Potentially Toxic Elements in Food Crops Grown on Urban Waste Dump-Sites A Case Study of Wakaliga Dump-Site, Kampala City, Uganda

ABSTRACT 

This study aimed at investigating the seasonal variation in the levels of contamination in soils, food crops, and domestic spring water samples from Wakaliga dumpsite in Kampala. Potentially Toxic Elements (PTE) and physicochemical properties were analysed during the dry season (May-July 2017) and the wet season (September-November 2017) and the results compared to standard limits set by international and local agencies. The results obtained indicated that some of the elements were found in concentration levels within permissible limits, while others were above permissible set limits in the soil, crops and domestic (spring) water samples. Zinc maximum levels of 3.41±0.01 mg/kg and 3.31±0.04 mg/kg d.w. were found in dry-season picked spinach and sugarcane, respectively, while the lowest concentrations of 0.98±0.02 mg/kg d.w. were found in wet-season cocoyam. Copper was found to exist below permissible limits for all the crops, soil and water; concentrations of lead in spinach were found to be within a permissible limit 2.31±0.005 mg/kg d.w. while all the others were below the permissible limits. Negligible amounts of cadmium were found in all the samples. All the soil samples indicated results below permissible levels of toxic metals in the arable soils as indicated by WHO (1996). The physiochemical parameters were all within acceptable limits for drinking water, as set by WHO and NEMA; Cadmium was not detected in the domestic water. The findings of this research indicated the presence of potentially toxic elements whose levels could be on the increase and found out that zinc concentrations were higher in both crops and soil for both the two harvesting seasons. This was followed by lead, which is attributable to the availability of discarded substances containing these elements in the environment. Proper waste management, control and disposal methods should be put in place to minimize exposure of toxic containing substances into the environment.

TABLE OF CONTENTS

TITLE PAGE....................................................................................................................................i

APPROVAL .....................................................................................................................................i

DECLARATION .............................................................................................................................ii

DEDICATION................................................................................................................................iii

ACKNOWLEDGEMENTS ............................................................................................................iv

LIST OF ACROMNYS ...................................................................................................................v

TABLE OF CONTENTS................................................................................................................vi

LIST OF TABLES ..........................................................................................................................xi

LIST OF FIGURES ....................................................................................................................... xii

ABSTRACT.................................................................................................................................. xiii

CHAPTER ONE ............................................................................................................................. 1

INTRODUCTION .......................................................................................................................... 1

1.1 Background ........................................................................................................................... 1

1.2 Problem Statement ................................................................................................................ 7

1.3 Objectives.............................................................................................................................. 7

1.3.1 General objective................................................................................................................ 7

1.3.2 Specific objectives.............................................................................................................. 7

1.4 Scope ..................................................................................................................................... 8

1.4.1 Content scope ..................................................................................................................... 8

1.4.2 Geographical Scope............................................................................................................ 8

1.4.3 Time Scope......................................................................................................................... 9

1.5 Significance of the study....................................................................................................... 9

CHAPTER TWO .......................................................................................................................... 11

LITERATURE REVIEW ............................................................................................................. 11

2.1 Introduction ......................................................................................................................... 11

2.2 Potentially Toxic Elements ( PTEs).................................................................................... 13

2.2.1 Zinc................................................................................................................................... 14

2.2.2 Occurrence of zinc ........................................................................................................... 14

2.2.3 Uses of zinc ...................................................................................................................... 14

2.2.4 Zinc pollution ................................................................................................................... 15

2.2.5 Zinc Toxicity.................................................................................................................... 15

2.3 Cadmium ............................................................................................................................. 16

2.3.1 Occurances....................................................................................................................... 16

2.3.2 Uses.................................................................................................................................. 16

2.3.3 Toxicity ............................................................................................................................ 17

2.3.4 Exposure ........................................................................................................................... 17

2.3.5 Copper.............................................................................................................................. 18

2.3.5.1 Occurances.................................................................................................................... 18

2.3.5.2 Uses............................................................................................................................... 18

2.3.5.3 Toxicity ......................................................................................................................... 19

2.4 Copper Exposure. ................................................................................................................ 19

2.4.1 Lead .................................................................................................................................. 19

2.4.2 Occurrence ....................................................................................................................... 20

2.4.3 Uses.................................................................................................................................. 20

2.4.4 Lead Toxicity ................................................................................................................... 20

2.4.5 Effects of potentially toxic elements ( PTEs) on the soil................................................. 21

2.4.6 Effects of potentially toxic elements (PTEs) on Plants.................................................... 22

2.4.7 Factors that affect the rate of heavy metal up take by plants........................................... 23

2.4.8 Soils and Soil Texture ...................................................................................................... 24

2.4.9 Spring water ..................................................................................................................... 26

2.5 Types of spring water.......................................................................................................... 27

2.5.1 Flow of spring water ........................................................................................................ 27

2.5.2 Classification of spring water........................................................................................... 27

2.5.3 Spring water content......................................................................................................... 27

2.5.4 Uses of spring water......................................................................................................... 28

2.5.5 Sacred springs .................................................................................................................. 28

2.6 The intolerable daily intake approach ................................................................................. 29

2.6.1 Transfer factor (TF).......................................................................................................... 31

