Spatial distribution and health risk assessment of nitrate contamination in groundwater from rural and peri-urban areas of Zanzibar, Tanzania
Résumé
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, their spatial distribution, compliance with drinking-water guidelines, and the associated non-carcinogenic human-health risk across 35 sampling wards in five districts of Unguja Island, Zanzibar. A total of 116 groundwater samples were collected from wells and boreholes (depths 1.03–28.43 m) during a single dry-season campaign in 2024 and analysed for nitrate (reported as NO₃ − ) by UV–Vis spectrophotometry, alongside physicochemical parameters (pH, electrical conductivity, temperature and chloride). Nitrate concentrations ranged from 0.3 to 331.6 mg/L NO₃ − (overall site-mean: 34.7 mg/L). The overall mean fell below both the World Health Organization (WHO)/Tanzania Bureau of Standards (TBS) guideline of 50 mg/L NO₃ − and the U. S. EPA maximum contaminant level of 10 mg/L as nitrate-nitrogen (equivalent to 44.3 mg/L NO₃ − ), but exceeded the stricter local Zanzibar Water Quality (JWQ) benchmark of 10 mg/L NO₃ − . Eight sites (22.9%) exceeded the WHO/TBS and EPA thresholds, whereas 23 sites (65.7%) exceeded the 10 mg/L JWQ benchmark. The highest concentrations occurred 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 recorded the highest district mean (65.5 mg/L), followed by West (42.1 mg/L) and South (40.5 mg/L). Because the data were non-normally distributed (Shapiro–Wilk p < 0.001), non-parametric tests were used: Kruskal–Wallis indicated marginally significant inter-district differences (H = 9.47, p = 0.050), with only North A exceeding Central after Dunn–Bonferroni correction. Spearman correlation showed no association between nitrate and well depth ( ρ = −0.085, p = 0.63) but a weak positive association with electrical conductivity ( ρ = +0.37, p = 0.029), consistent with shared anthropogenic and coastal influences. A screening-level hazard-quotient assessment indicated potential non-carcinogenic risk to infants at six sites (HQ > 1). The results call for district-specific monitoring, improved sanitation infrastructure, and regulation of fertilizer use to protect public health in Zanzibar.
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