ABSTRACT
Land snails (Achatina achatina) are readily consumed in Southeastern Nigeria, and they are often collected from the wild including cocoa plantations which are commonly sprayed with copper-based fungicides to combat the cocoa black-pod disease. This study was conducted to assess the possible accumulation of heavy metals (Cu, Cd, Pb, Fe, and Mn) in land snail species (Achatina achatina), soil, leaf litters and selected plants obtained from Ajata and Itunta cocoa plantations located in Bende and Ikwuano Local Government Areas, of Abia State respectively. Snail samples were collected during the rainy and dry seasons, in a two-by-three factorial design while the other samples were collected in a single-factor randomized complete block design. The snails were weighed and categorized into three sizes juvenile, medium and large and then dissected into three morphological parts, edible part (muscle), non-edible (intestine) and shell. The metal contents of the samples were determined using the Atomic Absorption Spectrophotometer, after acid digestion. The results showed that heavy metal concentrations in snail samples harvested from the cocoa plantations were higher than in those from the control site. The mean concentrations of the metals in the soil, leaves litter and the snails varied significantly (p≤0.05) in all locations in both seasons; with Ajata having the highest mean Cu concentrations. Pb contents of the various samples were generally below the detectable limits of the equipment used. Mean metal concentrations of the different snail sizes did not vary significantly (p≥0.05), whereas highly significant differences (p≤0.05) occurred in the mean Cu, Cd and Fe concentrations of the morphological parts examined. Cu and Cd concentrations in snail parts had the trend of intestine > muscle > shell; but Fe concentration had a different pattern: shell > intestine > muscle. Cu ranged from 0.72mg/kg in the shell to 54.68 mg/kg in the intestine, Cd varied from below detection level in the shell to 18.88 mg/kg in the intestine, while Fe ranged from 10.05 mg in the intestines to 200.66 mg/kg in the shells. Results on possible uptake and accumulation of the metals in snails tissues showed bio-accumulation factors (BAF) < 1 for all the metals in the muscles of the snails whereas the intestine contained metals with BAF>1, showing that the metals ingested from the soils or feed materials are mainly accumulated in the intestinal parts of the snail. The mean metal levels in the studied samples were mostly higher in the dry season. Nevertheless, the mean concentrations of the metals in the edible snail parts (muscle) across all sizes were lower than their permissible FAO/WHO levels. The hazard quotient (HQ) values for adults and children for all the metals analyzed were less than 1. The results also revealed that the estimated intakes for all ages and consumption amounts were lower than the Provisional Tolerable Dietary Intake (PTDI) values for all metals except Cd. Thus, consumption of snails obtained from the cocoa plantations investigated does not pose any significant toxicity risk to consumers. However, routine assessments of wild food especially snails collected from plantations where Cu-based fungicides is highly recommended. This will alert the public of possible heavy metal pollution concerns.
TABLE OF CONTENTS
Over
page i
Title Page
ii
Declaration iii
Certification iv
Dedication v
Acknowledgements vi
Table of
Content vii
List of
Tables xi
List of
Figures xiv
List of
Plates xv
Abstracts xvi
CHAPTER 1: INTRODUCTION
1.1 Environmental Heavy Metal
Pollution 1
1.2 Statement of Problem 3
1.3 Scope of the Study 4
1.4 Justification for the Study 5
1.5 Aim and Objectives of the
Study 6
1.6 Significance of Study 6
CHAPTER 2: LITERATURE
REVIEW 9
2.1 Cocoa Farming in Nigeria 9
2.2 Declining Cocoa Production
Capacity in Nigeria 10
2.3 Cocoa Farming and Use of
Pesticides 13
2.3.1 Environmental
contamination from the use of copper-based pesticides 15
2.4 Heavy Metals – A Cause for
Concern 19
2.5
Nature of Snail 27
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2.5.1 Habitat of snail
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32
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2.5.2 Food preferences of
snails
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32
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2.5.3 Environment for
snails’ survival
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35
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2.6 Heavy Metals
Accumulation by Snail
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37
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|
2.6.1 Partition of heavy
metals in different tissues
