International Journal of Sustainable Energy, 2013http://dx.doi.org/10.1080/14786451.2013.813027
Geochemistry of thermal springs around Lake Abhe,Western Djibouti
Bouh Housseina, D. Chandrasekharamb,c*, Varun Chandrasekharc and M. Jalludina
aLaboratoire de Géochimie du CERD, Route de l’Aéroport, BP: 486, Djibouti, Republic of Djibouti;bDepartment of Earth Sciences, Indian Institute of Technology, Mumbai 40006, India;
cGeoSyndicate Power Pvt. Ltd., Mumbai 400076, India
(Received 18 February 2013; final version received 3 June 2013)
The Republic of Djibouti, occupying an area of 23,180 km2, falls within the arid zone of EastAfrica and islocated above the ‘Horn of Africa’, adjacent to the Red Sea. This country has several thermal springs andfumaroles distributed over three regions – LakeAssal, Lake Hanle and LakeAbhe. The most characteristicfeature of LakeAbhe is the presence of several linear chains of travertine chimneys. The thermal waters aretypical of the Na-Cl type near neutral waters rich in CO2. These waters show an oxygen shift, indicatingreservoir temperatures >200◦C. The chemical signature of the thermal springs and the geology of theLake Abhe region are very similar to the Tendaho geothermal area of Ethiopia. The geology, temperaturegradient and its proximity to DamahAle volcanomake the LakeAbhe region a potential site for geothermalpower development.
Keywords: Djibouti; Lake Abhe; Tendaho; Afar; Damah Ale; geothermometer
1. Introduction
The Republic of Djibouti, occupying an area of 23,180 km2, falls within the arid zone of EastAfrica and is located above the ‘Horn of Africa’, adjacent to the Red Sea (Figure 1). It occupiesa strategic position in the eastern Africa, due to the presence of a natural harbour. The rainfall israndomly distributed along the coast and becomes monsoonal type towards the western part of thecountry. The average annual air temperature is around 40◦C and shows little seasonal variation.The population density of the country is about 34 persons/km2, most of them concentrated inDjibouti town. The country is bordered by Ethiopia in the west and Somalia in the SE. Althoughthe coastal plain is ∼ 25 km wide, the Ghoubet Kharab Bay at the Gulf of Tadjoura separates seafrom the SE–NW depression, where the land is 155m below the sea level. The western part ofthe country is dominated by several SE–NW trending ridges formed due to linear faulting, due tothe tectonic configuration of the AFAR triangle and are dominated by basaltic flows. Lake Abhe,located towards the south-western part of the country, receives drainage from the Awash Riverthat drains the Ethiopian Rift Valley. The entire country is covered by tertiary and Quaternaryvolcanic rocks and related deposits (Barberi et al. 1975).
Figure 1. Location map of Djibouti showing geothermal regions and location of thermal springs. Only major thermalsprings in Lake Abhe are shown. Inset shows Afar triangle and east African rift within Ethiopia. Location of Tendahogeothermal province is marked in the inset map (modified after Chandrasekharam and Chandrasekhar 2010).
The country is located within the Afar Depression (Figure 1), created by the extension of theRed Sea and Carlsberg Ridge over the continent that gives rise to the well-known East AfricanRift System extending down south to Kenya (Barberi et al. 1975). This region experiences intensetectonic and volcanic activities evident by the presence of active volcanoes like Erte Ala (BRGM1970). Vertical tectonic movements of the continental blocks gave rise to several N–S trendinghorst and grabens and endorheic drainage basins (e.g. Grand Bara, Abhè, Hanlé, Gobaad andAsal basins), formed during the Late Pleistocene till the present (Fontes et al. 1980; BRGM 1970;Gasses 1975; Aquater 1989). Two such basins are very active and characterised by high salinelakes, namely Lake Assal and Lake Abhe. Lake Abhe is a part of the Awash River Basin thatoriginates in the East African Rift plateau. In Djibouti, three distinct geothermal regions exist– represented by several thermal springs – Lake Assal, Lake Hanle and Lake Abhe (Figure 1).Considerable work has been carried out on the geothermal regions of LakeAssal and Lake Hanle(Sanjuan, Michard, and Michard 1990; Houmed Gaba 2002; Houssein, Sanjuan, and Michard
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Figure 2. Travertine mounds around Lake Abhe. Steam emanation can be seen from one of the travertine chimneys.These travertine chimneys are built over a thermalmanifestation. Eachmound cluster encloses several thermal emergences.
