Assessment of the impacts of groundwater pumpage on the future water supply sustainability in Zanzibar, Tanzania

Abstract

In the present study, a 42-year record of rainfall and temperature from Airport and a 43-year rainfall record from Kizimbani meteorological station were analyzed to understand how these climatic variables are affecting groundwater resources on the Island of Zanzibar, Tanzania. Water table fluctuation, abstraction volume and different Physico-chemical parameters such as chlorinity, nitrate, electrical conductivity and total dissolved solids were also studied. The balance between groundwater recharge and water abstraction rates and assess the impact of groundwater pumpage on water quality on the island of Zanzibar was estimated. Through the use of the water table fluctuation (WTF) method, this study estimated the local sustainable yield (SY) and integrated water balance (IWB) in Zanzibar. Rainfall records showed that Zanzibar Island receives a total mean annual rainfall of 1673 mm out of which 7% (equivalent to 1.79 x 10 6 m 3 /y) recharges the groundwater. Temperature variations indicated an incremental trend accompanied by low rainfall. The average estimated local sustainable yield was 0.72% while the integrated water balance showed a deficit of 39%. Furthermore, the total groundwater abstraction rate in the studied area was 2.49 x 10 6 m 3 /y, which is higher than the rate of recharge. This means that the groundwater resources are currently overexploited and if immediate action is not taken, the groundwater aquifers may be subjected to

pollution, collapse, and seawater intrusion. The effects of over-pumping are being manifested

by the levels of EC, Cl - , TDS, total hardness (TH) and nitrate that have shown an increasing trend with time. Due to the high variation of temperature, a controlled infiltration of harvested rainwater is suggested as a sustainable solution for salt intrusion as well the balance way of recharge and abstraction rate in Zanzibar and other islands.