DISTRIBUTION OF HEAVY METALS IN COCOA FARM SOILS IN THE WESTERN REGION OF GHANA BY JUSTICE EDUSEI ACKAH (10193655) A THESIS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL CHEMISTRY DEGREE OCTOBER 2012 University of Ghana http://ugspace.ug.edu.gh DISTRIBUTION OF HEAVY METALS IN COCOA FARM SOILS IN THE WESTERN REGION OF GHANA THIS THESIS IS SUBMITTED TO THE DEPARTMENT OF CHEMISTRY OF THE UNIVERSITY OF GHANA BY JUSTICE EDUSEI ACKAH (10193655) IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE AWARD OF MPHIL CHEMISTRY DEGREE DEPARTMENT OF CHEMISTRY UNIVERSITY OF GHANA OCTOBER 2012 University of Ghana http://ugspace.ug.edu.gh i DECLARATION It is hereby declared that this thesis is the outcome of research undertaken by Justice Edusei Ackah towards the award of MPhil Chemistry degree in the Department of Chemistry, University of Ghana, and has neither in part nor in whole been presented for another degree elsewhere. …..……………………………… Date…………..………… Justice Edusei Ackah (Student) ……….….……………………... Date…………………….. Professor Derick Carboo (Principal Supervisor) ….…………………………….. Date………….………… Dr. Eric K. Nartey (Co-supervisor) University of Ghana http://ugspace.ug.edu.gh ii DEDICATION To the Almighty God, and to the Ben Ackah family, Sefwi Bekwai. ………. JE Ackah University of Ghana http://ugspace.ug.edu.gh iii ACKNOWLEDGEMENTS I am grateful to my supervisors, Prof. Derick Carboo and Dr. Eric K. Nartey for their patience, guidance and invaluable contributions throughout the work. My sincere thanks also go to Mr Enock Dankyi, an assistant lecturer at the Department of Chemistry, for his support right from the inception to the culmination of this work. He has been very inspirational in this work. May the Almighty God richly bless you. I am greatly indebted to my father, Ben Ackah, who will go through anything to raise a pesewa to sponsor my education. I never anticipated a master’s degree this early. Mum Avic, your prayers have been answered. I pray that you all live long to enjoy the fruits from the seed you sowed in me. Also, I must not forget to pay tribute to my childhood friend, Justice Reginald Cobbinah a.k.a Kofi Soloku who volunteered as my escort to all the sampling sites. I, also, thank all COCOBOD/Agric officials in the Western region for the hospitality and immeasurable assistance given me during sampling, most especially Mr Akrofi (Enyinabrem), Victor Darko (Juabeso), Michael Amedzi (Enchi), Augustine Assaah (Asankragwa), Daniel K. Agyei (Samreboi), Nana Boah Kwasi (Chief of Nsoakrom, Debiso) and Theophilus Agbley (Ashiem, Sefwi Bekwai). May God bless you bountifully. My sincere gratitude goes to the Laboratory Technicians of ECOLAB, and of the Chemistry and Soil Science Departments of the University of Ghana especially Mr Prince Owusu, Mr Sarquah (ECOLAB), Mr Bob-Essien (Chemistry), Mr Julius A. Nartenor and Mr Bernard Anipa (Soil Science) for the assistance given me during the laboratory analysis. To my friends and fellow MPhil students, I say thank you for the roles you played in my life. The assistance from Michael Amponsah-Offeh, Jibro K. Bilham and Ishmael Duagbor is highly appreciated. My praises go to the Almighty God who does everything beautiful in his own time. University of Ghana http://ugspace.ug.edu.gh iv ABSTRACT Western Region is the largest cocoa producer in Ghana. Cocoa farmlands have over the past decades received heavy doses of agrochemical application to boost cocoa production. These agrochemicals, however, may contain heavy metals and it is therefore likely that the metals may have accumulated in the soils. Evaluating the total concentrations and understanding the distribution characteristics of heavy metals in cocoa growing soils can aid environmental managers and even help regulate the rate of agrochemical application. A study was therefore, carried out on some selected soils of major cocoa growing areas in Western Region of Ghana to determine the levels of cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) in the soils and also to determine some of the soil factors that control the distribution of the heavy metals in the soil. Eight soils (two Haplic Luvisols, three Ferric Acrisols, one Haplic Ferrasol and two Dystric Fluvisols) and their accompanying pristine soils as control were taken from adjacent natural forests sampled at depths of 0 – 10 cm, 10 – 30 cm, 30 – 50 cm, 50 – 80 cm and 80 – 100 cm. These soils were analysed for their particle size distribution, pH, organic carbon, cation exchange capacity, exchangeable bases, and total and bio-available Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn. The study indicated that the ΔpH which is pHKCl – pHH2O were all negative indicating that the soils generally had net negative charges on their colloidal surface. For all soils, clay content and pH increased with depth indicating co-migration of the two soil parameters whilst total organic carbon content decreased with depth. Cation exchange capacity, however, did not show any clear pattern with depth in the soils. The average abundance of heavy metals determined in these soils decreased as follows: Fe > Mn > Cr > Zn > Cu > Cd > Pb > Ni. The soils had low metal contents, less than or within the range of concentration for non-polluted soils and for European norms. However, total concentrations of Cd, Cu, Cr and Pb in the surface soils (0 – 10 cm) exceeded the thresholds for atmospheric fallout concentrations in University of Ghana http://ugspace.ug.edu.gh v top soil to 20 cm depth indicative of anthropogenic contamination. The lowest heavy metal contents were observed in the Haplic Luvisols while the highest metal loadings were in the Haplic Ferrasols and the Dystric Fluvisols. Depth function plots, ANOVA and correlation analyses indicated that clay influenced the distribution of Cr, Cu, Fe, Ni and Zn in the soils. Clay and total organic carbon controlled Cd distribution while pH and clay were associated with the distribution of Mn. Thus, clay had the most pronounce effect on the distribution of the metals in the soils. Accumulation-depletion ratios, enrichment factors and principal component analysis indicated that the distribution of Cd, Cu, Mn and Pb in the soils highlighted an anthropogenic pollution, most probably, from agrochemical inputs and/or from atmospheric deposition. Iron and Ni distributions were associated with lithogenic origin whereas Zn and Cr distribution were related to both anthropogenic and lithogenic contributions. University of Ghana http://ugspace.ug.edu.gh vi TABLE OF CONTENTS DECLARATION ………………………………………………..………………………….... i DEDICATION …..…………………………………………………………………………... ii ACKNOWLEDGEMENTS.....………………….…………….…………………………….. iii ABSTRACT..……………………………………..………………………….…………….... iv CHAPTER ONE 1.0 GENERAL INTRODUCTION ................................................................................. 1 1.1 Background ......................................................................................... 1 1.2 Problem statement ............................................................................... 3 1.3 Hypothesis .......................................................................................... 4 1.4 Objectives of the research ................................... ................................. 5 1.4.1 Aim ................................................................................................................... 5 1.4.2 Specific objectives ............................................................................................ 5 CHAPTER TWO 2.0 LITERATURE REVIEW ................................................................................. 6 2.1 Essentiality of elements ....................................................................... 6 2.2 Heavy metals ....................................................................................... 7 2.2.1 Cadmium .......................................................................................................... 8 University of Ghana http://ugspace.ug.edu.gh vii 2.2.2 Chromium………………………………………………………………..… 10 2.2.3 Copper ........................................................................................................... 11 2.2.4 Iron ................................................................................................................ 13 2.2.5 Manganese .................................................................................................... 15 2.2.6 Nickel ........................................................................................................... 16 2.2.7 Lead .............................................................................................................. 18 2.2.8 Zinc ............................................................................................................... 19 2.3 Heavy metals and soil ....................................................................... 2 0 2.3.1 Agrochemicals and heavy metals in soil ...................................................... 21 2.4 Heavy metal mobility ......................................... .............................. 24 2.4.1 Effect of clay content on heavy metal accumulation in soils ...................... 26 2.4.2 Effect of soil pH on heavy metal accumulation in soils .............................. 27 2.4.3 Effect of organic matter on heavy metal accumulation in soils ................... 28 2.4.4 Effect of cation exchange capacity on heavy metal accumulation in soils .. 30 2.5 Depth distribution of metals .................................................................................... 30 2.6 Analytical methods for soil analysis ........................................................................ 31 2.6.1 Inductively coupled plasma-atomic emission spectrometry (ICP-AES) ..... 32 2.6.2 Atomic absorption spectroscopy (AAS) ...................................................... 33 University of Ghana http://ugspace.ug.edu.gh viii 2.6.3 Flame photometry (FP) ............................................................................... 34 CHAPTER THREE 3.0 METHODOLOGY ................................................................................................ 35 3.1 Introduction ....................................................... .............................. 35 3.2 Selection of study area ............................................................................................. 35 3.3 Physiography of study site ................................................................ 38 3.4 Soil sampling .................. ................................................................ . 39 3.5 Sample preparation ................................................................................................... 42 3.5.1 Containers and cleaning process .................................................................. 42 3.6 Soil analysis ............................................................................................................. 43 3.6.1 Soil particle size analysis ............................................................................. 43 3.6.2 Soil pH ......................................................................................................... 45 3.6.3 Soil organic carbon ..................................................................................... 45 3.6.4 Cation exchange capacity and percent base saturation ................................ 46 3.6.5 Total elemental analysis .............................................................................. 47 3.6.6 Exchangeable heavy metal determination ................................................. ..48 3.7 Data analysis ................................................................................. .. 49 University of Ghana http://ugspace.ug.edu.gh ix CHAPTER FOUR 4.0 RESULTS ................................................................................................................. 