|Year : 2016 | Volume
| Issue : 2 | Page : 56-65
Geochemical Background of Some Potentially Toxic and Essential Trace Elements in Soils at the Nadowli District of the Upper West Region of Ghana
Emmanuel Arhin, Saeed M Zango, Belinda S Berdie
Department of Earth and Environmental Sciences, Faculty of Applied Sciences, University for Development Studies, Navrongo, Ghana
|Date of Web Publication||29-Sep-2016|
Dr. Emmanuel Arhin
Department of Earth and Environmental Sciences, Faculty of Applied Sciences, University for Development Studies, P.O. Box 24, Navrongo
Source of Support: None, Conflict of Interest: None
Introduction: Use of universal baseline values, such as continental crustal averages, to assess health issues from trace elements in environmental soils may be fraught with challenges because the method only considers unmineralized rocks and soils in the determination of average crustal abundances or background values. Legislated guideline values are also for specific geographic locations in the environments. None of these take into account the human activities at a particular local community as the environmental conditions have dire influence on trace element mobility, concentrations, and storage in the surface soils. Aim: The aim of this article therefore is to evaluate site-specific geochemical background concentrations of some potentially toxic trace elements in the artisanal mine area and farmland soils of Nadowli District. Materials and Methods: The method involved collection of 29 samples of trace element from soils up to the depth of 20 cm. These samples were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS) analytical technique. Results: The results of the trace element concentrations were statistically and graphically analyzed to isolate sets of background values that are better suited locally to identify and assess areas contaminated and depleted by trace elements. Local background values of 15.00 ppm was estimated for arsenic (As), 0.02 ppm for cadmium (Cd), 0.01 ppm for mercury (Hg), 35.0 ppm for zinc (Zn), 20.0 ppm for copper (Cu), and 0.40 ppm for selenium (Se). The study found that estimated local backgrounds for essential elements were in the range of the legislated guideline values and should be used to assess the environmental quality and health as well as develop environmental policies for environmental monitoring. The potentially toxic elements contrastingly have higher local background values for As and Cd and lower local background for Hg when compared with the legislated soil guideline values. Conclusion: In conclusion, for cleanup goals in environmental legislation and for the assessment of the impacts of trace elements on health in Nadowli District, these background values should be used.
Keywords: Crustal averages, health, legislated guideline value, local background, nadowli, trace element
|How to cite this article:|
Arhin E, Zango SM, Berdie BS. Geochemical Background of Some Potentially Toxic and Essential Trace Elements in Soils at the Nadowli District of the Upper West Region of Ghana. J Earth Environ Health Sci 2016;2:56-65
|How to cite this URL:|
Arhin E, Zango SM, Berdie BS. Geochemical Background of Some Potentially Toxic and Essential Trace Elements in Soils at the Nadowli District of the Upper West Region of Ghana. J Earth Environ Health Sci [serial online] 2016 [cited 2020 Sep 27];2:56-65. Available from: http://www.ijeehs.org/text.asp?2016/2/2/56/191402
| Introduction|| |
The storage and cycling of trace elements naturally and anthropogenically released into the environment have serious impacts on environmental quality and public health in the Nadowli District as they eat what they grow and drink from surface and groundwater. The soil contains essential, nonessential, and potentially toxic elements (PTEs) that affect human health. Their distributions and concentrations depend on the nature of geogenic materials, environmental conditions, and human activities. Elevated concentrations and depletions of trace elements, especially the PTEs and essential elements (EEs), in any environment may have serious detrimental impact on agricultural products, in addition to the adverse health impact on the ecosystem as a whole. identified arsenic (As) and cadmium (Cd) contaminations in the environment when compared with continental crustal averages whereas copper (Cu), nickel (Ni), and zinc (Zn) required for human metabolism showed depletions in soils of Nadowli District. The primary contaminants As and Cd released into the soils of some parts of Nadowli District possibly emanated from the artisanal mining activities or the geogenic materials that can cause a number of potentially serious problems when they occur in high concentrations. Similarly, in China. found heavy metal pollutions in soils from mines. also identified associations of health risks with urban soils because of Cu, lead (Pb), Zn, Cd, As, mercury (Hg), chromium (Cr), and Ni excesses compared with some accepted global values. Some heavy metals were found to exceed crustal abundances in agricultural soils in South China., Designing environmental policies to clean environmental risk areas will be effective if local activities that stand a chance to introduce new elements in the surface soils are factored into the determination of elemental averages.
