Given that the oxygen 18 is a conservative tracer, a preliminary assessment of mixture ratios between different water origins has been carried out by mass balance equation given the iso-topic signature of end-members (Equation 1):
[Eq. 1]
The deepest and the most confined part of the Continental Intercalaire aquifer is characterized by highly depleted δ18O values between –8.0 and –9.5 ‰. (Guendouz 1985, Edmunds et al., 2003). In Tunisia, the main origin of fossil water in the Djeffara aquifer is the discharge of the CI aquifer through the El Hamma-Chenchou faults, up to Medenine fault, through the frac-ture network of the Turonian and Senonian car-bonatic formations. Therefore, the oxygen-18 fingerprint of the fossil water for tunisian basin of the Djeffara system is taken equal to the δ18O value of the CI aquifer in the same area, equal to δ18O = –8.5‰ SMOW. For the Libyan basin of the Djeffara system, the signature of the fossil water has been considered equal to δ18O= –9.3‰ SMOW which is the most depleted value observed in this area.
The recent water, that means actual rainfall, has isotopic signature of δ18O = –4‰ SMOW (WISER-IAEA database) in Tunisia basin as well as in Libyan one.
The first remark highlighted by the calculation of the contribution of recent water is its large variation ranging between 0% and 98%
(Table 1). Lowest contribution of recent water to Djeffara recharge are calculated for the Triassic aquifer in Tunisian Dahar region and for the Middle Aquifer nearest the ElHamma fault where water show high depleted δ18O values, while highest recent water contribution are observed in the mio-plio-quaternary shallow aquifers.
The contribution of recent water to the recharge of the Upper and the Middle Aquifers of the Djeffara system is more important in Libyan basin. This can be explained by the discharge of the Continental Intercalaire aquifer through the El Hamma fault in the northern part of the Tunisian Djeffara basin. In fact, most of ground-water with depleted oxygen-18 values is located in this area near this geological feature where the fossil water contribution can reach more than 95% of the recharge. The contri -bution of fossil water to the Upper Aquifer decreases toward the south indicating that recent recharge of the upper aquifer is mainly occurring in the Libyan basin where the recent water contribution is more than 68% with a mean of 79% of the recharge. However, no direct relationship has been found with water table depth probably because this process is also influenced by local lithology and geologi-cal conditions. The Middle Aquifer is also more recharged in Libyan part of the system where the average of modern waters is ranging around 65%, this level sampled in Tunisian part is more influenced by old groundwater.
Stable isotopes trend to indicate that modern recharge of the Triassic Aquifer of the Djeffara system is of the same range in Tunisia and Libya (around 50%) and take place mainly
Aquifer level
Libya basin Tunisia basin Djeffara
min max mean σ min max mean σ min max mean σ
Upper 67,9 86,6 79,0 6,1 39,6 97,6 69,3 20,8 39,6 97,6 71,3 19,0 Middle 25,1 80,2 63,9 17,1 0,0 89,6 35,0 20,3 0,0 89,6 36,9 21,3 Lower 22,8 75,5 59,0 13,5 0,0 92,7 47,1 19,9 0,0 92,7 50,6 19,0 Table 1. Recent water contribution (%) to Djeffara aquifer recharge by mass balance equation
through the formation outcrops along the Oriental side of the Dahar Mountain.
Preliminary results suggest that the main recharge area of the whole system is located in the Libyan part of the basin as previously shown by piezometric mapping (SASS project in OSS, 2003).
4. Modelling of carbon-14 activities and estimation of renewal rates
Annual renewal rates of groundwater into unconfined aquifer can be estimated from radiotracers such us carbon-14 activities taking into account both (i) the annual input of the radio-tracers and (ii) the radioactive decay.
These components represent the two parame-ters of the models. The model of a well-mixed reservoir (Le Gal La Salle et al., 2001, Leduc et al., 1996) assumes that a complete mixing of groundwater issued from successive recharge events occurs within the aquifer, which is sup-posed to be at steady state, i.e. water loss equals water input.
As the CO2is well homogenised in each hemi-sphere (Fontes, 1983), variations of atmos-pheric carbon-14 activity measured in the northern hemisphere can be used in the study area. Before 1905, the carbon-14 activity shows little variation (Stuiver et al., 1991) and the atmospheric activity is assumed to have been constant at 100 pmc. Between 1905 and 1950, the consumption of fossil fuel generated a slight decrease in the radiocarbon atmospheric activity from 99.5 to 97.5 (Suess, 1971). The main changes in the carbon-14 activity are due to the aerial thermonuclear tests between 1953 and 1963 when the radioactivity of the atmos-phere rose dramatically, up to around 200 pmc in 1963 in the northern hemisphere (Levin et al., 1992). Since 1980, the mean annual radiocar-bon activity of the atmosphere has been assumed to have decreased exponentially (Levin et al., 1995).