1993; Jalludin 2010). The Lake Abhe’s geothermal region is characterised by several thermalsprings and fumaroles (Figure 3). The most characteristic feature of the Lake Abhe region is thepresence of a linear chain of travertine chimneys (Figure 2). These chimneys follow the regionaltectonic trend (E–W) and many of them discharge steam at their apex. The discharge temperatureof the thermal springs is 88–98◦C, with a flow rate of 18 L/s. The aim of this study is to presentpreliminary results of the investigation carried out on the thermal springs of Lake Abhe. Earlierwork on the thermal springs in the area was published by Chandrasekharam and Chandrasekhar(2010).
2. Regional geology
The first initiation of tectonic and volcanic activity related to the triple junction (Red Sea, Gulf ofAden and East African Rift) gave rise to a mafic volcanic complex near the Ali Sabieh area (Gallet al. 2010), followed by the out pouring of Adolei basaltic lava flows around 25–30Ma beforethe present. These basalts lie over a sedimentary basement (Jurassic limestone and Cretaceoussedimentary rocks). The Adolei basalts represent the initiation of the Arab-Nubian Rift Systemduring the Late Miocene period (Gaulier and Huchon 1990) (Figure 3). This is succeeded by aperiod of slow extension of theAfar Triple Junction that gave rise to the Mabla Rhyolites (15Ma)controlled by the regional sinister shear that reactivated the oldest N–S and N20◦E Ethiopianstructures giving rise to strike-slip faults and simultaneously triggering N160◦E trending normalfault system in the north. This phase was followed by the outpouring of Dalha basalts at about3.4–9Ma in the north and Somali basalts in the south. This event was followed by the Stratoidbasalts and Rhyolites and the Gulf basalts (1.5Ma), with the opening of the Gulf of Tadjourah(Black, Morton, and Rex 1974).
Sedimentary rocks are seen mainly in the coastal areas, in tectonic basins in the south-westernpart of the country, and along the main stream beds. During the Miocene–Pliocene period, sedi-mentary rocks (clays and alluviums) were inter-bedded within the Dalha basalt flows. Limestone,clays and diatomites are inter-bedded within the younger basalts flows and also fill the tectonicbasins. In the plains, the thickness of the sedimentary rocks can exceed several hundred of metresand in the stream bed it remains between some metres to some tens of metres. During the Quater-nary, marine sediments (coral limestone and limestone) were deposited in the coastal areas. Since100,000 years before present, five phases of major extension of lakes in the tectonic basins were
Figure 3. Geological map of Djibouti (modified after Jalludin and Razack 2004). (1) Sedimentary formation of Jurassicand Cretaceous, (2) Adolei basalts, (3) Mabla rhyolites, (4) Dalha and Somali basalts, (5) Stratoid basalts, (6) Stratoidrhyolites, (7) Gulf basalts, (8) recent volcanics and (9) Pleistocene sedimentary formation.
developed during the humid climates. During this period, limestone, diatomites and clay weredeposited (Gasse et al. 1980; CNRS-CNR 1973).
Lake Abhe is located at the south-western margin of the country, on the border with Ethiopia.Damah Ale volcano is located SW of Lake Abhe. This hypersaline lake is the western end of theGobaad Plain and the eastern end of the downstream valley of the Awash River, coming from theEthiopian plateau. The geology is characterised by the stratoid basalt plateaus limited by E–Wfault configuring the Gobaad Plain, filled by Quaternary and probably Late Pliocene sediments.
3. Sampling and analytical methods
During the field work between 2009 and 2010, 16 water samples were collected from the studyarea (Figure 4). These samples were filtered through a 0.45μm filter membrane and stored in two
Figure 4. Pictures showing one of the thermal springs of Lake Abhe. Field measurements and samples being collectedby Bouh. Small linear travertine (budding) mounds can be seen in the photo.
polyethylene bottles. For every sample, one of the bottles was acidified with suprapure HNO3
for the determination of cations and the other (unacidified) was used for the determination ofanions. These water samples include thermal, cold springs and bore well waters tapping the deepsedimentary and volcanic aquifers and water from LakeAbhe. Conductivity, pH and temperatureof the water samples were measured at the sampling site. The major chemical constituents inall the water samples were analysed in the geochemical laboratory of the Centre d’Etudes et deRecherche de Djibouti (CERD), Djibouti (Table 1), while δD and δ18O values were measured atthe ‘Laboratory d’Hydrogeologie d’Avignon’, France, with standard deviations of 2‰ and 0.2‰,respectively. In addition to this, chemical data on the Awash River, sea water, and oxygen andhydrogen isotope data of the thermal waters collected during 1998 (unpublished data availablewith CERD) were also utilised. The presence of CO2 in the vapour phase that is being emittedfrom the travertine chimneys was confirmed by the litmus paper test.
