Continental Intercalaire Aquifer Research Articles

REE and Y distribution and speciation in fossil water: The northwestern African Continental Intercalaire aquifer, Algerian Sahara

The Continental Intercalaire aquifer is a part of the Northwestern Sahara Aquifer System which, extending through Algeria, Tunisia and Libya with over one million km2, is one of the largest fossil aquifers in the world, with an age of about 775,000 years, water resource estimated at ∼31,000 × 109 m3 and negligible recharge rate at 1 × 109 m3/year. The fossil water of the Algerian Sahara, part of the Continental Intercalaire aquifer, was examined, for the first time, for its REE + Y composition, speciation, and geochemical significance in order to constrain the processes affecting the REE + Y distribution and fractionation pattern in a peculiar aquatic system.The fossil water of the Algerian Sahara has total mineralization in the 1170 and 2309 mg/l range and the prevailing hydrochemical facies is Na-Cl-SO4. The recorded REE + Y concentrations increase downstream, along with the dissolved silica content and temperature, as a function of water flow. The water samples are in equilibrium with 2:1 and 1:1 clay minerals with the exception of few upstream samples, characterized by alkaline pH values, that are in equilibrium with Na- and K-feldspars.The REE + Y speciation is mainly controlled by carbonate species. The Ln(CO3)+ species increase and the Ln(CO3)2− species decrease downstream as the carbonate rocks of the aquifer dissolve and the PCO2 equilibrating pressure increases. The LnSO4+ species are mostly LaSO4+ downstream the aquifer and, more in general, LaSO4+ largely prevails up to pH between 6.9 and 7.2.The low negative cerium anomalies affecting most of the samples might be explained by the CeO2 precipitation following Ce3+ transformation to Ce4+. Ce oxidation is probably promoted by Mn-minerals by oxidative scavenging, whereas positive Ce anomaly could be due to the preferential desorption of Ce3+ over Ce4+. Finally, the pronounced negative samarium anomaly observed in some samples may reflect depletion of organic matter and Sm adsorption onto clay minerals.

Evaluation of groundwater quality and hydrochemical characteristics in the shallow aquifer of El-Oued region (Algerian Sahara)

The water quality of the aquifer of the El-Oued region (Algerian Sahara) is deteriorating in water quality due to anthropogenic effects and over exploitation of the groundwater resources. This study assesses the quality of groundwater in the El-Oued shallow aquifer for domestic and irrigation purposes and highlights the various geochemical processes that contribute to the mineralization of this water. Twenty-five water samples were collected. The measured and analyzed parameters are: pH, EC, TDS, Na+, K+, Ca2+, Mg2+, Cl−, SO42−, NO3− and HCO3−. The assessment of water potability using the Water Quality Index (WQI) method shows two classes of water quality, namely, the very poor class (8%) and the unsuitable for drinking class (92%). Agricultural water quality was assessed using the parameters; electrical conductivity (EC), sodium adsorption ratio (SAR), sodium percentage (Na %), Kelly ratio (KR), permeability index (PI), magnesium hazard (MH), residual sodium carbonate (RSC) and chloride content. The results showed that the water of this shallow aquifer is unsuitable for irrigation based on EC, Cl− and USSL diagram. This aquifer discloses three facies of water: Cl–Na–Ca (12%), SO4–Na (36%) and SO4–Ca–Mg (52%). In addition, an analysis of physicochemical data obtained from water samples was conducted using correlation matrices, PCA, bivariate plots, and Gibbs diagrams; the saturation indices of the main minerals was also calculated. These analyses have shown that the main reactions responsible for the evolution of water types in the El-Oued shallow aquifer fall into four categories: (1) a mixing process in the waters of deep aquifers (the Complexe Terminal aquifer and the Continental Intercalaire aquifer); (2) the dissolution of salts (in particular, gypsum and halite); (3) cation exchange processes affecting the concentration of Na+, K+, Mg2+, and Ca2+; and (4) anthropogenic influences from domestic sewerage and agricultural activities.

