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Delineation of groundwater provenance in a coastal aquifer using statistical and isotopic methods, Southeast Tanzania

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Abstract

Rapid population growth and urbanization has placed a high demand on freshwater resources in southeast costal Tanzania. In this paper, we identify the various sources of groundwater and the major factors affecting the groundwater quality by means of multivariate statistical analyses, using chemical tracers and stable isotope signatures. The results from hierarchical cluster analyses show that the groundwater in the study area may be classified into four groups. A factor analysis indicates that groundwater composition is mainly affected by three processes, accounting for 80.6% of the total data variance: seawater intrusion, dilution of groundwater by recharge, and sewage infiltration. The hydrochemical facies of shallow groundwater was mostly of the Na–Ca–Cl type, although other water types were also observed. The deep groundwater samples were slightly to moderately mineralized and they were of the NaHCO3 type. This water type is induced mainly by dissolution of carbonate minerals and modified by ion exchange reactions. The signal from the stable isotope composition of the groundwater samples corresponded well with the major chemical characteristics. In the shallow groundwater, both high-nitrate and high-chloride concentrations were associated with localized stable isotope enrichments which offset the meteoric isotopic signature. This is inferred to be due to the contamination by influx of sewage, as well as intrusion by seawater. The depleted stable isotope values, which coincides with a chemical signature for the deep aquifer indicates that this deep groundwater is derived from infiltration in the recharge zone followed by slow lateral percolation. This study shows that a conceptual hydrogeochemical interpretation of the results from multivariate statistical analysis (using HCA and FA) on water chemistry, including isotopic data, provides a powerful tool for classifying the sources of groundwater and identifying the significant factors governing the groundwater quality. The derived knowledge generated by this study constitutes a conceptual framework for investigating groundwater characteristics. This is a prerequisite for developing a sound management plan, which is a requirement for ensuring a sustainable exploitation of the deep aquifer water resource in the coastal area of Tanzania.

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Acknowledgments

This research was financially supported by the Quota Scheme Programme and the Department of Geosciences, University of Oslo. We thank the local owners of the private wells and the Dar es Salaam Water Supply Authority (DAWASA) Manager for granting us access to the wells. The authors are extremely grateful to Mr. Addo Ndimbo (Ardhi University-Dar es Salaam, Tanzania) for his support during the sampling phase of this work. We are also indebted to technicians in the Department of Geosciences laboratory, University of Oslo, for their efficient assistance in the analysis of water samples. The authors are extremely grateful to Prof. Hans Martin Seip (Department of Chemistry, University of Oslo) for earlier comments, which are greatly appreciated. The constructive comments from the journal editors and anonymous reviewers followed by the language-review done by Ms. Amy Dale from GeoResearch Consulting, Skarnes, Norway, improved the quality of the manuscript.

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Authors and Affiliations

  1. Department of Geosciences, University of Oslo, Blindern, P.O. Box 1047, 0316, Oslo, Norway

    Said Suleiman Bakari & Per Aagaard

  2. Department of Chemistry, University of Oslo, Blindern, P.O. Box 1033, 0316, Oslo, Norway

    Rolf D. Vogt

  3. Ruden Aquifer Development Ltd, 1628, Engelsviken, Norway

    Fridtjov Ruden

  4. Department of Environmental Technology, Institute for Energy Technology, P.O. Box 40, 2027, Kjeller, Norway

    Ingar Johansen

  5. Department of Physical Sciences, The University of Dodoma, P.O. Box 259, Dodoma, Tanzania

    Said Ali Vuai

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  1. Said Suleiman Bakari
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  2. Per Aagaard
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  3. Rolf D. Vogt
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  4. Fridtjov Ruden
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  5. Ingar Johansen
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Correspondence to Said Suleiman Bakari.

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Bakari, S.S., Aagaard, P., Vogt, R.D. et al. Delineation of groundwater provenance in a coastal aquifer using statistical and isotopic methods, Southeast Tanzania. Environ Earth Sci 66, 889–902 (2012). https://doi.org/10.1007/s12665-011-1299-y

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