ABSTRACT
The concentration levels of trace metals in spinach and lettuce irrigated with sewage
water on agricultural lands in metropolitan Zaria were determined in a 3-month monitoring
programme (February − April, 2008). The investigation was carried out in the biggest sewage
water – irrigated vegetation in the city, which is a fadama land (flood plain) along river
Kubanni drainage basin. Samples of spinach, lettuce, sewage water, sewage sludge and soil
from the study area, as well as samples of tube-well water, soil, spinach and lettuce from a
control area were analysed for five trace metals (Zn, Ni, Cu, Pb and Cd) using atomic
absorption spectrophotometry (AAS). The physico-chemical characteristics of the sewage
water, tube-well water, sewage sludge and soil samples, and the fertility of the sewage sludge
were also evaluated. The results obtained showed that sewage water quality characteristics
were mostly beyond the permissible limits as recommended by the Food and Agriculture
Organisation (FAO) and United State Environmental Protection Agency (USEPA). The levels
of Cd, Cu, Pb, Cl− and NO3
− in sewage water samples analysed were also found to be higher
than the respective reference values for irrigation water.
Furthermore, the results indicated a substantial build-up of trace metals in soil and
vegetables that were grown with sewage water. The levels of Cd, Cu and Ni in soils, as well as
the levels of Cd and Pb in vegetables were beyond the permissible limits recommended by
FAO, USEPA and EU, suggesting that irrigation with sewage water resulted in an obvious
increase of trace metal content in soil (P > 0.05). In fact, metal transfer factors from soil to
spinach and lettuce were found significant for Pb, Ni and Cu. The mobility and bioavailability
of the five metals in sewage sludge and soil samples were also assessed using a six step
selective sequential extraction scheme for partitioning the metals into different chemical forms.
The results obtained showed that the residual fractions of both sewage sludge and soil samples
were the most abundant pool for all the trace metals examined. The mobility and the potential
bioavailability of the metals investigated were in the order: Pb > Zn > Cu > Ni > Cd and Ni >
Zn > Pb > Cd > Cu for soil and sewage sludge samples, respectively.
TABLE OF CONTENTS
Title page………………………………………………………………………….. ……. i
Declaration…………………………………………………………………………. ii
Certification………………………………………………………………………………. iii
Dedication…………………………………………………………………………. iv
Acknowledgement……………………………………………………………………….. v
Abstract……………………………………………………………………….. ….. vi
Table of contents………………………………………………………………… ……… viii
List of Tables……………………………………………………………………………. xii
List of Figures……………………………………………………………………… xv
List of Appendices………………………………………………………………… xvi
CHAPTER ONE
1.0 Introduction……………………………………………………………………….. 1
1.1 Background to the study…………………………………………………… . 1
1.2 The Research Problem…………………………………………………….. 3
1.3 Justification……………………………. ………………………………. ….. 5
1.4 Aim and Objectives…………………………………………………………….. 6
1.5 Scope and Limitation of the study……………………………………………….. 7
CHAPTER TWO
2.0 Literature Review………………………………………………………………….. 8
2.1 Trace Metals…………………………………………………………….. …… 8
2.2 Trace Metal Pollution of Sewage………………………………………………………… 8
2.3 Bioavailability of Trace Metals in Sewage…………………………………………… 10
2.4 Trace Metals in Sewage Sludge………………………………………………………. 11
2.5 Trace Metals in Soil…………………………………………………… …….. 12
2.6 Bioavailability of Trace Metals in Soil………………………………………. 13
2.7 Biomobility of Trace Metals in Soil………………………………………….. 14
2.8 Trace Metals in Vegetables……………………………………………………………15
2.9 Mechanism of Trace Metal uptake by Vegetables……………………………………17
2.10 Studies of Trace Metals in Vegetables in Nigeria…………………………………..18
2.11 Toxicity of Trace Metals……………………………………………………. . 19
CHAPTER THREE
3.0 Materials and Methods …………………………………………………………… 24
3.1 List of Apparatus and Equipment………….. … …. …. …. … …. … …. … .. … 24
3.2 List of Reagents and Chemicals………………………………………………….. 26
3.3 Description of the Study Area……………………………………………………….28