51 4.1 Introduction ..................................................... ............................... .. 51 4.2 Physicochemical characteristics of the soils ....................................... . 51 4.3 Heavy metal content and distribution in the soils studied ...................... 62 4.3.1 Validation of analytical method .................................................................... 62 4.3.2 Heavy metals in the studied soils .................................................................. 65 4.4 Physicochemical properties versus heavy metal content .......................................... 77 4.4.1 Clay content versus heavy metal distribution ............................................... 77 4.4.2 pH versus heavy metal distribution ............................................................... 80 4.4.3 Total organic carbon content (TOC) versus heavy metal distribution .......... 83 4.4.4 Cation exchange capacity (CEC) versus heavy metal distribution ............... 83 4.5 Discriminant analysis ........................................ .............................. .. 88 4.6 ANOVA analysis ................................................. .......................... ... 90 4.7 Correlation analysis .......................................... .............................. .. 92 4.7.1 Relationships among the heavy metals ........................................................ 92 4.7.2 Relationships between metal content and soil physicochemical properties .93 4.8 Anthropogenic versus lithogenic sources of heavy metals ............... .... 94 University of Ghana http://ugspace.ug.edu.gh x 4.8.1 Accumulation-depletion ratio ..................................................................... 94 4.8.2 Enrichment factor and anthropogenic/lithogenic contribution .................. 100 4.8.3 Principal Component Analysis (PCA) ....................................................... 108 CHAPTER FIVE 5.0 DISCUSSION ....................................................................................................... 111 CHAPTER SIX 6.0 CONCLUSION AND RECOMMENDATIONS .............................................. 121 6.1 Conclusion ................................................................................. ... 121 6.2 Recommendations .................................................................... ...... 123 REFERENCES ................................................................................................................. 124 APPENDICES .................................................................................................................. 153 University of Ghana http://ugspace.ug.edu.gh xi LIST OF TABLES Table 2.1: Recommended agrochemicals in Ghana ........................................................ 23 Table 3.1: Location and soil type of sampling sites ........................................................ 40 Table 4.1a: Soil physicochemical properties for Ferric Acrisols [Asankragwa (ASA), Ashiem (ASH) and Bogoso (BOG)] ............................................................ 55 Table 4.1b: Soil physicochemical properties for Haplic Ferrasol [Buako (BUA)], Haplic Luvisol [Debiso (DEB)] and Dystric Fluvisol [Enchi (ENC)] .................... 56 Table 4.1c: Soil physicochemical properties for Dystric Fluvisols [ENC and Samreboi (SAM)] and Haplic Luvisol [Juabeso (JUA)] ............................................... 57 Table 4.1d: Soil physicochemical properties for Dystric Fluvisols [SAM and pristine references (SEN)], Ferric Acrisol (pristine reference, ABA) and Haplic Luvisol (pristine reference, JUD) ................................................................. 58 Table 4.2: Soil physicochemical properties for the averaged cores .............................. 63 Table 4.3a: Measured and certified values for standard reference ISE 918 .................... 64 Table 4.3b: Measured and certified values for standard reference ISE 998 .................... 64 Table 4.4a: Total metal concentrations with corresponding exchangeable metal fractions for Ferric Acrisols (ASA, ASH and BOG) ................................................... 69 Table 4.4b: Total metal concentrations with corresponding exchangeable metal fractions for Haplic Ferrasol (BUA), Haplic Luvisol (DEB) and Dystric Fluvisol (ENC) ....................................................................................................................... 70 University of Ghana http://ugspace.ug.edu.gh xii Table 4.4c: Total metal concentrations with corresponding exchangeable metal fractions for Dystric FLuvisols (ENC and SAM) and Haplic Luvisol (JUA) ............. 71 Table 4.4d: Total metal concentrations with corresponding exchangeable metal fractions for Dystric FLuvisols (SAM and SEN), Ferric Acrisol (ABA) and Haplic Luvisol (JUD) ............................................................................................... 72 Table 4.5: Standard values of heavy metals for soils and atmospheric fallout ............. 73 Table 4.6: Mean total heavy metal concentrations with corresponding exchangeable metal fractions for the soils studied ............................................................. 76 Table 4.7: Discriminant analysis results for the investigated soils ............................... 89 Table 4.8a: Enrichment factors, EF1 (against the pristine reference value) and EF2 (against the value of the deepest horizon sampled), and anthropogenic contributions (%) for Cd, Pb, Cr and Ni in the studied soils; (bold: EF>2) ..................... 102 Table 4.8b: Enrichment factors, EF1 (against the pristine reference value) and EF2 (against the value of the deepest horizon sampled), and anthropogenic contributions (%) for Cu, Mn and Zn in the studied soils; (bold: EF>2) .......................... 103 Table 4.9: Principal component loadings (Varimax with Kaiser normalisation) for experimented variables in the soil samples (n = 48) ................................... 110 University of Ghana http://ugspace.ug.edu.gh xiii LIST OF FIGURES Figure 3.1: Map of Africa, Ghana and Western region indicating the sampling towns ... 36 Figure 3.2: A cocoa farm at Asankragwa (Ferric Accrisol) ............................................. 37 Figure 3.3: A cocoa farm at Enchi (Dystric Fluvisol) ...................................................... 37 Figure 3.4: Taking soil samples from ASH (Ashiem, Ferric Acrisol) with an auger ....... 41 Figure 3.5: Taking soil samples from SAM (Samreboi, Dystric Fluvisol) with an auger .41 Figure 3.6: Soil sample in a well labelled polypropylene zip-loc bag ............................. 42 Figure 4.1a: Relative distribution of physicochemical properties along the depth of each sampling site ............................................................................................................................. 53 Figure 4.1b: Relative distribution of physicochemical properties along the depth of each sampling site ............................................................................................................................. 54 Figure 4.2a: Proportional distribution of metals along the depths of each sampling site ............. 67 Figure 4.2b: Proportional distribution of metals along the depths of each sampling site ……..... 68 Figure 4.3: Comparing the average proportions of metals between the various farms ... 74 Figure 4.4a: Depth function plots illustrating the distribution of Cu, Fe, Mn and Zn with clay content ................................................................................................... 78 Figure 4.4b: Depth function plots illustrating the distribution of Cd, Cr, Ni and Pb with clay content .......................................................................................................... 79 Figure 4.5a: Depth function plots illustrating the distribution of Cu, Fe, Mn and Zn with pH (Soil:H2O = 1:1) ............................................................................................ 81 University of Ghana http://ugspace.ug.edu.gh xiv Figure 4.5b: Depth function plots illustrating the distribution of Cd, Cr, Ni and Pb with pH (Soil:H2O = 1:1) ............................................................................................. 82 Figure 4.6a: Depth function plots illustrating the distribution of Cu, Fe, Mn and Zn with total organic carbon content (TOC) ................................................................ 84 Figure 4.6b: Depth function plots illustrating the distribution of Cd, Cr, Ni and Pb with total organic carbon content (TOC) ................................................................ 85 Figure 4.7a: Depth function plots illustrating the distribution of Cu, Fe, Mn and Zn with cation exchange capacity (CEC) .................................................................... 86 Figure 4.7b: Depth function plots illustrating the distribution of Cd, Cr, Ni and Pb with cation exchange capacity (CEC) .................................................................... 87 Figure 4.8: Scatter plot showing between-group variations and the deviations of the individual samples from their group centroid or mean (within-group variations) ...................................................................................................... 90 Figure 4.9a: Relative accumulation ratio of heavy metals in the soils sites at different depths ............................................................................................................ 97 Figure 4.9b: Relative accumulation ratio of heavy metals in the soils at different depths ....................................................................................................................... 98 Figure 4.10: Average accumulation-depletion ratios of metals in the surface soils (0 – 10 cm) grouped according to soil types ............................................................ 100 Figure 4.11a: Lead enrichment in the different soil samples and lead anthropogenic proportion with reference to the pristine reference content in surface horizons .............................. 106 University of Ghana http://ugspace.ug.edu.gh xv Figure 4.11b: Chromium enrichment in the different soil samples and chromium anthropogenic proportion with reference to the pristine reference content in surface horizons surface soils (0 – 10 cm) ........................................................................................... 107 Figure 4.12: Principal component analysis loading plot for the rotated components ....... 