Recharge, groundwater flow pattern and contamination processes in an arid volcanic area: Insights from isotopic and geochemical tracers (Bara aquifer system, Republic of Djibouti)
Introduction
The republic of Djibouti (23,000 km2) is located in the Horn of Africa (Fig. 1), which is an emerged triple junction of the Red Sea, the Gulf of Aden and the East African Rift that correspond to large seismic, tectonic and volcanic activities (Barberi et al., 1975, Mlynarski and Zlotnicki, 2001).
On the other hand, the republic of Djibouti is subjected to an arid climate with an annual mean rainfall of 150 mm. This harsh climate explains the lack of permanent rivers, and has led the country to exploit the only available water resources, which are represented by groundwater in the volcanic aquifers (Jalludin and Razack, 1994). The continuous and drastic increases in water demand has led to an intensive exploitation of volcanic aquifers and has severely depleted its reserves and led to quality deterioration (Houssein and Jalludin, 1996), as found in several arid to semi-arid aquifers worldwide (e.g. Jimenez-Martinez et al., 2016).
The population of Djibouti relies heavily on groundwater resources for drinking water and irrigation purposes. Therefore, knowledge of the origin of groundwater resources and their renewal forms as well as the consideration of their vulnerability facing the anthropogenic pressure is essential to the rational management of volcano-sedimentary aquifers in Djibouti. However, few studies have been carried out on the hydrogeology (Jalludin and Razack, 1994, Jalludin and Razack, 2004) and hydrochemistry (Aboubaker et al., 2013) of these complex volcano-sedimentary aquifers. These studies address only briefly recharge conditions, groundwater residence times and groundwater flow pattern inside these aquifer systems. In order to fulfill this gap, environmental and radiogenic isotopes (δ2H, δ18O, 3H, δ13C, 14C, 87Sr/86Sr) as well as major ion chemistry combined with hydrogeological and geological data are applied to assess groundwater dynamics of the Bara aquifer systems located on the north of the East Africa Rift.
Furthermore, nitrate dual isotopes (δ15N–NO3− and δ18O–NO3−) are applied to identify the major sources of NO3− in groundwaters from this volcano-sedimentary aquifer. Indeed, the isotopic composition of dissolved nitrate in waters (δ15N–NO3− and δ18O–NO3−) has been used extensively to better constrain the sources of nitrate in groundwater (Edmunds and Gaye, 1997, Jackson et al., 2015, Kendall, 1998, Kendall et al., 2007, Stadler et al., 2008, Walvoord et al., 2003). However, to the best of our knowledge, no nitrate dual isotopes were used to investigate the origin of nitrate in groundwater in arid or semi-arid rift systems. The present study reports the first results of nitrogen isotopes of groundwater from semi-arid environment located in the East Africa Rift.
The results of this study will contribute to a comprehensive knowledge of the complex volcano-sedimentary aquifer systems in the Bara basin, by defining the groundwater flow pattern and the relations between aquifer compartments, assessing groundwater residence time and deciphering the origin of high nitrate content in semi-arid environment in the East Africa Rift areas. This study would serve as a valuable base for other integrated hydrodynamic, geochemical and isotopic studies of volcano-sedimentary aquifers in similar arid environments.
Section snippets
Climate and hydrology
The republic of Djibouti has a low precipitation regime, with annual mean rainfall of 150 mm. Overall, two seasons predominate: a cool season (winter) from October to April and a hot season (summer) from May to September. In winter, the climate is characterized by northeast trade winds coming from Saudi and Gulf of Aden and mean temperature comprised between 20 °C and 30 °C. In summer dominates equatorial westerly wind zone and mean temperatures comprised between 30 °C and 45 °C with high rate of
Material and methods
Twenty two borehole waters were collected in April–June 2014. Whenever possible, water level in boreholes was measured manually with a dip meter. Temperature (± 0.1 °C), pH (± 0.01 unit), electrical conductivity (± 1 μS/cm), redox potential (± 0.1 mV), and Dissolved Oxygen (± 0.1 mg O2/l) were measured on site using portable instruments, i.e. CheckTemp (Hanna), pH 610 (EutechInstruments), COND 610 (Eutech Instruments), WTW multi 3410, and YSI 550A DO Instrument respectively. Each was calibrated in the
Results and discussion
Samples were separated into categories according to i) the regional sub-area and possible sub-compartment considered, namely “Petit Bara”, “Grand Bara”, “Dadin” and “Mouloud” and ii) the aquifer type the borehole corresponded to, mostly “basalt” and “alluvium”. Two additional categories corresponding to boreholes lacking stratigraphic description or in specific sub-areas are proposed, namely “Grand Bara - central” (#12, located in the central Grand Bara plain) and “Petit Bara - central” (#7 and
Conclusion
This integrated study aimed at revealing the recharge conditions, groundwater age distribution, origin of contaminants and groundwater flow pattern inside the complex volcanic aquifer system of Bara (Djibouti). To this end, an integrated hydrodynamic, geochemical and isotopic approach was set up. Geochemistry showed an evolutionary trend from alluvium aquifers located in wadi-valleys and deltaic formations bordering the main sedimentary basins (Petit Bara and Grand Bara), with low salinity
Acknowledgements
This research work was financially supported by the United Nation Development Program (UNDP), the Centre d'Etudes et de Recherche de Djibouti (CERD), and the International Foundation for science, Stockholm, Sweden, through a grant no W/5800-1. We would like to thank Mr. Aden Atteyeh for fruitful discussion and Mr. Samaleh Ahmed Idriss and Abdi Abdillahi Djibril for their assistance in the field works. We would also like to thank two anonymous reviewers for their constructive comments that
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