each

year worldwide [12] and it has been estimated that a total of 1.8 million

tonnes have been produced globally since the 1940s [13]. Today, 4-5,000

tonnes are used globally each year for the control of malaria and visceral

leishmaniasis. India is the largest consumer. India, China, and North

Korea are the only countries that still produce and export. Production is

reportedly rising [14].

DDT is suspected to cause cancer. The NTP classifies it as "reasonably

anticipated to be a carcinogen," the International Agency for Research on

Cancer classifies it as a "possible" human carcinogen, and the EPA

classifies DDT, DDE, and DDD as class B2 "probable" carcinogens.

19

These evaluations are based mainly on the results of animal studies [13,15].

There is evidence from epidemiological studies (i.e. studies in human

populations) that indicates that DDT causes cancers of the liver [15, 16],

pancreas [15, 16] and breast [16]. There is mixed evidence that it contributes

to leukemia[16], lymphoma [16, 17] and testicular cancer [15,16,18].

Cyclodienes

Cyclodiene chlorinated insecticides are cyclic compounds having

methane-bridged structure substituted with chlorine atoms. These dienes

are prepared by Diels-Alder reaction. The starting material for the

preparation of cyclodiene is hexachlorocyclopentadiene [HCCP]. e.g.:

chlordene, chlordane, aldrin, isodrin, endrin and endosulfan.

Organophosphorus pesticides

Organo phosphorus pesticides are esters of phosphorus acids like

phosphinous, phosphinic, phosphonous, phosphonic, phosphorous and

phosphoric acids and their thio analogues. Organophosphorus compounds

show activity against variety of crop pests. These can be broadly

classified as insecticides, acaricides, nematicides, herbicides including

defoliants and plant growth regulators, fungicides and chemosterilants.

They degrade fast in nature. Malathion, methyl parathion, hinosan,

phorate are some of the pesticides, which belong to organophosphorus

group [19].

Carbamate insecticides

Carbamates are important group of synthetic compounds with high

insecticidal activity and a reasonable rate of biodegradation. The idea of

20

developing carbamate insecticides came from the alkaloid physostigmine,

an active ingredient of calaban beans. Based on the structure of this

alkaloid, attempts were made to synthesize a series of N-substituted

carbamate molecules for screening as insecticides.

The base for all carbamates is carbamic acid [structure 2.2] the mono

amide of carbon dioxide, which is unstable in free form but decomposes

to CO2 and NH3 .On the other hand; salts of carbamic acid, called

carbamates are very stable [structure 2.3]. One such stable salt is

ammonium carbamates, used as commercial insecticide and rodenticide

along with aluminium phosphide. Carbamic acid can be stabilized by

formation of a simple alkyl ester such as ethyl carbamate known as

urethane [structure 2.4]. This is used as bactericide and co solvent for

pesticides, but is carcinogenic in nature. In addition to alkyl esters, aryl

esters of carbamic acid are also prepared. One such compound is phenyl

carbamate [structure 2.5].

HO-CO-NH2 +NH4 O

- CO-NH2 C2H5O-CO-NH2 C 6 H 5 O-CO-NH2

[structure 2.2] [structure 2.3] [structure 2.4] [structure 2.5]

Some other examples of carbamate pesticides are carbaryl, carbofuran,

carbosulfan, aldicarb and oxamyl.

Carbofuran is one of the most toxic carbamate pesticides. It is

marketed under the trade names Furadan, by FMC Corporation and Curater,

among several others. It is used to control insects in a wide variety of field

crops, including potatoes, corn and soybeans. It is a systemic insecticide,

which means that the plant absorbs it through the roots, and from here the

plant distributes it throughout its organs where insecticidal concentrations

21

are attained. Carbofuran also has contact activity against pests.

Carbofuran usage has increased in recent years because it is one of the

few insecticides effective on soybean aphids, which have expanded their

range since 2002 to include most soybean-growing regions of

the U.S. The main global producer is the FMC Corporation.

Carbofuran is banned in Canada and the European Union. In 2008,

the United States Environmental Protection Agency (EPA) announced

that it intends to ban carbofuran [20]. In December of that year, FMC

Corp., the sole US manufacturer of carbofuran, announced that it had

voluntarily requested that the United States Environmental Protection

Agency cancel all but 6 of the previously allowed uses of that chemical as

a pesticide. With this change, carbofuran usage in the US would be

allowed only on maize, potatoes, pumpkins, sunflowers, pine seedlings

and spinach grown for seed [21]. However, in May 2009 EPA cancelled

all food tolerances, an action which amounts to a de facto ban on its use

on all crops grown for human consumption [22]. In Kenya farmers are

using carbofuran to kill lions and other predators [23, 24]. Kenya also is

considering banning carbofuran [25].

Pyrethroid insecticides

Natural pyrethroids are the most ideal and safer insecticides among

pesticides of natural origin. The production of natural pyrethrum extract is

limited because of the shortage of chrysanthemum cinerariaefolium plant,

which requires specific agro-climatic conditions to grow. Owing to the

high demand of pyrethrins, the chemists have all along been interested in

developing synthetic analogues with high potency, selectivity,

photostability and low mammalian toxicity. Pyrethroids are toxic to fish

22

and other aquatic organisms. At extremely small levels, such as 2 parts

per trillion [26] pyrethroids are lethal to mayflies, gadflies, and

invertebrates that constitute the base of many aquatic and terrestrial food

webs [27]. They are usually broken apart by sunlight and the atmosphere

in one or two days, and do not significantly affect ground waterquality

[28]. Chemical and spectroscopic evidences showed that cyclopentenyl

ring in the pyrethrin molecule was the most probable site of photo

induced oxidative decomposition.