110 University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE 1.0 GENERAL INTRODUCTION 1.1 BACKGROUND The natural occurrence of heavy metals is mostly from weathering of parent rocks and pedogenesis, and the anthropogenic inputs are associated with industrialization and agricultural activities such as fertilizer application and long-term application of wastewater in agricultural land (Baize and Sterckeman, 2001; Koch and Rotard, 2001; McLaughlin et al., 2000). In Ghana, agriculture is the main industrial activity and cocoa is the major cash crop grown. Ghana is the world’s second largest producer of cocoa beans and cocoa has, for many years, been the backbone of the country’s economy. Governments, therefore, never relent in the pursuit and implementation of measures to boost cocoa production (Osei, 2007; Dorman et al., 2004; Ahenkorah et al., 1982). The high output of cocoa beans in recent times in Ghana is due largely to the application of a wide variety of agrochemicals such as pesticides, herbicides and fertilizers which over the recent decades have been recommended (Appiah et al., 1997) and massively patronized by cocoa farmers (Vigneri, 2007). However, high application rates of fertilizers and fungicides have been shown to result in heavy metal accumulation in surface horizons making farmlands susceptible to heavy metal contamination (Faβbender and Bornemisza, 1987). These heavy metals often occur as cations which strongly interact with the soil matrix and may consequently become mobile as a result of changing environmental conditions (Facchinelli et al., 2001). Heavy metals such as Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn accumulate in the soil and do not only circulate in the soil ecosystem but also enter crops University of Ghana http://ugspace.ug.edu.gh 2 grown in contaminated soils thereby gradually ending up in the food chain (Ghrefat and Yusuf, 2006). Though some can be beneficial, heavy metals are the most dangerous contaminants for the environment and human beings especially when their levels exceed specific thresholds mainly due to the fact that they are non-degradable (Dyer, 2007; Bradl, 2005): Cadmium toxicity, for example, has side effects such as kidney dysfunction; Copper toxicity has been reported to cause liver cirrhosis (Graham and Cordano, 1976); Toxicity of zinc may lead to anaemia and lethargy (Fairweather-Tait, 1988); and lead toxicity manifestations include cancers, typically involving the skin, lung and bladder (Groten and VanBladeren, 1994; Gazza, 1990). The accumulation of heavy metals in agricultural soils is thus of increasing concern due to the food safety issues and potential health risks as well as their detrimental effects on soil ecosystems (McLaughlin et al., 1999). These concerns have attracted the attention of many countries who import agricultural products. Ghana’s cocoa beans thus need scrutiny as well as the soils in which they are cultivated. Evaluating the total concentrations and understanding the distribution characteristics of heavy metals in cocoa growing soils can aid environmental managers and even help regulate the rate of agrochemical application. Though the total metal concentrations may indicate the overall level of metals in soils, they provide no information regarding potential mobility and/or bioavailability of a particular element (Powell et al., 2005; Vijver et al., 2004). Heavy metals in soil can exist in various fractions showing varying availability. These include: (i) simple or complex ions in soil solution; (ii) exchangeable ions; (iii) ions linked to organic substances; (iv) ions occluded or co-precipitated with oxides, carbonates and phosphates, or other secondary minerals; and (v) ions in the crystalline lattice of primary minerals (Soon and Bates, 1982). Not all these forms are equally important from an ecological point of view. The (iii), (iv) and (v) fractions are University of Ghana http://ugspace.ug.edu.gh 3 very stable and hence unlikely to be released under weathering conditions whereas the soluble and exchangeable fractions are quite mobile (Kabata-Pendias, 1993). The mobile fraction may be taken up by plants or leached into the groundwater (Allen et al., 1995; Brummer et al., 1986; MahatTey et al., 1975). Accurate measurements of both the total and bio-available (mobile or exchangeable) concentrations of heavy metals are thus required to create a distinction between the concentrations that are in the soil and the proportion that may get to plants or enter the food chain. The physicochemical properties of soil also affect the distribution of heavy metals in soils. Clay content and type, soil pH, organic matter and cation exchange capacity are the predominant parameters controlling the accumulation and the availability of heavy metals in soil (Nyamangara and Mzezewa, 1999). For example, low pH of soil tends to increase the rate of desorption of heavy metals from soil particle surfaces into solutions that remain in the capillaries of the soil making them available for uptake by plants or leached into underground water. Each heavy metal undergoes differing reactions in the soil depending on the aforementioned factors and, consequently, the available metals concentrate to different degrees along the depth of the soil. It is necessary then to evaluate the relationship among these parameters and heavy metal accumulation in soil. 1.2 PROBLEM STATEMENT Agrochemicals have since the past decade been applied on almost all cocoa farms in Ghana with the sole aim of boosting crop yield through controlling pests and diseases, and weeds. The Western region is the largest producer of cocoa in Ghana, producing over 50% of the nation’s cocoa beans (Anim-Kwapong and Frimpong, 2005; Appiah et al., 1997). Cocoa farmlands in the region receive appreciable fertilizer and/or other agrochemical applications. University of Ghana http://ugspace.ug.edu.gh 4 The region is also the second highest producer of gold in Ghana and it hosts lots of small scale mining industries locally termed “galamsey”. Several studies have shown the invasive effect of mining activities on the environment (Akabzaa et al., 2005; Asante et al., 2005; Bonzongo et al., 2004). Most cocoa farming communities in the Western region are in proximity with active mining areas. Cocoa farms may, therefore, be exposed to a number of heavy metals as a result of the mining processes and agrochemical applications. A study of the concentrations of various heavy metals and their depth distribution in soils of cocoa farms in the Western region is, therefore, imperative to ascertain whether or not the levels of the metals are above the maximum contaminant levels. It is also equally important to establish the source of these metals if they indeed exist in the soils so as to formulate remedial measures. This thesis, among other things, sought to determine the levels of cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) in soils of cocoa farms. 1.3 HYPOTHESIS H0: Soils in cocoa farms in the Western region of Ghana are contaminated with heavy metals from anthropogenic inputs. H1: Soils in cocoa farms in the Western region of Ghana are not contaminated with heavy metals. University of Ghana http://ugspace.ug.edu.gh 5 1.4 OBJECTIVES OF THE RESEARCH 1.4.1 Aim The aim of this study is to establish the depth distribution of heavy metals in soils of cocoa farms in the Western region of Ghana. 1.4.2 Specific Objectives The specific objectives comprise the following: 1. To determine the total concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in soils of cocoa farms in the Western region of Ghana. 2. To determine the exchangeable (mobile) concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in soils of cocoa farms in the Western region of Ghana. 3. To establish the relationships among the heavy metals on one hand, and also between the heavy metals and physicochemical properties of the soils. 4. To determine the sources of the heavy metals in the soils. University of Ghana http://ugspace.ug.edu.gh 6 CHAPTER TWO 2.0 LITERATURE REVIEW 2.1 ESSENTIALITY OF ELEMENTS The ions in the soil solution and in the solid phase that are of primary interest to soil chemists are those essential or toxic to life and those important to soil development (which are also important for plants and animals). Resultantly, elements (including heavy metals) are classified essential or toxic by many studies (Gomes and Silva, 2007; Brady and Weil, 1999; Bohn et al., 1985). Several definitions exist for essential elements. According to Lindh (2005), an element is essential if: it is present in living tissues at a relatively constant concentration; it provokes similar structural and physiological anomalies in several species when removed from their organisms and these anomalies are prevented or cured by the supplementation of the element. For the World Health Organization (WHO, 2002), an element is considered essential to an organism when the reduction of its exposure below certain limit results consistently in a reduction in a physiologically important function, or when the element is an integral part of an organism structure performing a vital function in the organism. Sodium, F, Si, Cr, Ni, Co, As, Se, Cl and Sn are essential to only animals, Mo is essential to only plants while H, C, N, O, Mg, Ca, P, S, K, B, V, Mn, Fe, Zn and Cu are considered essential elements to all animals and plants. However, not all essential elements are needed in large quantities. H, C, N, O, Mg, Ca, Na, P, S and K are referred to as macro-elements because they are required in large quantities by humans whereas B, V, Mn, Fe, Zn, Cu, Mo, Si, Cr, Co, As, Se, Sn and I are required in small quantities and so are termed micro-elements. Chlorine is, however, considered macro and University of Ghana http://ugspace.ug.edu.gh 7 micro element to animals and plants, respectively (Bohn et al., 1985). Hydrogen, O, C, and N may be classified as major elements because they make up approximately 96% of the human body mass. Sodium, K, Ca, Mg, P, S, and Cl make up 3.78% of the body mass and thus classified minor elements (with their concentration being expressed in gkg−1). The remaining elements and others (about 70) are called trace elements (with their concentration being expressed in mgkg−1), (Bohn et al., 1985). The World Health Organization (WHO, 2002) considers the trace elements: Fe, Zn, Cu, Cr, I, Co, Mo and Se, essential to human health. In recent years, these trace elements, especially those termed heavy metals, in the soil have received attention as environmental contaminants because of their extended persistence and toxicity to many organisms (Hashem and Al-Obaid, 1996). 2.2 HEAVY METALS The term “heavy metal” has received several definitions from biologists to toxicologists thereby presenting no coherent scientific basis (Duffus, 2002). Most definitions are on the basis of density (specific gravity) (Morris, 1992; Lozet and Mathieu, 1991; Parker, 1989). However, no relationship can be found between density (specific gravity) and any of the various physicochemical concepts that have been used to define “heavy metals” and the toxicity or ecotoxicity attributed to “heavy metals” (Hodgson et al., 1988; Bennett, 1986; Phipps, 1981). Nevertheless, understanding bioavailability is the key to assessment of the potential toxicity of metallic elements and their compounds. Bioavailability depends on biological parameters and on the physicochemical properties of metallic elements, their ions, and their compounds. University of Ghana http://ugspace.ug.edu.gh 8 Heavy metals are totally non-degradable to non-toxic forms, although they may be ultimately transformed into insoluble and hence biologically unavailable forms. They enter the body through the breathing of air, drinking of water and through the eating of contaminated food. Heavy metals occur naturally in ecosystems with large variations in concentration. In modern times, anthropogenic sources of heavy metals have been introduced to the ecosystem. There are 35 metals of environmental concern because of occupational or residential exposure; twenty three of these are "heavy metals" and are antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium and zinc (Glanze, 1996). Some heavy metals are nutritionally essential for a healthy life whereas large amounts of any of them may cause acute or chronic toxicity (Coen et al., 2001; Vernet, 1992). It should, however, be stressed that metals like cadmium, lead, beryllium and mercury have no biological functions and are highly toxic disrupting bodily functions to a large extent (Phipps, 1981). They, therefore, need close scrutiny. The applications and properties of these heavy metals are as follows: 2.2.1 Cadmium Cadmium is produced mainly as a by-product from mining, smelting, and refining sulfidic ores of zinc, and, to a lesser degree, lead and copper (Fthenakis, 2004). Cadmium exists in low concentrations in all soils. It is spread by air and water (sewage sludge) far over sea and land, but especially in the vicinity of heavy industrial plants. Cadmium emission also occurs from the