PESTICIDE RESIDUES IN THE SURFACE RUNOFF

by

TAY TECK PIN

A thesis submitted

in p~tial fulfillment of the requirements for

the degree of Master of Environmental Science

Faculty of Resource Science and Technology

UNIVERSITI MALAYSIA SARAWAK

2005

ACKNOWLEDGEMENT

I wish to express my sincere thanks to my supervisor, Associate Professor Dr. Lau Seng

who has been patiently giving me guidance and advice in making this study a success.

My gratitude goes especially to his invaluable comments, discussion and constructive

criticism in reading my manuscript. My thanks also go to the officers of the Agriculture

Department who have helped in one way or another throughout my study. I also wish to

acknowledge Mr. Chai Lian Kuet and his staffs in the Agriculture Research Centre of

Agriculture Department Sarawak in Semnggok, Kuching for their technical assistance

offered. I appreciate their support and permission to use the laboratory facilities and

resources. My heart felt appreciation is also extended to Mr Wong who had kindly

allowed this study to be carried out in his farm. My special thanks also goes to Kuching

Water Board, Pepper Marketing Board, Kuching, Dr. Ling Teck Vee, UNIMAS lecturers

and laboratory assistants and all my friends who have given me their moral support and

assistance to me throughout my study. Last but not least lowe my success to my

beloved family who has given me moral support and considerations throughout my study

period.

r

ii

Pusat Khidmat Mate'umat Akademik UNIVEItSITI MALAYSIA SARAWAK

TABLE OF CONTENTS

Acknowledgement 11

Table of contents III

List of figures v

List of tables vi

List of plates VII

Abbreviations Vlll

Abstract ix

Abstrak. XI

Chapter 1 : Introduction

1.1 Problem statement 6

1.2 Objectives 8

Chapter 2: Literature review 9

2.1 Introduction

2.2 Diffusion 11

2.3 Volatilization 12

2.4 Adsorption 13

2.5 Leaching 16

2.6 Surface runoff 18

2.7 Degradation 24

Chapter 3 : Materials and Methods 28

3.1 Introduction

3.2 Surface runoff and rainfall monitoring and sampling 31

3.3 Determination of Soil texture and Carbon 35

3.3.1 Particle size distribution in soil (Chin 2000)

3.3.1.1 Procedure determining particle size 36

III

3.3.1.2 Sand fractions 37

3.3.1.3 Calculation

3.4 Principle and theory for pesticide extraction from water. 38

3.4.1 Sampling procedure and storage 39

3.4.2 Pesticide extraction procedures

3.4.2.1 Pesticide extraction from water

3.4.2.2 Pesticide extraction from soil 40

3.4.3 Gas Chromatography (GC) Analysis

3.4.3.1 Gas Chromatography and Gas Chromatography Mass 41

Spectrometer operating parameters

3.4.3.2 Pesticide identification

3.4.3.3 Calculations

3.5 Quality assurance 42

3.6 Statistical analysis 46

Chapter 4 : Results and discussion 47

4.1 Introduction

4.2 Influence of the time interval between pesticide application and

the first rainfall on pesticide runoff

4.2.1 Light rainfall intensity 48

4.2.2 Moderate rainfall intensity 52

4.2.3 Heavy rainfall intensity 56

4.2.4 Summary of pesticide wash out 60

4.3 Influence of rainfall intensity to the pesticide runoff 61

4.4 Pesticide accumulation in soil 66

75Chapter 5 : Conclusion

5.1 Limitation of the study 77

References 78

Appendix I Questiuonaires 84

Appendix II SPSS output 86

iv

LIST OF FIGURES

Figure 1.1

Figure 2.1

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Risks of pesticides

The fates of the pesticide in the agriculture field

Location site for the experimental plot

GC Chromatography of Standard solution for water

GC Chrmatography on water recovery for 5 ppb

GC Chromatography of Standard solution for soil

Figure 3.5 GC Chrmatography on soil recovery for 15 ppb.