For the model of a well-mixed reservoir with annual time steps, the radiocarbon activity of groundwater is calculated from the radioactive
decay of carbon-14 in solution and the annual input of carbon-14 as follows (Equation 2):
[Eq. 2]
where:
R: annual renewal rate; AGW: C-14 activity of groundwater; Ain: C-14 activity of input water;
λ: radioactive constant (1.21 × 10−4 a−1) and i: time by year between 1905 and sampling date. Equation 2 allows calculating the renewal rate of the last period ranging between 1905 and the sampling date.
In Djeffara aquifer, the input water is a mixture between rainfall (recent water) and leakage from Continental Intercalaire aquifer (fossil water) as calculated according to equation 1.
This process has taken place during such a long time that the reservoir can be considered as well-mixed. However, the use of this model over a long period of time might be limited by the assumption of steady state. Variations in the initial conditions assumed for the model can include changes in the atmospheric car-bon-14 activity before 1905, variations in the renewal rate itself or in the volume of the reser-voir. The estimated renewal rate remains reli-able as a high limit.
Samples showing a higher carbon-14 activity compared to the predicted value of input water could have been contaminated with modern atmospheric CO2in open wells, leading to an increase of the carbon-14 activity. Most of these samples have highly depleted δ18O (‰ SMOW) value indicating that they should have no C-14 activities or at least very low values. A first approach based on the comparison of C-14 activities and δ18O (‰ SMOW) values allows us to disregard non-representative samples.
Renewal rate estimated using the model of a well-mixed reservoir (Table 2) from the carbon-14 data varies by more than one order of mag-nitude from 0.004 to 2.5‰.
The renewal rate indicates a large spatial varia-tion according to local geological features.
Higher renewal values are observed for the Upper aquifer level with an annual mean rate of 0.335‰ (Table 2). For this level, the renewal rate in the Tunisian basin is ensured by fossil
groundwater with a contribution of 40% reach-ing a local maximum of 60% near the Elhamma fault as well as by local rainfall.
Renewal rates calculated for the Middle Aquifer vary between 0.000 and 0.075‰ and remain lower than for the Upper one, indicating that recharge processes are slower in the Tunisian basin as well as in the Libyan one. The recharge of this level in the Tunisian basin is mainly ensured by fossil water with a contribution of 65% reaching locally 100% near the Elhamma fault while in the Libyan basin, the fossil water contribution to recharge average around 35%
with a maximum local maximum of 75% in Aouled Mahmoud area.
The Triassic Aquifer shows more significant renewal rates ranging between 0 and 2.27‰
mainly due to local rainfall recharge through outcrops along the Dahar mountains. However, due to the weak amount of these local rainfalls, renewal rates remain lesser than those ensured by fossil water.
A frequencies analysis had been carried out in order to identify some homogeneous water types according to renewal rates. These water types plot on C-14 activities vs δ18O (‰ SMOW) diagrams along different mixing lines between the two-end-members used for the calculations of the mixing ratios. This approach has been applied for the Djeffara system and allowed to distinguish between three types of water (Fig. 7) that can be linked to three recharge mechanisms.
The first water type is characterized by a very low renewal rates ranging between 0.000 and 0.0292‰ with a mean value of 0.0132‰, mixing
processes on which appreciatively 57% is com-ing from modern water, have more effect on oxygen-18 values than on carbon-14 activities that remain very low.
The water type has more significant renewal rates, ranging between 0.032 and 0.13‰ with a mean value of 0.078‰, the recharge is balanced between modern water (52%) and fossil water (48%).
The third water type has relatively higher renewal rates ranging between 0.136 and 2.46‰ with a mean of 0.52‰, the contribution of modern waters to this renewal process is also about 61%.
5. General conclusions
The oxygen-18 signature of the groundwater emphasises the heterogeneity of the recharge processes that is also shown by the highly vari-able carbon-14 activity in the unconfined aquifer. The recent recharge is highly variable according to local lithology features, a spatial investigation shows that this recent recharge is more important in the Libyan basin of the Djef-fara aquifer along outcrops of different forma-tions.
