Consumers say it is the most expensive and thus the safest. However, this is not entirely true owing to the fact that groundwater, which is the major source of bottled drinking water, is not without contamination. The possible contamination threats could be attributed to natural or anthropogenic sources (Khatiwada et al., 2002; Ritter et al., 2002). The natural sources of contamination are contingent on the host rock interaction with the groundwater. This host rock interaction poses the proclivity for elevated levels of trace elements in groundwater, which subsequently affects the quality of the bottled drinking water.  Anthropogenic sources may emanate from industrial activities such as mining, agriculture inter alia. These activities involve the usage of chemicals in fertilizers on farms and chemicals in the extraction of precious minerals. Denuding agents such as rainfall could carry these chemicals which eventually percolate down to the groundwater, thereby increasing the levels of trace elements.

The elevated levels of trace elements in bottled drinking water can cause health related issues. For example, research has shown that continuous intake of lead (Pb) and cadmium (Cd) in drinking water may lead to liver, kidney, cardiovascular, reproductive, nervous system disorders and eventually death (Krajnc, 1987; Navas et al., 2007). Cleveland et al. (2008) further posited that prolonged intake of Pb in drinking water can lead to retarded growth, birth defects and brain damage in infants and foetuses. Deficiency in copper (Cu) can result in a number of diseases such as cold and flu, loss of skin tone, reproductive problems and fatigue (Jordana and Batista, 2004). High levels of arsenic (As) in drinking water can cause arsenical skin lesions, cancers and death (Das et al., 1996).

In Ghana, most bottled drinking water are sourced and bottled in Accra where high levels of trace elements such as have been reported in groundwater (Kortatsi et al., 2008). Therefore, researches related to elevated levels of trace elements in bottled drinking water are of enormous importance due to the concomitant health risks. In this study, we present As, Cd, Cu, Pb and Zn results from 10 selected bottled drinking water which are largely perceived to be of high quality by the Ghanaian populace.

Study area

The study is conducted in the Greater Accra region. All the water samples were sourced and bottled in the region. Accra is located in the south-eastern part of Ghana bordered by Central Region on the west, Volta Region on the east, Eastern Region on the north and the Gulf of Guinea on the south (Figure 1). It covers about 3,245 km² representing 1.4% of the total land area of Ghana. The population of the region was approximately 4,010,054 in 2010, accounting for 16.1% of Ghana’s total population (Ghana Statistical Services, 2012).

The area is predominantly underlain by the Pan African rocks composed of Precambrian Dahomeyan schists, granodiorites, granite gneisses and amphibolites to Late Precambrian Togo series comprising mainly of quartzite, phyllites, phylitones and quartz breccias (Mani, 1978; Kesse, 1985). Other formations observed are the Paleozoic Accraian sediment-sandstones, shales and interbedded sandstone-shales (Figure 1).

Sample collection and laboratory analysis

10 selected water samples (500-1500 ml) were bought from various shops in Accra and Tamale metropolis of Ghana in May, 2012. These water samples were iced and transported on the same day to the Chemistry laboratory of the University for Development Studies (UDS - Navrongo Campus) for laboratory analyses. The trace element analyses for Cd, Cu, Pb and Zn were conducted at the Water Quality Laboratory, Water Research Institute in Tamale using Atomic Absorption Spectrometer. Arsenic (As) however was analysed using the Hach EZ Arsenic High Range Test Kit.

Data analysis

Standard deviations, maximum and minimum values and mean were computed using Matlab software. Aquachem software was then used for graphical analysis.