It is unclear whether health impacts from PTEs have been assessed and with what background values. Archival reports however indicate that little to no work has been done in computing site-specific or local backgrounds for the different trace elements. Several conclusions drawn on impacts of trace elements in environmental soils particularly in Ghana were generally compared to global accepted background values and average crustal abundance that may not reflect actual values in local soil background values because of environmental activities and soil-forming processes. Average crustal abundances or background concentrations are estimates from global or regional averages and termed continental crustal averages., Environmental policies developed for environmental cleaning of environmental risk areas on the bases of the average crustal abundances, the statistical models, and national Environmental Protection Agency guideline or safe line values adopted from World Health Organization values may be fraught with some uncertainties and hence may not clean the environments properly. Attribute of the uncertainties are the result of physical, chemical, and soil-forming processes coupled with different anthropogenic activities at the different geographical locations despite similar lithologies. Background values of same geology will vary at developed and industrial areas in certain types of trace elements from a similar area in a developing area. Similarly, geochemistry of trace elements in a mine area will differ in concentration and distribution at agricultural land areas. It is possible to overly indicate strong pollution of a particular trace element in an area because of incorrect use of background values used as benchmarks in assessing enrichment and depletion of trace elements in soils with adverse effects on human life. Use of techniques that compare trace element concentrations with threshold and maximum acceptable concentration guidelines, provided in environmental protection legislation, may have associated difficulties because of differences in environmental conditions.
Healthcare is expensive and difficult to access in most areas in developing countries. The rural population especially in Ghana eat what they grow; the source of potable drinking water are from boreholes but majority of the rural folks drink from wells and streams. Preventing environmental-related diseases will not effectively work with accepted global background trace element values but rather will need local background values that factor variability of environmental activities as well as physical and chemical processes because of differences in climate. In spite of the limitations associated to the use of the accepted global values, their use has been able to provide some environmental guidelines that indicate severity of some trace elements contamination, but this is not free with some challenges. The weakness in this approach only considered the geochemical outliers in determining the backgrounds whereas it failed to factor human activities in the local area and the soil-forming processes that may influence migrations of elements in the secondary oxidize zones. Reimann et al. argued that graphical inspection using statistical and geographical displays to isolate sets of background data is far better suited for estimating the range of background variation and thresholds, action levels (e.g., maximum admissible concentrations values), or cleanup goals in environmental legislation.
Incidentally, background values used in most developing countries to establish environmental risk areas use crustal averages and safe line values for soils and sediments that disregard local activities and soil-forming processes in the geographical locations. Use of the global averages and guideline values are therefore justified in this context, if only they are to provide a basic indication of metal ions in soils or sediment. The distributions and concentrations of elements may vary depending on the environmental conditions and geogenic and anthropogenic activities (e.g., the mining operations and fertilizer-prone agricultural practice areas). Identifying the environmental risk areas in terms of PTE enrichments and EEs depletions localities is vital in areas where mining and agricultural practices can introduce new elements and also enhance element mobilization. Knowledge about the locations of the polluted and depleted areas in terms of toxic and EEs and the ability to define geochemical outliers or anomalies in surface soils sampled and analyzed for their trace elements contents could contribute to the reduction of environmental health issues and will allow an indication of sites with the highest environmental and health hazard to be demarcated. This article therefore seeks to evaluate site-specific geochemical background concentrations of some potentially toxic trace elements in the artisanal mine area and farmland soils of Nadowli District.