4. Results and discussion
4.1. Hydrochemistry
The analysed water sample data (Table 1) are plotted on the Piper diagram (Figure 5). Thermalwater, volcanic aquifer water (except sample #11) and the warm thermal spring are the Na-Cltype. Water from the deep sedimentary aquifer shows a wide variation in the water type and is ofNa-HCO3, Ca-HCO3 and the Na-Cl type. LakeAbhe water is of the Na-K-HCO3 type. The warmspring shows similar composition to the volcanic aquifer water. The composition of the thermalsprings and the waters pumped from the volcanic aquifers has similar chemical composition withrespect to major ions and as shown in later paragraphs of this paper, with respect to oxygen
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Table
1.Che
mical
andph
ysical
data
forthewater
samples
from
Djib
outi.
S.no
.Nam
eTy
peT
(◦C)
EC
pHCl−
HCO
− 3CO
2− 3SO
− 4Na+
K+
Ca2
+M
g2+
Li
Br
SiO
2δ18O
δD
1SP
2HS
9654
608.0
2072
18n
356
1100
3122
70.4
0.6
3.8
100.75
−3.48
−23.74
2SP
4HS
9860
308.0
2083
16n
350
1031
3222
10.6
0.6
4.0
111.4
−2.49
−22.18
3SG
3HS
9732
308.4
987
20n
361
488
1415
30.7
0.2
5.1
101.14
−3.42
−24.78
4SG
4HS
9432
708.2
972
20n
356
490
1415
70.1
0.1
4.1
n−3
.03
−25.67
5Lak
eAbh
éLAKE
2979
,100
9.9
23,462
77,763
31,140
9768
43,746
435
380.0
nn
n8.94
47.85
6Niin
leW
S68
2916
7.59
310
155
n20
030
021
.333
.42.8
0.27
nn
−1.9
−1.3
7AGNA
WS
4425
258.39
652.3
176.9
1217
2.9
563.3
12.1
101.2
0.01
nn
−3.27
−22.2
8Gorab
ous
SED
n7.85
3831
4n
74.5
625.5
6422
nn
n−0
.2−0
.99
KK
SED
n53
08.27
44.4
229.4
n50
.981
.73.9
19.6
15.6
nn
n−0
.69
−110
LLB
SED
n20
308.6
261.6
302.6
n41
2.1
445.5
6.3
16.4
12.9
nn
n−1
.16
−0.4
11Dou
doub
ololé
VOLC
n75
87.99
124.1
210.5
n84
.515
3.6
8.2
33.3
16.5
0.01
nn
nn
12R1
VOLC
n21
94n
566
146
n23
350
125
.129
.31.5
nn
n−2
.92
−20.4
13R2
VOLC
n21
26n
588
79n
247
489
18.4
33.5
0.6
nn
n−2
.86
−21.7
14R3
VOLC
n32
38n
700
273
n46
775
25.5
16.7
2.5
nn
n−2
.23
−17.7
15Han
lé2
VOLC
nn
n48
1.6
67.2
n33
3.8
367.2
7.5
91.4
0.6
nn
n−3
.52
−21.9
16TEAW
O4
VOLC
n21
408.43
519
117.1
1.2
220.8
481.9
24.6
14n
nn
n−2
.75
−18.4
17AwashRiver
nn
nn
nn
nn
nn
nn
nn
−0.70
3.2
18Se
awater
nn
nn
nn
nn
nn
nn
nn
1.55
6
HS,
hots
pringwater;L
AKE,L
akeAbh
ewater;W
S,warm
spring
water;S
ED,s
edim
entary
aquiferwater;V
OLC,v
olcanicaq
uiferwater,s
amples,c
oncentratio
nof
ions
inmg/
kg;n
,not
determ
ined
.
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International Journal of Sustainable Energy 7
Figure 5. Piper (1944) diagram showing the chemical type waters from Lake Abhe’s geothermal region.
and the hydrogen isotopic composition (Figure 8). This phenomenon demonstrates that the deepcirculating thermal waters are also flowing within the basalts of different ages. The position of thewarm springs in Figure 5 (and their temperature, Table 1) suggests that the same thermal watersare sometimes cooled and emerged as warm springs wherever the topography intersects the watertable.Mixing of volcanic aquifer waters with sedimentary aquifer waters is also occurring in somewells as evident from the position of one of the samples (sample #11, Table 1), shifting towardsthe sedimentary aquifer water plots in Figure 5.