production of artificial phosphate fertilizers and upon addition the element ends up in the soil (Jennings, 2005). University of Ghana http://ugspace.ug.edu.gh http://en.wikipedia.org/wiki/Lead http://en.wikipedia.org/wiki/Copper 9 Cadmium is today regarded as the most serious contaminant of the modern age. It is absorbed by many plants and sea creatures and, because of its toxicity, presents a major problem in foodstuffs (Concon, 1998; Das et al., 1997). Contamination through fertilisers becomes an increasing problem. Cadmium contamination cannot be removed from plants by washing them; it is distributed throughout the organism. It is often difficult to ascertain the cause of a Cd content in fruits or vegetables, as the substance in its natural form exists everywhere in the soil and is absorbed by the roots. It has been possible, however, to show that the increased Cd content in Central American cocoa was related to the specific local constituency of the soil. As opposed to African cocoa kernels, which contain 0.08-0.14 mg/kg, values from 0.18- 1.5 mg/kg are found in the fine cocoa varieties from Venezuela and Ecuador (Vitošević et al., 2007). Thus, site or vicinity or geographical location is an important factor when dealing with Cd contamination. Cadmium is a highly toxic element that accumulates in biologic systems and has a long half- life. Its toxicity is manifested by a variety of syndromes: side effects include kidney dysfunction (necrotic protein precipitation), hypertension, hepatic injury, reproductive toxicity, lung damage after inhalation exposure, and bone effects such as “itai-itai” sickness in Japan. The kidney is a critical target-organ for Cd accumulation, and the half-life of the element in this tissue is about 30 years (Robards and Worsfold, 1991; Concon, 1998). Cadmium is easily transferred from soil to plants, with absorption and accumulation of the element in plant to varying degrees. This process is favoured by low soil pH values, probably as a result of the increase in the exchangeable portion of Cd (Creaser and Purchase, 1991; Reilly, 1980). Cadmium accumulation is a continuous process that requires no particular threshold value of the element in soil and is influenced by physicochemical factors (Cabrera et al., 1994; Zurera et al., 1987; Haghiri, 1973) University of Ghana http://ugspace.ug.edu.gh 10 During weathering, Cd goes directly into soil solution and, although known to occur as Cd2+, it may also form complex ion such as CdCl+, CdOH+, CdCl3 -, Cd(OH)3 -, and Cd(OH)4 2-. Oxidation potential and pH are the principal factors controlling Cd ion mobility in soil. Under conditions of of strong oxidation, Cd is however, likely to form CdO and CdCO3, and is likely to be accumulated in phosphate and in biolith deposites (Kabata-Pendias, 2000). Cadmium concentrations in soil solutions are controlled by adsorption rather than precipitation until a threshold pH value is exceeded (Soon, 1981; Tiller et al., 1979). The solubility of CdCO3 and possibly Cd3(PO4)2 may control the Cd mobility in soil (Kabata- Pendias, 2000). 2.2.2 Chromium Chromium is mined as chromite (FeCr2O4) ore (Emsley, 2001) and it enters the air, water and soil in the Cr (III) and Cr (VI) forms through natural processes and human activities. According to Kotaś and Stasicka (2000), volcanic eruptions and erosion of Cr containing rocks constitute the natural sources whereas steel, leather and textile manufacturing are, among other things, the predominant human activities that increase Cr concentrations in the environment especially in water. Through coal combustion, however, Cr ends up in the air whereas waste disposal deposits Cr in soils. In soils, Cr2+ strongly attaches to soil particles and as a result does not move towards groundwater (Kotaś and Stasicka, 2000). The metal is known to enhance the action of insulin, the hormone critical to the metabolism and storage of carbohydrate, fat, and protein in the body (Emsley, 2001). Chromium toxicity symptoms, according to Stoecker (2001), include allergic dermatitis, skin lesions and University of Ghana http://ugspace.ug.edu.gh 11 increased incidence of lung cancer. Toxicity also causes respiratory problems, a lower ability to fight disease, birth defects, infertility and tumor formation (Myers et al., 1997). Chromium shows complex anionic and cationic ions in soil such as Cr(OH)2 +, CrO4 2- and CrO3 3-. Under progressive oxidation, Cr forms chromate ion (CrO4 2-) which is readily mobile and also is easily sorbed by clays and hydrous oxides. It has been shown that most of the soil Cr occurs as Cr3+ and it is within the mineral structures or forms of mixed Cr3+ and Fe3+ oxides. Cr3+ is is slightly mobile only in very acidic media, and at pH 5.5, it is almost completely precipitated. Consequently, its compounds are considered to be very stable in soils. Cr6+, on the other hand, is very unstable in soils and is easily mobilised in both acid and alkaline soils (Kabata-Pendias, 2000). 2.2.3 Copper Copper is a very common substance that occurs naturally in the environment. Copper enters food materials from soil through mineralisation by organic matter, food processing or environmental contamination, as in the application of agricultural inputs, such as Cu-based pesticides example fungicides. Kies and Harms (1989) state that adult human body contains about 1.5 ± 2.0 ppm of Cu which, according to Underwood (1977) and Schroeder (1973), is essential as a constituent of some metalloenzymes and required in haemoglobin synthesis and in the catalysis of metabolic oxidation. Symptoms of Cu deficiency in humans include bone demineralisation, depressed growth, depigmentation and gastro-intestinal disturbances. However, reports by Graham and Cordano (1976), Lucas (1974) and Somers (1974) indicate that Cu toxicity due to excessive intake causes liver cirrhosis, dermatitis, neurological disorders, brain damage, demyelization and renal disease. Copper toxicity is also usually characterized by metallic taste, chronic and intermittent nausea, headaches, abdominal University of Ghana http://ugspace.ug.edu.gh 12 cramping and diarrhea, irritation of the nose, mouth and eyes, dizziness and vomiting, and deposition of the element in the cornea (Garrow and James, 1993; Davidson et al., 1975). When Cu ends up in the soil, it strongly attaches to organic matter and soil minerals. As a result, it does not travel very far after release and it hardly ever enters groundwater. It does not break down in the environment and because of that it can accumulate in plants and animals when it is found in soils. On copper-rich soils, only a limited number of plants have a chance of survival. Due to the effects on plants, Cu is a serious threat to crop production on farmlands. The metal can seriously influence the productivity of certain farmlands, depending upon the acidity of the soil and the presence of organic matter (Patnaik, 1999). Copper forms quite easily soluble minerals in weathering processes and release Cu ions, especially in acid environments. These Cu ions can readily precipitate with various anions such as sulphide, carbonate and hydroxide. The metal is therefore rather immobile in soils and shows relatively little variation in total content in soil cores. The common characteristic of Cu distribution in soil cores is its accumulation in the top horizon. Its accumulation in surface soils thus reflects the bioaccumulation of the metal and also recent anthropogenic sources of the element (Kabata-Pendias, 2000). Adsorption, occlusion and coprecipitation, organic chelation and complexing, as well as microbial fixation constitute the processes controlling fixation of Cu by soil constituents. The specific sorption of Cu is related to the reaction with electron-pair donors, and therefore forms strong bonds of high covalency (Kabata-Pendias, 2000). Though the most mobile form of Cu in the soil is the cation with the valence of +2, several ionic species may also exist in the soil (Cu2+, CuO, Cu, CuCO3, Cu(CO3)2 2-, CuOH+, Cu(OH)3 -,Cu(OH)4 2- and Cu2(OH)2 2+). All these species may be adsorbed by soils but the hydroxide forms are the most readily adsorbed (Bodek et al., 1988). At high pH, more of the University of Ghana http://ugspace.ug.edu.gh 13 hydroxide species are formed due to the increase in OH- concentration and hence more of the Cu ions get adsorbed and/or precipitated. Consequently, the Cu content in the soil solution become considerably reduced making less available for plant uptake. All soil minerals are capable of adsorbing Cu ions but this property depends on the surface charge carried by the adsorbents which invariably is dependent on pH. Hence, the adsorption of Cu is a function of pH. Stevenson and Fitch (1981) indicate, in this regard, that the maximum amount of Cu2+ that can be bound to humic and fulvic acids is approximately equal to the content of acidic functional groups. 2.2.4 Iron Iron is released into the environment by natural processes such as weathering, erosion, forest fires and wind-blown dust. It is also released by human activities such as mining, agriculture, rusting of automobile parts and factory machinery among others. Iron is essential to most life forms and to normal human physiology. It is an integral part of many proteins and enzymes that maintain good health. Dallman (1986) explains that, in humans, Fe is an essential component of proteins and is involved in oxygen transport. It is also essential in the regulation of cell growth and differentiation (Andrews, 1999; Bothwell et al., 1979). Iron deficiency limits oxygen delivery to cells, resulting in fatigue and weakness, decreased ability to concentrate, hair loss, dizziness, headaches, brittle nails, apathy, depression and decreased immunity (Bhaskaram, 2001; Haas and Brownlie, 2001). In general, thus, adequate Fe in the body enhances oxygen distribution throughout the body, keeps the immune system healthy and helps the body produce energy. University of Ghana http://ugspace.ug.edu.gh http://ods.od.nih.gov/factsheets/iron.asp#en5 14 Despite the seemingly overwhelming importance of Fe, Corbett (1995) explains that excess amounts of Fe can result in toxicity and even death as it exerts its most profound effects on the cardiovascular system. Toxicity results in fatty necrosis of the myocardium, postarteriolar dilatation, increased capillary permeability, and reduced cardiac output (Greentree and Hall, 1995). Excess amount also interferes with clotting mechanisms, augmenting hemorrhagic processes (Myers et al., 1997; Goyer, 1996; Greentree and Hall, 1995; Osweiler et al.,1985) and has also been reported to cause thrombocytopenia (Hillman, 1995). Iron poisoning due to the ingestion of large quantities causes nausea, vomiting, damage to the lining of the intestinal tract, shock and liver failure, loss of appetite, fatigue, weight loss, headaches, bronze or gray hue to the skin, dizziness and shortness of breath. In recent years, excess Fe intake and storage, especially in men, has been implicated as a cause of heart disease and cancer and its overdose has been reported by Corbett (1995) to be one of the leading causes of fatality from toxicological agents in children younger than 6 years. The reactions of Fe in processes of weathering are dependent largely on Eh-pH system of the environment and on the stage of oxidation of the Fe compounds involved. The general rule governing the mobilization and fixation of Fe are that oxidizing and alkaline conditions promote the precipitation of Fe, whereas acid and reducing conditions promote the solution of Fe compounds. The released Fe readily precipitates as oxides and hydroxides, but it substitutes for Mg and Al in other minerals and often complexes with organic ligands (Kabata-Pendias, 2000). Both mineral and organic compounds of Fe are easily transformed in soils, and organic matter appears to have a significant influence on the formation of Fe oxides. These metals may be amorphous, semicrystalline, or crystalline, even under the same conditions. The University of Ghana http://ugspace.ug.edu.gh 15 content of soluble Fe in soils is extremely low in comparison with the total Fe content. Soluble inorganic forms include Fe3+, Fe(OH)2 +, FeOH2+, Fe2+, Fe(OH)3 - and Fe(OH)4 2-. In well aerated soils, however, Fe2+ contributes little to the total soluble inorganic Fe, except under high soil pH conditions. The concentration of Fe in soil solutions within common soil pH levels ranges from 30 to 550 µg/L, whereas in a very acid soil it can exceed 2000 µg/L. Acid soils are therefore higher in soluble inorganic Fe than are neutral and calcareous soils. Thus, Fe2+ cations when in acid anaerobic soils may become toxic, but in alkaline well- aerated soils, the low concentration of soluble Fe species may not meet plant requirements for this metal (Kabata-Pendias, 2000). 