| Subjects and Methods|| |
Location and geology
The portion of the Lawra Birimian Belt studied [Figure 1] is in northern Ghana, 700 km northwest of Accra, the national capital. The rocks here comprise metavolcanic rocks intruded by mafic granitoids and dolerites intruded by gabbro., In close contact with the metavolcanic rocks are metasedimentary units that consist of phyllite, sericite-schist, and metagraywacke that are locally intruded by felsic granitoids and mafic dykes.,
|Figure 1: Geology and location of the study area (modified from Ghana Geological Survey regional geology map, 2010)|
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The mafic granitoids comprise hornblende-rich varieties and are classified as ‘Dixcove’ or ‘belt’-type granitoids whereas the felsic granitoids contain mica-rich varieties and are known as “basin”-type granitoids. The belt-type granitoids are small discordant to semidiscordant, late or post-tectonic soda-rich hornblende-biotite granites, or granodiorites that grade into quartz diorite and hornblende diorite. On the contrary, the basin granitoids are large concordant and syntectonic batholithic granitoids commonly banded exhibiting black-and-white foliations. They are potash-rich and contain both biotite and muscovite, with the biotite dominating.
Regolith landform of the area is characterized by deep weathering profile with preserved preexisting and erosional surfaces. In association with these regolith landforms are widespread lateritization that has a surface veneer of pisoliths and depositional cover of exotic origin. Regolith materials at lowland terrains are sheetwash deposits moved downslope during the flash floods. These regolith units originate from redistributed sediments that differ from the weathered materials of the underlying rocks. The regolith units cover at the upland areas contain degraded weathered rocks and materials that decrease in fragment size downslope.
The topography is generally low, undulating with isolated hills at some places. Some of the hills are capped by hardpans with the slopes marked by scree, consisting of small fragments of visibly mineralized and altered rock that decrease in fragment size downslope.
The climate of the area is Guinea Savannah with an annual rainfall range of 600–1200 mm. Short, single rainy season with long period of dryness characterizes the area. Total monthly rainfall slowly increases from March and peaks in August after which there is a sharp decrease of rain after October. The average monthly rainfall and temperature estimates are 986 mm and 28.6°C, respectively.
History of artisanal mining at the study area
Records indicate that artisanal mining was practiced as early as the fourth century and the indigenous population of Ghana got more involved when the Europeans arrived in 1471., Extraction of gold was done by amalgamation technique that uses Hg to obtain gold from the auriferous gravels and veins. This practice is still in use.,, The source materials on which the artisanal miners work for gold come from alluvial and colluvium deposits along rivers/streams and transported talus accumulations at base of hills, auriferous-exposed quartz veins, and terrestrial soils.
At the study area, gold is inexpertly mined mainly from auriferous quartz veins and alluvial deposits along rivers, streams, and terrestrial soils [Figure 2]. The mining principles do not follow any professional mining methods. The artisanal miners only follow the supposedly mineralized quartz veins or auriferous gravels [Figure 3] with no assessment of the gold (Au) grades, manually excavating the expected quartz vein or gravel along the waterways. These are messily done that degrade the environment and also enhance element mobility. The apparent excavated gold-bearing ore is then processed by crushing and grinding as the initial preparation to extract the gold contents. The crushed and ground gold-bearing ore undergo separation process using “sluice” gravimetric method [Figure 4]. The gold is extracted from the concentrate by adding Hg to form gold amalgam that is normally roasted in open air to obtain “raw gold” [Figure 5]. The inept mining and extraction processes influence the concentrations and distributions of the trace elements.
|Figure 2: Unprofessional mining methods of artisanal workers degrading the environment|
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|Figure 3: Environmental degradation as a result of quartz vein and auriferous gravel mining by artisanal miners|
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|Figure 5: Use of mercury (Hg) during gold extraction from auriferous concentrates|
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Sampling and analysis
The study involved field and laboratory methods of which laboratory chemical analysis of sample results were treated using multivariate and cumulative frequency curves methods. The fieldwork involved collection of 29 soil samples at a depth of 20 cm (because landscape evolution, climate change, and human interactions may influence the geochemistry) along 200 m × 100 m grid system. The collected samples were taken from lands known to host gold mineralization and are characterized by ancient illicit mining operation workings and the area that are cultivated lands with farm houses. Sample weight of 1 kg was collected from each 20-cm-diameter hole dug up to 20 cm depth. Sampling information such as soil type, lithology of sampling environment, and possible weathering and geomorphic histories were recorded to aid in the establishment of environmental impacts on possible natural and anthropogenic contaminations.