2.2.3. Miscellaneous insecticides

Nitromethylene heterocycles

Soloway et al. screened a series of nitromethylene heterocyclics and

found that 2-(nitro methylene) tetra hydro 1, 3- thiazin analogues

exhibited good insecticidal activity with low mammalian toxicity and less

persistence. But these compounds are photolabile. To make a photo stable

product, a formyl derivative of nitro methylene was synthesized with

reasonable photo stability and wide spectrum of insecticidal activity.

These compounds also exhibited activity against insects resistant to other

pesticides.

2.3. Chemicals containing metals in use as pesticides

Some of the pesticides, which contain metals, are listed in Table 2.1.

23

Table 2.1.Chemicals containing metals in use as pesticides: [29]

Chemical Metal composition of

Product Crops

Insecticides

Copperaceto-arsenite

(Paris green) 2.3% As, 39% Cu

Apples and cherries,

vegetables and small fruits

Calcium arsenate 0.8-26% As Fruit and vegetables

Lead arsenate 4.2-9.1% As,

11-26% Pb

Apples, Cherries, Peaches,

vegetables

Mercuric chloride 6% Hg Cruciferous crops

Zinc sulphate 20% Zn Peaches

Fungicides

Copper sulphate-

Calcium salts

(Bordeaux and

Burgundy mixtures)

4-6% Cu Fruit and vegetables

Fixed copper salts 2-56% Cu Fruit and vegetables

Ferbam 0.5-12% Fe Vegetables

Fruit

Maneb 1-17% Mn Fruit and vegetables

Mancozeb 16% Mn, 2% Zn Fruit and vegetables

Methyl and phenyl

Mercuric salts 0.6-6% Hg Seed treatment

Phenyl mercuric

Acetate

6% Hg Apples

Zinels and ziram 1-18% Zn Vegetables

Fruit

Calcium arsenite 30%As Vegetables

Sodium arsenite 26% As Vegetables

24

2.4 Use of Pesticides in Indian Agriculture

The pesticides have proved to be effective tools to protect the crops from

tremendous losses caused by pests and diseases and paved the way for

green revolution. In modern agricultural technology, benefit of all the

important inputs will be easily lost if the crop is not well protected from

pests and diseases High yielding varieties are often more prone to pests

and diseases and hence require a “pesticide umbrella” for an optimum

yield.

Pesticide science is barely 60 years old in India. Prior to 1930, little

pesticides were used. But the discovery of insecticidal properties of DDT

completely revolutionized the pest control operations. Upto 1966, most of

the pesticides used were in public sector but from 1967 onwards when

high yielding varieties responsive to fertilizers and irrigation were

introduced, the pesticide consumption in agriculture increased

significantly.

2.4.1. Agricultural production and use of pesticides in India

However, total pesticide consumption has gone up to 119.2 thousand

tones in 1990. The agricultural production and amount of pesticides used

annually from 1960 to 1985 are given in Table 2.2.

25

Table 2.2. Year-wise agricultural production & use of pesticides in

India [30]

Year

Total

production of

food grain

(MT)

Pesticides used in

agriculture

(1000 T)

Pesticides in

public health

(1000 T)

Total

pesticides

consumption

(1000 T)

1960 80 2.0 21.0 23.0

1968 74 7.4 9.6 17.1

1970 108 10.2 8.8 19.0

1975 121 43.4 15.2 58.6

1980 126 40.6 15.5 56.1

1981 129 48.0 16.8 64.8

1982 133 53.0 19.3 72.3

1983 129 64.0 17.8 81.8

1984 151 68.0 20.6 88.6

1985 153 72.0 19.2 91.2

If we look at the consumption of pesticides in different states of India,

Andra Pradesh is found to be using maximum pesticides followed by

Tamil Nadu, U.P., Punjab and West Bengal. But on the basis of per

hectare cropped area, the highest amount of pesticide is used in Tamil

Nadu (1.8Kg) followed by Punjab (0.87 Kg), Andra Pradesh (0.83 Kg)

and Haryana (0.8kg). The increase in pesticide consumption per unit

cropped area may be due to the development of resistance and over-

26

utilization of pesticides due to the lack of proper knowledge about the

hazards

2.4.2. Crop wise consumption of pesticides

The amount of pesticides applied to various crops at farmers level is

shown in Table 2.3. This shows the severity of pests on different crops.

Table 2.3. Crop wise consumption of pesticides

Crop Cropped area

covered (%)

Pesticides used

(%)

Cost (Rs) (Crores)

1980 1986

Cotton 5 54 139 250

Rice 24 17 102 100

Vegetables and fruits 3 13 39 100

Plantation 2 8 15 40

Cereals, millets,

pulses and oil seeds

58 2 15 30

Sugarcane 2 3 19 40

Others 6 3 14 20

Total cost 343 540

2.4.3. Consumption of pesticides in different states of India

If we look at the consumption of pesticides in different states of India

(Table 2.4), Anthra Pradesh is found to be using maximum amount of

pesticides, which is followed by Tamil Nadu, UP, Punjab and West

Bengal. But on the basis of per hectre cropped area, the highest amount of

pesticide is used in Tamil Nadu (1.8Kg) followed by Punjab (0.87 Kg),

Anthra Pradesh (0.83 Kg) and Haryana (0.8 Kg). The increase in pesticide

27

consumption per unit cropped area may be due to the development of

resistance and over-utilization of pesticide due to lack of proper

knowledge about the hazards and the requirement of pesticides.