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40
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2.6.2 Use of snail as
bioindicators
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44
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2.7 Food Chain - Snail to
Man
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47
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2.8 Snail Industry and
Market in Nigeria
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48
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2.8 Significance of Permissible Limits in Public Health
50
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|
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CHAPTER 3: MATERIALS AND
METHODS
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53
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3.1 Study Area
|
53
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3.2 Experimental Design
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56
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3.3 Sample Collection
3.3.1 Collection of wild
snail feed materials, leaf litter and soil from the cocoa
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56
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plantations
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56
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3.3.2 Collection of snail
species growing in the cocoa plantation
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57
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3.4 Sample
Preparation/Preservation
|
62
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3.5 Sample Digestion for
Determination of Heavy Metals
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64
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3.5.2 Determination of heavy
metals contents of soil samples
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64
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3.5.3 Quality
control/quality assurance
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64
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3.6 Statistical Analysis
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65
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|
3.7 Evaluation of Metal
Bioaccumulation by Snails
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65
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3.8 Estimation of Potential
Metal Intake and Risks from Consumption of Snails
|
66
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3.8.1 Estimation of
potential metal intake from snail consumption
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66
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3.8.2 Estimation of potential
health risk from consumption of snail 66
CHAPTER 4: RESULTS
AND DISCUSSION 69
4.1 Results 69
4.1.1 Heavy metal levels of soil
during the dry and rainy seasons 69
4.1.2 Metal levels of leaves
litter during the dry and rainy seasons 71
4.1.3 Metal levels in cocoa pods
during the dry and rainy seasons 73
4.1.4 Metals levels of chromoleana
odorata leaves during the dry and rainy seasons 76
4.1.5 Metals levels of banana (musa spp) fruits obtained from the study sites 79
4.1.6 Metal levels of musa spp (banana) leaves obtained from the study sites 81
4.1.7 Metal contents of pawpaw leaves (carica papaya) obtained from the study
sites 84
4.1.8 Metal contents of carica papaya (pawpaw) fruits obtained from the study sites 86
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4.1.9 Metal contents of costus afar leaves obtained from the
study sites
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88
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4.1.10 Metal contents of achatina achatina spp obtained from
the study sites
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91
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4.1.11 Metal contents of
different snail sizes collected in the dry season
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94
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4.1.12 Metal contents of
different snail sizes collected in the rainy season
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96
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4.1.13 Metal contents of
large-sized snails’ morphological parts (rainy season)
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98
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4.1.14 Metal
contents of large-sized snails’ morphological parts (rainy season) 101
4.1.15 Metal contents of medium-sized snails’
morphological parts (dry season)
103
4.1.16 Metal contents of medium-sized snails’
morphological parts (rainy season)
105
4.1.17:
Metal contents on juvenile-sized snails’ morphological parts (dry season) 108