3.4.0 Sampling and Preservation………………………………………………….. 33
3.4.1 Determination of sample sizes ………………………………………. …….33
3.4.2 Sampling Points………………………………………………………….. . 34
3.4.3 Sample Collection and Preservation…………………………………………………34
3.4.4 Sample Labeling……………………………… …………………………. . 36
3.5 Sample Pretreatment…………………………………………………………… 36
3.6 Analysis of Physico-Chemical Parameters……………………………………… 37
3.6.1 Physicochemical Analysis of Sewage water and tube-well water samples .37
3.6.2 Physicochemical Analysis of Sewage Sludge Samples . . .. .. .. .. .. .. .. .. .. .. 43
3.6.3 Physicochemical Analysis of Soil samples ……………………………………….48
3.7 Analysis Using Flame Atomic Absorption Spectrophotorneter……………….. 48
3.7.1 Principles…………………………………………………………………. 48
3.7.2 Preparation of Stock Solutions………………………………………….. .49
3.7.3 Calibration Curve.,……………………………………………………………. 50
3.7.4 Digestion of Samples………………………………………………………….. 51
3.7.5 Method Validation (Spiking Experiment)……………………………………. 52
3.8 Sequential Extraction of trace metals in sewage sludge and soil samples .55
3.9 Statistical Analysis…………………………………………………………… 58
CHAPTER FOUR
4.0 Results and Discussion …………………………………………………. .59
4.1 Quality Assurance………………………………………………………… 59
4.2 Physico-Chemical Parameters of Water Samples……………………………….60
4.3 Trace Metal Content of Water Samples…………………………………………..65
4.4 Physico-Chemical Parameters of Sewage Sludge……………………………… 74
4.5 Trace Metal Content of Sewage Sludge Samples……………………………….85
4.6 Physico-Chemical Parameters of Soil Samples……………………………….. 92
4.7 Trace Metal Content of Soil Samples………. …………………………….….100
4.8 Trace Metal Content of Vegetable Samples…………………………………..103
4.9 Chemical Fractionation of Trace Metals in Sewage Sludge samples ……110
4.10 Chemical Fractionation of Trace Metals in Contaminated Soil samples .128
4.11 Transfer and Contamination Factors for Trace Metals… . . .. . . .. . .. . . .. . .. . .143
CHAPTER FIVE
5.0 Conclusion and Recommendations . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 153
5.1 Conclusion……………………………………………………………… 153
5.2 Recommendation……………………………………………………………….154
References……………………………………………………………………….155
Appendices …………………………………………………………………….. 174
CHAPTER ONE
1.0 INTRODUCTION
1.1. Background
Rapid population growth and extensive damage from natural disasters have intensified
the worldwide struggle for adequate food supplies. Thus, the urgency of increasing food
production becomes patently obvious while one of the basic inputs required for achieving this
still remains water. If adequate supply and control of water are not ensured, it is very difficult
to grow crops and vegetables more than once a year. Conversely, to achieve optimum
production of these farm produce, adequate use of available natural resources or development
of new natural resource becomes necessary if starvation and malnutrition and their related
problems, are to be minimized.
Many parts of the world where rainfall is not adequate have so far developed new
methods of harnessing natural resources, which may be either surface dams or reservoirs for
their domestic, irrigation and livestock activities. Similarly, places where rainfall is seasonal
and the aforementioned facilities are also not available, sewage water from drainage basins or
ponds (pools of stagnant water) have been put to use for small-scale irrigation to supplement
rainy season farming. The use of this facility for irrigating farm lands has long been noted in
northern Nigeria where rainfall is seasonal.
The use of sewage water on farmlands provided copious resource products (e.g. carrot,
lettuce, onion and other vegetables) and a sustainable livelihood to several number of people in
Northern Nigeria. On the other hand, use of this delicate resource can result into numerous
health and environmental problems. Obviously, the chemical status of this delicate resource
would have its influence on the receiving lands, which might possibly reflect on the produce
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cultivated on such lands and subsequently ends up within the inner system of humans and
becomes a danger to life.