Pesticide concentration extracted from the surface runoff under the light rainfall

intensity at different time intervals

Pesticide concentration extracted from the surface runoff under the

moderate rainfall intensity at different time intervals.

Pesticide concentration extracted from the surface runoff under the heavy

rainfall intensity at different time intervals

Pesticide concentration washed out pattern for the three simulated rainfall

intensities

Amount of pesticide concentration in soil throughout the experiment period

v

LIST OF TABLES

Table 2.1 Pesticide mobility

Table 3.1 List of pesticides used by the farmers in the survey

Table 3.2 Rainfall intensity from the Department of Meteorology, Kuching

Table 3.3 Settling time for silt and sand

Table 3.4 GC and GC Mass Spectrometer operating parameters

Table 3.5 Pesticide Residues recoveries from spiked soil and water samples

Table 3.6 Physical properties and fate characteristics of chlorpyrifos

Table 4.1 Pesticide concentration extracted from the surface runoff under different

simulated rainfall intensities

Table 4.2 Pesticide concentration detected in the soil

Table 4.3 Pesticide concemration detected in the soil

VI

LIST OF PLATES

Plate 3.1

Plate 3.2

Plate 3.3

Plate 3.4

Plate 3.5

Plate 3.6

The vegetable beds in the experimental plot

The experimental plot

Water sample collection point

Water meter used in the experiment

Knapsack sprayer

The sprinkler head

vii

ABBREVIATIONS

OP Organophosphate

GC Gas chromatography

ANOVA Analysis of variance

FPD Flame photometric detector

Vlll

Abstract

The purpose of this study is to investigate the amount of pesticide washed out under the

influence of different rainfall intensities and different time intervals between the pesticide

application and the first rainfalL Three different rainfall intensities were simulated on an

experimental plot and administered in different time intervals after pesticide application.

Both the surface runoffs and the soil samples were taken for pesticide analysis in the

laboratory. The extracted pesticide from both the water and the soil samples were later

determined by gas chromatography with flame phosphorus detector. Generally, the

results show a similar trend of pesticide wash out in the surface runoff. There was no

significant different in the wash out of pesticide in the surface runoff among the three

rainfall intensities studied. All the three rainfall intensities had shown that the l-h and 2­

h time intervals after the pesticide application, were more susceptible to being washed out

in the runoff. It was observed that there was a significant different (p<0.05) in the

pesticide amount between the short hour time intervals and the long hour time intervals

runoff. Parallel to most studies, most of the pesticide was found to have accumulated in

the soil after high frequencies of pesticide applications. The soil in the light rainfall

intensity had adsorbed the most amount of pesticide. Comparatively, the soil under the

heavy rainfall intensity had the least amount of pesticide adsorbed among the three

rainfall intensities. Nonetheless, the amount of pesticide found in the soil of all the three

rainfall intensities was ranging from 1.06 mg/kg to 9.44 mg/kg. The observation had

concluded that in the long hour time intervals degradation processes and the

crystallization of the pesticide on the soil particles had accounted for the lower amount of

pesticide washed out in the surface runoff. Thus, for the amount of pesticide washed out

ix

in the surface runoff, the time interval between the pesticide application and the first

rainfall was the major factor.

x

I

Abstrak

Tujuan penyelidikan ini adalah untuk mengkaji mobiliti raeun perosak dalam aliran air

permukaan di bawah pengaruh kelebatan hujan dan jangka masa di antara menyembur

raeun dan hujan pe·rtama di kebun sayur. Tiga tahap kelebatan hujan yang berlainan

disimulasi dan diberikan kepada tapak eksperimen dengan jangka masa yang berbeza

selepas penyemburan raeun. Sampel air aliran permukaan dan sampel tanah diambil

untuk analisis raeun perosak di makmal. Raeun perosak yang diekstrak dianalisis

melalui kromatograji gas dengan pengesan nyalaan fosforus. Pada amnya, keputusan

telah menunjukkan satu trend yang sama bagi rae un perosak yang terhakis dalam air