4.2. Mineral Saturation Index
Saturation Index (SI) of the mineral is used to understand the saturation status of the solution withrespect to a particular mineral phase. The SI of water samples is calculated using the WATQFcomputer program (Ball and Nordstrom 1991) and is shown in Table 2. The SI values of thegeothermal thermal water suggest the saturation with aragonite and calcite and are under satu-rated with respect to gypsum and anhydrite. Extensive occurrence of linearly deposited travertinemounds demonstrates the availability of a large amount of CO2 to the circulating waters. As aresult, the waters (with an exception to the water occurring in the volcanic aquifers) are saturatedwith respect to CaCO3 minerals like aragonite and calcite (Table 2). Emission of steam, alongwith CO2, through the travertine chimneys suggests that CO2 is a part of the geothermal systemsand is mixing with other waters, such as those occurring in the sedimentary aquifer and warmsprings. The Lake Abhe waters are supersaturated with carbonate minerals (Table 2), and as aresult the Ca is removed from the waters as calcium carbonate resulting in the enrichment of Nain the waters that is making the water to fall in the Na-HCO3 field in Figure 5.
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Table 2. SI for waters from Djibouti.
S. no. Type Name Anhydrite Aragonite Calcite Dolomite Gypsum
Water samples of the study area were plotted in the Cl-SO4-HCO3 diagram (Giggenbach 1988),to select suitable thermal water samples for geothermometry. In Figure 6, the thermal waters plotnear the Cl mature field (of Giggenbach) and show a shift towards the SO4 field due to the possiblepresence of SO2 in the vapour phase. However, these thermal waters seem to be fast ascending asindicated by lowHCO3/Cl ratio (much less unity), suggesting absence of the mixing with the nearsurface ground or other waters under consideration in this area. Water samples form the volcanicaquifers, which indicate enrichment of SO4 and some amount of HCO3 as compared with the
Figure 6. Cl-SO4-HCO3 diagram showing the position of waters from Lake Abhe. The field marked at the Cl-apexindicated matured waters of Giggenbach (1988). For symbols see Figure 5.
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thermal water. Sample #11 deviates from other samples, and thus falls in the HCO3 field andshows a higher HCO3 content relative to other samples. The warm springs also fall within the Clfield indicating their similarity to the thermal waters and volcanic aquifer waters. The waters fromLake Abhe and the sedimentary aquifer fall in the HCO3 field indicating their typical meteoricorigin.
4.4. Geothermometry
The position of all the thermal springs in Figure 6 (Giggenbach 1988) suggests that these springsare emerging to the surface without much mixing with waters of other origin. Their shift towardsthe SO4 domain in Figure 6 only supports mixing with volcanic gases and or with magmaticwaters. According to Giggenbach (1988), such springs are good candidates for estimating thereservoir temperature. The reservoir temperatures calculated using the cation thermometers arelisted in Table 3. The reservoir temperatures thus vary from 123◦C to 152◦C.
Furthermore, to understand the thermal waters’ equilibrium status based on Na-K and K-√Mg
thermometers recommended by Giggenbach (1988), the thermal water samples were plotted inthe tri-linear Na-K-
√Mg temperature diagram (Figure 7), where the thermal water samples give
temperatures ranging from 80◦C to 180◦C. The reservoir temperatures obtained by this diagramconcur with those calculated using the cations listed in Table 3. In fact the volcanic aquiferwaters also give temperatures similar to the thermal springs. The behaviour of volcanic aquiferwaters in all the figures indicates that the deep tube wells may actually be tapping geother-mal waters ascending along the fractures or faults in the volcanic flows. It is interesting to notethe position of one sample from the volcanic aquifer within the full-equilibrium field (Figure 7)indicating a temperature of 180◦C.
4.5. Hydrogen and oxygen isotopes
The oxygen and hydrogen isotopic compositions of water samples were plotted on the δ18O vsδD diagram (Figure 8). The meteoric water line in Figure 8 represents the rain water compositionover Ethiopia and is drawn based on the database available with the International Atomic EnergyAgency (IAEA 2007). The water of Lake Abhe shows evaporation character that is expected dueto its location within an arid zone with limited precipitation. The high temperature character ofthe thermal reservoir is further supported by the position of the thermal waters with respect tothe local meteoric water line in Figure 8 where the plots show ‘oxygen shift’. The minimumtemperature needed for oxygen shift, according to Nuti (1991) is 200◦C. In order to compare theLake Abhe data with an established data from a neighbouring geothermal site, the isotope datafrom Tendaho geothermal field (Ali 2005), located and also plotted in Figure 8, shows a distinctoxygen shift and this field having reservoir temperature about 250◦C. The thermal water samples
Table 3. Geothermometry of thermal waters of Lake Abhe.
