The lifespan of the complexe terminal aquifer, Algerian-Tunisian Sahara

The lifespan of the complexe terminal aquifer, Algerian-Tunisian Sahara

Journal of African Earth Sciences, Vol. 20, No. 3, pp. 751-756, 1999 0 1999 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0899.53...

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Journal of African Earth Sciences, Vol. 20, No. 3, pp. 751-756, 1999 0 1999 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0899.5362/99 $- see front matter

Pergamon

Pll:SO899-5362(99)00043-3

TECHNICAL

NOTE

The lifespan of the Complexe Terminal Aquifer, Algerian-Tunisian Sahara (La durke de vie de I’aquifke du Complexe terminal, Sahara algkro-tunisien)

C. S. SWEZEY * BP Amoco

Exploration,

P.O. Box 3092,

Houston,

Texas 77253-3092,

USA

ABSTRACT-The Complexe Terminal Aquifer is a multi-layered aquifer that supports a thriving date industry in the Algerian-Tunisian Sahara. Although there are numerous uncertainties regarding the exact size of this aquifer and the amount of water in place, it is certain that more water is being removed from the aquifer than is replenished by recharge. Calculations of aquifer volume, discharge rates, and recharge rates indicate that the lifespan of the Complexe Terminal Aquifer is limited, and that depletion of the aquifer will probably occur sometime well before the year 2266. @1999 Elsevier Science Limited. All rights reserved. Rl%UM5-Le Complexe terminal est un aquifere multicouche qui entretient une industrie active de dates dans le Sahara algero-tunisien. Bien que de nombreuses incertitudes subsistent concernant la taille exacte de cet aquifere et la quantite d’eau en place, il est certain que la quantite d’eau enlevee de I’aquifere est superieure a celle apportee par recharge. Les calculs du volume de I’aquifere et des taux de d&charge et de recharge indiquent que la duree de vie du Complexe terminal est limitee, et que son epuisement se produira probablement bien avant I’annee 2266. @1999 Elsevier Science Limited. All rights reserved. (Received

15/2/98:

revised version received

ENGLISH

INTRODUCTION The Complexe

Terminal

aquifer

in Algeria

thriving

date industry

of the Sahara

Aquifer

and Tunisia

Desert

AND

supports

along the northern

(Castany, a

margin

(Figs 1 and 2). Stratigraphi-

SETTING 1982).

areal extent 2), the total with

Although

volume

precision.

state

the

Upper

thick,” while Castany

Formation Middle

(Table

Zebbag

l),

with

Formation

to the Miocene the

Late

forming

Beglia

Cretaceous an aquiclude

beneath the aquifer and the Miocene-Pleistocene Segui Formation forming the overlying aquiclude

*email: [email protected]

Tunisia

that

the aquifer

the Chott

Djerid

350,000

of the aquifer

any great

(1987)

Formation

the aquifer

of approximately

tally, the aquifer extends from the Late Cretaceous Zebbag

1217198)

VERSION

HYDROLOGICAL

is a multi-layered that

717198: accepted

Roberts

aquifer

(1982)

is not known and Mitchell

is “up to 400

m

says that in southern

is 700-900

and 1100

covers an km* (Table

m thick beneath

m thick

beneath

the

Chott Rharsa (Fig. 2). Despite these uncertainties, it is certain that more water is removed from the aquifer than is replenished by recharge (Castany, 1982), and that the aquifer will eventually be depleted if this trend continues.