dapat diperhatikan. Seeara statistik. terdapat perbezaan yang signifikan bagi raeun

perosak yang dikesan dalam air aliran permukaan untuk jangka masa pendek dan jangka

masa panjang di antara masa menyembur raeun dan hujan pertama turun. Di antara

ketiga-tiga tahap kelebatan hujan, tempoh jangka masa pendek (satu jam dan dua jam)

selepas memberi racun perosak, air hujan dapat melarut resap terbanyak raeun perosak

ke dalam air aliran permukaan. Selari dengan hasil penyelidikan lain, tanah didapati

menyerap kebanyakan raeun perosak yang diberi. Tanah menyerap raeun perosak yang

terbanyak di bawah eksperimen kelebatan hujan renyai berbanding dengan kelebatan

~ I j

«

I ! 1 1

f j, I ~

I hujan Ie bat. Walau bagaimanapun jumlah raeun perosak yang terdapat dalam tanah

untuk ketiga-tiga kelebatan hujan adalah di antara julat J. 06 mglkg dan 9.44 mglkg.

Pemerhatian mendapati proses kereputan sebelum hujan turun serta raeun mengalami

penghabluran merupakan faktor utama raeun perosak kurang dikesan dalam air aliran

permukaan untuk e/u,perimen tempoh masa panjang. Jadi, tempoh masa di antara masa

menyembur raeun perosak dan hujan pertama turun merupakan faktor utama dalam

raeun perosak terhakis dalam air aliran permukaan di kebun sayur.

xi

Chapter 1

Introduction

In Malaysia, a total area of 36, 938.08 ha are cultivated with vegetables and the production for the year

2001 was 683, 426.73 metric tons (Anon. 2002). Thus, vegetable growing has developed from

backyard gardening to a commercial scale. However, in Sarawak only 0.4% of the total land use of

12,325,402 ha. are used for vegetable farming (Anon. 2002). Data from Agriculture Department of

Sarawak (Anon. 2002) shows that majority of the vegetable fanns are concentrated in the Kuching

Division.

The concentration of the fann in an area will definitely attract diseases and pests. As food is available

for pests especially when the pest management program is not effectively implemented, thus the

presence of pests in a fann is unavoidable. To make matter worse, if the vegetable waste is not

properly disposed off, then this will be the breeding ground for pest regeneration. In the course of

protecting the crops from the infestation of many insect pests and improve production, fanners have

resorted to chemical control. Chemical pesticides are consistently used to control them so as to give

fast and consistent prod.uction. According to the Agriculture Department in Sarawak (Anon, 2002),

the import of insecticides for the year 2001 was RM 30,262,903. Pesticide applications for the

intensively cultivated farms are unavoidable as both the fanners and the consumers would like to see

quality products. Vegetables with no traces of pest attack can fetch good price and on the other hand

satisfY the consumers' psychological fear of contracting diseases from the pests invested vegetables. It

is obvious that pesticides have played an important role in stabilizing food supply, maintaining

product quality, especially appearance, and enhancing agricultural productivity (Takagi and Ueji

1997). Besides, they also pointed out that the field productivity was increased by 35% when I

pesticides were used. In the rice production, the labour productivity became 10 times higher during

the 35-year period from 1950 to 1985 by using herbicides in Japan.

Excessive use of pesticides in the vegetable farms has caused adverse effects on the consumers as well

as the environment. The damages that it inflicted range from micro-organisms in the soil and water to

human who use it. All aspects on earth like river, ground water, soil, air and our drinking water are

badly affected by the pesticides. Indiscriminate applications of pesticides will not only cause the

decrease in diversity and number of wild life especially the non-target organisms, it may cause the

targeted organisms to be'come more resistant.

Pesticides residues are harmful to the micro as well as macro organisms in the soiL It has chain effects

on the food web. For example, Beyer and Gish (1980) found that the pesticides accumulated in

earthworms were hazardous to some sensitive bird species, Takagi and Ueji (1997) has illustrated in

the diagram below (Fig. 1.1) the potential risk of pesticides.

