Spatial Distribution and Health Risk Implications of Nitrate Contamination in Groundwater from Rural and Peri-Urban Areas of Zanzibar, Tanzania

Nitrate contamination of groundwater is a growing concern in tropical island settings, where shallow aquifers serve as the primary drinking water source. This study assessed nitrate concentrations, spatial distribution patterns, and compliance with drinking water guidelines across 35 sampling wards in five districts of Unguja Island, Zanzibar. Groundwater samples were collected from wells and boreholes (depths 1.03-28.43 m) and analyzed for nitrate using ion spectrophotometry alongside physicochemical parameters (pH, electrical conductivity, temperature and chloride). Nitrate concentrations ranged from 0.3 to 331.6 mg/L (overall mean: 34.7 mg/L). While the overall mean fell below the World Health Organization (WHO) and Tanzania Bureau of Standards (TBS) threshold of 50 mg/L, it exceeded the U.S. Approximately 22.9% of sites exceeded the WHO limit, and 65.7% exceeded the EPA limit. Approximately 22.9% of sites exceeded the WHO limit, and 65.7% exceeded the EPA limit. The highest concentrations were recorded at Kivunge (170.1 ± 132.89 mg/L), Mtoni (134.9 ± 131.8 mg/L), and Pwani Mchangani (112.58 ± 97.28 mg/L). North A district had the highest mean concentration (65.5 mg/L), followed by West (42.1 mg/L) and South (40.5 mg/L). Statistical analysis revealed significant inter-district differences (p < 0.05) but weak correlations between nitrate and well depth (r = -0.012) or electrical conductivity (r = 0.104), suggesting localized anthropogenic sources rather than natural hydrogeological controls. The results call for district-specific monitoring programs, improved sanitation infrastructure, and regulation of fertilizer use to protect public health in Zanzibar. The results call for district-specific monitoring programs, improved sanitation infrastructure, and regulation of fertilizer use to protect public health in Zanzibar.