Assessment of vulnerability to seawater intrusion for the coastal aquifer of Dar es salaam (Tanzania)

Groundwater represents worldwide the main reserve of freshwater for human needs. The increasing demand combined with the use of unsustainable systems for groundwater management leads to a remarkable decay in its quality and quantity, especially in many cities of the Global South, where the fast urban growing frequently bring to environmental critical issues as the case of seawater intrusion for coastal aquifers. Dar es Salaam, in Sub-Saharan Africa, is one of these cities, with more than 4 million of inhabitants and a population growth rate of about 8 per cent per year. Due to the urban pressure on the aquifer, the city is now facing a considerable saline intrusion along the coastline, with depletion of groundwater resources and contamination of pumping wells. Aim of this paper is to apply and test the GALDIT method for the assessment of coastal aquifer vulnerability to seawater intrusion, starting from data and measures collected from 2011 to 2013 during the “Adapting to Climate Change in Coastal Dar es Salaam (ACC-DAR) project. The GALDIT is a weights and rating based method that consider six parameters to compute a vulnerability to seawater intrusion index according to a specific numerical ranking. The key parameters are the following ones: Groundwater occurrence, Aquifer hydraulic conductivity, Level of the groundwater above the mean sea level, Distance from the shore, Impact of existing status of seawater intrusion in the area and Thickness of the aquifer. Providing different weights and ratings the GALDIT method allows to evaluate the relative impact of each factor, starting from different hydrogeological settings. The results for Dar es Salaam study case show the presence of all the classes (Low, Moderate and High), among which the widest area has a moderate vulnerability to seawater intrusion. The higher class of vulnerability is rather more concentrated in the northern part of the city. In this context, the assessment of coastal aquifer vulnerability to seawater intrusion in the area of Dar es Salaam might turn out to be a helpful basic information in support of groundwater local policies, for a proper and more sustainable use of the resource.