Recharge, groundwater flow pattern and contamination processes in an arid volcanic area: Insights from isotopic and geochemical tracers (Bara aquifer system, Republic of Djibouti)

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Highlights

  • Investigating an alluvium-basaltic aquifer system in arid eastern Africa.

  • Groundwater flow pattern and mixing revealed by geochemistry and isotopes.

  • Consistent recharge from wadis and downward circulation to the basalts.

  • Absence of a deep volcanic CO2 contribution to basalt groundwater.

  • Soil, manure and caliche nitrate sources evidenced by isotopic nitrate.

Abstract

Fractured volcanic aquifers are the main water resources in the arid climate of the Republic of Djibouti. Nonetheless, these strategic reservoirs are overexploited and their comprehensive understanding is therefore a pre-requisite for a sustainable use. A geochemical and isotopic survey, including major ion chemistry, 2H, 18O, 13C, 3H, 87Sr/86Sr, 15N was performed and combined with existing 14C data to study recharge, contamination processes and groundwater flow patterns inside and between the compartments of a complex aquifer system composed by basaltic rocks and by alluvium located in Petit Bara, Grand Bara, and Mouloud areas (Southwest of Djibouti). A main feature was the common trend from a fresh Na-Cl-HCO3 water type (alluvium groundwaters) to an intermediate water type (alluvium and basalt groundwaters) and finally to a Na-Cl-SO4 water type (most mineralized basalt groundwater). Elementary and isotopic nitrate evidenced and located anthropogenic and geogenic origins of nitrate. Alluvium groundwaters had δ2H and δ18O signature of modern precipitation while basalt groundwaters were significantly depleted and enriched in δ13C due to water-rock interactions. Modern radiocarbon and tritium were evidenced in the alluvium groundwaters, while recalculated radiocarbon ages located recharge of the basalt groundwaters in the early to mid-Holocene. These features revealed a common evolutionary pattern, with a recharge from wadi-rivers to the alluvium and a downward circulation to the basalt through major faults, combined with a mixing with a more geochemically evolved groundwater. Accordingly, highly saline groundwater at the outlet of the Petit Bara plain was found to be diluted by modern recharge in the alluvium. Two major basaltic aquifer compartments were found to be connected (Grand Bara and Mouloud), revealing a global northeastward flowpath below the endorheic Grand Bara plain.

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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