Water is the most essential liquid for maintaining life on the earth and its sustenance. According to Miller (1997) about 97% water exists in oceans that is not potable, and only 3% is fresh water (2.97% is comprised by glaciers and ice caps, and 0.3% is available as a surface and ground water). Safe drinking water is a fundamental need for good health; it is also a basic human right. In many parts of the world, fresh water is already an inadequate resource.

In the next century, it is anticipitated to be even more limiting due to factors such as increased population, urbanization and other environmental degradations. Nevertheless, there is a general belief that groundwater is purer and safer than the surface water due to the protective qualities of the soil cover. On the other hand, groundwater can be considered as a renewable resource depending on its usage and consumption.

Furthermore, there is a general knowledge gap regarding which chemicals are present in various products related to motorized transports, detergents, building constructions materials including paints cements etc, solid and liquid wastes such as obsolete electronic devices, and how big is their amounts. Thus, current knowledge about the spread of these chemicals in to the environment is also insufficient. This creates a kind of motivation for in-depth investigation about toxicants entering different ecosystems.

Announced that the groundwater is never chemically pure, as at any given time, it is invariably exposed to some of the minerals, which in turn results into minerals dissolution. 1

Generally, groundwater quality refers to its chemical, physical, biological, and isotopic characteristics. Usually, the quality of groundwater is controlled by several factors including rock types, climate, and human activities 2. As the groundwater is polluted anthropogenically, groundwater pollution becomes a serious global problem. Thus the analysis of the water quality is very important to preserve and protect the natural eco system.

The biological and physico-chemical composition of groundwater varies depending upon several factors for instances; contact time of recharge water with the surrounding soil or rock material, frequency of precipitation (which will leach out different materials) and duration of rainwater in the soil before reaching the respective aquifers. Nevertheless, anthropogenic release of pollutants to the ground is a potential source of groundwater contamination. This is because; pollutants released on to the ground, most probably find their way down to the soil profiles into the groundwater aquifers.

Groundwater contamination is very common in water sources located within unplanned settlements characterized with dense population and poor or no sanitation facilities. Consequently, as recharge water moves down the soil profile, it picks up a wide range of chemical species, such as magnesium, calcium, and chlorides. This process deteriorates the quality of the recharged water 3.

Groundwater quality determines its suitability for different purposes such as, domestic, drinking, agricultural, and even industrial purposes. Thereby, the knowledge on the groundwater quality as well as its quantity is very important. 4

Accordingly, this study aimed at characterizing ground water quality in Southern and Northern region of Zanzibar Island based selected physico-chemical parameters (Turbidity, Eh, ORP, pH, EC, TDS, sulphate, bromide, nitrate, nitrite, and fluoride) so as to find out the perception applicability of a general belief that groundwater (due to the protective qualities of the soil cover) is always pure and safer source of water.5

Twenty-nine community water sources (dug wells and bore wells) were investigated during an initial field survey and laboratory analysis, some wells were specifically chosen to find some association of the measured parameters with the location of wells. The groundwater samples were collected in November 2014 leading to twenty-nine sets of groundwater sampling and analysis. The groundwater samples were collected based on standard procedures in 500 mL capacity bottles. The positions of the wells were marked using Global Positioning System (GPS) as depicted in figure 1. Before the sampling, all bottles were soaked with diluted nitric acid solution, washed with distilled water, and again prior to each sampling, each bottle was washed with the water to be sampled. In the case of bore wells, the water samples were collected after pumping the water for sufficient time to collect the formation water. The collected samples were then transported and stored in laboratory for analysis. Handling of the samples at different stages was in accordance with the protocol given in the standard EPA method 200.7.

Prior to anions analysis, all samples were filtered using 0.45 µm Millipore filter paper. Concentrations of bromide, sulfate, nitrate, nitrite, and fluoride, in the groundwater samples were analyzed using an Ion Chromatograph (IC Metrohm 883 with column type Metrosep A Supp 5 - 150/4.0) along with appropriate standards. Calibration chromatogram for the seven analyzed parameters (figure 2), and chromatogram for nitrite and fluoride (figure 3 and figure 4) all are well presented in this paper.

The general summary for the descriptive statistics of the twenty-nine analyzed water samples is depicted on table 1.

Generally, almost all the measured parameters were within the WHO guidelines for drinking water. However, for nitrate (7%) and bromide (10.3%) exceeded the WHO limit respectively. The finding suggests for the establishment of guidelines, which outline better management framework for safe drinking water that comprises important key good health preventive components.

The authors of this manuscript would like to express thanks to His Majesty Government of Brunei Darussalam for funding this project, the State University of Zanzibar for permission for study leave. Special thanks to the staff of the faculty of Science of University Brunei Darussalam for their assistance. To mention few are, Hjh Faridah, Azzlin, Hjh Rasiah, Hj Jamil, Najib, and other UBD chemistry laboratory staffs for providing the necessary facilities and technical support they offer for the successful completion of the project.