The results of the trace elements (in mg/l) in all water samples are presented in Table 1.  For BonAqua Premium drinking water, the trace element values were observed as; 0 (As), <0.002 (Cd), 0.067 (Cu), <0.005 (Pb) and <0.005 (Zn). AquaSplash purified non-carbonated drinking water recorded values ranging from below detection level (BDL) for (As), <0.002 (Cd), 0.075 (Cu), <0.005 (Pb) to <0.005 (Zn). Values observed in Safina natural mineral water were BDL for (As), <0.002 (Cd), 0.0674 (Cu), <0.005 (Pb) to <0.005 (Zn). Bel-Aqua natural mineral water also recorded similar values; BDL for (As), <0.002 (Cd), 0.0731 (Cu), <0.005 (Pb) to <0.005 (Zn). Results for EcoSpa natural mineral water are BDL for (As), <0.002 (Cd), 0.0924 (Cu), <0.005 (Pb) and <0.005 (Zn). US drinking water yielded; BDL for (As), <0.002 (Cd), 0.0888 (Cu), <0.005 (Pb) and <0.005 (Zn). Special Ice natural mineral water had values varied from BDL for (As), <0.002 (Cd), 0.0593 (Cu), <0.005 (Pb) to <0.005 (Zn). Everpure purified drinking water recorded BDL for (As), <0.002 (Cd), 0.0843 (Cu), <0.005 (Pb) and <0.005 (Zn)). Privada natural mineral water gave BDL for (As), <0.002 (Cd), 0.0954 (Cu), <0.005 (Pb) and <0.005 (Zn). The other bottled drinking water, Voltic natural mineral water yielded values from BDL for (As), <0.002 (Cd), 0.0848 (Cu), <0.005 (Pb) to <0.005 (Zn). According to   the   World   Health   Organisation   (WHO)  guideline, standards for As, Cd, Cu, Pb and Zn are 0.01, 0.003, 2, 0.01 and 3 respectively.

The statistical results of the trace element analyses conducted on the 10 selected  popular  brands  of  bottled drinking water are presented in Table 2.

From the Table 2, As was below detection limit, whereas Cd was <0.002 mg/l. The detection limit for Cu, Pb  and  Zn   was  <0.005   mg/l.  In   all  samples,  mean,median and standard deviation, minimum and maximum levels for As, Cd, Pb and Zn were below detection limit. However, for Cu, the minimum and maximum values were 0.0593 (mg/l) and 0.0954 (mg/l) respectively. Additionally, the mean, median and standard deviation for Cu were 0.0788 (mg/l), 0.079 (mg/l) and 0.0121 (mg/l) respectively.

All the trace elements analysed in the selected bottled drinking water fell below the guidelines stipulated by the WHO (Table 1). This suggests that the trace elements As, Cd, Cu, Pb and Zn will not pose any health-related problems to Ghanaian bottled water consumers.

Nonetheless, Cu has elevated values compared to the other trace elements in all the water samples (Figure 2). Privada natural mineral water recorded the highest Cu concentration (0.0954 mg/l) closely followed by EcoSpa natural mineral water (0.0924 mg/l) and US drinking water (0.0888 mg/l) as illustrated in Table 1 and Figure 2. Although the Cu content of all analysed water samples were within the WHO guidelines, the basis of the guideline value was to give a threshold to prevent acute gastrointestinal effects of copper in vulnerable members of the population and provide an adequate margin of safety in populations with normal copper homeostasis (WHO, 2011). Additionally, ingestion of Cu in higher doses can cause neurological complications, hyper-tension, liver and kidney dysfunctions (Krishna and Govil, 2004; Khan et al., 2010). Bottled drinking water in Ghana contain higher copper concentration (median value of 0.0797 mg/l) when compared with European bottled water with a median value of 0.000057 mg/l (Misund et al., 1999). Cu usually occurs in areas that are stained with iron oxide but could also occur in areas with little iron oxide contents (Langendonck et al., 2013). The outcrops in these areas are usually highly silicified and kaolinized (Lindgren et al., 1910). Considering the geological setting of Accra which is mainly composed of low grade metamorphic rocks and silicified sedimentary rocks of the Dahomeyan formation with little iron oxide (Nyarku et al., 2011), it is possible the  rocks  may  have  played  a  vital role in the elevated level of Cu in the groundwater. Another important source of Cu in Accra could be due to the industrialized nature of the city compared to other cities in Ghana. Most plumbing systems in the city use Cu pipes which in time can corrode when in contact with water. The corroded Cu may later percolate down to groundwater through fractures, fissures and faults thereby increasing the natural Cu content of the groundwater. This, when tapped for drinking and other domestic purposes, can pose serious health effects.

