Elsevier

Applied Geochemistry

Volume 97, October 2018, Pages 301-321
Applied Geochemistry

Geochemical, multi-isotopic studies and geothermal potential evaluation of the complex Djibouti volcanic aquifer (republic of Djibouti)

https://doi.org/10.1016/j.apgeochem.2018.07.019Get rights and content

Highlights

  • Several geothermometrical approach have been used to estimated the reservoir temperature.

  • Seawater, fossil saline and fresh including thermal waters are the main components.

  • Boron isotope is higher than seawater both on saline and fresh waters.

  • Sulfate and strontium isotope show the interaction with sulfate minerals and basalts.

  • Dual isotope on nitrate molecule reveals the presence of manure contamination.

Abstract

The complex Djibouti volcanic aquifer system was studied to improve understanding of the recharge conditions of the Awrlofoul low-enthalpy geothermal system located in the middle of the aquifer. Forty-four thermal and non-thermal groundwater samples were analyzed to determine their major chemical compositions, trace element compositions, and multi-isotopic compositions (δ2H(H2O), δ18O(H2O), δ18O(SO4), δ34S(SO4), δ13C(DIC), 14C, 87Sr/86Sr, δ11B, δ15N(NO3), and δ18O(NO3)). Statistical analysis (Hierarchical Cluster Analysis and Principal Component Analysis) of chemical composition identified three main water groups, two affected by salinization (C1 and C2) and one fresh water group useful for drinking (C3). The latter group includes thermal water from the Awrlofoul geothermal field. This separation into three different water groups is also clear on a Langelier-Ludwig plot and is confirmed by analysis of historical chemical data over the last 30 years. The main causes of salinization are contamination of the fresh groundwater either by recent seawater intrusions (C2) or mixing with Ca-Cl fossil saline water (C1). The C1 waters are also highly affected by Mg/Ca-Na clay exchange. As expected, the 11B/10B isotope ratio of the intruded salt water, both recent and fossil, was much higher than that of seawater (δ11B up to +55‰). Unexpectedly, groundwater of meteoric origin (i.e., unaffected by a seawater intrusion), also showed a δ11B higher than that of seawater (46.3‰ < δ11B < 51.3‰). That the unexpectedly high δ11B values are likely due to 10B sequestration resulting from interaction with clay and/or carbonate precipitation is demonstrated by activity diagrams and saturation indices. The C1 water group is also affected by nitrate contamination (56.8 ± 19.2 mg/l). That the nitrate contamination is likely due to manure contamination is indicated by comparing the dual isotopic composition of nitrate to the boron isotope ratios. The isotopic composition of sulfate highlighted the importance of SO2-disproportionation to the local sulfate minerals that interacted with the meteoric recharge, while the strontium isotope ratios showed the importance of the seawater-basalt interaction with the fossil saline water component. The results of the mixing analysis using chemical composition, δ13C(DIC), and 14C data by geochemical software (NetpathXL) confirmed the presence of ternary mixing with at least three sources (seawater, meteoric, and fossil) in the waters with the highest chloride concentrations. The estimation of groundwater age by 14C was complicated by overexploitation (as testified by the lumped parameters approach). However, the fossil saline water component was dated back to the Holocene Humid Period.

To estimate the temperature of the Awrlofoul low-enthalpy geothermal system, a multi-method geothermometric approach was applied. Chemical (mainly SiO2) and isotope (sulfate-water oxygen fractionation) geothermometers were employed together with multiple mineral equilibria. These different geothermometric approaches estimated a temperature range of 102 °C–140 °C for the geothermal reservoir, with a mean temperature of about 110 °C.

Finally, a conceptual model was proposed for the Awrlofoul low-enthalpy geothermal system on the basis of the geochemical and isotopic data of the thermal and non-thermal groundwaters combined with the geology and hydrogeology of the study area.

Introduction

The republic of Djibouti is one of several African countries located on the East African Rift System (EARS) (Fig. 1). As in other rifting zones, the activity of the East African Rift System corresponds to large seismic, tectonic, and volcanic activities (Mlynarski and Zlotnicki, 2001). This unique geodynamical environment puts the republic of Djibouti in an excellent position for the development of geothermal energy.

As a consequence of the tectonic activity in the rifting system, the level of complexity of the hydrogeological setting is highly variable in the EARS (Mechal et al., 2017). The complexity of the coastal aquifer system in the EARS is a result of the heterogeneity of these aquifers combined with present day and/or past seawater intrusion, climate change impacts, and anthropogenic pressures.