Journal of African Earth Sciences 751

C. S. SWEZEY

500

26”N

0

8”W

I

,

J

Figure 1. Map of North Africa (after US Army Map Service, 1943, 1969; Director of Military Survey, Ministry of Defence UK, 1969, 19 70). Topographic elevations are given in metres. Flow paths within the Complexe Terminal Aquifer (,I are from Cas tan y ( 19821 .-. -__ . and Roberts and Mitchell (1987). Bold rectangle denotes area shown in Fig. 2. Cross-hatch/g: topography below sea level; stippling: sand sea. Figure 1. Carte de Ii9 frique du Nord. Les altitudes topographiques sont don&es en metres. Les trajectoires dUcoulement a l’interieur de I’aquifere du Complexe terminal (+) sont represent&es d’apres Castany (1982) et Roberts et Mitchell 11987). Le rectangle en gras indique la zone montree sur la Fig. 2. La Croix hachure: topographie dessous le niveau de la mer; pointiller: mer du sable.

Most recharge of the aquifer occurs at outcrops around the margins of the Grand Erg Oriental (Fig. 11, although there is some minor recharge by direct infiltration from the Grand Erg Oriental itself (Table 3). Once within the aquifer, water flows towards a series of continental sabkhas (‘chotts’) at 33-34ON latitude (Figs 1 and 2). Discharge from the aquifer occurs by vertical percolation at the sabkha surfaces, upwelling at natural springs and spring mounds (‘aioun’), and upwelling from water wells (Table 4). Most of this discharge is concentrated in southern Tunisia around the Chott Djerid, where natural upwelling occurs at a relatively constant rate throughout the year (Coque, 1962). Variations in evaporation values, however, cause the Chott Djerid surface

752 Journal of Africen Earth Sciences

to experience a seasonal cycle of winter and summer desiccation.

flooding

CALCULATIONS Assuming an aquifer recharge rate of 18.5 m3 s’ and an aquifer discharge rate of 21.5 m3 s-’ (values for the year 1970, Tables 3 and 41, water is lost from the aquifer (and not recharged) at a rate of 3.0 m3 s-l. From these numbers it is possible to obtain a rough calculation of the lifespan of the Complexe Terminal Aquifer. If the aquifer has an areal extent of 350,000 km2 (Table 2) and an average thickness of 400 m, then the aquifer volume is 1.4 x IO” m3. Given an average aquifer porosity of 20% (Table 21, then the aquifer would

The lifespan of the Complexe Terminal Aquifer, Algerian- Tunisian Sahara

-

- 34”30’N

Gulf of Gabes

%

Figure 2. Map of the southern Tunisian chotts (after Swezey, 1996, 1997). Topographic elevations are given in metres. Crosshatching: chott (sabkha); CF: Chott el Franig; D: DOW; DED: Draa ei Djerid; Ed: El Hamma du Djerid; Eg: El Hamma de Gabes; Eo: El Oudiane; F: Fatnassa; G: Gafsa; Gb: Gabes; K: Kebili; N: Nefta; RM: Redjem Maatoug; SBH: Sidi Bou Helal; T: Tozeur. topographiques sont donnees en metres. Figure 2. Carte des chotts sud-tunisiens fd’apres Swezey, 1996, 1997). Les altitudes

contain 2.8 x 1O’O m3 of water, if the total porosity was filled with water. On the other hand, if the average thickness of the aquifer is 1000 m, then the aquifer volume is 3.5 x IO” m3. Given an average aquifer porosity of 20%, then the aquifer would contain 7.0 x 1 O’O m3 of water, if the total porosity was filled with water. Using these calculated amounts of water in the aquifer, and an aquifer depletion rate of 3.0 m3 s-l, the following estimates are obtained for the lifespan of the aquifer: i12.8 x 1 O’O m3 of water in the aquifer = aquifer depletion 296 years from the year 1970 (i.e. depletion by the year 2266). ii) 7.0 x IO’O m3 of water in the aquifer = aquifer depletion 740 years from the year 1970 (i.e. depletion by the year 27 10).