Hydrochemical and isotopic characteristics of groundwater in the Continental Intercalaire aquifer system: Insights from Mzab Ridge and surrounding regions, North of the Algerian Sahara

Study regionNorth of Sahara, Algeria. Study focusThe effects of water/rock interactions and hydrodynamic conditions on the characteristics of groundwater in the Continental Intercalaire aquifer (CI) are demonstrated using hydrochemical and isotopic data from the Mzab Ridge. New hydrological insightsThe results confirm that CI groundwater in the oriental basin is highly mineralized (1010 > EC > 3500 μS/cm) and is of the SO4-Cl-Na type. However, it is poorly mineralized (340 > EC > 960) in the occidental basin and predominantly belongs to the HCO3-Ca type. The important contribution of evaporite minerals has been deduced using saturation indices and minor element ratios of Sr/Ca and Br/Cl. This is explained by lithological heterogeneities within the CI aquifer and corroborates the observations from regional piezometric features which suggest a groundwater divide located between 31.55° and 31.57 °N latitude. δ18O-NO3 and δ15N-NO3 analyses show that NO3 has a predominantly natural origin, which is soil nitrification even for the high concentrations exceeding 50 mg/L. CI groundwaters are highly depleted in both δ18O and δ2H compared to modern rainfall isotopic signatures. The most depleted ones are those situated in the Great Oriental Erg sub-basin. Isotopic values as low as δ18O=−6‰ can be taken as the limit for groundwater that is typically old. In the El Golea region, isotopic enrichment seems to indicate mixing between old groundwater and evaporation-affected groundwater infiltrating through the dunes of the Great Occidental Erg.

Krypton-81 dating of the deep Continental Intercalaire aquifer with implications for chlorine-36 dating

Deep groundwater samples from the Continental Intercalaire (CI) aquifer in the Northern Tunisian Sahara have been analyzed for noble gases 3He, 4He, Ne and 81Kr, and for 14C to better constrain the groundwater residence time of this large transboundary aquifer. Its significant radiogenic 4He content and background-level 14C both indicate water older than a few tens of thousands of years. Distinct helium concentrations and Ne/He ratios suggest different groundwater flow paths through the Tozeur and Kebili regions. 81Kr is applied for the first time on the CI aquifer, providing more direct evidence for the presence of old groundwater with 81Kr/Kr ratios at 63% – 15% of the atmospheric value, corresponding to residence times of 150 – 630 ka. These newly obtained 81Kr ages enable a re-evaluation of the previously reported data from the Tozeur region in order to better constrain the input values of 36Cl/Cl and Cl. The initial 36Cl/Cl ratio is found to be 4 – 6 times larger than previously assumed, and the initial Cl concentration 6 – 15 times smaller. The results of 81Kr dating and the recalibrated 36Cl dating define an eastward age progression across >500 km of flow path from the Algerian to Tunisian sections. This study exemplifies the utility of 81Kr not only as an independent and conservative chronometer, but also as a supplemental measurement to better calibrate the parameters required for other age tracers, including cosmogenic 36Cl and radiogenic 4He.

Geothermometry and geochemistry of groundwater in the Continental Intercalaire aquifer, southeastern Algeria: Insights from cations, silica and SO4–H2O isotope geothermometers

Characterization of geochemistry and geothermometry of the groundwater from the Continental Intercalaire (CI) aquifer, one of the largest aquifers in the world, stretching over one million km2 surface area shared between Algeria, Tunisia and Libya, was conducted in southeastern Algeria nearby the border with Tunisia. Thirty-two water samples were collected from boreholes to analyze the physicochemical parameters and stable isotopes (δ18O(H2O), δ18O(SO4), δ 34S(SO4)) and determine the origin of water mineralization of the CI aquifer. Water temperature was assessed using several geothermometers, such as cations, silica and SO4–H2O stable isotopes. The CI aquifer displayed a discharge temperature varying from 45 to 65.1 °C due to water circulation at deeper depths ranging from 1000 to 2200 m. The Results show that CI water contains high total dissolved solids (TDS) and neutral to slightly alkaline pH. The most frequent water types were Na–Cl–SO4 and Ca–SO4 indicating the geological formation nature of CI aquifer which is mainly composed of evaporites. The application of Na–K and Na–K–Ca geothermometers yielded unreliable temperatures. However, Na–Li geothermometer resulted in relatively plausible temperatures ranging from 74.69 to 147.83 °C. Quartz and CaSO4–H2O isotope geothermometer are the most suitable methods for temperature estimation, due to their attainment of equilibrium resulting in temperatures ranging from 62 to 93 °C, and validation by the multiple mineral equilibrium approach. The present study demonstrated a successful application of CaSO4–H2O isotope geothermometer in a low enthalpy (<100 °C) geothermal system. The results corroborate previous findings on the same CI aquifer at the Tunisian side and on carbonate-evaporite geothermal reservoirs. Furthermore, geothermal potential of the CI aquifer has been highlighted suggesting its use as a source of renewable energy which could be applied in heating greenhouses and/or generating electricity.