4.1.18 Metal contents of juvenile-sized snail’s
morphological parts (rainy season)
110
4.1.19 Aggregates of metal content of snails’ muscle,
intestine and shell
113
4.1.20 Effect of seasonal variation on metal levels in
snails
120
4.1.21
Bioaccumulation of the metals by snails 126
4.1.22 Estimated metal intakes
from consumption of snails 132
4.1.23 Estimation of potential
health risk from consumption of snails 135
4.2 Discussion
137
4.2.1 Distribution
of metals in the soil collected from the plantations 137
4.2.2 Distribution
of metals in the leaves, and fruits collected from the plantations. 139
4.2.3 Distribution
of metals in snails collected from the cocoa plantations 143
4.2.4 Contribution
to knowledge 149
CHAPTER 5: CONCLUSION
AND RECOMMENDATIONS
5.1 Conclusion 151
5.2 Recommendations 153
REFERENCES 155
APPENDICES
191
LIST OF TABLES
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|
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2.1:
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Cocoa Production Output and Export in Nigeria, and the
Prices
(1970-2010)
12
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2.2:
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Status
of Some Pesticides Used by Cocoa Farmers in Nigeria 18
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2.3:
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Feed
Sources of Snails 34
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3.1:
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Number
of Samples Collected /Studied in the Dry Season 61
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3.2
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Number
of Samples Collected/Studied in the Rainy Season 61
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4.1:
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Heavy Metal Levels
of Soil in Cocoa Plantations at Ajata, Itunta
and
Control Sites During the Dry and Rainy Seasons 70
|
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4.2:
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Heavy Metal Levels of Cocoa Leaves Litter at the Study Areas
During
the
Dry and Rainy Seasons 72
|
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4.3:
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Heavy Metal Levels
in Cocoa Pods of Cocoa Plantations at Ajata,
Itunta
and Control Sites During the Dry and Rainy Seasons 75
|
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4.4:
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Heavy Metals Levels
in Chromoleana Odorata Leaves
Obtained from the Study
Areas During the Dry and Rainy Season 78
|
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4.5:
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Heavy Metals Levels
in Banana (Musa Spp) Fruits Obtained from the
Study
Areas in Dry and Rainy Seasons 80
|
|
4.7:
|
Heavy Metals Contents of Pawpaw Leaves (Carica Papaya) in the Study
Areas
During the Dry and Rainy Seasons 85
|
|
4.8:
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Heavy Metal Contents of
Pawpaw (Carica Papaya) Fruits (Dry
And Rainy
|
Seasons) 87
4.9:
Metal Contents of Costus Afar Leaves During the Dry
and Rainy Seasons 90
4.10: Metals Concentrations of Achatina Achatina Species Obtained from
Ajata, Itunta, and Control
Sites During the Dry and Rainy Seasons 93
4.11: Variations
in Metal Contents of Different Sizes of Snails Obtained
from Ajata,
Itunta, and Control Sites During the Dry Season 95
4.12: Variations
in Metal Contents of Different Sizes of Snails Obtained
from Ajata,
Itunta, and Control Sites During the Dry Season 97
4.13: Metal
Concentrations in Morphological Parts of Large Snails from
Study Areas During
the Dry Season 100
4.14: Metal
Concentrations in Morphological Parts of Large Snails from the
Study Areas During
the Rainy Season 102
4.15: Metal
Concentrations in Morphological Parts of Medium-Sized Snails
During the Dry
Season 104
4.16 : Metal Concentrations
in Morphological Parts of Medium-Sized Snails
During the Rainy
Season 107
4.17: Metal Contents of Morphological Parts of
Juvenile-Sized Snails During
the Dry Season 109
4.18: Metal
Contents of Morphological Parts of Juvenile-Sized Snails Obtained from Ajata, Itunta and Control Sites
During the Rainy Season 112
4.19: Seasonal
Comparison of Metals Levels in Snail Samples from Ajata
Plantation 121
4.20: Seasonal Comparison of Metal Levels in
Snails from Itunta Plantation 123
4.21: Seasonal Comparison of Heavy Metal Levels in
Snails from Control Site 125
4.22: Mean Bioaccumulation Factor of Metals in
Snail Parts Considering Soil in the Dry Season 128
4.23: Mean Bioaccumulation Factor of Metals in
Snail Parts Considering Soil
in the Rainy Season 129