Silica (Fournier 1973)Na/K Na-K-Ca
Surface No. Maximum (Fournier and Na/K (Fournier andS. no Name temperature (◦C) steam lost steam lost Potter 1979) (Fournier 1979) Truesdell 1973)
Figure 7. Na-K and K-√Mg geothermometers of Giggenbach (1988).
from Djibouti also show a similar shift as Tendaho (Figure 8). The oxygen and hydrogen isotopiccompositions of the thermal waters collected in the year 1998 were also plotted in Figure 8. Thoseplots also show an oxygen shift but show a slight variation with respect to δD. Thus, the LakeAbhe’s geothermal region can be considered as a high temperature geothermal region and hasvery high prospects for developing the field into a major power generating field. This is furthersupported by the high temperature gradients (80–100◦C/km; Zan et al. 1990) recorded in the boreholes drilled in the Hanle geothermal region located north east of Lake Abhe (Figure 1).
4.6. A conceptual model
In order to understand the evolution of the geothermal system of Lake Abhe, a conceptualschematic model was developed with the available data and is shown in Figure 9. The topographicprofile was prepared based on the topographic map of Djibouti and the Ethiopian Plateau pro-file (Chandrasekharam and Chandrasekhar 2010). According to the model, CO2–SO4 rich Na-Cl(primary) magmatic and meteoric waters ascend into the Lake Abhe area interacting with theCa-HCO3 water of Lake Abhe precipitating travertine deposits all around the region. Part of thiswater appears to be flooding the bottom of the Lake where the water reacts with Ca-HCO3 watersbeing drained by the Awash River precipitating CaCO3 (water is supersaturated with carbonateminerals; Table 3) leaving the lake water rich in Na and HCO3. The primary waters flow alongthe fractures and flow contact in the basalts give rise to volcanic aquifers rich in Na-Cl waters(Figure 5). The deeper Ca-HCO3 sedimentary waters are partly mixed with the Na-Cl waterswithin the basalt formations. Bore wells tapping this waters show mixing phenomena (Figure 5).The presence ofDamahAle volcano on thewestern bank of LakeAbhe supports the presentmodel.Thus, the geothermal region of this lake appears to be of a high temperature geothermal systemlike Tendaho and needs detailed investigation through MT investigation, followed by drilling ofa deep exploratory bore well.
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Figure 8. Oxygen–hydrogen isotope diagram showing the location of the water samples from the study area. A, AwashRiver samples; S, Red Sea water. A and S data are from CERD, Djibouti. Solid circles represent thermal waters collectedin 1998 by CERD. The meteoric line represents rain over East Africa (IAEA 2007). Isotope data for thermal waters fromTendaho geothermal site (Ali 2005) are also plotted for comparison. For other symbols refer Figure 5.
Figure 9. Schematic evolutionarymodel for LakeAbhe’s geothermal and other waters. The cross-section is drawn basedon that published earlier by Chandrasekharam and Chandrasekhar (2010) and the topographic maps of Djibouti. AwashRiver flows south of Damah Ale before joining Lake Abhe.
5. Conclusion
It is interesting to observe that the geochemical behaviour of thermal waters of LakeAbhe is verysimilar to those thermal waters of the Tendaho geothermal province in Ethiopia (Giovanni et al.1998). The Tendaho geothermal province is located NW of the Lake Abhe’s geothermal regionand falls within the Afar depression. The Tendaho geothermal province has been developed anda pilot power plant is generating 5MWe of electricity and has the capacity to generate >20MWe(Meseret and Solomon 2010). Considering the flow rate of the thermal springs (18 L/s) and the
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transmissivity of the Stratoid basalt aquifer (20–800m2/h; Jalludin and Razack 2004) and themodelled reservoir conditions similar to that of the Tendaho geothermal field (Battistelli et al.2002), the Lake Abhe’s geothermal region should be able to generate a minimum of 15–20MWe(∼ 120 × 106 kWh) with a group of 3–4 wells tapping both basaltic and sedimentary geothermalaquifers.
Acknowledgements
The authors thank Mr. Hemant Kumar Singh, IIT Bombay for his help in drafting the figures and editing the manuscript.
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