Within the EARS, the Djibouti volcanic aquifer system supplies drinking water to the capital city of the republic of Djibouti, where 58% of the population resides. This complex aquifer system faces increasing anthropogenic pressures due to the combined effects of population growth and persistent droughts, exacerbating already existing water quality and quantity issues. Located in the middle of the Djibouti volcanic aquifer system (Fig. 1), the Awrlofoul geothermal field (AGF), has been identified as a potential site for geothermal development. Boreholes drilled in the AGF in the early 1990s for drinking water produced groundwater with a temperature range of about 50 °C–70 °C. However, due to the lack of geothermal activity on the surface, the AGF has not yet been fully characterized.

Only a few studies have been carried out on the hydrochemistry (Houssein and Jalludin, 1996; Bouh, 2006) of the aquifers that comprise the Djibouti volcanic aquifer system. A recent study (Ahmed et al., 2017) addressed the geochemical processes that may control the salinization of the coastal aquifer of the Djibouti aquifer system. However, this study tackles neither the recharge conditions nor the groundwater residence times inside this aquifer. The present study seeks to address these issues as well as study all the volcanic aquifers that constitute the Djibouti aquifer system and characterize the hydrothermal activity of the AGF. To that end, detailed geochemical investigations were carried out on all thermal and non-thermal waters from the Djibouti volcanic aquifer system. The strategy developed for the study of this hydro system is a multi-tracer approach combining stable and radiogenic isotopes (δ2H, δ18O, δ18O-SO42−, δ34S-SO42−, δ13C, 14C, 87Sr/86Sr, δ11B, δ15N(NO3), and δ18O(NO3)) as well as major, minor, and trace ion chemistry. To the best of our knowledge, the present study is the second to combine δ11B and N isotopes to track the origin and fate of the nitrate in groundwater from coastal-arid aquifers (Re and Sacchi, 2017 in Morocco).

In sum, the main goals of this study are as follows: (1) to classify the groundwater composition into genetic groups, (2) to characterize the main geochemical processes that explain the thermal and non-thermal water geochemistry of the Djibouti volcanic aquifer system and understand its geochemical evolution, (3) to estimate the temperature in the Awrlofoul geothermal reservoir through chemical and isotope geothermometry as well as a mineral equilibrium approach, and (4) to propose a conceptual model for the Awrlofoul geothermal system on the basis of the geochemical and isotopic study of the thermal waters from the AGF combined with geological and tectonic information as well as regional hydrogeology.

The results obtained in this study improve our understanding of the behavior of the Awrlofoul geothermal system and are useful for planning future management of this geothermal system in the East Africa Rift.

Section snippets

Climate and hydrogeological setting

The republic of Djibouti has a low precipitation regime, with an annual mean rainfall of 150 mm. 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 the Gulf of Aden and a mean temperature between 20 °C and 30 °C. In summer an equatorial westerly wind zone dominates, and mean temperatures rise to between 30 °C and 45 °C with a high rate of

Sampling and analytical methods

Forty-four borehole water samples were collected in April–June 2016. Temperature (±0.1 °C), pH (±0.01 unit), electrical conductivity (EC; ±1 μS/cm), redox potential (±0.1 mV), and dissolved oxygen (±0.1 mg O2/l) were measured on site using portable, in-field calibrated instruments: CheckTemp (Hanna), pH 610 (EutechInstruments), COND 610 (Eutech Instruments), WTW multi 3410, and YSI 550A DO instruments, respectively.

Water samples were collected in polyethylene containers after filtration through

Statistical analysis

In this study, 23 variables (EC, Ca, Mg, Na, K, HCO3, Cl, SO4, NO3, Li, F, Br, SiO2, B, Al, Fe, Ni, Cu, Zn, As, Se, Rb, Sr) in 44 groundwater samples were analyzed using Q-mode HCA and PCA.

The HCA results are presented as a dendrogram (Fig. S3 in Supplementary Material). The HCA allowed us to distinguish three groundwater clusters in the aquifer system studied (Cluster 1, Cluster 2, and Cluster 3). Cluster 1 (C1) and Cluster 2 (C2) include groundwater from coastal boreholes, while Cluster 3

Conclusions

Hydrogeochemical and multi-isotope investigations of groundwater samples collected from the Djibouti volcanic aquifer system revealed the main geochemical processes that explain the geochemical evolution of several genetic groups (C1, C2, and C3). Two main causes of salinization were identified: recent seawater intrusion (C2 water group) and mixing with fossil saline water (C1 water group). Statistical treatment of chemical data was found to be a valid tool to discriminate between the different

Acknowledgements

This work was financially supported by the Centre d’Etudes et de Recherche de Djibouti (CERD). We are thankful to the Korea Institute of Geoscience and Mineral Resources (KIGAM) and the Korea Basic Science Institute (KBSI), South Korea for the analytical support. We would also like to thank Prof. Michael Kersten, the Editor-in-Chief, Dr Marcello Liotta, the Associate editor and three anonymous reviewers for their constructive comments that improved the manuscript.

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