Table 1. General stratigraphy of southern Tunisia (after Ben Ferjani et al., 1990) Table 1. Stratigraphie du sud de la Tunisie (d’apres Ben Ferjani et al., 1990) Formation

Predominant sand,

lithologies

mud

Segui

mudstone

Age Quaternary MioceneEarly Pleistocene

Beglia

sandstone

Miocene

Sehib

mudstone

Oligocene

DjebslSeugdal

gypsum/limestone

Eocene

Metlaoui

limestone, mudstone,

Pakocene-Eocene

El Haria

shale

Abiod

limestone

Late Cretaceous

Aleg

limestone

Late Cretaceous

Late CretaceousEarly Palseocene

Upper Zebbag

limestone,

Middle

shale,

Zebbag

phosphate, gypsum

dolomite

limestone,

gypsum

Late Cretaceous Late Cretaceous

Table 2. General characteristics of the Complexe Terminal Aquifer Table 2. CharacWistiques g&&ales de I’aquifere du Complexe terminal Area

350,000

km*

Roberts and Mitchell (1987)

Porosity

15-20%

Pallas (1972)

Salinity

1-3 g I-’

Gueddari (I 980)

Storage coefficients

0.0005-O.

Transmissivity

0.004-0.300

1500

Castany (1982)

m* s-’ Castany (1982)

Journal of African Earth Sciences 753

C. S. SWEZEY Table 3. Recharge locations and rates for the Complexe Terminal Aquifer (Castany, 1982) Table 3. Localisation des zones de recharge et taux de recharge de I’aquifere du Complexe terminal (Castany, 1982) Chebka du Mzab area (western margin of Grand Erg Oriental)

6.5 m3 .s’ 5.3 m3 s’

Al Hammadat al Hamra and Matmata Plateau (eastern margin of Grand Erg Oriental) Tademait Plateau, Tidikelt Plateau and Tassili N’Ajjer (southern margin of Grand Erg Oriental)

area

2.7 m3 .s’

2.4 m3 se’

Southern Piedmont of the Atlas Mountains (northern margin of Grand Erg Oriental)

0.2 m3 s’

Infiltration from the Grand Erg Oriental (exceptional rainfalls) Total

18.5 m3 se’

Table 4. Discharge locations and rates for the Complexe Terminal Aquifer (Castany, 1982) Table 4. Localisation des zones de decharge et taux de decharge de I’aquifere du Complexe terminal (Castany, 1982) Rates for the year 1956: Vertical

percolation

Springs

and spring mounds

6.6 m3 s’

at sabkha surface

3.35 m3 5.’

(aioun)

8.5 m3 s-’

Water wells Total

18.45

m3 s”

Rates for the year 1970: Vertical

percolation

Springs

and spring mounds

6.5 m3 s-’

at sabkha surface

2.4 m3 se’

(aioun)

12.6 m3 s.’

Water wells Total DISCUSSION

AND CONCLUSION

The calculations given above assume that the aquifer was filled completely in the year 1970, that all of the water within the aquifer may be pumped out of the aquifer, and that the rate of aquifer discharge remains at the value calculated for the year 1970. In reality, however, these assumptions are not valid because the aquifer was probably not filled completely in 1970 and because some water will be trapped within pores in the rock and will not be able to leave the aquifer. Also, many more wells have been drilled into the aquifer since 1970 (Castany, 19821 and this trend is likely to continue in the near future, increasing the rates of aquifer discharge. Therefore, the dates estimated above give maximum lengths of time

754 Journal of African Earth Sciences

21.5 m3 s“

before aquifer depletion, and depletion of the Complexe Terminal Aquifer should be expected to occur sometime well before the year 27 10 and probably before the year 2266. Additional field work in the Sahara should lead to more refined calculations and to a more precise estimate of the aquifer lifespan.