4.24: Mean Bioaccumulation Factor of Metals in
Snail Parts Considering
Plant Litter in
the Dry Season 130
4.25: Mean
Bioaccumulation Factor of Metals in Snail Parts Considering Plant
Litters in the
Rainy Season 131
4.26: Estimated Metal Dietary Intakes from the
Consumption of Snail 134
4.27: Target Hazard Quotient (THQ) for the Snail
Edible Parts (Muscle) 136
LIST OF FIGURES
2.1:
Cocoa-producing States in Nigeria
11
3.1: Location
of the Cocoa Plantations Investigated in Abia State 55
4. 1: Aggregate Metal Concentrations in the Snails’
Morphological Parts
at Ajata Cocoa Plantation (all sizes) 115
4.2: Aggregate
Metal concentrations in the snails’ Morphological Parts at Itunta
Cocoa Plantation
(all sizes). 117
4.3: Aggregate
Metal Concentrations in the Snails’ Morphological Parts at the
Control Site Cocoa
plantation (all sizes) 119
LIST OF PLATES
2. 1: Cocoa Tree
with Fruits Showing Early Signs of Black Pod Disease 14
2. 2 Achatina achatina 29
2. 3 Sectional
Diagram of the Anatomy of Snail
31
3.1: Ajata Cocoa
Plantation Showing the Stand Spacing and Ground Litter 58
3.2: Musa Spp Tree
Growing Beside a Cocoa Tree at Ajata Cocoa Plantation 58
3.3: Collection of
Soil Samples in Triplicates at Itunta Using the Soil Auger 59
3.4: Bagged Soil
Samples in Waterproof Materials Before Laboratory Analyses 59
3.5: Dry Season
Snail Sample Collected from Itunta Plantation 60
3.6: Rainy Season
Snail Sample Collected from Ajata Cocoa Plantation 60
3.7: Measurement of
the Snail Weight Using a Digital Weighing Balance 63
CHAPTER 1
INTRODUCTION
1.1 ENVIRONMENTAL HEAVY METAL POLLUTION
Heavy metal contamination of the
environment has increased, especially in the past 2 - 3 decades. Some metals,
including lead, mercury and cadmium, are very harmful to humans and many other
biotas; their presence in the environment elicits concern (Falandysz et al., 2017; Baroudi et al., 2020; Deng et al., 2022; Zergui et al.,
2023). In addition, recent research reveals that both beneficial and highly
toxic metals can be transferred from polluted soils to plants and animals in
those areas. Several studies have explored this phenomenon (Félix et al., 2015; Li et al., 2022). In addition, some organisms, including plants and
animals, can accumulate some chemical substances taken up from soils even when
the elements occur at low concentrations in the soils (Baroudi et al., 2020; Zhao et al., 2022; Zhang et al.,
2023).
Cocoa (Theobroma cacao Linn), the primary source of raw material for the
chocolate industry, is a critical cash crop for most of the countries in the
sub-regions of West and Central Africa, mainly Cote d Ivoire, Ghana, Nigeria
and Cameroun (Famuyiwa et al., 2012;
Amon-Armah et al., 2021;
Amponsah-Doku et al., 2022). Cocoa is
a primary foreign exchange earner supporting the provision of socio-economic
infrastructure in these countries. In terms of employment, the cocoa industry
has been estimated to employ, in some cases, as much as 60% of the national
agricultural labour force (Aikpokpodion
et al., 2012). Cocoa is one of the earliest cash crops cultivated in
Nigeria; it was first seen in South America a thousand years ago in the humid
and tropical climate mixed with high rainfall. It was introduced to Nigeria and
other countries in West Africa during the 19th century (Oyekale et al., 2009).
Pest and diseases
cause severe yield losses to cocoa production worldwide. While nonchemical
solutions for managing pests and diseases in cocoa are recommended,
agrochemicals are still necessary;
this pattern is anticipated to persist for the foreseeable future, as stated by
Alloway (1995), Parra-Paitan et al.
(2023), Asante et al. (2022) and
Osei-Gyabaah et al. (2023). Farmers
use copper-based fungicides such as Bordeaux mixture, Kocide 101, Nordox,
Champion, Ridomic, and Caocobre Sandoz. However, these leave behind significant
amounts of copper residues, which can accumulate in cocoa trees. Although
copper in the soil is necessary for proper functioning of biological systems in
plants, animals, and humans, it can become toxic in excessive amounts (Alloway,
1995; Adejori and Akinnagbe, 2022; Acheampong et al., 2023).
Snails have long been used to
study pollutant accumulation (Elias and Bernot, 2017; Ejidike and Afolayan,
2010; Joseph et al., s2021; Baroudi et al., 2020). Studies have shown that
contaminants such as heavy metals in snails are dependent on the availability
of pollutants in the environment (food and soil), exposure period, the
accumulation capacity of snails, the bioavailability and the intensity of the
transfer of the contaminant (Gimbert et
al., 2006; Baroudi et al., 2020;
Zhang et al., 2022). Therefore,
snails have been used as sensitive environmental pollution indicators (Baroudi et al., 2020; Kakade et al., 2023).