Table 2.4. Consumption of pesticides in different states of India [31]

State / Union territory

Consumption

1974-75 1986-87

Total Per ha (Kg) Total Per ha (Kg)

Andhra Pradesh 10,030 0.7 12,000 0.83

Assam 155 0.1 650 0.42

Bihar 3,000 0.3 3,000 0.3

Gujarat 800 0.1 5,000 0.63

Haryana 2,400 0.5 3,800 0.80

Himachal Pradesh 315 0.3 300 0.3

Jammu and Kashmir 78 0.1 500 0.64

Karnataka 2,472 0.2 4,200 0.34

Kerala 586 0.2 1,200 0.40

Madhya Pradesh 3,422 0.2 4,000 0.23

Maharashtra 3,500 0.2 4,100 0.23

Manipur 17 0.1 48 0.28

Meghalaya 15 0.1 50 0.33

Nagaland 6 0.1 20 0.33

Orissa 1,000 0.1 1,600 0.16

Punjab 3,300 0.6 4,800 0.87

Rajasthan 1,000 0.1 2,600 0.26

Tamil Nadu 1,070 0.2 11,600 1.8

Uttar Pradesh 6,000 0.3 120 0.6

West Bengal 1,800 0.2 5,000 0.56

Others 254

28

2.5. Environmental Implications of Pesticide Use

2.5.1. Harmful effects of pesticides

Pesticides are inherently poisonous chemicals. One has to take proper

precautions and care during their use. Accidental casualties from the

misuse of pesticides are often reported from several places in India.

Contamination of food commodities with insecticides like parathion,

DDT, BHC, endrin etc.are matter of serious concern. Beginning with

merely two or three laboratories in the mid sixties, today we have 30 well-

equipped laboratories (ICAR, CSIR, ICMR, Agricultural Universities)

engaged in pesticide residues analysis.

Owing to Issues regarding the harmful effects of pesticides on human

beings and other living organisms, a Pesticide Information Project of

Cooperative Extension Offices of Cornell University, Oregon State

University, the University of Idaho, and the University of California at

Davis and the Institute for Environmental Toxicology, Michigan State

University was carried out [32,33] [12,13]. The environmental protection

agency (EPA) has initiated a ban on all granular formulations of

carbofuran, which became effective on September 1, 1994. The ban was

established to protect birds because birds ingested carbofuran granules,

which resemble grain seeds. There is no ban on liquid formulations of

carbofuran, which are classified as Restricted Use Pesticides (RUP)

because of their acute oral and inhalation toxicity to humans. The harmful

nature of the liquid formulations forced the manufacturers to give the

signal word "Danger." Granular formulations were given the signal word

"Caution" [34] [14]. The ban was justified by the Toxicological Effects

such as Acute toxicity. Risks from exposure to carbofuran are especially

29

high for persons with asthma, diabetes, cardiovascular disease,

mechanical obstruction of the gastrointestinal or urogenital tracts, or those

in vagotonic states [35] [15].

The toxicity of carbofuran was reviewed by Gupta etc. al [36] [16]. The

review describes the contamination of food, water, and air with the pesticide.

Adverse health effects in humans, animals, wildlife, and fish are also

reviewed. The literature on chemical properties, acute toxicity data,

poisoning incidences, pharmacokinetics, and mechanism of toxicity of

carbofuran is briefly discussed giving emphasis to its two major metabolites

(3-hydroxycarbofuran and 3-ketocarbofuran) on overall toxicity.

Biochemical (cholinergic and noncholinergic), hematological, and

immunological effects induced by carbofuran are discussed in detail.

Carbofuran causes burns to the skin or eyes, therefore eye protection is

necessary when handling carbofuran. A respirator should be worn by

applicators of carbofuran. It also causes cholinesterase inhibition in both

humans and test animals [37-39] [17-19].

The subchronic toxicity of carbofuran in rats was investigated by Brki´c et al.

[40] [20]. The results of the studies revealed that the carbofuran administered

caused a significant decrease in water consumption as well as in brain,

serum and erythrocyte cholinesterase activities. Statistically significant

increases in serum enzyme activities were found in the test animals. The

haematological data showed that carbofuran had no significant effect on

Hb concentration and total RBC, but total WBC showed a significant

statistical decrease. The histopathological changes in liver and kidneys

were observed but were followed by cell regeneration. Reproductive

effects noted were: weak mutagenic effects, carcinogenic effects in

30

animals, organ toxicity, and is responsible for many ill fates in humans

and animals. Other ecological effects noticed are effects on birds (Lethal

dose in Japanese quail is 746 ppm) [41, 42] [21, 22].

Effects on aquatic organisms, carbofuran may be teratogenic to frogs

(Toxicol). Lett. 22(1):7- 13.1984). It is very toxic to trout, Coho salmon,

perch (Hdbk. Acut. Tox. Chem. Fish & Aqua. Invert. 1980), bluegills),

and catfish (Toxicology 23(4):337-345.1982). The 96-hour LD50 for fish

is 150 ug/L. [43] [23]. The acute Toxicity of Carbofuran to selected

species of aquatic and terrestrial organisms (the guppy Poecilia reticulata

Peters, the water flea Daphnia magna Straus and the green algae

Raphidocelis subcapitata Korsikov), and to a terrestrial organism (a white

mustard Sinapis albaLinné) was investigated by Radka DOBŠÍKOVÁ

[44] [24]. The results of the study [44] [24] revealed that Daphnia magna

is very sensitive to carbofuran residues. This study indicated the need to

protect the natural surface waters from the release of pesticides like

carbofuran even if it occurs accidentally.

Some recent development in pest control chemicals are highlighted

below:

2.5.2. Development of resistance in insect pests

Many pests will initially be very susceptible to pesticides, but some with

slight variations in their genetic makeup are resistant and therefore

survive to reproduce. Through natural selection, the pests may eventually

become very resistant to the pesticide. The indiscriminate use of

pesticides has led to the emergence of more virulent pests that have

developed an in- built resistance to some of the frequently used

31

chemicals. This problem has culminated in an outbreak of several

secondary pests to the extent that several minor pests have assumed the

status of major pests. In India, resistance in insects to pyrethroid group of

insecticides has created problems in the control of Heliothis armigera,

Plutella xylostella and Spodoptera litura. This problem can be managed

by suitably integrating synthetic and botanical pesticides along with other

cultural and biological control measures in integrated pest management

programmes.

