Geochemical, multi-isotopic studies and geothermal potential evaluation of the complex Djibouti volcanic aquifer (republic of Djibouti)
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.
References (104)
- et al.
Assessment of chemical quality of groundwater in coastal volcano sedimentary aquifer of Djibouti, Horn of Africa
J. Afr. Earth Sci.
(2017) - et al.
The chemistry of geothermal waters in Iceland III: chemical geothermometry in geothermal investigations
Geochem. Cosmochim. Acta
(1983) - et al.
Hydrochemistry and geothermometrical modeling of low-temperature Panticosa geothermal system (Spain)
J. Volcanol. Geoth. Res.
(2012) - et al.
Recharge, groundwater flow pattern and contamination processes in an arid volcanic area: insights from isotopic and geochemical tracers (Bara aquifer system, Republic of Djibouti)
J. Geochem. Explor.
(2017) - et al.
Geochemical study of the Sakalol – Harralol geothermal field (Republic of Djibouti): evidences of a low enthalpy aquifer between Manda-Inakir and Asal rift settings
J. Volcanol. Geoth. Res.
(2017) - et al.
The geothermal resources of the Republic of Djibouti — I: hydrogeochemistry of the Obock coastal hot springs
J. Geochem. Explor.
(2015) - et al.
The geothermal resources of the Republic of Djibouti – II: geochemical study of the Lake Abhe geothermal field
J. Geochem. Explor.
(2015) - et al.
Magma genesis in an ongoing rifting zone: the Tadjoura Gulf (Afar area)
Geochem. Cosmochim. Acta
(1993) - et al.
Thermoddem: a geochemical database focused on low temperature water/rock interactions and waste materials
Appl. Geochem.
(2012) Application of Brine Differentiation and Langelier-Ludwig plots to fresh-to-brine waters from sedimentary basins: diagnostic potentials and limits
J. Geochem. Explor.
(2011)
Oxygen isotope equilibrium in sulfate-water systems: a revision of geothermometric applications in low-enthalpy systems
J. Geochem. Explor.
Continental rift evolution: from rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa
Earth Sci. Rev.
Structural settings of hydrothermal outflow: fracture permeability maintained by fault propagation and interaction
J. Volcanol. Geoth. Res.
Hydrothermal carbonate chimneys from a continental rift (Afar Rift): mineralogy, geochemistry and mode of formation
Chem. Geol.
Hydro-climate changes over southwestern Arabia and the Horn of Africa during the last glacial–interglacial transition: a pollen record from the Gulf of Aden
Rev. Palaeobot. Palynol.
Chemical geothermometers and mixing model for geothermal systems
Geothermics
Palaeoenvironments and palaeohydrology of a tropical closed lake (L. Asal, Djibouti) since 10000 yrs B.P
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Geothermal solute equilibria derivation of Na–K–Mg–Ca geoindicators
Geochem. Cosmochim. Acta
Amorphous silica solubility and the thermodynamic properties of H4SiO4 in the range 0° to 350°C at Psat
Geochem. Cosmochim. Acta
Application of geochemistry to resource assessment and geothermal development projects
Hydrogeochemical evaluation of thermal, mineral and cold waters between Bursa city and Mount Uludağ in the South Marmara region of Turkey
Geothermics
Assessment and modelling of geothermal reservoirs (small utilization schemes)
Geothermics
Boron and lithium isotopes as groundwater tracers: a study at the fresh kills landfill, staten Island, New York, USA
Appl. Geochem.
The salinity of Djibouti's aquifer
J. Afr. Earth Sci.
Chemical and isotopic characteristics of geothermal fluids from Sulphur Springs, Saint Lucia
J. Volcanol. Geoth. Res.
The effects of extensional structures on the heat transport mechanism: an example from the Ortakçı geothermal field (Büyük Menderes Graben, SW Turkey)
J. Afr. Earth Sci.
Extreme boron isotope ratios in groundwater
Procedia Earth and Planetary Science
Boron isotope ratios of surface waters in Guadeloupe, Lesser Antilles
Appl. Geochem.
Fluid circulation in the active emerged Asal rift (east Africa, Djibouti) inferred from self-Potential and Telluric – telluric Prospecting
Tectonophysics
Catalog of geothermal play types based on geologic controls
Renew. Sustain. Energy Rev.
Sulfur and strontium isotopic study of epithermal mineralization: a case study from the SE Afar Rift, Djibouti
Ore Geol. Rev.
Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals, gases and an aqueous phase
Geochem. Cosmochim. Acta
Origine des substances dissoutes dans les eaux des sources thermales et des forages de la région Asal-Goubhet (République de Djibouti)
J. Volcanol. Geoth. Res.
Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments
Chem. Geol.
Understanding the origin and fate of nitrate in groundwater of semi-arid environments
J. Arid Environ.
Fluid/mineral equilibrium calculations for geothermal fluids and chemical geothermometry
Geothermics
Salinization and saline environments
Hydrochemistry of a complex volcano-sedimentary aquifer using major ions and environmental isotopes data: Dalha basalts aquifer, southwest of Republic of Djibouti
Environ. Earth Sci.
Etude hydrogéologique et hydrochimique de la partie septentrionale de la République de Dibouti. Djibouti
Caractérisation hydrogéochimique et qualité des eaux souterraines du système aquifère volcano-sédimentaire côtier sous climat aride de Djibouti (Afrique de l'Est)
Geochemistry, Groundwater and Pollution
The Geochemist's Workbench® - Release 7
The geothermal zone of Lake Assal (F.T.A.I.), Geochemical and experimental studies
Geothermics
Etude de l’aquifère basaltique de Djibouti et des aquifères adjacents: Approche hydrochimique et isotopique
Boron isotope geochemistry of Na-bicarbonate, Na-chloride and Ca-chloride waters from the Northern Apennine Foredeep basin: other pieces of the sedimentary basin puzzle
Geofluids
Seawater intrusion in the Guanahacabibes Peninsula (Pinar del Rio Province, western Cuba): effects on karst development and water isotope composition
Environmental Earth Sciences
Etude de la contamination fécale de la nappe de Djibouti – République de Djibouti
Etude de préfaisabilité géothermique – Awrlofoul – République de Djibouti
Tectonics of the westernmost Gulf of aden and the Gulf of Tadjoura from submersible observations
Nature
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