Sangodoyin (1993) studied on five institutional farms where intensive agriculture was practised in

southwest Nigeria. It was found that the streams, ponds and wells were slightly polluted by the

pesticides applied to adjacent land. The concentrations of pesticide residue in drinking water were up

by 15 times higher than the standard of 10 IJgL'! set by the United States Environmental Protection

Agency (USEPA).

2

Acute I...-H_um_a_n_b_ei_n_g_s---, ----~..I Toxicity Chronic

Environment Pollution ~_

Carcinogenicity Teratogenicity

Biological concentration

River water Ground water Drinking water

Air

soil

Decline of wild life increase in resistance

Disturbance ofecosystems

Source: Takagi and Ueji (1997)

Figure 1.1: Risks of pesticides

Undoubtedly, pesticides have adverse effects on human health through either direct or indirect contact.

Wheeler (1998) conducted a study to explore the economic and health impacts of prolonged pesticide

use in the Mekong Delta in 1996. The study revealed that 41.8% had experienced headaches, 26.2%

had dizziness and 31.4% had experienced skin irritation. Ramasamy and Aras (1988) in a survey

found that various pesticide poisoning were recorded from the estate workers, rice farmers, small

holders and vegetable growers. A significant statistical correlation (r=0.87) was established between

pesticide use and incidences of poisoning. Mulla et al. (1981) reported that Parathion p=s oxidation

on airborne particulates may be a contributing factor in field workers toxicity cases.

Pesticides that contaminated water are adversely affecting the health of aquatic life. It causes deforms

and abnormal growth among the aquatic life. Pesando et al. (2004) found that three organochlorine

pesticides (dieldrin, methoxychlor (MXC) and Lindane) had decreased the rate of fertilization in sea

3

urchin. However, the pesticides also increased the rate in polyspenny, delayed or blocked the first

mitoic divisions and altered early embryonic development. This could probably be through changing

the intracellular biochemical pathways that control first mitoic divisions and early development. All

the three pesticides were found to affect the early embryonic development. Among the pesticides

studied MXC appeared to be the most potent compound to disrupt development in plutei.

Pesticide in aquatic environment has a considerable impact on fish such as salmon that lives in fresh

water in the early life cycle and later, migrates to the sea. A study by Waring and Moore (2004)

showed that there was a significant physiological effect on salmon smolts in atratzine polluted fresh

water at the early stage of life. In the study, the salmon smolts were exposed to different

concentrations of atrazine in fresh water and found that a significant reduction of gill Na~K+ ATPase

activity and elevated plasma cortisol concentrations and monovalent ion concentrations. The fish that

were exposed to high atrazine concentrations (6.5 to 22.7 Ilg/L) had a high mortality rate after being

released in the seawater, It was believed that the fish were unable to regulate ion flux activity within

the gills of the smolts. For those fish that survived the sea challenge had shown some physiological

stress. This may be a hazard for salmon undergoing smoltification and subsequent migration into

seawater.

Pesticides that move from targeted areas are health hazard to mankind and detrimental to ecosystem.

According to the statistics from the Department of Agriculture in Sarawak (DOA, 2002), about RM 30

million has been spent on the import of fungicides and herbicides in 2000. Usually, the pesticides will

degrade itself under the natural environment. Due to the increase in the resistance of the pest, the

application of pesticides has increased in dosage as well as in the application frequency. Before the

pesticides have time to degrade, another round of spray is applied. The excessive pesticide is either

gone with the harvest of the vegetable or finds its way to the water body. Off target movement of