Pest resistance to a pesticide is commonly managed through pesticide

rotation, which involves alternating among pesticide classes with different

modes of action to delay the onset of or mitigate existing pest resistance [45].

2.5.3. Development of leads from nature

Nature has always provided leads for the development of new dyes, drugs

and pesticides. The synthetic pyrethroids, carbamates, the JH mimics and

many plant growth regulators are the result of follow up of such leads.

The continued search for new pest control chemicals resulted in the

discovery of many unknown pesticides of plant origin. So far, several

thousands of natural products / biochemicals extracted from plants have

been screened at random. Among these, only a few have shown good

potential as pesticides. These include natural pyrethrins, rotenoids and

nicotinoids etc. A few compounds with moderate insecticidal activity like

coumarins, unsaturated isobutyl amides have also been discovered.

The neem tree (Azadirachta indica), a versatile plant contains some

biologically active chemicals called limonoids. Scientists have isolated

several limonoids from seeds, bark and leaves of neem. These limonoids

32

have different modes of action. It has been possible for neem products to

control more than 200 species of insects, mites and nematodes. Despite

advances in technology and understanding of biological systems, drug

discovery is still a lengthy, "expensive, difficult, and inefficient process"

with low rate of new therapeutic discovery [46].

2.5.4. Pesticides in soil

Pesticide in soil can reduce soil fertility. According to a report by US

researchers, the pesticides developed for boosting the crop yield were

found to be acting in the reverse way in the long term [47]. A survey of

orchard and vineyard soils in Ontario, Canada has revealed the extent of

accumulation and persistence of many inorganic pesticides and some

organo chlorine and organophosphorus pesticides. The concentration of

arsenic (As), lead (Pb), mercury (Hg) and copper (Cu) are elevated as a

result of prolonged use of these elements as fungicides. Residues of

organo chlorine pesticides like DDT are found in substantial quantity in

soil. Presence of organophosphorus pesticides in traces is also reported.

Pesticide behavior in soil can be simulated in a computer model to study

the rate of leaching through the upper layer of soil in the case of a potato

crop growing in a Sandy loam soil. Calculated leaching is dependent on

decomposition rates and adsorption strengths, and ranges from virtually

none to 10% of the dosage or more for compounds with high persistence

and mobility. Both decomposition and uptake by plants are important

factors in reducing leaching.

Application of PMA sprays to an apple orchard followed by soil residue

examination shows that Hg residues are almost entirely confined to the

33

top five centimeters of the soil. Dieldrin lost from the soil is found in leaves,

which absorb the pesticide from the vapour phase. The nitro aniline herbicide

benefin readily volatilizes and undergoes photodecomposition.

2.5.5. Pesticides in water

Pesticide content of water will be of great importance in determining the

use of that water. Factors involved are the relative occurrences and levels

of pesticides in solution, adsorbed on solids, or present in the flora and

fauna. An example of the importance of water quality is revealed and

reported in the occurrence of severe growth regulator injury in glasshouse

tomatoes. The injury is characterized by failure of leaf expansion, in

rolling of leaf venation and margin morphology, along with fruit

deformation. The injury could be traced to the domestic water supply,

which had been contaminated by effluent from a factory manufacturing 2,

3, 6-TBA.

Fish and other aquatic biota may be harmed by pesticide-contaminated

water [48]. Pesticide surface runoff into rivers and streams can be highly

lethal to aquatic life, sometimes killing all the fish in a particular stream

[49]. Application of herbicides to bodies of water can cause fish

kills when the dead plants rot and use up the water's oxygen, suffocating

the fish [48]. Some herbicides, such as copper sulfite, that are applied to

water to kill plants are toxic to fish and other water animals at

concentrations similar to those used to kill the plants[48]. Repeated

exposure to sublethal doses of some pesticides can cause physiological

and behavioral changes in fish that reduce populations, such as

abandonment of nests and broods, decreased immunity to disease, and

increased failure to avoid predators [48]. Application of herbicides to

34

bodies of water can kill off plants on which fish depend for their habitat

[48]. Pesticides can accumulate in bodies of water to levels that kill

off zooplankton, the main source of food for young fish [50]. Pesticides can

kill off the insects on which some fish feed, causing the fish to travel farther

in search of food and exposing them to greater risk from predators [48].

2.6. Carbofuran

2.6.1. Structure of Carbofuran

Carbofuran (2, 3–Dihydro–2, 2–Dimethyl–7 Benzofuranyl methyl carbamates)

Is an important soil insecticide grouped under carbamates with the

molecular formula C12 H15 O3N (Structure 3.6). The structural

formula of carbofuran is given below;

Chemical Structure 2.6. Carbofuran

Persistence and bio-availability of carbofuran in soil is decided by

processes like adsorption - desorption, movement and leaching,

degradation and metabolism. A brief review of the research work done

on the above aspect of carbofuran and its interaction with soil components

is given below.

35

2.6.2. Adsorption - desorption.

For better understanding and good environmental management, the

sorption behavior of pesticides in soils is highly useful. The adsorption

and adsorption of carbofuran in two types of Malaysian Soils [clay (high

organic carbon content) and sandy clay (low organic carbon content)] was

investigated by Farahani et. al. [51]. The recovery rate of carbofuran from

sandy clay with low organic carbon content was higher. This means that

percentage adsorption of carbofuran is higher in clay than in sandy clay

soils. They also found that at low adsorption of carbofuran decreased with

increase in temperature initial concentration (10 μg). However, at higher

initial concentrations (eg. 50 μg), the influence of temperature on

adsorption is not significant.