2.2.5 Manganese Manganese occurs principally as pyrolusite (MnO2) and to a lesser extent as rhodochrosite (MnCO3) (Emsley, 2001). Manganese is naturally present in rocks, soil, water, and food. It is naturally present in food, with the highest concentrations typically found in nuts, cereals, legumes, fruits, vegetables, grains, and tea. It is also present at low levels in drinking water (Agency for Toxic Substances and Disease Registry (ATSDR), 2000; Pennington et al, 1986). Manganese is required for growth, development and maintenance of health. It is necessary for skeletal system development, energy metabolism, activation of certain enzymes, nervous system function, immunological system function, reproductive hormone function and is an antioxidant that protects cells from damage due to free radicals (IOM, 2001; ATSDR, 2000). Despite its use, Mn is toxic at high levels and causes various adverse effects in the respiratory tract and in the brains. Studies in animals have shown that very high levels of Mn in food or University of Ghana http://ugspace.ug.edu.gh 16 water can cause changes in the brain. This suggests that high levels of Mn in food or water might cause brain injury but it does not appear that this is of concern to people exposed to the normal amounts of Mn in food, water or air. The USEPA has, however, determined that Mn is not classifiable as a human carcinogen (ATSDR, 2000). In soils, the negatively charged Mn(OH)4 2- and MnO2 2- are responsible for the high degree of association of Mn concretions with some heavy metals, in particular with Co, Ni, Cu, Zn,Pb, Ba, Tl, W and Mo. Additionally, the oxidation of As,. Cr, V, Se, Hg and Pu by Mn oxide is likely to control the redox behaviour of these elements in soils (Bartlett, 1986). The solubility of Mn in soils is highly dependent on the pH and redox potential. Therefore, the most common reactions occurring in soils are oxidation-reduction and hydrolysis. The solubility of soil Mn is of significance since the plant supply of Mn depends mainly on the soluble Mn pool in the soil. In well-drained soils, the solubility of Mn always increases with the increase of soil acidity. However, the ability of Mn to form anionic complexes and to complex with organic ligands may contribute to increased Mn solubility in the alkaline pH range. 2.2.6 Nickel Organic matter has a strong ability to adsorb nickel and as a result coal and oil contain considerable amounts of Ni. Nickel is released into the air by power plants and trash incinerators. It is also released by the combustion of fuel by automobiles. The larger part of all Ni compounds that are released to the environment are adsorbed to sediment or soil particles and become immobile. The element is not known to accumulate in plants or animals. University of Ghana http://ugspace.ug.edu.gh 17 As a result, it does not biomagnify up the food chain (Yaman, 2000; Rasmussen et al., 1988; Grandjean, 1984). Nickel plays important roles in the biology of microorganisms and plants: Urease (an enzyme which assists in the hydrolysis of urea) contains Ni; the NiFe-hydrogenases contain Ni in addition to iron-sulfur clusters; a nickel-tetrapyrrole coenzyme, F430, is present in the methyl coenzyme M reductase which powers methanogenicarchaea; one of the carbon monoxide dehydrogenase enzymes consists of an Fe-Ni-S cluster; and other Ni-containing enzymes include a class of superoxide dismutase and a glyoxalase (Szilagyi et al., 2004; Thornalley, 2003). Excess of Ni can be very dangerous. Symptoms of Ni toxicity include skin rash (called nickel dermatitis), nausea, dizziness, diarrhea, headache, vomiting, chest pain, weakness and coughing. Contact with nickel vapor can lead to swelling of the brain and liver; degeneration of the liver; irritation to the eyes, throat and nose; and various types of cancer. The most common harmful health effect of Ni in humans is an allergic reaction (Grandjean, 1984). Nickel is easily mobilised during weathering and then is coprecipitated mainly with Fe and Mn oxides (Kabata-Pendias, 2000). However, unlike Mn2+ and Fe2+, Ni2+ is relatively stable in aqueous solutions and is capable of migration over a long distance. In surface soil horizons, Ni appears to occur mainly in organically bound forms, a part of which may be an easily soluble chelate (Bloomfield, 1981). Nickel distribution in soil cores is related either to organic matter or to amorphous oxides and clay fractions, depending on soil types. Concentrations of Ni in natural solutions of soil horizons of different soils vary from 3 and 25 µg/L at the boundary and at the centre of the affected area, respectively (Anderson et al., 1973). Information on Ni ionic species in the soil University of Ghana http://ugspace.ug.edu.gh http://en.wikipedia.org/wiki/Urease http://en.wikipedia.org/wiki/Urea http://en.wikipedia.org/wiki/Hydrogenase http://en.wikipedia.org/wiki/Iron-sulfur_cluster http://en.wikipedia.org/wiki/F430 http://en.wikipedia.org/wiki/Coenzyme_M http://en.wikipedia.org/wiki/Methanogen http://en.wikipedia.org/wiki/Methanogen http://en.wikipedia.org/wiki/Superoxide_dismutase http://en.wikipedia.org/wiki/Glyoxalase 18 solution is rather limited but the Ni species described by Garrels and Christ (1965) such as Ni2+, NiOH+, HNiO2 -, Ni(OH)3 - are likely to occur when the Ni is not completely chelated. Generally, the solubility of soil Ni is inversely related to the soil pH. The Ni sorption on Fe and Mn oxides is especially pH dependent probably because the NiOH+ is preferentially sorbed and also because the surface charge on the sorbent is affected by pH (Bodek et al., 1988). 2.2.7 Lead Lead pollution in the environment is primarily known to be sourced from industrial production processes and their emissions, road traffic with leaded petrol, the smoke and dust emissons of coal and gas-fired power stations, the laying of lead sheets by roofers as well as the use of paints and anti-rust agents (Ve´ron et al., 1999). Basically, as a result of their comparatively high affinity for proteins, Pb ions, when ingested, bond with the haemoglobin and the plasma protein of the blood. This leads to inhibition of the synthesis of red blood cells and thus of the vital transport of oxygen. If the bonding capacity is exceeded, Pb passes into the bone-marrow, liver and kidneys. Chronic intoxication leads to severe complications such as encephalopathies in the central nervous system (CNS), disturbances in kidney and liver functions progressing as far as necrosis, damage to the reproductive organs, anaemia and many metabolic deficiency symptoms (Vitošević et al., 2007). Little is known about the excretion of Pb, once it has been absorbed. Thus, a large percentage of the metal accumulates in the body (ATSDR, 2000). Lead is the least mobile of all heavy metals. A high soil pH may precipitate Pb as hydroxide, phosphate or carbonate. The metal forms Pb2+ though its oxidation state is +4. The natural Pb University of Ghana http://ugspace.ug.edu.gh 19 content is strongly related to the bedrock and this is supported by the relatively low concentration in natural soil solutions. For instance, in a study of heavily Pb polluted soils, the formation of pyromorphite, Pb5Cl(PO4)3, was observed. It was also observed that the concentration of the mineral was mainly close to the grass (Agrostis capillaris) and hence indicated an influence of the rhizoshere on the process of its formation (Kabata-Pendias, 2000). Therefore, natural systems are known to contribute to the formation and distribution of Pb. 2.2.8 Zinc Zinc ores include Zinc blende or sphalerite (a form of zinc sulfide), wurzite, smithsonite and hemimorphite (Emsley, 2001). It is the 24th most abundant element in the earth crust and it is no surprise it occurs in air, water and soil due to natural processes such as weathering and erosion. The addition of Zn through human activities such as mining, coal and waste combustion and steel processing has increased its concentration in the environment. Zinc metal is included in most single tablets and it is believed to possess anti-oxidant properties which protect against premature aging of the skin and muscles of the body (ATSDR, 2000). However, zinc deficiency usually results from poor diet, alcoholism and malabsorption. Zinc deficiency symptoms include decreased sense of taste and smell, dwarfism, hypogonadism and dermatitis whereas toxicity of Zn may lead to electrolyte imbalance, nausea, anaemia, birth defects and lethargy (Dibley 2001; Garrow and James, 1993; Fairweather-Tait, 1988; Prasad, 1984 and 1976). University of Ghana http://ugspace.ug.edu.gh http://www.lenntech.com/water-FAQ.htm 20 Zinc accumulates in surface soils as it is easily adsorbed by mineral and organic components. On weathering, Zn2+ is released. However, ZnO2 2-, ZnO2 -, Zn(OH)3 -, ZnCl+ and ZnHCO3 + are some of the other ionic species in which Zn may exist in the soil. Though the factors controlling the mobility of Zn are similar to that of Cu, the metal appears to occur in more readily soluble forms (Kabata-Pendias, 2000). Two different mechanisms of Zn adsorption are known. Firstly, in acid media, it is related to cation exchange sites, and in alkaline media it is considered to be chemisorptions and is highly influenced by organic ligands (Lindsay, 1972). Nucleation of Zn hydroxide on clay surfaces may produce a strongly pH-dependent retention of Zn whereas the adsorption of Zn2+ can be reduced at lower pH (< 7) by competing cations and this will result in easy mobilisation and leaching of Zn from light acid soils (McBride and Blasiak, 1979). 2.3 HEAVY METALS AND SOIL Soils contain trace levels of metals due mostly to the natural abundances of some metals (McLean and Bledsoe, 1992). Soil, being the interface between the atmosphere and the earth crust as well as the substrate for natural and agricultural ecosystems, is open to inputs of heavy metals from many sources. Concentration of metals in uncontaminated soils has been primarily related to the geology of the parent material from which the soil is formed (McLean and Bledsoe, 1992). Relatively, pristine soils normally contain low background levels of heavy metals as compared to soils in areas where agricultural, industrial or municipal wastes are land-applied as fertilizer. Thus, depending on the surrounding geological environment and anthropogenic and natural activities occurring or once occurred, many soils contain a wide range of heavy metals with varying concentration ranges. University of Ghana http://ugspace.ug.edu.gh 21 Principally, the sources of heavy metals in the soil are natural processes such as weathering and, also human activities such as mining and application of fertilizers and pesticides. Mining, manufacturing and the use of synthetic products (e.g. pesticides, paints, batteries, industrial waste, and land application of industrial or domestic sludge) have resulted in heavy metal contamination of urban and agricultural soils. Potentially, contaminated soils may occur at old landfill sites, particularly those that accepted industrial wastes, old farms that used insecticides, fields that had past applications of waste water or municipal sludge, areas in or around mining waste piles and tailings, industrial areas where chemicals may have been dumped on the ground, or in areas downwind from industrial sites (Akabzaa et al., 2005; Asante et al., 2005; Bonzongo et al., 2004; Vernet, 1992; Ahenkorah et al., 1982). 