VERSION FRANCAISE

INTRODUCTION ET CONTEXTE HYDROGtOLOGIQUE Le Complexe terminal est un aquifere algerotunisien multicouche qui entretient une industrie active de dates (Figs 1 et 2). Du point de vue

The lifespan of the Complexe Terminal Aquifer, Algerian- Tunisian Sahara stratigraphique, I’aquifere s’btend de la formation Zebbag superieure datee du C&ace terminal a la formation miocbne de Beglia (Tableau 1). La formation de Zebbag forme un aquiclude sous I’aquifere, tandis que la formation mio-pleistocene de Segui forme I’aquiclude sus-jacent (Castany, 1982). Bien que I’aquifere couvre une superficie approximative de 350.000 km2 (Tableau 21, son volume total n’est pas connu avec une grande precision. Roberts et Mitchell (I 987) supposent que I’aquifere peut faire jusqu’a 400 m d’bpaisseur, tandis que Castany (1982) declare que dans le sud tunisien, I’aquifere a une dpaisseur de 700 a 900 m sous le Chott Djerid et de 1100 m sous le Chott Rharsa (Fig. 2). En depit de ces incertitudes, il est certain que davantage d’eau est retiree de I’aquifere qu’il n’en est apporte par recharge (Castany, 19821, et que I’aquifere sera probablement epuise si cette tendance continue. La recharge de I’aquifere s’effectue principalement au niveau d’affleurements situ& autour des marges du Grand Erg Oriental (Fig. 1 I, bien qu’une recharge mineure se produise aussi directement par infiltration a partir du Grand Erg Oriental luimeme (Tableau 3). Une fois dans I’aquifere, I’eau s’ecoule vers des sabkhas continentaux (‘chotts’) a 33-34” de latitude Nord (Figs 1 et 2). La decharge de I’aquifere se produit par percolation verticale au niveau des surfaces de sabkhas, par remontee d’eau au niveau de sources naturelles et de monts (‘aioun’), et par remontee a partir de puits (Tableau 4). La plupart de cette decharge est concentree dans le sud-tunisien autour du Chott Djerid, oi! I”upwelling’ nature1 se produit a un taux relativement constant au tours de I’annee (Coque, 1962). Cependant, des variations du taux d’evaporation font que la surface du Chott Djerid subie un cycle saisonnier de deluge hivernal et de dessication estivale.

lOi m3 d’eau, si la porosite totale est remplie d’eau. En revanche, si I’bpaisseur de I’aquifere est de 1000 m, son volume est de 3,5 x IO” m3. Pour une porosite moyenne de 20%, I’aquifere contiendrait 7,0x 1 O’O m3 d’eau, si la porosite totale est remplie d’eau. En utilisant les quantites d’eau contenues par I’aquifere calculees plus haut et un taux d’epuisement de 3,0 m3 s-l, les estimations suivantes sont obtenues pour la duree de vie de I’aquifere: i) 2,8 x IO” m3 d’eau dans I’aquifere equivaut a un Bpuisement de I’aquifere en 296 ans B partir de I’annee 1970 (i.e. en I’annee 2266). iil 7,0 x 1O’O m3 d’eau dans I’aquifere Bquivaut a un Bpuisement de I’aquifere en 740 ans a partir de I’annee 1970 (i.e., en I’annee 2710).

DISCUSSION

ET CONCLUSION

Ces calculs supposent que I’aquifere Btait entierement plein en I’annee 1970, que toute l’eau contenue dans I’aquifere peut etre retiree, et que le taux de d&charge de I’aquifere demeure a la valeur calculee pour I’annee 1970. En realite, cependant, ces suppositions ne sont pas valables parce-que I’aquifere n’etait probablement pas plein en I’annee 1970 et parce-qu’une certaine quantite d’eau sera piegee dans les pores de la roche et ne pourra pas quitter I’aquifere. De plus, beaucoup d’autres puits ont 6th fores dans I’aquifere depuis 1970 (Castany, 1982) et cette tendance continuera probablement dans le futur proche, ce qui augmente les taux de decharge de I’aquifere. De ce fait, les dates estimees plus haut donnent un laps de temps maximum avant I’epuisement de I’aquifere, qui devrait done se produire bien avant I’annee 2710 et probablement avant I’annee 2266. D’autres etudes de terrain dans le Sahara devraient conduire a des calculs plus precis et a une meilleure estimation de la duree de vie de I’aquifere.

