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Hydrogeological and Hydrogeochemical Investigation of a Coastal Aquifer in Dar-es-Salaam, Tanzania

2008; Brill; Volume: 21; Issue: 1 Linguagem: Inglês

10.1163/2031356x-02101012

2031-356X

lbrahimu Chikira Mjemah, Kristine Walraeuens,

Transboundary Water Resource Management

Water, the lifeblood of Mother Earth has always been important for the existence of human life from time immemorial.The shortage of good quality water from surface sources has made groundwater to be an important source in many urban areas, including Dar-es-Salaam.The Dares-Salaam City is the largest urban centre in Tanzania, with a population of about 3 million.Due to population growth in the area, to get a safe and reliable water source to meet the demand has been a problem since many years.The primary objective of this study was to investigate groundwater quantity and quality by focusing on the hydrogeological characteristics of the unconsolidated sediments of quaternary age, which form the major aquifer in the coastal area of Dar-es-Salaam region.The study was based on data acquired from previous studies and fieldwork campaigns conducted between 2004 and 2006.The study area comprises three major parts, distinguished by the geological formations outcropping: the central coastal plain with Quaternary fluviatile-deltaic sediments, the deltaic Mio-Pliocene clay-bound sands and gravels in the northwest and southeast and the Lower Miocene fluviatile sandstones of Pugu Hills in the west of the study area.The groundwater reservoir in Dar-es-Salaam area, in the coastal plain, mainly consists of two aquifers: the unconfined and the semi-confined aquifer.These aquifers are mainly consisting of sands and are separated by a clay aquitard.The groundwater recharge is considered to be both of regional and local type, and is contributed by the faults present in the study area as well as by the nature of the sandy soil type within the coastal plain.The major source of renewable groundwater in the aquifer is rainfall.Hence, long term average recharge to the aquifer was estimated using the balance method ofThornthwaite and Mather (1957) and is quantified at 122 mm a year.The results of the water level measurements from 84 boreholes show that the groundwater flow generally is from Kisarawe and Pugu Hills areas toward the Indian Ocean.The general direction of the groundwater flow is west to east.This was also confirmed in the developed groundwater model of the area.The depths to water level in the study area range from less than 10 m in the centre of the coastal plain to about 50 min the southeast and northwest of the basin.The Dar-es-Salaam area has four main rivers namely Mzinga, Kizinga, Msimbazi and Mbezi and several seasonal streams.During the fieldwork campaign, the river discharge measurements were taken in the same period in June 2004 and July 2006, after the end of the long rainy season which usually occurs between March and May, in the three perennial rivers Mzinga, Kizinga and Msimbazi.Mbezi River north of the Morogoro Road is usually dry except for flash floods during the rainy periods.The results show that the aquifer contributes to the flow of the main rivers (Mzinga, Kizinga and Msimbazi), keeping them flowing during the dry period.The lithology of the drainage basin proved to play a major role in river flow: Mbezi River is located within the clay-