2.3.1 Agrochemicals and Heavy Metals in Soil According to Foster Wheeler Environmental Corporation (1998), a wide variety of unsafe metals may exist in fertilizers and fungicides which may include: arsenic, lead, cadmium, copper and mercury. This is validated by many studies (Dubey and Townsend, 2004; Wilcke and Dӧhler, 1995; Faβbender and Bornemisza, 1987; Lepp et al., 1984; Cordero and Ramirez, 1979). Giuffre et al. (1997) also found out that continuous application of fertilizers to the soil may increase the heavy metal concentration thereby exceeding the natural abundances in soils, and transfer of these metals to the human food chain despite the fact that these heavy metals may be present in minute quantities in fertilizers. Nevertheless, fertilizers are basically chemicals applied to the soil to promote plant growth (McLaughlin et al., 1996). The main nutrients (macronutrients) present in fertilizers include University of Ghana http://ugspace.ug.edu.gh 22 nitrogen, phosphorus and potassium. Other secondary macronutrients which include sulphur, calcium and magnesium are sometimes added in minute quantities. Additionally, there are micronutrients which are added in smaller amounts such as boron, chlorine, manganese, iron, zinc, copper, molybdenum, selenium and cobalt (Fertilizer Industry Federation of Australia (FIFA), 2008; Mills and Jones, 1996). Commercial fertilizers have been used for decades and will continue to be used for years to come. Some commercial fertilizers, however, are made from recycled hazardous waste materials including highly toxic metals and chemicals produced for public use (Heckman and Barbour, 2005). Phosphorus fertilizers contain varying amounts of heavy metals and other rare earth elements as contaminants. These contaminants may either come from the phosphate rock ores or other ingredients used in the phosphate fertilizer industry. According to Taylor (1997), application of phosphate fertilizers is the main pathway through which Cd accumulates in the soil. He also noticed that the concentration of up to 100 mg/kg of Cd in phosphate minerals increased the contamination of soil with Cd in New Zealand. Syers et al. (1986) and Trueman (1965) found similar observations in soils of Nauru and the Christmas Island, respectively. Hence, even low annual accumulations of metals may finally build up undesired concentrations in soils, especially where fertilizers with high heavy metals or rare earth element concentrations are used. In Ghana, two main types of fertilizer formulations are used in the Hi-Tech Programme undertaken by Ghana Cocoa Board. These fertilizers have been tested and approved to be used on cocoa by Cocoa Research Institute of Ghana (CRIG). They include: granular fertilizers – “Asaasewura” and “Cocofeed”, and liquid fertilizer – “Sidalco”. Table 2.1 presents fertilizers, fungicides and insecticides (including their active ingredients) that are recommended by Ghana Cocoa Board. University of Ghana http://ugspace.ug.edu.gh http://en.wikipedia.org/wiki/Nauru 23 Table 2.1: Recommended agrochemicals in Ghana Agrochemical type Trade name Active ingredient (a.i.) Fertilizer Asasewura NPK: 0-22-18 + 9CaO + 6S+ 5MgO Cocofeed NPK: 0-30-20 Sidalco NPK NPK:15-15 -15 + 2MgO + 3Zn Fungicide Funguran 300 g/L of Cu metal as cupric hydroxide. Champion Cupric hydroxide Nordox Cuprous oxide, Cr2O Ridomil Gold Plus Metalaxyl and copper oxychloride Fungikill 35% Copper + 15% Metaxyl Insecticide Confidor 200 g/L Imidacloprid Actara 240 g/L Thiamethoxam (a.i.) + 0.03% 1, 2- benzisothiazolin-3-one as a preservative. Akatemaster 27 g Bifenthrin Source: Ghana Cocoa Board, 2011. Recommendations on the application of these agrochemicals by Ghana Cocoa Board, are as follows: For the granular fertilizers, 1 kg to a hectare of cocoa farm should be applied; spraying of fungicides must be done at three weekly intervals for six to nine times in the crop season (the crop year begins in October, when purchases of the main crop begin, while the smaller mid-crop cycle starts in July); from August, spraying should be done against capsids using the recommended insecticides only – spraying should be done once in August, September, October and December (Ghana Cocoa Board, 2011). From Table 2.1, agrochemicals applied in cocoa farms in Ghana have some heavy metals in them. University of Ghana http://ugspace.ug.edu.gh 24 The levels of metals in the soil have direct effect on plants (Hall and Robarge, 2004; Kabata- Pendias, 2000). For example, variation in soil nutrient levels, including metals, influences plant species composition and growth (Etherington, 1982). Tudureanu and Phillips (2004) reported that Cd can accumulate in plants and not have any effect on the plants but will be toxic to animals and humans that eat them. Soils can be contaminated by the heavy metals which will bioaccumulate in plants and animals eventually making their way to humans through the food chain of humans (Frink, 1996). The transport of heavy metals in the soil is not only dependent on the properties of the metals but mostly on the physicochemical properties of the soil, viz. clay content, pH, soil organic matter content, cation exchange capacity and mineralogical composition. 2.4 HEAVY METAL MOBILITY Generally, metals added to soil will stay at the soil surface. Movement to groundwater, surface water, or the atmosphere is minimal as long as, according to McLean and Bledsoe (1992), the retention capacity of the soil is not exceeded. Normally, metals do not travel downward from the soil surface to any great extent. Their movement in soil is directly related to the surface chemistry of the soil matrix and soil solution (Sposito, 1989). Thus, when metals are introduced at the soil surface, downward transportation does not occur to any great extent unless the metal retention capacity of the soil is overloaded, or metal interaction with the associated waste matrix enhances mobility. At the same time, metals participate in chemical reactions with the soil solid phase. The concentration of metals in the soil solution, at any given time, is governed by a number of interrelated processes, including inorganic and organic complexation, oxidation/reduction University of Ghana http://ugspace.ug.edu.gh 25 reactions, precipitation/dissolution reactions, and adsorption/desorption reactions (Wilcke et al., 1996; Kabata-Pendias, 1993). Changes in soil environmental conditions over time, such as the degradation of the organic waste matrix, changes in pH, redox potential, or soil solution composition, due to various remediation schemes or to natural weathering processes, also may enhance metal mobility. Metals associated with the aqueous phase of soils are subject to movement with soil water, and may be transported through the vadose zone to groundwater. Metals, unlike the hazardous organics, cannot be degraded. However, some metals, such as Cr, can be transformed to other oxidation states in soil, reducing their mobility and toxicity (Kabata- Pendias, 1993; Bohn et al., 1985). In general, the mobility of metals from soil to plants is a function of the physical and chemical properties of soil and plant species, and it is altered by environmental and human factors (Wilcke et al., 1998; Cabrera et al., 1992; Basque et al., 1990; Haghiri, 1973). Adsorption processes are also affected by the form of the metal added to the soil and by the solvent introduced along with the metal. These interactions can either increase or decrease the movement of metals in soil water. Soils with heavier textures and higher pHs are effective in attenuating metals, while sandy soils and/or soils with low pH do not retain the metals effectively (McLean and Bledsoe, 1992). Korte et al. (1976), in their research, observed that Pb and Cu are the least mobile cationic metals whereas Cr is considered quite mobile. The principal soil surface that controls the mobility of metals in soils and natural water, according to Blume and Schwertmann (1969), and Jenne (1968), is Fe and Mn oxides. University of Ghana http://ugspace.ug.edu.gh 26 2.4.1 Effect of Clay content on Heavy Metal Accumulation in Soils Heavy metals tend to accumulate in the clay fraction of most soil cores (Lee et al., 1997; Boon and Soltanpour, 1992). Boon and Soltanpour (1992) conclude that the concentration of heavy metals in soil is sometimes dependent on clay content because clay-sized particles have a large number of ionic binding sites due to the larger surface area. The effect of clay mineralogy on heavy metals geochemistry has been shown by many studies (Andras et al., 2009; Onweremadu, 2008; Amusan and Adeniyi, 2005; Sipos and Némeh, 2001; DeMatos et al., 2001; Kabata-Pendias, 1993; McBride, 1991). In several studies, kaolinite was found to be a very good sorbent of the majority of the heavy metals considered in those studies (Gupta and Bhattacharyya, 2008; Wahba and Zaghloul, 2007). A sorption/desorption study of heavy metals on competing clays also showed that Cu, Pb and Zn were preferentially fixed on smectites and that Pb was also fixed on illite (Brigatti et al., 1996; Rybicka et al., 1995; Griffin and Shimp, 1978; Griffin and Au, 1977). A study by Matini et al. (2011) on the clay mineralogy responsible for the vertical distribution of Pb, Zn and Cu in the core of an abandoned treatment plant in Mfouati (south east of Congo-Brazzaville) revealed that Kaolinite (1:1 type clay minerals) was more present in all the soil cores in high amount than chlorite and smectite and could control the vertical migration of Pb, Zn and Cu. Clay particles are usually negatively charged. This is a very important factor influencing sorption properties of the soil. There are at least two major possibilities as to how these charges are formed (Loughnan, 1969). Firstly, the hydroxyl groups which exist on the edges and on the outer layers of minerals can dispose of hydrogen which is bonded with oxygen probably covalently, not very tight. This is a pH-dependent process and the ability to split the hydrogen atom decreases when pH decreases. When pH is above 6 hydrogen may easily be University of Ghana http://ugspace.ug.edu.gh 27 replaced by other ions like Ca2+, Al3+, Pb2+, Cd2+. The second process of creating negative charges is connected to the isomorphous ion replacement in the minerals. In the silica tetrahedral, Al3+ can replace the silicon ion Si4+ because these two have a similar ionic radius, whereas Mg2+ and Fe2+ can exist in the octahedral layers instead of Al3+. The negative charge, which appears as a result of isomorphous ion replacement, is not pH-independent and therefore quite persistent. The ability to create negative charges is highest for 2:1 type of clays (Brown, 1998; Dobrzański and Zawadzki, 1993). Clays thus usually act as adsorbents and play an important role in ion exchange reactions (Barrow, 1999; Brigatti et al., 1996). 