Sample preparation and laboratory chemical analysis
The collected field samples were sun dried and later sieved to <2-mm particle size fractions. These sieved samples were then vaporized using laser cells. The vaporized samples were introduced to ME-MS 41 instrument manufactured by ALS that uses ICP-MS method for ultra-level trace elements in samples. The introductions of vaporized samples were done through a peristaltic pump, nebulizer, and spray chamber. There is a torch in the ICP-MS instrument that generates plasma that serves as the ion source that converts the atoms to be analyzed to individual ions in the samples. The sample ions are then detected after passing over the mass filter wherein the individual sample ions are either detected by direct current measurements on the ion collector or the ions generate secondary electrons that are propagated in the multiplier. The concentrations of trace elements in the samples are then measured.
Quality control and quality assurance of analytical data
Quality control-certified reference materials (CRMs) to monitor accuracy were inserted in the batch of samples to ALS-Chemex in Kumasi, Ghana. The CRM (GBM 398-4, GBM 900-10, and GBM 901-5) samples used were sourced from Geostats Pty Ltd., O’Connor, Australia. The precision of the analytical data was also evaluated from the field duplicate sample results. The average trace elements recovery from the CRM was in the range of 85–96% and percentage precision was 9.3. The combined outcomes of CRM and field duplicate samples were considered acceptable.
| Results and Discussions|| |
The adequacy of samples collected and analyzed to estimate the background values particularly for As, Hg, and Cd were tested using Kaiser–Meyer–Olkin (KMO) measure of sampling adequacy and Bartlett’s test. The results are shown in [Table 1]. According to Field, the value of KMO should be greater than 0.5 before samples for a study can be considered adequate. The test performed on the 29 samples of the trace element, produced a KMO value of 0.55. This value is greater than 0.5 and thus makes the 29 samples acceptable for the study. Summary statistics showing the basic statistics that allow comparison of means (μ) of trace elements with the continental crustal averages or abundances (Bn) is presented in [Table 2]. The trace element concentrations appear mixed; some show enrichments and others depletion when measured averages are compared with the continental crustal averages for soil. The minimum As concentration measured in soils is 6.4 ppm that is over 250% in excess of the crustal average. The mean average of As is about 25.0 ppm [Table 2] that suggest moderate-to-strong and even extreme pollution at some places if the maximum concentration of 101.0 ppm is considered. The mean for Hg is 0.02 that is lower than the average crustal value. On the whole, this is satisfactory for Hg storage and concentration in the area but there are some localities with extremely high concentrations of about 0.17 ppm. Identifying these areas for soil cleanup and remediation processes may be essential. Pb and Cd concentrations in soil samples are low (i.e., 8.17 ppm for Pb and 0.02 ppm for Cd) with values below the crustal averages of 14.00 ppm for Pb and 0.15 ppm for Cd. The EEs Zn and Cu appear depleted in soils. As seen in [Table 2], the measured means of Zn and Cu are 27.20 and 28.04 ppm as against 70.00 and 60 ppm established for continental crustal values. The deficiency of the EEs may not be a problem because the routes of exposure to the crops determine their development. If the crops take up sufficient concentrations of these elements then the low concentrations in the soil may not pose a problem. In [Table 2], the selenium (Se) average is 0.51 as against the continental crustal average of 0.05 ppm and has a maximum value of 1.2 ppm. This suggests its prevalence in the District that requires some investigations to assess the crops uptake to avoid crop development problems.
|Table 1: Measure of sample adequacy using Kaiser–Meyer–Olkin (KMO) and Bartlett’s test on collected samples of trace element|
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|Table 2: Summary statistics of trace elements in soil samples at parts of Nadowli District|
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The bivariate analysis between two variables for the purpose of determining the empirical relationship between the trace elements is shown in [Table 3]. As, Cd, Zn, and Cu show moderate-to-strong correlations with Hg and Se. A weak-to-moderate correlation exists between Pb and molybdenum (Mo). Common in the area are granitic intrusions occurring in the volcanic and metasedimentary lithologies that are enriched in Mo. The moderate correlation between Pb and Mo presumes the existence of Pb in the area, which can be detrimental to human health depending on exposure and concentrations. The low concentrations of Pb in analyzed samples does not imply their absence in the area. They are either concealed by iron (Fe)-oxyhydroxides/oxides and clay minerals and could be undetectable by the analytical method used. Its moderate correlation with Mo makes Mo a good pathfinder element to define possible localities with Pb-related health problems. From [Table 2], some trace elements show depletion in soils compared with the continental crustal averages of world soils. However, the positive correlations of some elements presented in [Table 3] provide useful guide to locate their occurrence. The occurrence and concentrations of trace elements in soils depend on chemical, physical, and environmental conditions.