Arsenic (As) is one of the key elements of greatest health concern in many natural waters causing large scale health effects (Smedley and Kinniburgh, 2002). Geologic materials largely composed of As are abundant in the earth’s crust and are usually present in natural waters less than 0.001-0.002 mg/l (Das et al., 1996; WHO, 2011). In waters, particularly groundwater, where there are sulfide mineral deposits or sedimentary deposits deriving from volcanic rocks, the concentrations can be quite elevated (Mandal and Suzuki, 2002; Nriagu et al., 2007). However, in our study area, there are no volcanic rocks and this peradventure accounts for the very low levels of As in all the selected bottled drinking waters. It is also released during mining and spread through the environment (Das et al., 1996). There is little mining activities in Accra and consequently these mining activities have little or no effect on the groundwater. The WHO and Ghana Standards Board (GSB) recommend a limit of 0.01mg/l and 0.05mg/l of arsenic in drinking water, respectively. In this study, As in all samples is found to be below detection limit and is within the WHO and GSB guidelines or recommended limits (Table 2).

Naturally, a very large amount of Cadmium (Cd) is released into ground waters and rivers through weathering of volcanic rocks (Hem, 1972; Pinot et al., 2000). Hiatt and Huff (1975), suggested that if the under-lying rocks in a particular catchment are predominantly sedimentary either than igneous and metamorphic, then there is the tendency for Cd enrichment in the groundwater. Fertilizers produced from phosphate ores also constitute a major source of diffuse cadmium pollution of soil and groundwater (Loganathan et al., 2003; Pan et al., 2010; Lenntech, 2012; Tiwari et al., 2013).  The  World  Health  Organization  and  the Ghana Standards Board recommend a limit of 0.003 mg/l of Cadmium in drinking water. In all the analysed water samples, the Cd levels are <0.002 mg/l. In Accra, agricultural activities are limited compared to other cities and where they are, stringent regulations are put in place regarding the use of fertilizers during cropping (Boateng, 2002). Therefore, the low levels of Cd in bottled drinking water in Accra could be due to the absence of volcanic rocks, narrow contact of sedimentary outcrops and limited agricultural activities.

Pb and Zn were also found to be below detection limit (Table 2). Pb is generally released from sulphide-bearing ores during mining activities (Reimanne and Decarital, 1998). The apparently low values detected for these two elements especially Pb culminates from the fact that there are little mining activities in the Dahomeyan formation exposed in Accra. Therefore, the groundwater still preserves its natural content for these elements with little or no contamination. Zn however, is not of health concern at levels found in drinking water; the World Health Organization has a guideline of 5mg/l for zinc in water based on taste. Water containing zinc at concentrations  in   excess   of  3–5  mg/l    may   appear opalescent and develop a greasy film on boiling, it also imparts an undesirable astringent taste to water (WHO, 2011). All the selected bottled drinking water are generally within the standards postulated by the WHO.

The overall compliance rate of Ghanaian bottled drinking water revealed by this study for As, Cd, Cu, Pb, and Zn relative to their respective WHO drinking water quality guidelines is overwhelmingly hundred per cent (100%). The bottled drinking water are therefore of high quality for consumption. In the light of bottled water gaining ever-increasing preference for general drinking water purposes, it is recommended that further research be conducted periodically to analyse for a wider range of trace and rare earth elements since natural concentrations of such elements can reach surprisingly high levels over time. The overall performance of Ghanaian bottled waters, obtained in this study is indeed excellent. However, more extensive surveillance of the bottled water industry as well  as  stricter  regulations  should  be developed and enforced to ensure that this admirable compliance rate is maintained. Additionally, the Bottled Water Industry should employ safe and hygienic conditions during the sourcing and packaging of the water in order to avoid contamination.

The authors have not declared any conflict of interests.

The authors are grateful to Musah Saeed Zango for his advice throughout this research. We thank the staff of the Chemistry Laboratory of the University for Development Studies (UDS - Navrongo Campus) and staff of the Water Quality Laboratory of the Water Research Institute in Tamale for laboratory analyses. Nsiah Bediako Daniel is also appreciated for his help in sample collection.