Multivariate statistical analysis for fluoride occurrence in groundwater in the Northern region of Ghana

https://doi.org/10.1016/j.jconhyd.2012.08.002Get rights and content

Abstract

The presence of excess fluoride in groundwater in the Northern region of Ghana has resulted in the closure of many boreholes for drinking water supply to avoid the incidence of fluorosis and other related health effects. The fluoride concentration in 357 groundwater samples from the area ranged between 0.0 and 11.6 mg/L, with a mean value of 1.13 mg/L. Piper graphical classification, correlation coefficients, principal component analysis (PCA) and thermodynamic calculations were used as an approach to gain insight into the groundwater chemical composition and to help understand the dominant mechanisms influencing the occurrence of high fluoride waters. Spatial join procedure was used to examine the relationship between the underlying geology of the study area and fluoride distribution. Six groundwater types were identified for the area: Ca–Mg–HCO3, Ca–Mg–SO4, Na–Cl, Na–SO4, Na–HCO3 and mixed water type. PCA performed on the groundwater chemical data resulted in 4 principal components (PCs) explaining 72% of the data variance. The PCs represented the predominant processes controlling the groundwater chemistry in the study area which include; mineral dissolution reactions, ion exchange processes and evapotranspiration processes. PHREEQC calculations for saturation indices for the groundwater samples indicated they were largely saturated with respect to calcite and under-saturated with respect to fluorite, suggesting that dissolution of fluorite may be occurring in the areas where it is present. A review of the PCA results and an evaluation of the equilibrium state of the groundwater based on the saturation indices, suggest that some of the processes controlling the overall groundwater chemistry in the study area also influenced the fluoride enrichment. These predominant processes include the dissolution of the mineral fluorite, anion exchange processes (F/OH) involving clay minerals and evapotranspiration processes. Elevated fluoride levels in the study area were found to occur predominantly in the Saboba and Cheriponi districts and also in the Yendi, Nanumba North and South districts. These areas are underlain by the Middle Voltain formation (Obossom and Oti beds), comprising mainly of sandstone, limestone, conglomerate, shale, arkose and mudstone. Results of the hydrochemical analysis show that aside from the boreholes with elevated concentrations of fluoride (beyond 1.5 mg/L), groundwater in the study area based on the parameters analyzed is generally chemically acceptable and suitable for domestic use.

Highlights

► Groundwater from parts of Northern region of Ghana has excess fluoride which can cause fluorosis. ► The source and mechanism of fluoride enrichment has not been well studied. ► Principal component analysis, Person's correlation and thermodynamic calculations helped explain the enrichment mechanism. ► Fluorite dissolution, F/OH anion exchange and evapotranspiration processes were found as predominant mechanisms. ► The study results may help in planning strategies for safe drinking water for the population at risk.

Introduction

Over 90% of rural domestic water requirements in the Northern region of Ghana is being met from groundwater sources, due to its many advantages (CWSA-NR, 2007, Gyau-Boakye and Dapaah-Siakwan, 2000). The presence of fluoride in groundwater in parts of the region at levels above 1.5 mg/L, the WHO guideline value has however, resulted in the closure of many otherwise successful boreholes in order to avoid the incidence of fluorosis (CWSA-NR, 2007, WHO, 2008).

Fluoride is known to have both beneficial and detrimental effects to human health (Biswas et al., 2007, Fawell et al., 2006, Karro et al., 2006, WHO, 2008). Ingestion of low concentrations of fluoride (about 1 mg/L) in drinking water can prevent the incidence of dental caries. Intake of excess fluoride for long periods can, however, result in the incidence of dental and skeletal fluorosis as well as other adverse health effects including growth retardation, loss of mobility, changes in DNA structure, lowering of IQ of children and even death when doses reach very high levels (about 250 mg/L) (Apambire et al., 1997, Brunt et al., 2004, Fawell et al., 2006, Reddy et al., 2010, Shomar et al., 2004).

A limited fluorosis survey conducted in six selected communities in the eastern corridor of the Northern region of Ghana, where the presence of high fluoride in groundwater is most prominent, revealed the incidence of dental fluorosis (tooth mottling). The prevalence rate in some of the communities was found to be as high as 61%. The severe type of dental fluorosis based on Dean's fluorosis index classification criteria, which is characterized by brown stains, discrete or confluent pitting of the dental enamel was observed. The possible emergence of skeletal fluorosis (bone deformation and painful brittle joints in older people) was also observed in some of the affected communities (Ayugane, 2008, Mandinic et al., 2010).