|Table 3: Correlation coefficient between trace elements in soils at Nadowli District|
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Results showing the multivariate statistical analysis that examines relationships among multiple variables at the same time is presented in [Table 4]. Rigorous statistical analysis grouped the interrelations of the elements into three components that may be results of natural and human activities. Also presented in [Table 5] is the total variance explained based on trace elements loadings into components. This explains the initial trace elements extracted from the samples and are displayed based on element loadings in terms of components. The display of trace element distributions in soil samples at the study area compared with continental crustal abundances (Bn) and few accepted legislated guideline values are shown in [Figure 6]. From [Table 4], the first component that accounts for about 30% of total variance [Table 5] contains Cu, Se, Zn, cobalt (Co), and Cr, with some influence from Mo, As, and Hg. The geochemical association of elements in the first component may be a result of weathering-controlled fractionation of underlying rocks based on their continental crustal abundances, and chemical and environmental afflicted processes. The second component is the As factor with strong association with Hg and Cd. This component also accounts for 23% of the total variance and may represent the artisanal mine areas. The third component represents 17% of the total variance explained consisting of Mo, Hg, and Pb. The three components accounted for 70% of total variance explained, of trace elements released to surface soils from the underlying rocks and elements introduced to the environment via human activities. Chemicals such as Hg used in gold amalgamation by the artisanal mine workers, fertilizers and herbicides used by farmers, and pesticides used at homes against mosquitoes introduce some trace elements into the environment. The concentrations and distributions of trace elements introduced to the environment add to the naturally released elements from the underlying rocks. The nature of the regolith, the aquifer characteristics, and the degree of human activities may result in enrichments and depletions of the trace elements. It is understandable that levels of trace element can vary from location to location, because of the differences in soil types formed from variable parent materials and differences in environmental conditions. This means that the use of continental crustal averages for soils should differ for different geographical locations because of variations in compositional melts. These accepted global averages may not be able to draw any precise and concise conclusions on environmental health issues but are able to scientifically indicate the degree of contamination.
|Table 4: Component matrix analysis showing different element associations|
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|Figure 6: Box plots displaying distributions of potentially toxic elements (PTEs) and essential elements in soils at Nadowli District|
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However, correct monitoring of trace element concentrations in soils will be most useful with the establishments of local background values to maintain environmental quality while providing basic data necessary for contamination management policies. As seen in [Figure 6], the legislated accepted guidelines for soils are lower than the continental crustal averages for Hg and Cu. This suggests some form of oversimplification in the determination of the crustal abundances in dealing with environmental samples because it precludes additions of trace elements from anthropogenic activities. For some trace elements, the legislated guideline values coincide with the lower quartile (e.g., As and Zn). For others (e.g., Hg, Cu, and Se), it is within or above the interquartile range. The inconsistencies in baseline values between the legislated guideline values and universal crustal averages may be the differences in lithological materials and geochemical processes as well as the environmental conditions and human activities at different geographical location. The estimations of background values for selected PTEs (As, Cd, and Hg) and some EEs (Cu, Se, and Zn) at the study area are also presented in [Figure 7] and [Figure 8], respectively.
|Figure 7: Background estimations of As, Cd, and Hg in soils at the study area|
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|Figure 8: Background estimations of Zn, Se, and Cu in soils at the study area|
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The local background values estimated for some trace elements from the cumulative frequency plots [Figure 7] and [Figure 8] are As (15 ppm), Cd (0.02 ppm), Hg (0.01 ppm), Cu (20 ppm), Se (0.4 ppm), and Zn (35 ppm). The several points of inflexions shown on the ogive of the cumulative curve for PTEs and EEs indicate the different actions that controlled the weathering and fractionation of elements during their migration to store and concentrate in the surface environment. The released trace element concentrations and distributions will be influenced by the geology, chemical and physical processes, and prevailing human activities. The dissimilar human activities from one location to another globally make the use of universal baseline or background values inappropriate typically for trace elements and health assessment. Local background values compared with some legislated agricultural soils and continental crustal averages is presented in [Figure 9].
