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Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

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Abstract

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl− near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO3 and Ca–Mg–HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na–HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.

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Acknowledgments

The authors thank BTC (Belgian Technical Cooperation) for granting a PhD scholarship to Yohana Mtoni. The authors are grateful to the editor Olaf Kolditz and two anonymous reviewers for their comments, which greatly contributed to improving the manuscript.

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Authors and Affiliations

  1. Laboratory for Applied Geology and Hydrogeology, Ghent University, Krijgslaan 281 S8, 9000, Ghent, Belgium

    Yohana Mtoni, Charles Bakundukize, Marc Van Camp, Kristine Martens & Kristine Walraevens

  2. National Environment Management Council (NEMC), P.O. Box 63154, Dar es Salaam, Tanzania

    Yohana Mtoni

  3. Sokoine University of Agriculture (SUA), P.O. Box 3038, Morogoro, Tanzania

    Ibrahimu Chikira Mjemah

  4. Department of Earth Sciences, University of Burundi, P.O. Box 2700, Bujumbura, Burundi

    Charles Bakundukize

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  1. Yohana Mtoni
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  2. Ibrahimu Chikira Mjemah
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Correspondence to Yohana Mtoni.

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Mtoni, Y., Mjemah, I.C., Bakundukize, C. et al. Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania. Environ Earth Sci 70, 1091–1111 (2013). https://doi.org/10.1007/s12665-012-2197-7

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