2.4.2 Effect of Soil pH on Heavy Metal Accumulation in Soils The pH of soil affects several mechanisms of metal retention by soils according to several studies (Li and Wu, 1999; Peles et al., 1998; Chen et al., 1997; McLean and Bledsoe, 1992; Cataldo et al., 1981). In general, adsorption of cationic metals increases with increasing pH. A study by Harter (1983) of Pb, Ni, Zn, and Cu concluded that the retention of metals did not significantly increase until the pH was greater than 7. Li and Wu (1999) explained that as soil pH decreased, heavy metals were desorbed from organic and clay particles, entered the soil solution and became more mobile. However, when the pH was higher (pH > 7) heavy metals remained adsorbed, and those in solution precipitated out in the form of salts (Chen et al., 1997). Consequently, variability in pH affects the amount of heavy metal assimilated by plants. For example, John and VanLaerhoven (1972) showed that higher pH resulted in lower Cd uptake. Peles et al. (1998) concluded in his study that the addition of lime to contaminated soils (essentially increasing the pH) decreased the uptake of heavy metals by Ammbrosia trifida, while in unlimed soils, University of Ghana http://ugspace.ug.edu.gh 28 uptake increased – it accumulated 2.5 µgCd/g of tissue in limed soils in contrast to a 13.6 µgCd/g of tissue in unlimed soils. Johnson (1992) observed that soils became more acid when excess hydrogen (H) and aluminium (Al) ions replaced basic cations such as Ca, Mg, K, and Na on the surface of clays and soil humus. The basic cations were often leached below the root zone, leaving H and Al behind because they were more strongly attached to the negative charges on the soil surface. Conversely, any processes (such as liming, weathering and recycling of cations by deep- rooted plants which brought cations to the surface and incorporated them in the topsoil) that would encourage high levels of the exchangeable base forming cations (Ca, Mg, K, Na) would contribute toward an increase in alkalinity. His research again revealed that when the soil pH was too high, deficiencies of Fe, Mn and other micronutrients occurred. This observation is buttressed by a study by Gauch (1972) which noted that the concentration of Fe in a soil solution was markedly affected by pH, since pH values of 7 or higher drastically reduced the availability of Fe to plants because of the precipitation of Fe in the soil. McLean and Bledsoe (1992) attributed this pH dependence of adsorption reactions of cationic metals partly to the preferential adsorption of the hydrolyzed metal species in comparison to the free metal ion. The proportion of hydrolyzed metal species increases with increasing pH. 2.4.3 Effect of Organic Matter on Heavy Metal Accumulation in Soils Soil organic matter (SOM) is a term generally used to represent the organic constituents in soils including undecayed plant and animal tissues, their partial decomposition products, and soil biomass. Thus, this term includes: identifiable, high-molecular weight organic materials such as polysaccharides and proteins, simpler substances such as sugars, amino acids, and University of Ghana http://ugspace.ug.edu.gh 29 other small molecules and humic substances (Jobbagy and Jackson, 2000; Stevenson, 1992; Schulten et al., 1991). Principally, SOM is frequently said to consist of humic substances and nonhumic substances. Nonhumic substances are all those materials that can be placed in one of the categories of discrete compounds such as sugars, amino acids, fats and so on. Humic substances are the other, unidentifiable components. Soil Organic Matter may range in soils from 0.1% in desert soils to 90% in organic soils. Humic substances make up approximately 85-90% of the total organic carbon in soils (Giesking, 1975; Grim, 1968). Humic substances consist of a heterogeneous mixture of compounds for which no single structural formula will suffice. There is no strict chemical formula for these materials, though substantial evidence exists that humic materials consist of a skeleton of alkyl/aromatic units cross-linked mainly by oxygen and nitrogen groups with the major functional groups being carboxylic acid, phenolic and alcoholic hydroxyls, ketone and quinone groups (Schulten et al., 1991). Humic substances are traditionally defined according to their solubilities. Fulvic acids are those organic materials that are soluble in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values < 2. Humin is the fraction of natural organic material that is insoluble in water at all pH values (Grim, 1968). The existence of humic material in soils strongly influences sorption of chemicals (Stevenson, 1992). Humic and fulvic acids can exist in a dissociated form and thus are negatively charged. The main sources of these charges are carboxylic and phenolic groups in which hydrogen can be replaced by metal ions. This source of negative charges in soil colloids is strongly pH-dependent so the sorption of heavy metals in organic soils or in soils with relatively high organic content is mostly pH dependent. The Cation exchange capacity University of Ghana http://ugspace.ug.edu.gh 30 (CEC) is also very high for soil organic matter, especially for fulvic acids according to clay minerals (Stevenson, 1992). 2.4.4 Effect of Cation Exchange Capacity on Heavy Metal Accumulation in Soils Cation exchange capacity (CEC) is simply a measure of the quantity of negatively charged sites on soil surfaces that can retain positively charged ions (cations) such as Ca2+, Mg2+, Na+ and K+. It may range from 2.0 cmol/kg for sand to > 50 cmol/kg for some clay, and humus 100-300 cmol/kg under certain soil conditions. Thus, CEC is influenced by pH, clay and organic matter content. A study by Johnson (1992) concluded that the nutrient holding capacity of soil is largely determined by the cation exchange capacity (CEC). The larger the CEC number, the more cations the soil can hold as a result of the many negative charges available on the soil surface. Thus, soils with higher CEC values are more likely to attenuate cations than those with lower values. In view of this, soils rich in plant nutrients but with very high CEC may not support plant growth due to the almost unavailable or slow release of nutrients into soil solution for uptake by roots of plants. Conversely, soils rich in plant nutrients but with very low CEC values may have most of their nutrients in the soil solution available for uptake by plants but may be greatly at risk of heavy leaching. 2.5 DEPTH DISTRIBUTION OF METALS Depth distribution of metals in soil cores is indicative of weathering and soil genesis and anthropogenic pollution (Jin et al., 2005). Generally, the distribution of heavy metals in University of Ghana http://ugspace.ug.edu.gh 31 soil is influenced by the nature of parent materials, climatic conditions and their relative mobility depending on soil parameters such as mineralogy, texture, and classification of soil (Krishna and Govil, 2007; Filipinski and Grupe, 1990). Heavy metals are more strongly sorbed to Fe oxides (Brummer et al., 1986) and, thus, depending on the distribution of Fe oxides along the depths of tropical soils, the consequence would be a different depth distribution of pedo-/geogenic metals in soil cores of the tropical and the temperate zones, and even in soil cores within the tropics. Most soil cores have an A horizon, which is primarily topsoil composed of decaying organic matter such as leaves and grass, and a B horizon, which is composed of smaller clay-sized particles. In general, heavy metal concentrations are higher in the B horizons than in the A horizons (Lee et al., 1997). According to Boon and Soltanpour (1992) and Khan and Frankland (1983), due to the immobilization of heavy metals, there is little leaching through the soil core. Immobilization, however, can increase the Cd concentration of soil with a concomitant toxicity of the contaminated soil. 2.6 ANALYTICAL METHODS FOR SOIL ANALYSIS Elements in soils can be determined in the laboratory using the following fixed laboratory assays: Atomic Absorption Spectroscopy (AAS), Atomic Fluorescence Spectroscopy (AFS), Graphite Furnace Atomic Absorption Spectroscopy (GFAAS), Hydride Generation Atomic Absorption Spectroscopy (HGAAS), Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), X−ray fluorescence (XRF), Electron Microprobe (EM), Flame Photometer (FP) and Instrumental Neutron Activation Analysis (INAA). These instruments accurately measure elements in University of Ghana http://ugspace.ug.edu.gh 32 environmental sample to parts per billion (ppb) concentrations i.e. µg L-1 and µg kg-1 solid samples respectively (Melamed, 2005). The choice of a particular technique, however, depends on factors such as speed of analysis, availability of the instrument, technical expertise of the analyst or technician and the cost of analysis among others (Skoog et al., 1998). In this study, ICP-AES was used to determine the total and exchangeable concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn whereas, for the concentrations of exchangeable bases, AAS (for Ca and Mg) and FP (for Na and K) were used. Before any element is determined with any of these instruments, pre-treatment of sample with acidic extraction (acidic oxidation digestion) or with target reagents is required. The significance of pre-treatment is that all elemental species is converted into the inorganic form for easier detection and measurement. These laboratory assays measure elements accurately but they are expensive to operate and maintain. They are also bulky, requiring fully equipped and staffed laboratories to maintain and operate. Below is the list of the different instruments employed in this study and their operations: 2.6.1 Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) Inductively Coupled Plasma-Atomic Emission Spectrometry is a hyphenated analytical technique which measures characteristic emission spectra by optical spectrometry. Samples are nebulized and the resulting aerosol is transported to the plasma torch. Element-specific emission spectra are produced by radio-frequency inductively coupled plasma. The spectra are dispersed by a grating spectrometer, and the intensities of the emission lines are monitored by photosensitive devices (Jeffery et al., 1989). University of Ghana http://ugspace.ug.edu.gh 33 Background correction is required for trace element determination. Background is measured adjacent to analyte lines on samples during analysis. The position selected for the background-intensity measurement, on either or both sides of the analytical line, is determined by the complexity of the spectrum adjacent to the analyte line (Jeffery et al., 1989). Alternatively, users may choose multivariate calibration methods. In this case, point selections for background correction are superfluous since whole spectral regions are processed (Jeffery et al., 1989). 