2.6.2 Contamination/ pollution index........................................................................................ 31

2.6.3 pH : Water dissociates by a very slight extent into H+ and OH- ions. It has been.......... 32

2.6.4 Electrical Conductivity..................................................................................................... 32

2.6.5 Chemical Oxygen Demand .............................................................................................. 32

2.6.6 Total suspended solid ....................................................................................................... 33

CHAPTER THREE ...................................................................................................................... 34

MATERIALS AND METHODS.................................................................................................. 34

3.1 Research Design.................................................................................................................. 34

3.1.1 Chemical Analysis............................................................................................................ 35

3.1.2 Analytical procedure ........................................................................................................ 35

3.1.3 pH of the water samples................................................................................................... 36

3.1.4 Conductivity ..................................................................................................................... 36

3.1.5 Chemical oxygen demand (COD).................................................................................... 36

3.1.6 Total Suspended Solids (TSS).......................................................................................... 36

3.2 Sampling in the wet land cultivation study sites................................................................. 37

3.2.1 Soil sampling.................................................................................................................... 38

3.2.2 Water sampling ................................................................................................................ 38

3.2.3 Plant sampling and chemical Analysis............................................................................. 38

a) Sugar cane ...................................................................................................................... 38

b) Green vegetables(Spinach)............................................................................................. 41

3.2.4 Soil sampling and chemical analysis................................................................................ 41

3.2.5 Water sampling and chemical Analysis ........................................................................... 42

3.2.6 Potentially toxic element chemical analysis..................................................................... 43

3.2.7 Instrumentation................................................................................................................. 43

3.2.8 Quality control.................................................................................................................. 46

3.2.9 Quality assurance ............................................................................................................. 46

3.3 Statistical analysis............................................................................................................... 47

3.3.1 Ethical consideration........................................................................................................ 47

3.4 Limitations .......................................................................................................................... 47

CHAPTER FOUR......................................................................................................................... 48

RESULTS AND DISCUSSION ................................................................................................... 48

4.1 Physiochemical parameters. ................................................................................................ 48

4.1.1 Zinc (Zn) levels in wet and dry season spinach (Spinach) soil. ....................................... 50

4.1.2 Zinc (Zn) levels in wet and dry season (cocoyam) soil.................................................... 51

4.1.3 Zinc levels in wet and dry season (sugarcane) soil. ......................................................... 51

4.1.4 Zinc levels in wet and dry season (maize) soil................................................................. 52

4.1.5 Cu Levels in wet and dry season spinach (Dodoo) soil ................................................... 52

4.1.6 Cu levels in wet and dry season (cocoyam) Soil.............................................................. 53

4.1.7 Copper levels in wet and dry season (sugarcane ) soils ................................................... 56

4.1.8 Copper in wet and dry season (maize) soils..................................................................... 57

4.1.9 Pb Levels in wet and dry season spinach (Spinach) soil.................................................. 57

4.1.10 Pb level in wet and dry season (cocoyom) soil .............................................................. 58

4.1.11 Lead levels in wet and dry season (sugar cane) soil ...................................................... 58

4.1.12 Lead levels in wet and dry season (maize)soil............................................................... 59

4.1.13 Cadmium level in wet and dry Season Spinach (Spinach) soil...................................... 59

4.1.14 Cd level on wet and dry season (cocoyam) soil ............................................................. 59

4.1.15 Cd level on wet and dry season (sugare cane) soil......................................................... 60

4.1.16 Cd Level in wet and dry season (maize ) soil ................................................................ 60

4.1.17 Zinc level in wet and dry season spinach (Spinach) ...................................................... 61

4.1.18 Zinc level in wet and dry season cocoyam..................................................................... 61

4.1.19 Zinc levels in wet and dry season sugarcane.................................................................. 62

4.1.20 Zinc level in wet and dry season maize.......................................................................... 63

4.2 Copper level in wet and dry season spinach (Spinach)....................................................... 63

4.2.1 Copper level in wet and dry season cocoyam. ................................................................. 64

4.2.2 Copper level in wet and dry season sugarcane................................................................. 65

4.2.3 Copper level in wet and dry season maize. ...................................................................... 65

4.2.4 Lead levels in wet and dry season spinach (Spinach) ...................................................... 66

4.2.5 Pb levels in wet and dry season cocoyam. ....................................................................... 66

4.2.6 Pb level in Wet and Dry Season Sugarcane ..................................................................... 67

4.2.7 Pb level in maize corn in dry and wet season .................................................................. 68

4.2.8 Cd levels in Spinach (Spinach) in dry and wet season..................................................... 69

4.2.9 Cadmium levels in dry and wet seasons cocoyam........................................................... 69

4.2.10 Cd level in sugarcane in wet and dry seasons ................................................................ 70

4.2.11 Cd levels in wet and dry season maize........................................................................... 71

4.2.12 General Discussion......................................................................................................... 73

CHAPTER FIVE .......................................................................................................................... 76

CONCLUSION AND RECOMMENDATIONS ......................................................................... 76

5.1 Conclusions......................................................................................................................... 76

5.1 Recommendations............................................................................................................... 77

REFERENCES ............................................................................................................................. 79