The adsorption of carbofuran in wet zone soils of Sri Lanka was analysed

by Janitha et. al [52]. As the use of carbofuran in the wet zone of Sri

Lanka is very high, the authors [52] have analysed the pesticide soil

sorption coefficient (Kd, (L/kg) for all types of soils in Sri Lanka by using

HPLC. Kd for carbofuran was highest in Wagura soil which has the

highest soil organic carbon (SOC) content of 8.5 L/kg soil and the Kd was

lowest for the Pugoda soil series which has the lowest SOC content of 0.2

L/kg soil. The variation in Kd values indicates the different sorption

capacities of the soils for carbofuran and the vast range of Kd indicates

the differences in the SOC content in different wet zone soil series in Sri

Lanka.

Janitha et. al [53] also studied the sorption behavior of two commonly

used pesticides (carbofuran and diuron) in 43 surface soils samples from

dry and wet zones of Sri Lanka. For carbofuran, the Kd (L/kg) values

36

varied from 0.11 to 4.1 (mean, 0.83; median, 0.62) and Koc ranged from

7.3 to 120.6 (mean, 41.65; median, 36.1), whereas for diuron Kd values

varied from 0.5 to 75 (mean, 9.6; median, 5.15) and Koc ranged from 55.3

to 962 (mean, 407; median, 328). A comparison of sorption data [53] on

these tropical soils with published studies (mostly European and north

American soils) showed that the ranges of sorption coefficients from Sri

Lankan soils were within the wide range of Koc values reported in the

literature. However, these values for both pesticides in soils from dry

zones of Sri Lanka were consistently higher (up to two times) than those

from the wet zone. The wide range of Kocvalues in Sri Lankan soils may

be due to the possible difference in the nature of soil organic carbon,

which needs to be further investigated.

Various factors such as the effects of exchangeable cations (H+ and Na

+),

autoclaving, organic matter, cationic and anionic surfactants, and

temperature on the adsorption of carbofuran on to two different types of

soils are studied by R. P. Singh et. al [54]. They found that the amount of

carbofuran adsorbed in all cases was higher in Jhansi red loam soil than in

Pilibhit sandy loam soil and was related to organic matter content, clay

content, CaCO3 content, surface area, and cation-exchange capacity of the

soils. Carbofuran adsorption was better correlated with clay content than

with organic matter content of soils.

The evaluation of pesticide-soil system interactions as a function of soil

properties was analysed by Ali and Wilkins [55]. The controlled release

granules made from lignin and poly (vinyl chloride) have reduced losses

in delivery of soil pesticides, such as carbofuran, leading to lower

application rates in the field and less leaching hazard. According to the

37

studies of Johnson et.al., [56] carbofuran is expected to partition into the

water from soil.

Hamaker et. al., [57] reported that there is stronger adsorption of picloram

and 2, 4, 5, - T by Iron (Fe) and Aluminium (Al) oxides by clay minerals.

They further stated that although crystallised and amorphous Fe and Al

hydroxides are generally poor adsorbents, they are important in the

sorption of weak acidic compounds. Adsorption mechanism of carbamate

was reported to be through low energy (<80 KJ mol-1

) bonding and was

considered to be due to hydrogen bonds (Bailey et al. [58]. Helling et al [59]

reported that organic matter, pH, clay content, cation exchange capacity

and field moisture capacity are the major factors that influence the

behavior of the pesticides in soil. Felsot and Wilson [60] have correlated

the adsorption of carbofuran positively with organic matter content.

(R2=0.96) and CEC (R

2=0.83). Garg [61] studied the adsorption,

desorption and degradation of carbofuran and bendiocarb in soil and

found that organic matter and clay content are the important factors that

control the availability of carbofuran through adsorption-desorption

mechanisms and degradation.

Rajukkannu and Sree Ramulu [62] conducted a detailed study on the

adsorption of carbofuran in red, alluvial, black and laterite soils. They

found that the adsorption isotherm of carbofuran follows a Freundlich’s

adsorption equation X/M=KC1/n

where X/M = Quantity of carbofuran

adsorbed per unit weight of soil or clay, C=equilibrium concentration,

K= constant and n=constant which indicate energy of adsorption. The K

value for red, black, alluvial, and laterite soils were 9.09,15.14,0.56 and

70.79 respectively. The variation in clay content, organic matter and pH

38

were attributed for this observation. In correlation studies, organic matter

was highly correlated with Kd (ratio of the quantity of a compound

adsorbed per unit weight of the adsorbant to the equilibrium

concentration) values (0.98) while pH and EC were found to be

negatively correlated.

Cation exchange capacity and sesquioxides had a known significant

relationship with Kd values. The Kd values were highest in laterite soil

followed sequentially by black, alluvial and red soils.

Garg and Agnihotri [63] evaluated the adsorption of carbofuran and

bendiocarb on alluvial soil, red soil, black soil and forest soils and in

reference clays like bentonite vermiculite and kaolinite at pH 5, 7 and 9.

They found that the amount of pesticide adsorbed was related to the clay

content and organic carbon and decreased in the order bentonite>

vermiculite > kaolinite clays. The adsorption increased with increasing

pH values of soil. The infrared spectra showed shifting of bonds on

adsorption of insecticides in 2920, 1745,1225 cm-1

indicating that – CH2,

C=0 & - C - O – C groups were involved in adsorption. Only about 18-

43% of the adsorbed carbofuran could be desorbed.

Somasundaran et. al., [64] studied the effect of manuring on the

persistence and degradation of carbofuran and found that in continuously

manured soil, persistence of carbofuran was drastically increased.

According to them organic matter is a major adsorbent in the soil and the

persistence of the carbofuran was positively correlated with its content.

Adhikari and Ray [65] observed that adsorption isotherm of carbaryl on a

model soil was ‘S’ shaped and followed the Freundlich’s equation.

39

X/M= KC1/n

Equation 2.1.

The adsorption was mainly dependent on the availability of adsorbent

surface and was found to be physical in nature.