4

Pusat Khidmat Maklumat Akademik UN'VERSfTI MALAYSIA SA~AWAK

pesticides has been the center of debates for ecological and human health. The fragile ecosystems are

usually sited next to the agricultural production zone whereby protection of these resources can be

achieved by minimizing the pesticide runoff. Furthermore, pesticide concentration in drinking water

from surface water body as well as underground water source must be kept below toxic level. Sujatha

et al. (1999) in their study on the distribution pattern of Endosulfan and Malathion in the Cochin

estuary, India found that these two pesticides were in high concentration (13.0l3 !lIlI) in the

premonsoon. They also discovered that Endosulfan when oxidized is converted to metabolite which is

more deleterious in the aquatic environment. In another study, Begum and Vijayaghavan (1999)

studied the biochemical aspects of Clarias batrachus (Linnaeus) when exposed to organophosphate

insecticide Rogor. Sublethal levels of Rogor can disrupt carbohydrate metabolism in C. batratchus.

Rogor reduces oxidative metabolism in the muscle tissue of C. batrachus and it subsequently

increases the lactate content in muscle tissue. The accumulation of Rogor in the muscle tissue after a

continuous exposure of 8 days makes the muscle unfit for human consumption. Furthermore,

pesticides, which are persistent, are a threat to aquatic life. A small concentration of pesticide in the

water will not kill the water creatures but biological accumulation of the toxic in the carnivorous fish

and birds are found to contain higher pesticides concentration. Therefore their high capacities for bio­

accumulation can be a t\1reat to ecosystems and human health (Doong et al. 2002). In 1994 Miliadis

pointed out that pesticide residues reach aquatic environment through direct run-off, leaching, careless

disposal of empty containers, equipment washing and others.

Pesticide is versatile in its movement from the designated area even during applieation. Several

pathways have been identified for pesticides that are applied in the field. They are either lost to the

atmosphere through volatilization or transported long distances from their sites of application whereas

others are carried away in the surface runoff or photo degraded by sunlight. Hung et al.(2004)

mentioned that organophosphates(OP) are relatively soluble in water that makes its availability in

5

aquatic environment unavoidable especially through surface runoff, sprays and soil leachate. Sujatha

et al. (1999) stated that the movement of these pesticides to the aquatic environment is dependent on

the processes such as solubility, precipitation, volatilization, leaching, surface erosion, sorption and

others. In his study it was found that Malathion was more soluble in water and thus more mobile than

Endosulfan. It was found out that these pesticides were detected in the soil of the agricultural farm

even though its half life in soi I was quite short. However the persistence of these chemicals can last to

a year in dry sandy soils (ASTDR, 2000). The non selective nature of the chemicals that are toxic to

both the vertebrates and invertebrates, has raised concern to non target organisms. Llaser and

Gonzalez (2001) had found that 5.4 Ilg/L of methiocarb was discovered in ground water sample and

for 3-hydroxycarfuran it was 18 flg/L in a surface water sample in an agricultural zone of the Yaqui

Valley. Through leaching the pesticides had found their way to the surface water as well as

underground water. It is important to understand the pesticide runoff processes from the agricultural

fields in order to be safe and effectively use the chemicals continuously and to keep the applied

chemicals at the site. Furthermore, surface waters have been a source of drinking water for all

consumers and measures must be taken to keep the pesticide concentrations below toxic levels.

1.1 Problem Statement

Pesticides are commonly used to control pests in agriculture but excessive and uncontrolled spraying

will lead to polluting the environment Many studies like Heim et al. (2002), Guo et al. (2004),

Nakano et al. (2004), Rovedatti et al. (2001) had confirmed that there was a substantial amount of

pesticides leaching in the surface runoff. Furthermore the degradation of these pesticides, most of

them, are more toxic than the parent compounds. Several studies (Kimbrough and Litke 1996,

Battaglm and Fairchild,2002, Neumann et al.2001) had investigated the amount of pesticides leached