The presence and distribution of elevated concentrations of fluoride in groundwater and the related health impacts in other parts of Ghana, particularly the Bongo and Bolgatanga districts in the Upper East region, have also been reported and well studied (Anongura, 1995, Apambire et al., 1997). The fluoride concentrations in the area were found to range between 0.11 and 4.60 mg/L. The fluoride enrichment was also found to be most probably associated with the dissolution of the mineral fluorite (CaF2), found in the Bongo granites in the area, and also from both the dissolution and anion exchange from micaceous minerals and their altered clay products.

Even though groundwater remain the most important source for rural water supply in the Northern region of Ghana, little is known about the natural and any anthropogenic factors that control the groundwater chemistry, and hence the groundwater quality and fluoride contamination. The aim of this paper is to study the groundwater chemistry in the Northern region with a focus on the occurrence and genesis of high fluoride waters in the eastern corridor of the region. Piper graphical classification, Pearson's correlation, principal component analysis (PCA) and thermodynamic calculations were used as an approach to gain an insight into the groundwater chemical composition and the dominant mechanisms influencing the occurrence of high fluoride waters. Spatial join procedure was used to examine the relationship between fluoride and the underlying geology of the study area.

Section snippets

Factors controlling fluoride enrichment in groundwater

Fluoride occurs in practically all natural groundwaters, in concentrations varying from trace to as high as 2800 mg/L in environments such as the Soda Lakes of the East African Rift System. The dominant factors that control the concentration of fluoride in natural groundwater include: the geological setting and types of rocks/minerals traversed by groundwater, solubility of fluorine-bearing minerals in the aquifer matrix and the amount of leachable fluorine they contain, anion exchange capacity

The study area

The eastern corridor of the Northern region of Ghana (the study area) is located between latitudes 8° 30″ and 10° 30″ N and longitudes 1° 0″ W and 1° 0″ E. It comprises eleven administrative districts (West Mamprusi, East Mamprusi, Bunkpurugu-Yunyo, Gushegu, Karaga, Saboba/Cheriponi, Zabzugu Tatale, Yendi, Nanumba North and South (Fig. 1)), and covers an area of about 27,900 km2 with a population of about 1,150,000.

Temperatures in the Northern region of Ghana are relatively high, ranging from a

Descriptive statistics

A univariate statistical analysis of the hydrochemical data of the groundwater samples collected from the area, which include the lower quartile (Q1), median (Q2), upper quartile (Q3) and the inter-quartile range (IQR) are presented in Table 1.

The concentration of fluoride in the groundwater samples ranged from 0.0 to 11.6 mg/L, with a mean value of 1.13 mg/L and a standard deviation of 1.24 mg/L. A relatively high percentage (23%) of the samples were found to have fluoride concentrations

Genesis of high and low fluoride groundwaters in the eastern corridor of Northern region

The presence of fluoride at concentrations up to 11.6 mg/L in the groundwater samples suggests that favorable conditions exist for the dissolution of fluorine‐bearing minerals that may be present in the study area, as well as other processes which results in fluoride enrichment. A review of the PCA results, the equilibrium state of the groundwater based on the saturation indices and the direct relationship between fluoride and calcium concentrations (Fig. 2) highlights the following with regard

Conclusions

Twenty three percent (23%) of 357 groundwater samples from the eastern corridor of the Northern region of Ghana for which the physico-chemical parameters were studied were found to have fluoride concentrations exceeding 1.5 mg/L, the WHO guideline for drinking water, with concentrations as high as 11.6 mg/L. Human consumption of water from these wells can result in the incidence of fluorosis. Fifty two percent (52%) of the groundwater samples were within the acceptable fluoride concentration

Acknowledgment

The authors of the paper are greatly indebted to the UNESCO—IHE UPaRF project for the financial sponsorship which made this study possible. The authors are also very grateful to the management and staff of the following organizations: (i) Community Water and Sanitation Agency, Tamale and Accra, Ghana, (ii) Water Research Institute, Tamale and Accra, Ghana, (iii) Church of Christ Rural Water Project, Yendi, Ghana, (iv) UNIHYDRO Ltd, Accra, Ghana and many other organizations and individuals who

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