2.6.2 Atomic Absorption Spectroscopy (AAS) It is a technique in which the absorption of light by free gaseous atoms in a flame or furnace is used to measure the concentration of atoms. Atomic Absorption Spectroscopy is based on absorption of monochromatic light by a cloud of atoms of the analyte metal. In AAS, a liquid sample is aspirated into a nebulizer system. The sample then mixes with an oxidant gas which is drawn under pressure into a burner to form an aerosol. The flame which uses either air- acetylene or nitrous-oxide acetylene operates at a temperature of 2400 0C and 2800 0C respectively. Within the flame, the aerosol undergoes processes such as evaporation of the solvent and excitation of the gaseous metallic element. To determine the concentration of the analyte, a light beam from a lamp usually a Hollow Cathode Lamp (HCL) whose cathode is made of the element being determined is passed through the flame. A photomultiplier tube attached to the AAS can detect the amount of reduction of the light intensity due to absorption (absorbance) by the analyte. The absorption is proportional to the concentration of the metal ions following the Beer-Lambert Law (Skoog et al., 1998). University of Ghana http://ugspace.ug.edu.gh 34 2.6.3 Flame Photometry (FP) Flame Photometry is simply a modification of flame test. In the instrumental technique of flame photometry, a monochromator replaces the coloured glass filter, and a photocell detector/readout replaces our eye. Also, the burner design is more sophisticated in that the sample is continuously fed into the flame by aspiration. Since each element emits its own characteristic line spectrum, qualitative analysis can be performed here by observing what wavelengths are emitted and comparing these with various standards. However, since the detector is capable of measuring light intensity, qualitative analysis, as well as, quantitative analysis is possible: the intensity of the emitted light increases with concentration. In short, FP is an atomic technique which measures the wavelength and intensity of light emitted by atoms in a flame resulting from the drop from the excited state (formed due to absorption of energy from the flame) to lower states. Flame photometry, thus, uses flame atomic emission and a filter to quantify elements like Li, Na, K and Ca in liquid samples. No light source is required since the energy imparted to the atoms comes from the flame and, hence, FP is different from AAS. University of Ghana http://ugspace.ug.edu.gh 35 CHAPTER THREE 3.0 METHODOLOGY 3.1 INTRODUCTION Effective pest control and fertilizer application in cocoa production constitute the predominant factors that may lead to heavy metal pollution of cocoa growing soils (Vigneri, 2007). Agrochemicals’ application on cocoa farmlands as well as mining activities in and around cocoa communities in the Western Region of Ghana makes soils in this area susceptible to heavy metal contamination as many studies in areas under similar conditions have shown (Faβbender and Bornemisza, 1987). This chapter describes the fieldworks, procedures for sample collection, sample preparation, sample treatment and analysis carried out to ascertain whether long years of agrochemical application may have led to contamination of some selected cocoa growing soils in Western Region. 3.2 SELECTION OF STUDY AREA Though some overlap, there exist differences in the type of soils in the Western Region. In this study, four major soil types were considered. These are Ferric Acrisol, Dystric Fluvisol, Haplic Luvisol and Haplic Ferrasol, with the selection being based on predominance. In all, the study covered eight (8) major cocoa farming communities in the Western Region of Ghana (Fig. 3.1). The farming communities were selected based on purposive and stratified sampling to reflect the soils’geographical abundance in the Western Region, agrochemicals’ application, nearness to mining sites and volume of cocoa production. University of Ghana http://ugspace.ug.edu.gh 36 Figure 3.1: Map of Africa, Ghana and Western region indicating the sampling towns University of Ghana http://ugspace.ug.edu.gh 37 Figure 3.2: A cocoa farm at Asankragwa (Ferric Acrisol) Figure 3.3: A cocoa farm at Enchi (Dystric Fluvisol) University of Ghana http://ugspace.ug.edu.gh 38 3.3 PHYSIOGRAPHY OF STUDY AREA The Western Region of Ghana has over 75% of its vegetation within the high forest zone of Ghana. Agriculture thus, is the biggest industrial activity employing a large majority of workers in the region. The climate is characterized by moderate temperatures ranging from 22 oC at nightfall to 34 oC during the day. The region is the wettest part of Ghana with a double maxima rainfall pattern averaging 1600 mm per annum. The two rainfall peaks fall between May–July and September/October. In addition to the two major rainy seasons, the region also experiences intermittent minor rains all the year round. This high rainfall regime creates much moisture culminating in high relative humidity, ranging from 70 to 90% in most parts of the region. It is a major cocoa growing region and because of the high humidity, the crops are prone to fungal attack, particularly the Black Pod disease. A lot of fungicides are therefore used. The region is characterised by Tarkwaian and Birrimian rocks. These rocks are believed to have resulted from folding, faulting, metamorphosis, igneous activity, erosion and sedimentary process giving rise to the region’s gold belts that exist today (Brash, 1962). Most soils in the region developed over lower Birrimian phyllites and greywacke, such as Nzima- boi, which consists of yellowish brown silty clay loam, with few quartz gravels and stones overlying yellowish red, very gravelly and stony silty clay (Nzima series), while others developed over Tarkwaian sandstones, quartz and phyllites (such as the Juaso and Bompata series – Juaso series occur on summits and upper slopes. The profile consists of deep, dark reddish brown, sandy clay loam topsoil. This overlies dark red or red sandy clay loam having abundant ironstone concretion and quartz gravels, overlying decomposing rock, and Bompata series consist of deep, reddish brown sandy loam topsoil overlying very deep, medium sub- angular blocky, red clay; grading into red, firm clay with few ironstone concretions and University of Ghana http://ugspace.ug.edu.gh 39 quartz gravels) (Brammer, 1962). Others also developed over alluvial deposits as both small and large flowing water bodies are prevalent in the region. The predominant soil association is Nzima-Boi and the predominant soil type is the Acrisols. These soils have great agriculture value – they support all forms of arable crops such as cocoa, coffee, oil palm and cassava, and hence the region’s overwhelming success in cocoa production. 3.4. SOIL SAMPLING Soil samples were collected from the study area in October 2011. Samples were collected from the eight (8) cocoa farming communities with history of at least ten years of agrochemical application. Samples from Asankragwa and Bogoso were close to small scale mining sites but the rest were not. Additionally, soil samples were taken from three (3) natural forests (sites ABA, JUD and SEN) as pristine reference not directly affected by agrochemical inputs (Table 3.1). Each chosen farm was divided into grids using poles (sticks) on one side and some cocoa trees perpendicular to the poles. Labels of A1, A2, A3, B1 etc were used to identify the quadrants formed. The various quadrant names were written on pieces of papers, folded, thoroughly mixed and five (5) were picked (Carter, 1993). Soil samples were taken at increasing defined depths of 0-10, 10-30, 30-50, 50-80 and 80-100 cm at each quadrant. Five (5) soil core samples from each of the randomly selected quadrants at the respective aforementioned depths were taken with a pre-calibrated 1-metre auger. The core samples from each of the corresponding same depths of the five quadrants were bulked to form a composite sample. These culminated in five composite core samples per farm. Samples were then put into well labelled polypropylene zip-loc bags (Figs. 3.3 - 3.6). In order to distinguish the different depths of a site from each other, 1, 2, 3, 4 and 5 were used to indicate depths 0- University of Ghana http://ugspace.ug.edu.gh 40 10, 10-30, 30-50, 50-80 and 80-100 cm, respectively. However, at some sites, not all the depths were accessible to be sampled due to the presence of gravels and stones at deeper depths. A Global Positioning System (GPS) device was used to map out the sampling sites (Table 3.1). All samples were then transported to the Ecological laboratory of the University of Ghana, Accra for sample preparation. Table 3.1: Location and soil type of sampling sites Farm (code) Code Interpretation Location Latitude Longitude αSoil association αSoil type ABA* ACf Pristine Asankragwa 05o45.940’N 002o28.219’W Nzima – boi ACf ASA Asankragwa Asankragwa 05o45.960’N 002o28.236’W Nzima – boi ACf ASH Ashiem Ashiem 06o07.495’N 002o20.168’W Nzima – boi ACf BOG Bogoso Bogoso 05o36.032’N 002o03.557’W Juaso–bompata ACf BUA Buako Buako 06o22.631’N 002o33.182’W Sefwi FRh DEB Debiso Debiso 06o40.098’N 003o07.436’W Subin LVh ENC Enchi Enchi 05o48.364’N 002o49.483’W Alluvial FLd JUA Juabeso Juabeso 06o21.516’N 002o50.024’W Subin LVh JUD* LVh Pristine Debiso 06o40.277’N 002o50.423’W Subin LVh SAM Samreboi Samreboi 05o37.534’N 002o31.460’W Alluvial FLd SEN* FLd Pristine Enchi 05o48.228’N 002o49.650’W Alluvial FLd * = natural forest as pristine reference; ACf = Ferric Acrisol; FRh = Haplic Ferrasol; LVh = Haplic Luvisol; FLd = Dystric Fluvisol; α. sources: 1. Ahenkorah (1981); 2. Adu and Mensah-Ansah (1969). University of Ghana http://ugspace.ug.edu.gh 41 Figure 3.4: Taking soil samples from ASH (Ashiem, Ferric Acrisol) with an auger Figure 3.5: Taking soil samples from SAM(Samreboi, Dystric Fluvisol) with an auger University of Ghana http://ugspace.ug.edu.gh 42 Figure 3.6: Soil sample in a well labelled polypropylene zip-loc bag 3.5 SAMPLE PREPARATION The soils were air-dried at room temperature for three (3) weeks. They were then disaggregated using porcelain pestle and mortar, and sieved with a 2-mm nylon mesh to give the fine earth fraction. The fine earth fraction (< 2mm) was then used for the various analytical determinations. 3.5.1 Containers and Cleaning Process All glassware and high density polyethylene containers to be used in the analytical determinations were immersed in a warm liquid soap bath for two days. They were then rinsed with deionised-water (DI-water) and left immersed in 10% HNO3 at room temperature for three days. Flasks were again rinsed three times with DI-water and afterwards immersed University of Ghana http://ugspace.ug.edu.gh 43 in 50% HNO3 bath at 90 ºC for 24 hours. They were further rinsed with DI-water several times and placed overnight in a clean oven at 60 ºC, then removed from the oven and allowed to cool down. They were then double bagged in new polyethylene bags and stored under room temperature. 3.6 SOIL ANALYSES 3.6.1 Soil Particle Size Analysis Forty grams of the fine earth fraction of the soil was weighed into a plastic bottle and 100 mL of 5% calgon (sodium hexametaphosphate) solution was added. The content of the bottle was then shaken on a mechanical shaker for 2 hours after which it was transferred into a 1.0 litre measuring cylinder and topped up to the mark with distilled water. The suspension was then agitated with a plunger and five minutes thereafter, the density of the suspension (silt and clay) was taken using a hydrometer. The hydrometer reading of the suspension was taken again after eight hours (clay). The temperatures of the suspensions, T1 and T2, were respectively recorded during the 5 minute and 8 hour hydrometer readings. The contents of the cylinder after the eight hour reading were emptied onto a 47-μm sieve. The sand retained on the sieve was then washed off into a moisture can and dried at 105 oC for 24 hours, after which the dry weight of the sand was recorded (FAO, 1974; Day, 1965). Blank sample hydrometer readings at five minutes and eight hours were also taken for the 5% calgon solution topped up to 1.0 L. The particle size distribution was then determined using the formulae below. Temperature of the suspensions at T1 and T2 = 28 °C University of Ghana http://ugspace.ug.edu.gh 44 % Clay and Silt = 5 minute reading − correction for temperature oven dry mass of soil sample ×100% ........ Equation 3.1 % Clay = 8 hour reading − correction for temperature oven dry mass of soil sample ×100% ....................... Equation 3.2 % Silt = % (Clay and Silt) - % Clay ...................................................................... Equation 3.3 % Sand= oven dry weight of particles retained on the 47 μm sieve oven dry mass of soil sample ×100% ..... Equation 3.4 Temperature effect on density of the soil particles was accounted for using the relation provided by Day (1965): for every 1 °C increase in temperature, above 19.5 °C, there is an increase of 0.3 in the density of the particles in suspension. Hence, increase in weight = (T2 – T1) × 0.3 = (