CALCULS En considerant un taux de recharge de I’aquifere de 18,5 m3 s-l et un taux de decharge de 21,5 m3 s-l (valeurs pour I’annee 1970, Tableaux 3 et 41, I’eau n’est pas rechargee mais perdue par I’aquifere a un taux de 3,0 m3 s-l. A partir de ces chiffres, il est possible d’obtenir un calcul approximatif de la durde de vie de I’aquifere du Complexe terminal. Si la surface de I’aquifere est de 350.000 km2 (Tableau 2) et son epaisseur moyenne de 400 m, I’aquifere a un volume de 1,4 x 10” m3. En considerant que I’aquifere a une porosite moyenne de 20% (Tableau 21, I’aquifere contiendrait 2,8 x

ACKNOWLEDGEMENTS The idea of this article developed while the author was studying Quaternary deposits of southern Tunisia for a doctoral dissertation, directed by G. Kocurek (The University of Texas at Austin, USA) and with substantial guidance from M. Deynoux (CGS-CNRS, Strasbourg, France). Conversations with B. Mahler helped clarify many ideas on hydrology. An earlier version of this article benefitted from comments by B. Kirkland George. Thanks to I. Bourasseau for his assistance with translations. Editorial Handling - G. W. McNeil1 & J. McManus

Journal of African Earth Sciences 755

C. S. SWEZEY

REFERENCES Ben Ferjani, A., Burollet, P., Mejri, F., 1990. Petroleum Geology of Tunisia. Entreprise Tunisienne d’Activites Petrolieres (ETAP), Tunis, 194p. Castany, G., 1982. Bassin sedimentaire du Sahara septentrional (Algerie -Tunisie) - Aquiferes du Continental intercalaire et du Complexe terminal. Bulletin Bureau Recherches Geologiques Mini&es (BRGM), Serie 2, 3, 127-l 47. Coque, Ft., 1962. La Tunisie Presaharienne. Armand Colin, Paris, 476~. Director of Military Survey, Ministry of Defence UK, 1969. Topographic Map of Africa, Series 2201, Sheet 8 (Sabhah), Edition 4 -GSGS, scale 1:2 000 000. Director of Military Survey, Ministry of Defence UK, 1970. Topographic Map of Africa, Series 2201, Sheet 2 (Alger), Edition 5 -GSGS, scale 1:2 000 000. Gueddari, M., 1980. Geochimie des sels et des saumures du Chott el Jerid (Sud Tunisien). These, Doctorat de 3Bme Cycle,

756 Journal of African Earth Sciences

Universite Paul Sabatier de Toulouse, France, 131 p. Pallas, P., 1972. Water resources in the northern Sahara. Nature Resources 8, 9-l 7. Roberts, C., Mitchell, C., 1987. Spring mounds in southern Tunisia. In: Frostick, L., Reid, I. (Eds.), Desert Sediments: Ancient and Modern. Geological Society London, Special Publication 35, 321-334. Swezey, C., 1996. Structural controls on Quaternary depocentres within the Chotts Trough region of southern Tunisia. Journal African Earth Sciences 22, 335-347. Swezey, C., 1997. Climatic and tectonic controls on Quatenary eolian sedimentary sequences of the Chott Rharsa Basin, southern Tunisia. Ph.D. Dissertation, The University of Texas at Austin, USA, 243~. US Army Map Service, 1943. Topographic Map of Africa, Series 2201, Sheet 7 (Sahara), Edition 2 -AMSB, scale 1:2 000 000. US Army Map Service, 1969. Topographic Map of Africa, Series 2201, Sheet 3 (Tunis), Edition 4 -TPC, scale 1:2 000 000.