Groundwater quality assessment in the urban-west region of Zanzibar Island.

Abstract

This paper highlights the levels of anions (nitrate, nitrite, sulfate, bromide, chloride, and fluoride) and cations (potassium, sodium, magnesium, and calcium) in selected springs and groundwater sources in the urban-west region of Zanzibar Island. The levels of total dissolved solids (TDS) and sodium adsorption ratio (SAR) were also studied. Thirty water samples were collected in December 2012 from various types of water sources, which included closed hand-dug wells (CHDW), open hand-dug wells (OHDW), springwater (SW), public bore wells (PBW), and bore wells owned by private individuals (BWP), and analyzed after filtration and sometimes dilution. The cations were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES). The anions were analyzed by chemically suppressed ion chromatography (IC). The ranges of the levels of the investigated parameters were as follows: Na 13.68-3,656 mg L(-1), K 2.66-583 mg L(-1), Mg 0.63-131.10 mg L(-1), Ca 16.79-189.9 mg L(-1), Cl(-) 8.61-4,340.97 mg L(-1), F(-) 0-1.02 mg L(-1), Br(-) 0-10.88 mg L(-1), NO₃(-) 0.18-342.4 mg L(-1), NO₂(-) 0-1.39, SO₄(2-) 4.43-534.02 mg L(-1), TDS 7-6,380 mg L(-1), and SAR 0.63-50. Except fluoride, most of the studied parameters in the water samples had concentrations beyond the permissible limits of the World Health Organization (WHO). The elevated concentrations are a result of seepage of contaminated water from on-site septic tanks, pit latrines, landfill leachates, fertilizer applications, and domestic effluents. These results should alert domestic water stakeholders in Zanzibar to the urgent task of initiating a quick mitigation response to control these alarming water risks.