Research
Gaps:
Several studies
have evaluated the effects of pollutants on different species of snails and
their uptake of pollutants from terrestrial and aquatic environment (Leomanni et al.,
2015; Ma et al., 2014; Adedeji et
al., 2011; Guiller and Madec, 2010; Baurand et al., 2014; Pérez-Rafael
et al., 2014, Qaysi et al 2022;
Anagha et al., 2022). However, there
is not much data available on the levels of heavy metals in the snail Achatina achatina of the Nigerian
environment and of Abia State in particular where this snail species abound in
cocoa plantations where copper-based fungicides are regularly applied. Research
gaps include
• the
impact of using copper-based fungicides in cocoa plantations on soil and plant
species where snails live and feed on
• levels
of the heavy metals in the morphological parts of snails living in cocoa
plantations
• accumulation
of metals by snails inhabiting cocoa plantations in relation to their sizes or
age
Therefore, studying the impact of
copper fungicides use on lower invertebrates, like snails, in cocoa plantations
is crucial and will provide valuable data on the copper and other metals uptake
by the snails and the likelihood of the metals to be bio-accumulated by the snails
in their morphological parts and shells. In addition, such a study will also
provide data that will enable the estimation of human intake of copper and
other metals from the consumption of snails collected from such farms or
plantations.
1.2 STATEMENT OF PROBLEM
Cocoa (Theobroma cacao Linn),
is a valuable crop grown in the humid lowland tropics of Nigeria. Small-scale
farmers mainly cultivate this crop, and it has gained significant popularity
over time. Nigeria became the second-largest producer of cocoa in the world by
1965. The major constraint to cocoa production usually encountered by farmers
is persistent heavy yield loss due to cocoa disease, particularly the black pod
disease caused by Phytophthora palmivora,
and in some years, the yield loss due to this disease can be as high as 80
% (Nzegbule, 2003).
In combating cocoa diseases,
copper-based fungicides, which are very effective against fungal
disease-causing organisms, are sprayed by cocoa farmers for prevention and control.
However, many fungicides contain high levels of toxic heavy metals. Therefore,
excessive and continuous use of these substances can lead to fatal
environmental consequences, including harm to soil fauna and non-target
organisms that may be exposed to the toxicants, as noted in a study by Nwankwo et al. (2015).
In Nigeria, people gather snails
mainly from forests, farmlands, and plantations like cocoa farms for
consumption or to sell in nearby rural or urban markets. Snail sales constitute
a good source of income for many rural dwellers. Snails gathered from the wild,
such as cocoa plantations, may have been subjected to high levels of copper due
to the use of fungicides for pest and disease management. This is because
snails are soildwelling and eat plants, making them susceptible to
environmental pollutants from either the soil or vegetation. There is concern
that when snails are gathered from coppercontaminated sites, they may have
accumulated metal that could be passed on to higher animals up the food chain.
1.3 SCOPE OF THE STUDY
The study
focused on copper because it is an active ingredient of the pesticides commonly
used by cocoa farmers. However, other metals like cadmium, lead, iron, and
manganese which some literature have shown to be impurities associated with
some local pesticides were also assessed in this study. The study was carried
out in Abia State which is one of the major cocoa producing states of Nigeria.
A local snail species, Achatina achatina (Linnaeus, 1758) was used to
study heavy metal levels at Ajata Bende and
Itunta Ikwuano cocoa plantations. Snail species samples were also
collected from an area free of copper-based spray in Abia State, their heavy
metal levels were determined and used to compare to those of the cocoa plantations.
Levels of the heavy metals and their variations in the morphological parts of
the snail species were determined. The observed levels of metals were used to
determine the snails' capacity to absorb and accumulate metals in both their
edible and non-edible components. This was carried out by calculating the
bioaccumulation factor (BAF) for the morphological parts of the snail (edible
part, non-edible part, and the snail shells). Similarly,
the health risks associated with the intake of the metals from the consumption
of metals from the snails were assessed by estimating the daily intake and
potential hazards, if any.