The model of a well-mixed reservoir has been applied to C-14 activities for each aquifer of the Djeffara system in order to estimate renewal rate of groundwater. The C-14 activities of input water were weighted according to recent/fossil Aquifer
level
Libya basin Tunisia basin Djeffara
min max mean σ min max mean σ min max mean σ
Upper 0,1025 0,2581 0,1536 0,0561 0,0020 2,4639 0,3840 0,5810 0,0020 2,4639 0,3346 0,5810 Middle 0,0066 0,2255 0,0512 0,0730 0,0000 0,7496 0,1392 0,2044 0,0000 0,7496 0,1236 0,2028 Lower 0,0040 2,2737 0,1799 0,4974 0,0000 0,7930 0,1455 0,1891 0,0000 2,2737 0,1640 0,3828
Table 2. Renewal rate (‰) of Djeffara aquifer system by well-mixed reservoir model
ratio as calculated by oxygen-18 values. Pre-dicted C-14 activities are in good agreement with measured data of the groundwater in the studied aquifer. The multi-tracers approach appears to be necessary to identify ground-water with carbon-14 data that are not repre-sentative of residence time.
The renewal rate of the Upper Aquifer of the Djefarra system is the higher one but remains of weak range, probably because it is directly related to recent water input, i.e. rainfall rates, in an area submitted to arid climate conditions with low rainfall averages. The Middle Aquifer and the Lower Aquifer of the Djeffara system has renewal rates of similar range.
However, renewal rates over the entire Djeffara basin show very high variability, ranging from 0.004 to 2.5‰.
Further investigations of stable isotopes and of renewal rates according to different parameters such as temperature could help to refine recharge mechanisms as well as mixing processes between different water origins.
6. References
Besbes M., Bouhlila R., Pallas P., Pizzi G., Ayoub A., Babasy M., El Barouni S., Horriche F., 2005, Survey of Water Resources in the Djef-fara Aquifer System Part II Construction and calibration of the groundwater flow and TDS transport model – Report of Observa-toire du Sahel et du Sahara
Dubief, J., 1959, Le climat du Sahara. Mém.
Hors-série. Inst. Rech. Sah. Tome 1. Alger, 312 pages
Edmunds, W.M., Shand, P., Guendouz, A., Moulla, A.S., Mamou, A., Zouari, K., 1997, Recharge characteristics and groundwater quality of the Grand Erg Oriental basin, Final report. EC (Avicenne) Contract CT93AVI0015, BGS Technical ReportWD/97/46R, Hydroge-ology series
Edmunds, W.M., Guendouz, A., Mamou, A., Moulla, A.S., Shand, P., & Zouari, K., 2003, Groundwater evolution in the Continental Intercalaire aquifer of southern Algeria and Tunisia: trace element and isotopic
indica-Rmean=0,013‰
r2 = 0,454 Rmean=0,078‰
r2 = 0,8134 Rmean=0,517‰
r2 = 0,7594
0 10 20 30 40 50 60 70 80 90 100
-10 -9 -8 -7 -6 -5 -4 -3
į18O (‰ SMOW)
carbon-14 activity (pcm)
Upper Aquifer Middle Aquifer Triassic Aquifer
High renewal rate: 0,136 - 2 ,464 ‰ Medium renewal rate: 0,032-0,130‰
Low renewal rate: 0-0,0292‰
Fig. 7. C-14 Activities vs δ18O (‰ SMOW) according to renewal rates
tors, Applied Geochemistry, Vol. 18, No. 6, pp. 805-822.
Fontes, J.-Ch., 1983, Dating of groundwater. In:
Guidebook on Nuclear Techniques in HydrologyTechnical Reports Series No. 91, IAEA, Vienna pp. 285–317
Guendouz, A., 1985, Contribution a l’étude géo-chimique et isotopique des nappes pro-fondes du Sahara Nord-Est septentrional, Algérie. Thèse doctorat 3éme cycle. Univ.
Paris-Sud, Orsay, 243 p.
Leduc, C., Taupin, J.-D. and Le Gal La Salle, C., 1996, Estimation de la recharge de la nappe phréatique du Continental Terminal (Niamey, Niger) à partir des teneurs en tri-tium. C. R. Acad. Sci. Paris323, pp. 599–605.