Crepeion et. al. [66] had shown that the addition of coal to soil at 4:1

soil: coal ratio retained 94.7% of applied carbofuran as compared to soil

alone which had 48.5% retention.

Moisture content of the coal was positively correlated with retention.

Achik et. al. [67] studied adsorption-desorption and movement of

carbofuran in two types of soil viz. clay and clay loams and observed that

adsorption in both the soils followed Freundich’s equation. The adsorption

constant ‘K’ for clay soil, which had higher organic matter content, was

grater than that for loamy clay soil.

Achik et. al. [68] found that carbofuran molecule tended to desorb and

remain in solution .The estimated values for desorption coefficient of

carbofuran were always significantly higher than the estimated values for

adsorption coefficient, irrespective of the soil. This indicated that

carbofuran was not strongly adsorbed and showed desorption tendency

regardless of the type of the soil.

2.6.3. Movement and leaching

Many workers studied the movement and leaching of a wide variety of

pesticides. The adsorption and movement of carbofuran (a systemic

nematicide) in soil columns was analysed by Kumari et. al. [69] taking

two Indian soils (clay loam and silt loam) of alluvial origin. Equilibrium

adsorption coefficient (K) values measured using a batch-slurry technique

40

follows the order clay loam >silt loam soil. A larger amount of water was

needed for leaching the carbofuran to 152 cm in clay loam soil than in silt

loam soil. Carbofuran appeared to increase in drier soils and in finer

textured soils. The influence of soil structure, percolation speed and

product dose on the movement of carbofuran in soils was analysed by

Achik et. al. [70]. Under natural conditions, drained water of a clay soil

rich in organic matter was shown to be systematically more highly

contaminated than that of a loamy clay soil. Although these factors

modify insecticide transfer, it was, however, observed theoretically that

the soil having the stronger adsorption capacity did not retain the product

well.

The downward movement of carbofuran in two Malaysian soil types was

studied using soil columns by Farahani et. al. [71] The columns were

filled with disturbed and undisturbed soils of either the Bagan Datoh soil

(clay) or the Labu soil (sandy clay). The average total percentage of

carbofuran in the leachate of the undisturbed Labu soil after 14 days of

watering (80.8%) was approximately similar to that of the total amount

from the disturbed soil (81.4%). However, carbofuran leaching was

observed in the disturbed soil after the fourth day of watering whereas for

the undisturbed soil, leaching occurred after the first watering. The study

showed that less leaching occurred in soil columns with high organic

content.

R.P. Singh et. al. [72], determined the adsorption and movement of

carbofuran on four divergent textured Indian soils at a fixed volume

fraction (fs = 0.1) of methanol/water mixtures using batch equilibrium and

soil thin layer chromatography (soil TLC) techniques. The measured

41

equilibrium adsorption isotherms for silt loam (FSL and ASL) and loam

(KL) soils were L-shaped and for sandy loam (BSL) soil S-shaped, all

being well fitted by the Freundlich isotherm. A higher adsorption of

carbofuran was observed on FSL followed by ASL, KL and BSL soils as

anticipated by the values obtained for the Freundlich constant, KF and

partition coefficient, KD. The affinity of carbofuran towards organic

carbon, organic matter and clay content of the soils was evaluated by

calculating the KOC, KOM and KC values. The negative magnitude of the

Gibbs' free energy (ΔG0) indicated the spontaneity of the adsorption of

carbofuran onto the soils studied. The leaching index (LI) of carbofuran

calculated for the soils studied indicated its high potential to leach into

shallow aquifers and ground water.

Bowling [73] studied the lateral movement of carbofuran and reported

that carbofuran applied to flooded rice moved laterally at the rate of 22.5

cm in 48 hours in quantities toxic to leaf hopper. Felsot and Wilson [60]

found that cabofuran was very mobile in agricultural Soils. They studied

and reported a direct relationship between adsorption of the insecticides

and its mobility in soil. According to Moreali and Bladen [74] elution

curves obtained by a method using undisturbed soil columns (1 m deep)

showed the leaching characteristics of carbofuran are likely to

contaminate the soil leading to real contamination hazards.

Dibakar Sahu and Agnihotri [75] found that when 0.6 kg ai/ ha of

cabofuran was applied, no detectable residue was observed in sub soil

samples at 15 – 30 cm. They suggested that carbofuran residue are rather

tightly adsorbed on the soil particles and are not available for leaching

through percolating water.

42

Moreale and Bladen [76] observed that due to increasing utilization of

carbofuran, relatively high water solubility, persistence in soils of low pH

(less than 5.5) and low adsorption by soil colloids, the migration of this

chemical to relatively deep water table can occur. Residual amounts were

fairly high and the possibility of water table contamination depends

primarily on the volume of water present (dilution effect). Rajukannu and

Sree Ramulu [77] conducted leachate studies of carbofuran in 5 types of

soil using soil columns and classified carbofuran as mobile (Class 1V) in

red, black and alluvial soils while in laterite soil it was ranked

“moderately mobile” (class 111). The leaching of carbofuran was very

rapid in red, black and alluvial soils and the leachate contained 55, 66.5

and 41.5% of the added carbofuran In laterite soils, a major portion of the

added carbofuran was retained in the 1st

column section (0 -10 cm) and

movement was not detected beyond the 3rd

column section (20—30 cm).

Copin et. al. [78] in a field trial studied the leaching loss of carbofuran in

cornfield analysis and observed that no residue was detected at > 10 cm

depth. Residue of carbofuran in the top layer was smaller and decreased to

undetectable limit within 3 months.

Lee et. al. [79] studied the fate of cabofuran in soil and found that

carbofuran is highly mobile in soils. It moved to a depth of 70 cm under

natural precipitation and evaporation condition in 135 days after the

application. The concentration throughout the profile was > 0.02 mg kg -1

soil. Somasundaramm and Coats [80] studied the mobility of the pesticide

using soil thin layer chromatography technique. They found that mobility

of carbofuran was less in clay loams and silty clay loams compared to

sandy loam and loamy sand. From the soil properties, they concluded that

43

the low mobility in clay loams and silty loams was due to its low pH, high

organic matter and high clay content compared to sandy loams.