Causes and risk evaluation of oil and brine contamination in the Lower Cretaceous Continental Intercalaire aquifer in the Kebili region of southern Tunisia using chemical fingerprinting techniques

In the Kebili region of southern Tunisia, there is increasing demand of water from the Lower Cretaceous Continental Intercalaire (CI) aquifer and the Upper Cretaceous-Miocene Complex Terminal (CT) aquifer. The CI aquifer, given limited low recharge of water and increasing amounts of water extraction, has suffered intense overexploitation since the year 2000. Currently, the sustainability of CI resources is threatened by oil and brine contamination detected at a number of water wells in the Kebili region.Hydrocarbon pollution of the aquifers seems to be ubiquitous because the groundwaters sampled in El Fedjej and Nefzaoua basins exhibit bad water quality according to a number of toxicity indices. Geochemical data indicate that, on a regional scale, groundwater quality, salinity, and dissolved element concentrations are best correlated to petroleum contamination-extraction and to aquifer vulnerability to human perturbations rather than to multiple interactions within the hydrogeological system of the region.The analyses of petroleum compounds in sampled waters indicate that these waters are most consistent with increasing concentrations of organic pollutants; the organic matter is crude and unaltered, testifying to continuous flows of contaminants. The brine contamination, however, is limited to the Djemna water well where the water exhibits a salinity of 20 g L−1. Combined, these findings suggest that groundwater composition in CI water wells in Kebili field is influenced by the migration of hydrocarbons and brine-enriched waters through fractures, and (or) by aquifer decompression.

Geochemical, isotopic and statistical monitoring of groundwater quality: Assessment of the potential environmental impacts of the highly polluted CI water in Southwestern Tunisia

Groundwater chemistry depends on a number of natural and anthropogenic processes. Its evolution under natural conditions or in response to human perturbations remains of major concern. Thus, the main purpose of this study is to evaluate the groundwater quality deterioration in Southwestern Tunisia where the increasing water demand is mainly supplied by the deep confined aquifers of SASS (Système Aquifère du Sahara Septentrional) multi-layered system. The permanent exploitation of the deepest Continental Intercalaire (CI) aquifer leaded to increasing water quality deterioration. Recently, the hydrogeological regime of the aquifer system undergoes a tremendous change resulting in hydrocarbon (HC) contamination of the largest aquifer in Southern Tunisia. A mixing of petroleum substances and CI thermal water is currently pumped from a number of CI water boreholes in Kebili region owing to a spectacular on shore hydrocarbon seepage at SW Tunisia. This contamination is attributed to natural migration by continuous upward petroleum flows via permeable faults, plans of weakness and/or leakage through drilling wells (spilled oil).The most noteworthy finding of the current study is that the measured chemical composition of CI aquifer water reveals a significant wider distribution than expected. The impacts of the HC pollution are expressed by scattered geochemical, isotopic and statistical data. The hydrogeochemical results indicate that in the Western field, the CI is characterized by low mineralized water (TDS from 0.6 to 2.7 g/l) while towards the East, the CI water becomes mostly polluted with salinity ranging between 3 and 20 g/l and cadmium (Cd) concentrations between 12 and 90 μg/l these values are widely above the permissible limits for both human consumption and agriculture activities. Similarly, the environmental isotopic fingerprinting indicates that CI thermal waters in the contaminated field are influenced by mixing with deep groundwater. The mixing process seems to be facilitated by regional tectonic features such as deep fractures and major faults in the contaminated area. Correspondingly, the statistical analysis (Principal Component Analysis (ACP) and Hierarchical Cluster Analysis (HCA)) confirm, in turn, the scattered results suggesting the dissimilarity of groundwater composition between the contaminated boreholes and the sampled waters from the western side.