The possibility of trace metal transfer from sewage water and sludge to humans through
the food chain in many cities in Northern Nigeria is of great concern. The toxic trace metals
entering the ecosystem may lead to geoaccumulation, bioaccumulation, and biomagnifications.
Trace metals like Fe, Cu, Zn, Ni and other trace elements are important for proper functioning
of biological systems and their deficiency or excess could lead to a number of disorders (Ward,
1995). Food chain contamination by trace metals has become a burning issue in recent years
because of their potential accumulation in bio-systems through contaminated foods, water, soil
and air. Therefore, a better understanding of trace metal sources, their accumulation in the
soil and the effect of their presence in water, soil and plant systems seem to be particularly
important issue of present-day research on risk assessments (Rajesh et al., 2004).
Most of our water resources are gradually becoming polluted due to the addition of
foreign materials from the surroundings. These include organic matter of plant and animal
origin, land surface washing, and industrial effluents and domestic sewage (KSPCB, 2002).
Rapid urbanization with improper environmental planning often leads to discharge of domestic
sewage into lakes and rivers. The lakes and rivers have a complex and fragile ecosystem, as
they do not have self-cleaning ability and therefore readily accumulate pollutants. River
Kubanni drainage basin is no exception being one of the drainage basins that drain Southern
Zaria, through which domestic sewage, refuse and wastes from abattoir find their ways into the
river basin. Trade wastes (from auto-mechanics, hotels, and road side car-wash operators) are
also directly or indirectly drained into the river basin.
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Fadama land (Flood plain) along Kubanni River basin in Zaria city is noted for
agricultural irrigation practices both for subsistence and small scale farming during dry seasons
for many years. The River is a seasonal river, with intermittent flows occurring in flashes
mainly during rainy season (Eigbefo, 1978; KDSG, 2000) and, thus chiefly becoming sewage –
bearing river in dry seasons. This has eventually forced the farmers to be using the raw sewage
water for irrigation due to dearth of fresh water from the River during dry seasons.
However, it has been reported that sewage water of municipal origin contains
appreciable amount of major essential plant nutrients and therefore the fertility levels of soil
are improved considerably under sewage irrigation of vegetables (Rajesh et al., 2004). But the
application of raw sewage water in soil may give rise to accumulation of trace metals such as
Pb, Cd, Ni, Cu, Zn, Hg and Cr in the top soil (Williams et al., 1980) and hence in the tissues of
plants grown on them (Day et al., 1979). As such, this may become important pathways of
human exposure to metal contamination. It is with this consideration that it becomes necessary
to investigate the level of trace metals in vegetable crops grown with sewage water. Although
water is commonly employed as a pollution indicator of trace metals, sewage sludge can also
provide deeper insight into threat to water contamination. Indeed sewage sludge is ready sink
or reservoir of pollutants including trace metals where they accumulate to pollution level
(Onyari et al., 2003).
1.2 The Research Problem
The Kubanni River is one of the most important rivers draining Zaria and its environ.
Much considerations and attention have been given to this river because of the numerous
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farming and irrigation activities that take place seasonally along its course, as well as its
economic importance to the inhabitants. A great number of Zaria peasants have farms along
the course of the River, from which they obtain their food stuffs (such as maize, rice,
sugarcane and vegetables such as tomato, garden egg, carrot, lettuce, cabbage, onions etc).
Many of the farming activities take place during dry season by irrigation. Unfortunately, the
River is prone to pollution due to all forms of human activities that take place around it.