Achik et. al. [67] conducted an experiment on the movement of carbofuran in

soil and influence of soil type and aggregate size on the movement of

carbofuran. The results showed that carbofuran leaching in clay soil was

grater than loamy clay and the product moved through the soil more

rapidly and the total quantity exported by percolating water was higher

in clay soil than in loamy clay soil .The fine aggregate fraction retarded

the movement of carbofuran and elimination of the fine aggregate fraction

increased the movement of the product in the soil.

Kladivko et. al., [81] conducted an experiment with subsurface tile drains

installed on low organic matter and poorly structured silt loams soil under

typical agricultural management practices. They found that 0.8 to 14.1

g/ha or 0.05 to 0.94% of the soil-applied carbofuran was lost annually in

the sub-surface drain flow from the soil.

Daniel et. al., [82] conducted an experiment on the dissolution rate of

granular carbofuran and found that carbofuran concentration decreased

rapidly as a function of the distance, such that even after 72 hours there

was only 1 mg ml-1.

of carbofuran at a distance of 2cm from the surface of

the granule applied. This data is indicative of the low molecular diffusion

rate of carbofuran in soil and hence could not be considered as a major

factor in defining pesticide mobility.

2.6.4. Degradation and metabolism

Carbofuran is highly soluble in water (700 mg/l, 25°C) and therefore it

has high potential for water contamination [83]. Its half life in soil is 30 to

44

120 days and in vegetation it is 4 days when applied to roots and longer

than 4 days if applied to the leaves. It undergoes hydrolysis in alkaline

soils [84] and also breakdown in soil and groundwater. Otieno et. al., [85]

have noted the soil and water contamination with carbofuran residues in

agricultural farmlands in Kenya. The two of its toxic metabolites

3-hydroxycarbofuran and 3-ketocarbofuran, were noted in soil and water.

The studies indicated the risk in using the water samples as drinking water

for animals. Carbofuran degrades in water to carbofuran phenol by base

catalysed hydrolysis [86]. The rate of aqueous hydrolysis of carbofuran

increases dramatically with increasing pH.

According to the study of Bailey et al., [87] at a pH of 3, 80-95% of

carbofuran was recovered, how ever when the pH was 10 only 65% of the

original amount was recovered after 1 hour, 35% remained after 3 hours,

and 10% remained after 6 hours.

The biodegradation of carbofuran in the soil was investigated by

Plangklang et. al. [88] using Burkholderia cepacia PCL3 (GenBank

accession number of EF990634), a carbofuran degrader isolated from

phytoremediated rhizosphere soil. Short half-lives (t1/2) of carbofuran of

3–4 d in BSM were obtained using the isolate PCL3 in both free and

immobilized cell forms. PCL3 could be reused twice without loss in their

abilities to degrade carbofuran in BSM, which suggested an advantage of

using immobilized cell over free cell. Repeated application of carbofuran

to soils can result in enhanced rates of microbial degradation was

analysed by Harris et al. [89] and Turco et. al. [90]. According to Parkin

et. al. [91] enhanced degradation of a soil-applied pesticide may occur

when a population of soil microorganisms is repeatedly exposed to a

45

chemical and adapts by developing the ability to catabolize the chemical.

The accelerated degrdation of carbofuran in pretreated soils was analysed

by D. L. Suett [92] and Raha [93]. The rapid breakdown was related to

changes in the populations of soil micro-organisms rather than to

differences in soil properties.

The Investigation of pyrolysis behavior of carbofuran by pyrolysis-gas

chromatography–mass spectrometry was carried out by Wang et. al., [94].

It was found that carbofuran was decomposed with the temperature

increase. A large number of mono aromatics and polycyclic aromatic

hydrocarbons (PAHs) were formed when the temperature was higher than

750◦C, and the concentration of aromatic hydrocarbons were higher at

higher temperature.

Kandaswamy et. al., [95] studied a Helminthosporium sp, which would

degrade grater than 70% of a 20-ppm amount of carbofuran within 14

days. Strains of Aspergillums Niger and Trichoderma viridae were less

active in degradation. All the three isolate formed hydroxyl carbofuran as

an intermediate.

Greenhalgh and Relangur [96] observed that when carbofuran was applied

to a humic mesisol, 3 OH carbofuran and 3-keto carbofuran were detected

as transformation products. 3-hydroxy carbofuran reached maximum

concentration in 1-7 days and 3-keto carbofuran attained maximum value

in 16-36 days.

Garg and Agnihotri [63] studied the degradation rate of carbofuran in five

soils and observed that half-life period of carbofuran showed wide

46

variation between different soils. In case of laterite soil, half-life period

was 69 days compared to 8 days in black soils.

Brahmaprakash and Sethunathan [97] reported that degradation of

carbofuran occurred in soils mainly by hydrolysis and 3 OH carbofuran

was found to be the chief metabolite in soil.

Arunachalam and Lekshmanan [98] studied the decomposition of labeled

carbofuran under four different conditions viz. Sterile flooded (SF), sterile

non flooded (SNF), non sterile flooded (NSF) and no sterile non flooded

(NSNF) and observed that in SF and SNF soil, more than 75% of the

added carbofuran was recovered as a residue after 60 days. In NSF and

NSNF soil, more than 75% of the added carbofuran was metabolized

during the same period with 3 keto- carbofuran and carbofuran 7 phenols

as major metabolized during incubation. Jae-Koo Lee et. al., [99] found

that after the application of carbofuran in soil, 3 keto carbofuran phenol

was the major metabolite and carbofuran phenol, 3 hydroxy carbofuran

and 3 hydroxy carbofuran phenol were found as minor metabolites.