6

into rivers in a calendar year from a large, multi-field heterogeneous areas where different crops and

agricultural practices as well as non agricultural areas are included. These are the monitoring

programs on the pesticide concentrations in the receiving water bodies. A number of models have been

developed to estimate the pesticide concentrations in the runoff. However, they all failed to relate the

influence of rainfall intensity with the pesticide applied and will the amount of pesticide leached be

influenced by the time interval between the pesticide application and the first rainfall? Is there any

method to predict the washed out pesticide given the factors like rainfall intensity and the time

difference before the first rainfall? Climatic patterns drive the transport of the runoff soil, water and

pesticide leaving the target areas. Rainfall amount, intensity and duration as well as the proximity of

the rainfall to pesticide application are of concern. The time period between the pesticide application

and the beginning rain event is a crucial factor (Wauchope, 1995). The question is how much of the

applied pesticide is washed away by water? How long after pesticide application will there be the

least amount of pesticide being washed away by the rainwater? How much of the pesticide is retained

in the soil? What rainfall intensity will have the most concentration of the pesticide being washed in

the surface runoff? The answers to these questions will help the farmers and the enforcement

authorities to introduce steps with an aim to reduce the pesticide loss through runoff. Thus, it is

essential to perform a study to actually quantifying pesticide concentration in the surface runoff based

on the rainfall intensity and the time interval before the first rainfall.

This study will enable us to understand what controls and influences pesticide runoff. Hopefully the

findings can be extrapolated to establish a relationship between the pesticide used and rainfall intensity

where the runoff flows into the river systems near the vegetable farms. Mitigation measures can be

drawn up to arrest the p~sticide runoff into surface water with the knowledge of factors influencing the

runoff. Remedies can be taken to reduce if not overcome the contamination of the environment. The

findings can be used as the reference for the Agriculture Department to formulate stringent rules or

7

monitoring programme on the vegetable farms. The findings provide information for the departments

concerned like Water Board Department, Fishery Department and Department of Environment to plan

and implement steps to safeguard the natural resources.

1.2 Objectives

The objective of the project was to determine the proportion of the applied pesticide that would be

washed out from cultivated soil to the aquatic environment through surface runoff.

The specific objectives of the study were:

a)

b)

c)

To determine the amount of pesticide that can be washed out in relation to the time interval

between the pesticide application and the first rainfall;

To determine the influence of rainfall intensity on the amount of pesticide washed out;

To determine the pesticide concentration accumulated in the soil.

" '!'. w

8

I ·

Chapter 2

Literature review

2.1 Introduction

Pesticides are divided into several classes. Among the three important classes are: organochlorine,

organ phosphate and pyrethoid compounds. Some of these compounds especially organochlorine are

known to be resistant to· biodegradation whereas organophosphate can be degraded rapidly depending

on their formulations, method/technique of application, climate and the stages of plant growth.

Pesticide that differs in the chemical and physical properties has different persistence and mobility in

the environment. The properties of concern are solubility, adsorption, persistence and volatilization.

There are a number of channels where pesticide can move into after it was applied to the field. The

understanding on the pesticide fate processes can help every pesticide applicator to ensure that

applications are not only effective but also safe for the environment. The possible fate of pesticide

after it is applied to the agricultural field is shown in Figure 2.1. (Iowa State University, 1999)

9

----------------------- -----~-..--..

Source: Iowa State University, ] 999

Figure 2.1: The fates of pesticide in the agriculture field

Brady (1974) discovered that the release of pesticide in the field in 5 possible fates.

a. volatilization

b. adsorption

c. leaching downward into the soil

d. chemical reactions within or on the soil surface and

e. degradation by soil organisms.

The mobility of the pesticide can be in the form of solution or adsorbed on the migrating particulate

matter or by volatilization. Two mechanisms that influence the mobility of pesticide through soil in

the solution stage are diffusion and mass-flow processes.

10

T!

2.2 Diffusion

Diffusion is the process by which matter is transported as a result of random molecular motions caused

by their thermal energy. This can be seen as the movement from positions of a higher concentration to

positions of a lower concentration position. On the other hand mass flow is the movement of the

pesticide carrier caused by the external forces. Diffusion determines the distribution pattern of

pesticide in soil. Fick's law of diffusion is represented by equation(1).