Le Gal La Salle, C., Marlin C., Leduc C., Taupin J. D., Massault M., Favreau G., 2001, Renewal rate estimation of groundwater based on radioactive tracers (3H, 14C) in an unconfined aquifer in a semi-arid area, Iullemeden Basin, Niger. Journal of Hydrol-ogy Volume 254, Issues 1-4, 10 December 2001, Pages 145-156.
Levin, I., Bösinger, R., Bonani, G., Francey, R.J., Kromer, B., Münnich, K.O., Stuter, M., Triv-ett, N.B.A. and Wölfli, W., 1992, Radiocarbon in atmospheric carbon dioxide distribution and trends. In: Taylor, R.E., Long, A. and Kra, R.S., Editors, 1992. Radiocarbon After Two Decades, pp. 503–518 Springer, Berlin.
Levin, I., Graul, R. and Trivett, N.B.A., 1995, Long term observations of atmospheric CO2 and carbon isotopes at continental sites in Germany. Tellus47B, pp. 23–34.
OSS (2003): Système Aquifère du Sahara Sep-tentrional. Volume 4 : Modèle Mathéma-tique. Projet SASS ; Rapport interne.
Annexes. 229 p.
Stuiver, M., Braziunas, T.F., Becker, B. and Kromer, B., 1991, Climatic, solar oceanic and geomagnetic influences on the late-glacial and Holocene atmospheric 14C/12C change.
Quat. Res.35, pp.1–24
Suess, H., 1971, Climatic changes and the atmospheric radiocarbon. Palaeogeogr.
Paleoclimatol. Plaeoecol.10, pp. 199–202.
1. Introduction
La plaine côtière de la Djeffara s’étend en Tuni-sie (Mamou, 1990), et en Libye (Macdonald, 1994), sur une superficie de près de 40 000 km².
Cette région est d’une importance capitale pour la Libye dont plus de la moitié de la population y est active, de même qu’elle abrite dans sa partie tunisienne, près du dixième de la popu-lation du pays et des activités économiques névralgiques comme le tourisme, les industries chimiques de Gabès et l’agriculture. Le système aquifère de la Djeffara qui s’étend sous cette plaine, a connu une intense évolution liée au développement démographique et écono-mique de la région. L’étude hydrogéologique de ce système aquifère, menée en collabora-tion étroite entre l’Observatoire du Sahara et du Sahel (OSS) et les deux pays concernés (Libye et Tunisie) a pour but d’affiner et de préciser ses liaisons d’alimentation avec le Système Aqui-fère Saharien à travers l’exutoire tunisien et le Djebel Nafusa en Libye, les risques d’intru-sion saline de l’eau de mer et les éventuels échanges d’influence à travers la frontière, à la lumière des options de planification des eaux du système.
L’évaluation des différents apports à l’alimen-tation du système aquifère de la Djeffara est d’autant plus nécessaire que les prélèvements sont l’objet d’une croissance impressionnante et affichent des signes de surexploitation mani-feste (importantes baisses piézométriques, tarissement de l’artésianisme et des sources, intrusion d’eau de mer le long de la côte
libyenne). Les deux pays sont ainsi fortement intéressés par le bilan en eau de ce système aquifère et par l’ampleur de l’influence que peut exercer l’exploitation dans l’un des deux pays sur l’autre.
L’intrusion saline de l’eau de mer est déjà un fait constaté dans les alentours de Tripoli et constitue un danger majeur dont le développe-ment est intimedéveloppe-ment lié à l’intensification de l’exploitation de ce système aquifère.
A travers l’intérêt que suscite une bonne connaissance du fonctionnement hydrodyna-mique et chihydrodyna-mique du système aquifères de la Djeffara, la quantification des différents termes de son bilan en eau et les perspectives de mieux orienter la planification de l’exploitation vers la minimisation des risques en vue d’as-surer la durabilité de la ressource en eau et ses différents usages, Cette étude est un cas typi-quement instructif sur l’approche adoptée par l’OSS. Cette approche a permis, en plus des résultats scientifiques obtenus, la mise en place d’un mécanisme permettant aux partenaires concernés par l’exploitation des ressources en eau transfrontalières de s’échanger l’informa-tion et d’harmoniser les visions quant à l’éva-luation de l’état de la ressource en eau et à la planification de son exploitation future. Ce mécanisme est initié par le développement d’ou tils performants, permettant de mieux maî-triser l’information hétérogène disponible dans les deux pays, comme la cartographie numé risée, la base de données commune et le sys -tème d’information géographique (Abdous, 2004).
Cette approche trouve dans la modélisation du