2.6.5. Persistence

Owing to its low tendency to volatilize from water or moist soils, very

low concentrations are noted in air. The investigators [100] noted only

0.03 to 0.66 ug/m3 after a 44-hour sampling period following its an

application of 44% active ingredient carbofuran. In a sandy loam soil

Harris [101], employing bioassay techniques, found that the carbofuran

was detectable up to 16 weeks after the application. He also noticed the

persistence of this carbamate insecticide for a longer period in muck soil

compared to sandy loam soil. Gupta and Devan [102] observed much

47

shorter persistence for carbofuran in soils under Indian condition. The

residue persisted only for 35 days when 0.4 to 1.33 kg ai/ha was applied

to soil. Das et. al., [103] found a high persistence for carbofuran in laterite

and black soils of Kerala as shown by the mortality of Aphis craccivora

released from cowpea (Vigina unguiculata) grown in the treated soil.

Green Halgh and Relangur [96] observed the half life of carbofuran to be

15 – 30 days when applied to a humic mesisol at the rate of 2.24 and 4.48

kg ai/ha.

Gordan et. al., (1982) [104] in field and lab studies on the persistence of

soil applied carbofuran identified soil moisture as the most important

factor in carbofuran persistence. He observed no carbofuran residue at the

depth of more than 7.5 cm, 22 weeks after application.

Copin et. al., [78] found that residue of carbofuran applied on the surface

in the top soil layer to be very small and became undetectable with in

three months.

Getzin and Shanks [105] measured the persistence of C14

carbonyl

carbofuran in pacific North West soil. The half life was > 2 weeks in all

cases and > 15 weeks in five different soils. The carbofuran decay curve

always possessed an initial lag phase where soil mixing enhanced

insecticide decline.

The Persistence of carbofuran in marine sand and water was investigated

by Campbell et. al., [106]. Marine sand and seawater samples were

collected from Laysan Island in the Hawaiian Islands National Wildlife

Refuge, where a small area was contaminated by the carbamate

insecticide carbofuran. Carbofuran was detected at microg g (-1)

levels in

48

the Laysan sand. The photolysis of carbofuran was faster in seawater than

in distilled deionized water when it was exposed to 300 nm light (t1/2, 0.1

vs. 3.1 h) and to direct sunlight (t1/2, 7.5 vs. 41.6 h).

The persistence of carbofuran applied to surface (oxidised) and sub-

surface (reduced) layers of a flooded soil was studied using radiolabelled

insecticides by Soudamini Panda et. al., [107]. In one experiment, these

compounds were placed in the surface (2–5 mm) and sub-surface (10–15 cm)

layers of 10-day flooded soil columns. The decreased stability of surface-

applied carbofuran was attributed to be due to a relatively higher pH in

the surface layer and in the floodwater, which was in immediate contact

with the surface layer.

The persistence of carbofuran in two Malaysian soils namely the Bagan

Datoh and Labu soils was studied under laboratory conditions at a

constant temperature of 30 C by Farahani et. al., [108]. It was observed

that the half-lives of carbofuran in the Labu soil samples (which are low

in organic matter content) at 100%, 90% and 60% field capacity were

57.28, 38.51 and 115.52 days respectively. However the corresponding

half-lives of carbofuran in the Bagan Datoh samples (which are high in

organic matter content) at 100%, 90% and 60% field capacity were

192.54, 141.46 and 203.87 days respectively. The degradation of

carbofuran followed a first order kinetic reaction. The results of this study

showed that soil moisture content, micro-organisms and the organic

matter content (OM) affected the degradation of carbofuran in both soils.

Achik et. al., [109] studied the carbofuran (Curater 5G) behavior in two

drained cornfield soils, clay and loamy clay, for 2 successive years.

Different dissipations were observed in each soil for the same time period

49

(8 weeks in 1985, 9 weeks in 1986). The authors found drained water

from organic matter-rich soil to possess a higher carbofuran content, with

7.1–13.7 and 2.5–5.0% of the applied dose for clay and loamy-clay soils,

respectively. The major part of these percentages arose from the drained

waters associated with rainfall occurring during the first 2–3 weeks after

application. Laboratory experiments confirmed the influence of the soil

structure and its properties on carbofuran adsorption, and consequently on

carbofuran leaching under field conditions.

To conclude, the literature survey conducted above revealed that the

following were the factors that control the adsorption, desorption,

movement, leaching, degradation and metabolism of the pesticide residue

in the soil; factors such as organic matter, clay content, CEC, field

moisture capacity (water holding capacity (WHC)) influenced the

behaviour of the pesticide in the soil. The mobility of carbofuran in the

agricultural soil was found to be greater. The availability of carbofuran in

the soil, adsorption and desorption mechanisms and degradation

behaviour are mainly decided by the organic matter and clay content of

the soil. Correlation studies conducted by various scientists revealed a

direct relation between organic matter and adsorption of the pesticides,

while and inverse relation was obtained with respect to pH and electrical

conductivity. In the case of laterite soil, black, alluvial, and red soils, it

was found that adsorption increased with decreasing pH. The continuous

manuring technique, employed in agriculture, was found to be drastically

increasing the presence of carbofuran in the soil due to two main reasons.

First of all, the organic matter in the soil being a major adsorbent of

carbofuran enhances the persistance of carbofuran. Secondly, the higher

50

manure content checks the microbial activity that degrades carbofuran,

which also increases the tendency of carbofuran to persist in the soil. This

caused the prolonged persistence of carbofuran in the soil subjected to

continuous manuring technique. The retention of carbofuran in the soil

was found to be very much dependent on the moisture content of the soil.

With increasing depth of the soil, the amount of carbofuran reduced, as

expected. The degradation rate of carbofuran was directly related to the

high pH of the soil and inversely related to the organic matter content.

51

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