Environmental pollution is one of the most investigated subjects of research at present
time. Most industrial and municipal wastes end up in rivers, lakes and sea. Hence, the idea of
having continuous survey of water quality receives wide spread attention. In recent years,
because of industrialization and increasing population, water pollution has become an
important problem all over the world (USEPA, 1984). Inorganic materials have proved to be
much more persistent than organic matter as sewage pollutants, since the former are not readily
removed by in situ oxidation, and, may render the water unsuitable for irrigation. Thus, it is
important to investigate the state and quality of sewage and its sludge in terms of trace metal
load and its influence on the receiving vegetation along the river course, as well as the
vegetables that are grown with the sewage water. The human activities that are likely to cause
pollution of the Kubanni River include:
(i) Disposal of domestic wastes
(ii) Food processing
(iii) Biological wastes from Zango Village and the abattoir
(iv) Domestic and artisenal/micro industrial intrusion, e.t.c.
These entire wastes eventually drained into the Kubanni River. There is, therefore, the
need to monitor periodically the Kubanni waterway for trace metals. It is for this reason that
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the sewage water, which is being put to use for irrigation during dry seasons, its sludge, soil of
the irrigated sewage vegetation and the vegetables that are grown with sewage water would be
screened for trace metal content.
Although there is some information on the levels of trace metals on soil and some
vegetables grown on the bank of River Kubanni in the literature, there are no comprehensive
studies at one time on the levels of trace metals in sewage water, soil, sewage sludge and
vegetables from the River bank. Thus, there is need to carryout such a comprehensive
investigation on the sewage water, sludge, soils and vegetables grown on the river bank, so as
to evaluate the trace metal health status of the vegetables and to have some insight into the
impact of sewage water used for irrigation activities in the area.
1.3 Justification
Studies have shown that uptake and accumulation of metals by different plant species
depend on several reasons (Lokeshwari and Chandrappa, 2006). Unlike other pollutants that
are visible, trace metals may bioaccumulate in vegetable crops to toxic levels unnoticed.
Because the Kubanni River is a sewage-bearing river in the dry season and is used for
growing vegetable crops, it becomes imperative to periodically screen all the compartments of
the River bank, most especially crops, for their trace metal status. Thus, it is important to
investigate the state and quality of sewage water and its sludge in terms of metallic load and its
influences on the receiving vegetation along the bank of the River. Considering the production
level and consumption of the vegetables, significant level of trace metals in the vegetables may
lead to ingestion of toxic metals by consumers at unacceptable levels.
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1.4 Aim and Objectives
The aim of this study is to investigate the trace metal health status of vegetable crops
grown with sewage water along Kubanni River bank, Zaria, Nigeria. This will be achieved by
pursuing the following objectives:
a. Determination of the physico-chemical parameters of sewage water and sludge from
the study area as well as tube-well water, which is also used by the farmers that are not
close to the River to grow vegetables as control.
b. Determination of the physico-chemical parameters of the River bank soil on which the
vegetable crops are grown with sewage water as well as the soil on which tube well
water is used as control.
c. Determination of the concentrations of Zn, Ni, Cu, Pb and Cd in sewage water, sludge,
soils and vegetables (spinach and lettuce) in the study area.
d. Determination of the same metals in tube-well water, soils and vegetables (spinach and
lettuce) in the control area.
e. Assessment of fertilizer capacity of sewage sludge of the study area.
f. Determination of chemical fractionation of Zn, Ni, Cu, Pb, and Cd in sewage sludge
and soils in order to assess the mobility/availability of the metal from soils to
vegetables.
g. Determination of water and soil contamination risks based on the individual and global
trace metal contamination factors.
h. Establishment of trace metals pollution status of sewage water, sludge, soils and
vegetables (spinach and lettuce) by comparison of results to international limits for
trace metals.
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1.5 Scope and Limitation of the Study
The study concentrates on the sewage water irrigation of fadama land along Kubanni
River basin and is aimed at determining concentration levels of trace metals in the four
environmental compartments of the study area namely, sewage water, sewage sludge, soil and
vegetables (spinach and lettuce). Physico-chemical parameters of sewage water, sewage sludge
and soils were also analyzed. Cost was the majr factor in limiting length of investigation and
sample size in the study. Thus, samples of the four compartment of the study area were
collected during the dry season only. The AAS analysis was done at the price of fifty naira
(N50.00) per element per sample. This limited the number of sampling points as well.
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