ASSESSMENT OF THE IMPACTS OF GROUNDWATER ABSTRACTION ON WATER QUALITY AND QUANTITY IN MAGHRIB B DISTRICT IN ZANZIBAR (UNGUJA) ISLAND

Abstract

Zanzibar Island is experiencing a shortage in its water supply, primarily relying on groundwater sources such as shallow wells, boreholes, and cave wells situated on coastal aquifers. The surge in population and the push for tourism growth have intensified the water demand, prompting increased extraction from boreholes. Groundwater abstraction leads to over-exploitation of aquifers and poses risks of pollution, collapse, and seawater intrusion. The main objective of this study was to assess the impacts of groundwater abstraction on groundwater quality and quantity to develop water resources management strategies for sustainable water supply in Zanzibar. This study presents a 40-year rainfall record and temperature analysis to determine how these climatic variables affect groundwater resources in the Island of Zanzibar, Tanzania. In addition, the water table fluctuation, abstraction volume, and different Physio-chemical parameters such as chloride, nitrate, electrical conductivity, and total dissolved solids were also studied. The balance between groundwater recharge and water abstraction rates on the island of Zanzibar was estimated. The water table fluctuation (WTF) method estimated the recharge rate. MODFLOW software was used to analyze and model the aquifer characteristics. Rainfall records showed that Zanzibar Island receives a mean annual rainfall of 1743.1 mm/yr. Temperature variations indicated an incremental trend accompanied by low rainfall. The results also showed that the total groundwater abstraction rate was 1.97×106 m3/yr compared to the recharge rate of about 1.73×106 m3/yr. This means that the groundwater resources are currently over-exploited, and if immediate action is not taken, the groundwater aquifers may be subjected to pollution, collapse, and seawater intrusion. This study compared water quality data from ZAWA against the WHO standards. Most of the water quality parameters were within the WHO limits. However, an increasing trend was observed, especially for parameters related to seawater intrusion. The increasing contaminant trends in Kianga, Bweni, and Chunga may be attributed to long-term groundwater abstraction. The water quality in all boreholes is considered good according to the Water Quality Index classification. Additionally, the study models the aquifer characteristics and found that the water outflow rate is higher than the inflow rate based on the water balance analysis. The results of this study provide crucial insights for water resource managers in Zanzibar, where there is a pressing need for sustainable water resource management due to the imbalance of an outflow and inflow rate