J _Dec ex (1)

Where J is the quantiity of transfer per unit cross sectional area per unit time, D is the diffusion

coefficient, C is the concentration, and x is the space coordinate measured normal to the section. But

when diffusion occurs i~ water, the Fick's law can be represent by equation (2).

C _Dec (2)t ex

The diffusion of pesticide can occur both in the vapour and in the non vapour phase. The non vapour

phase occurs in solution or at the air water or air solid interface. So volatile fumigants diffuse rapidly

through a porous media except when the water content is high. Factors such as the soil and the

diffusion coefficient, solubility, vapour density, adsorption, bulk density, soil water content and

porosity influence the mobility of the pesticides in the soil.

11

r 2.3 Volatilization

Vapour phase movement can be a source of distributing certain pesticide throughout the soil profile

and eventually lost through surface evaporation. Factors like vapour pressure of the pesticide in the

soil and its rate of movement to the evaporating surface will influence the volatilization rate of

pesticide. However, temperature also plays an important role in determining the saturation vapour

pressure of every pesticide.

Volatilization of pesticide in soil is greatly influenced by water. Water acts as a carrier and when it

evaporates, the surface layer will produce a pulling effect on the water column in the soil by capillary

action. This will accelerate the pesticide loss by evaporation.

Scholtz et al. (2002) studied volatilization using a pesticide emission model (PEM) on agricultural soil

and crops. They found that PEM was useful to predict the volatilization of pesticide applied to

agricultural soils and crops through various methods including soil incorporation, surface spraying or

on furrow at the time of planting.

Pesticide lost through volatilization is greater when the soil has low content of organic matter. The

organic matter content in the soil has an inverse relationship with the rate of pesticide volatilization.

Bedos et al.(2000) in their wind tunnel system to study the rate of pesticide volatilization from soil

proved that pesticides volatilization to the air is closely link to environmental, physico-chemical and

technical factor. Burn et al. (1978) reported that volatilization is frequently the major way in which

initial loss of pesticide occurs. The high temperature and the hot sun seem to be the encouraging

factors for the dissipation of the pesticides. However, very little studies have been done on this initial

loss to ascertain how much and how fast the pesticides have escaped into the air.

12

2.4 Adsorption

Mass flow can be understood through the external forces that acts on water, air or soil particles which

serves as a carrier for the pesticide. The pesticide must be either dissolved, suspended or in the form

of emulsion in water before it is carried away. The amount of pesticide to be transported by mass flow

of water in the soil will depend on the sorption characteristics of the pesticide with soil as well as the

land gradient and the speed of water flow. The sorption power of pesticide will determine the distance

of movement and the maximum pesticide concentration being transported. Sorption is the ability of

the ions or molecules adhere or attracted to the soil particle surfaces. This is the electrical attraction

between charged particles that is pesticide molecules and soil particles. The pesticide molecules that

are positively charged are attracted to and can bind to negatively charged particles of clay or organic

matter. Sorption is sensitive to the characteristics on the soil surfaces like organic matter content, pH,

soil particle size distribution, temperature and moisture content. It is said that there is an inverse

relationship exist between adsorption and movement of particles by water through soil. The soil

particles, when adsorbed with the pesticide act as a carrier in the water or air. So the quantity being

moved by the soil particles will depend on the amount adsorbed by transporting soil. Wind as an

erosion agent can move the adsorbed particles over long distances when compared with the water.

Adsorption is the proces.s whereby a substance is accumulated at or near an interface which is relative

to its concentration in the bulk solution. It is due to the electrical binding force that exists between

pesticide molecules and soil particles. Benjamin (1992) pointed out that molecules nearest to the

surface acted differently from the bulk solution. These adsorbed molecules which "half in solution

and half out" to reside in the solid surface will either attach to the solid or form a separate phase

different from the dissolved phase as a solid or gas. When the amount of the adsorbate at the surface

to that in solution has reached equilibrium, an adsorption isotherm is formed. Benjamin (1992)

13