ORCID
0000-0003-3345-4423
Abstract
Groundwater is the primary freshwater source in Zanzibar, sustaining domest ic, agricultural, and tourism needs. Increasing demand has intensified the pressure on this limited resource and has led to over-abstraction and contamination. Amid the growing demand and contamination pressures, this study examines the quality of groundwater in Zanzibar to clarify current conditions and emerging risks, and inform resource management and public health priority interventions. Particularly, the present hydro-chemical and bacteriological quality and variability was examined for both Unguja and Pemba Islands. A total of seventy-two samples were collected using a stratified random sampling approach, with the first field campaign focusing on physical and chemical properties and the second campaign focusing more on the bacteriological quality of groundwater resources in Unguja. Gibbs plots show groundwater chemistry is predominantly controlled by rock weathering, with a minor subset reflecting recent recharge in Unguja.
The findings indicate that both the physico-chemical and bacteriological quality of the groundwater is relatively good except for a few locations. In Unguja, electrical conductivity (EC) ranged from 274 to 3700 µS/cm, while total dissolved solids (TDS) ranged from 170 to 1960 mg/L. In Pemba, EC values were lower and more stable (173–1373 µS/cm), as were TDS levels (90–780 mg/L). pH values in Unguja ranged from 6.67 to 8.28, indicating slightly alkaline water, while Pemba had a lower average pH of 6.96. Total hardness in Unguja varied from 72 to 780 mg/L, with some sites exceeding safe allowable limits.
Corrosiveness assessment carried out using the Langelier Saturation Index (LSI) revealed that, 95% of the samples in Unguja and 90% of samples in Pemba were undersaturated (LSI < 0), indicating the presence of corrosive water with potential to damage water infrastructure. Only two sites Kiwengwa (Unguja, LSI = +0.36) and Kangagani (Pemba, LSI = +0.23) were supersaturated, suggesting scaling potential. Bacteriological analysis shows that out of the tested sites, 47% exceeded WHO and Tanzania Bureau of Standards (TBS) limits for fecal coliforms (0 CFU/100 mL), with recorded values of up to 3 CFU/100 mL.
Generally, the findings indicate that there is large variability in key water quality parameters such as pH, electrical conductivity (EC), and total dissolved solids (TDS) between the two islands, with Unguja generally exhibiting higher values and greater variability, often attributed to geological factors and human activity, while Pemba's groundwater is more stable and less mineralized. Major contamination sources identified were inadequate sanitation infrastructure, unprotected borehole casings, and proximity of latrines to boreholes. Further, during the field survey, we noted numerous unregistered private wells on private residential premises; their water quality remains unknown due to access constraints that impede routine monitoring.
The study recommends the development of guidelines for private well drilling to ensure safe siting, construction, and sealing. Further, there is a need to register all private wells with requirements for baseline laboratory tests of key parameters at the registration stage.
Recommended Citation
Munishi, S., & Kimati, A. (2026). Hydro-chemical and Bacteriological Assessment of Zanzibar Groundwater Resources. Tanzania Journal of Engineering and Technology (TJET), 45(2), 35-54. https://doi.org/10.65085/2619-8789.1226
Publisher Name
University of Dar es Salaam