ORIGINAL ARTICLE Estimation of depth to fresh–salt water interface and its implications for sustainable groundwater resource management: a case study of the Coastal strip of Dar es Salaam, Tanzania Yohana Mtoni • Ibrahimu Chikira Mjemah • Kristine Martens • Charles Bakundukize • Paul Enock Mtoni • Kristine Walraevens Received: 16 March 2014 / Accepted: 15 November 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting overex- ploitation is entailing the risk of seawater intrusion. This problem has been systematically investigated for the first time, and appears to be wide-spread. Determining the depth of the fresh–salt water interface was deemed crucial in the view of sustainable exploitation of the Dar es Salaam quaternary coastal aquifer system. Vertical electrical soundings (VES) and horizontal resistivity profiles have shown a decrease of resistivity with depth and toward the coastline. Cross sections have shown the presence of salt water in the aquifer near the coastline, and the boundary between fresh and saline waters is mapped. The surface prospection resistivity data showed comparable results as reflected in hydrochemical/lithological cross sections and resistivity well logs. Salt water was found mostly in the area within 2 km of the coastline, and the depth to the interface in this zone is ranging from 1.3 to 20 m. Away from the coastline, the interface (if present) is at greater depth. VES executed at Kurasini (4 km from the sea) showed the fresh–salt water interface at a greater depth (43 m). Keywords Coastal aquifer Á Salinization Á Depth of fresh–salt water interface Á Groundwater management Introduction Background of the study In many countries with coastal areas, increase of human settlements, industrial development, coupled with tourist activities, and development of urban farming has led to over-utilization of coastal aquifers. Such over-utilization commonly results in a rise of the fresh–salt water interface and concomitantly salinization of coastal aquifers. Salt- water intrusion is possibly the most common contamina- tion problem in aquifers, and a major constraint imposed on groundwater utilization (Bear et al. 1999; Voudouris et al. 2010). In densely populated coastal areas, the demand for freshwater is enormous. Struggles to meet this demand lead to a danger of groundwater contamination by seawater intrusion which occurs when rates of extraction of groundwater near the shoreline are raised. Numerous coastal aquifer salinization problems are known around the world: for example, in Japan (Hosokawa et al. 1990), USA (Krause and Clarke 2001; Kaushal et al. 2005), Germany (Diersch et al. 1984; Diersch and Nillert 1987), the Neth- erlands (Huisman 1954; Bear et al. 1999; Goes et al. 2009), Israel (Dagan and Bear 1968; Schmorak and Mercado 1969), Nigeria (Obikoya and Bennell 2012), Tanzania (Mjemah et al. 2009, 2012; Mtoni et al. 2011, 2012a, b, c, 2013; Bakari et al. 2012; Van Camp et al. 2013), Mozambique (Adonis 2007), Libya (Al Farrah et al. 2011; Y. Mtoni (&) National Environment Management Council (NEMC), P.O. Box 63154, Dar es Salaam, Tanzania e-mail: mtoni.yohana@gmail.com I. C. Mjemah Á P. E. Mtoni Sokoine University of Agriculture (SUA), P.O. Box 3038, Morogoro, Tanzania K. Martens Á K. Walraevens Laboratory for Applied Geology and Hydrogeology, Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium C. Bakundukize Department of Earth Sciences, University of Burundi, P.O. Box 2700, Bujumbura, Burundi 123 Environ Earth Sci DOI 10.1007/s12665-014-3887-0