|Figure 9: Comparison of local background, universal legislated agricultural soils, and continental crustal averages|
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From [Figure 9], there is a mixed representation of local soil background values compared with the legislated soil values for agricultural lands. The estimated local background values in soils for the EEs, Cu and Zn, seem to be close or fall in the legislated accepted values in agricultural lands. As seen in [Figure 9], local background for Cu is 9% off the legislated value whereas local background for Zn falls into the accepted guideline range of 3–79 ppm. The Se legislated guideline value in agricultural soil is 0.3 approximately and that is 33% lower off the estimated local background value of 0.4 ppm. For agricultural purposes, <0.5 is good for crop production; hence, the local background value of 0.4 ppm estimated is appropriate as Nadowli is an agrarian community. The PTEs, As and Cd, have local background values higher than the accepted guideline values in soils. This suggests that the entire area is contaminated with these elements. Using the legislated guideline value for As for this area, the entire area will be marked out to be unsafe for human settlement. However, this might not present any health issues as the impact of As will depend on exposure and pathways. In addition, the estimated local background is in the range of the accepted guideline value of 10.38 ppm that has been used for agricultural activities without any health issues. This significantly explains the usefulness of the local background values over the use of global accepted averages. Environmental policy to stop all farm activities in this area is appropriate if continental crustal average of world soil for As is employed. Similarly, the continental Hg crustal average (0.085 ppm) in soil is different from legislated Hg content in agricultural soils (0.04 ppm) and this is higher than the calculated background value (0.01 ppm). The local background value is lower than the legislated value in soils. So some potential areas may not be identified as high Hg-risk zones if the accepted legislated guideline value of 0.04 ppm is used for agricultural soils. Crop uptake of Hg in soils over long periods and its exposure to humans may present some health problems. Therefore, maintaining and assessing environmental soil quality and health require the establishment of local background values that incorporate local geology and environmental conditions to provide basic data necessary for contamination management policies.
| Conclusions|| |
There are strong correlations among PTEs, As, Cd, and Hg, and EEs, Zn, Cu, and Se. The storage and concentrations of these trace elements depended on the underlying geology, chemical and physical processes, as well as human activities in the area. There was close association in background values between the EEs when the legislated soil and the local estimated background values in measured soil samples were compared. The estimated local backgrounds for the EEs were in the range or close to the legislated guideline values and can be used to assess the environmental quality and health as well as develop environmental policies on their applications for environmental monitoring.
The PTEs contrastingly have higher local background values for As and Cd and lower local background for Hg when values were compared with the legislated soil guideline values. Use of the universal legislated background values without considerations particularly to human activities will yield results that may not be representative to the area and could lead to incidence of some public health cases unattended to. Low universal guideline values may present inappropriate contamination policies that will bring fear and panic. The minimum As value is 6.40 ppm that is greater than the average crustal value of 1.80 ppm. This shows enrichment. The local background for Hg is 0.01 ppm. This is lower than the legislated guideline value but equal to the continental crustal average. Some areas still have high Hg levels up to 0.17 ppm. Therefore, to avoid the inconsistencies in the use of universal legislated guideline values and the continental crustal abundances, the study estimated the local background values for As, Cd, Hg, Zn, Cu, and Se to be 15.00, 0.02, 0.01, 20.00, 0.40, and 35.00 ppm, respectively. The authors recommend the use of these local background values for environmental health assessment and suggest further work at the high-level PTEs environment to identify the causal agents for the high storage and concentration levels.
The authors wish to thank the International Medical Geology Association (IMGA) for their moral support and West African Exploration Initiatives whose assistance during their quest of knowledge contributed financially in gathering and analyzing the data. They are also grateful to IMGA-Ghana Chapter for their in-kind support. Finally, to those whose names are not mentioned but contributed one way or the other, they say a big thank you.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]