1.4 JUSTIFICATION FOR THE STUDY
Abia State is one of the major
cocoa-producing States in Nigeria. Cocoa farmers in Abia State use Cu-based
fungicides such as Bordeaux mixture, Kocide 101 and Ridomic to control cocoa
diseases since these diseases impact yields negatively. The use of large
quantities of Cu-based fungicide in cocoa farms, a common practice, could
result in soil contamination by Cu and its subsequent transfer through the food
chain to man. Edible plants growing within the cocoa farms as well as animals
such as snails are vulnerable to exposure to such contaminants and could even
bio-accumulate the toxic substances, thereby exposing higher animals in the
food chain to high doses of harmful chemicals. The primary drinking water
sources in rural areas come from nearby soil and surface water bodies. However,
harmful compounds can contaminate these sources through storm runoff after
rainfall. Snails living in cocoa plantations risk being affected by these
compounds due to their habitat, eating habits, mobility, and hibernation.
Snails are a significant source of meat for middle- and low-income earners and
may contribute to the transfer of copper within cocoa plantations and people.
Furthermore, high-income earners prefer wild snails as they are regarded as
healthy and natural.
1.5 AIM AND OBJECTIVES OF THE STUDY
This study aimed to support
quality control in the snail industry and reduce the risk of heavy metal
contamination by providing reliable information on the potential for copper
contamination of these land snails and possible toxicological effects. The
specific objectives of this study were:
i. To
assess the levels of Cu, Cd, Pb, Fe, and Mn on feed materials of land snails
occurring in the wild in cocoa plantations;
ii. To determine the levels of Cu, Cd, Pb, Fe, and Mn in different
sizes of snail species harvested from the cocoa plantations; iii. To determine the variation in the levels
of Cu, Cd, Pb, Fe, and Mn in selected morphological parts of snail species
harvested from the cocoa plantation where
Cu-based fungicides were used; iv. To assess the seasonal variation in the
levels of Cu, Cd, Pb, Fe, and Mn in snail species harvested from the cocoa
plantations.
1.6 SIGNIFICANCE OF STUDY
The significance of this study
lies in its potential to inform agricultural practices, influence policies,
safeguard public health, and contribute to the broader scientific understanding
of the environmental and health implications of fungicide use in cocoa
plantations. These include:
•
Human
health implications: The study investigated the potential transfer of heavy
metals from fungicide-treated cocoa plantations to land snails and some flora
species, which are part of the human food chain. Heavy metal exposure can have
serious health consequences, including neurological and developmental issues,
organ damage, and other adverse effects.
•
Environmental
impact: It shed light on the environmental impact of fungicide use in cocoa
plantations, particularly in terms of heavy metal contamination. Understanding
how these chemicals affect non-target organisms like land snails is crucial for
sustainable agricultural practices.
•
Cocoa
industry practices: The study provides insights into the agricultural
practices in cocoa plantations, specifically the use of fungicides. This
information can be valuable for industry stakeholders, policymakers, and
researchers looking to develop sustainable and environmentally friendly
agricultural practices.
•
Food
Safety Concerns: Lands snails from fungicide-treated cocoa plantations were
found to contain elevated levels of heavy metals compared to control, raising
concerns about the safety of the food supply. This information is crucial for
regulatory bodies and agricultural authorities to ensure the safety of the food
chain.
Regional relevance: Conducting the study in Abia State, Nigeria,
added regional specificity to the findings. Different regions may have distinct
agricultural practices, soil compositions, and environmental conditions, so
understanding the local context is essential for designing effective mitigation
strategies.
Policy implications: The study might have implications for
agricultural and environmental policies in Nigeria. Significantly higher levels
of heavy metals observed in samples from the studied cocoa plantations may lead
to recommendations for changes in farming practices, regulations, or the
development of alternative methods to protect crops without negatively
impacting the environment and human health.
Public awareness:
Findings from this study contributes to public awareness regarding the
potential risks associated with consuming land snails from certain agricultural
areas. This information empowers consumers to make informed choices about their
food sources.
Scientific contribution: The study adds to
the body of scientific knowledge concerning the impact of agricultural
practices on both the environment and human health. This contributes to the
broader understanding of the complex interactions between agriculture, ecology,
and public health.
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