Groundwater flowpaths and residence times inferred by 14C, 36Cl and 4He isotopes in the Continental Intercalaire aquifer (North-Western Africa)

In a semi-arid to arid climate context, dependency on groundwater resources may lead to overexploitation and deterioration of water quality. The Continental Intercalaire (CI) aquifer is one such continental-scale aquifer (more than a million of km2), which is mainly confined, poorly recharged but intensely abstracted. To date, the management of this resource relies on hydrogeological modelling and key parameters such as recharge/discharge rate and groundwater dynamics. We use a combination of residence time indicators (14C, 36Cl, 4He) and stable isotopes of water (2H and 18O) to give greater constraint on the groundwater residence time in the CI. In previous studies, 14C measurements and steady state modelling indicate a residence time of less than 100 ka whereas in others, 36Cl measurements and transient scenarios modelling suggest a longer residence time (>500 ka).In this study, most of the 14C measurements are below the limit of detection, establishing residence times greater than 40 ka and confirming the necessity of strict sampling protocols to exclude all air and AMS measurements when low 14C concentrations are expected.In the Tunisian recharge area, detectable 14C indicate sporadic recharge episodes (3–7 ka and 29–43 ka), whereas 4He and 36Cl concentrations in central areas suggest very old (<2 Ma) groundwaters. In these central areas, chlorine concentration can reach more than 2 g/l. Since 36Cl concentrations are up to 4 time less than the initial input, we are confident there is no excessive deep 36Cl production.We characterise five distinct flowpaths reaching the Tunisian discharge area using their isotopic signatures. According to our mixing model, the average contribution from the main recharge area, the Algerian Atlas Mountains, is around 88%. This value is close to hydrogeological models. Conversely, the contribution from the Dahar Mountains is lower than in the hydrogeological modelling (2% against 10%) whereas the Tinhert shows a greater contribution (10% against 1%).Increase of abstraction from the CI can potentially activate the circulation of old brackish groundwaters and dramatically decrease the water quality in the whole system.

Assessing hydrothermal groundwater flow path using Kohonen's SOM, geochemical data, and groundwater temperature cooling trend.

Assessing groundwater flow path in a thermal aquifer, such as El Hamma aquifer, southeastern Tunisia, and its lateral communication with the adjacent Jeffara-Gabes aquifers, is a very complex operation which requires the integration of several approaches to understand and explain the reality of phenomenon. In this study, geochemical and isotopic data, Kohonen self-organizing map, temperature cooling trend, and kriging techniques were used to assess groundwater flow path in hydrothermal aquifer of El Hamma-Gabes, Tunisia. For this objective, 32 sampled wells are analyzed for major ions, electric conductivity, pH, total dissolved solids, and stables isotopes (δ2H and δ18O). Geochemical diagrams reveal that groundwater chemistry was controlled by evaporation, and rock-water interaction with a dominant water facies was Cl·SO4-Na·Ca-Mg. Kriging techniques were used to highlight groundwater flow path. Kohonen self-organizing map shows that the waters are clustered into three classes according to chemical and isotopic composition. These clusters represent a hydrothermal groundwater class from the Continental Intercalaire aquifer, a shallow groundwater class corresponding to Jeffara-Gabes aquifer and mixed water class. Groundwater cooling trend and stable isotopes indicate that groundwater flow is toward west to east part of study area, indicating a recharge of Jeffara aquifer from El Hamma thermal aquifer.

Incrustation ability of geothermal waters of the Continental Intercalaire aquifer in the Grand Erg Oriental basin (Algeria and South Tunisia)

The chemical composition of groundwater from wells tapping the Continental Intercalaire (CI) aquifer in the Grand Erg Oriental basin and the detailed analysis of saturation indices enable identification of the mineralization origin. CI groundwater is characterized by a Ca–SO4/Cl abundant water type and a Na–Cl second type. Chemical pattern is controlled by evaporites dissolution (halite, anhydrite and gypsum). Recently, the decrease in these CI geothermal water wells performance is explained by an incrustation in the well screen. To study the scaling process, we used the degree of calcite saturation, hardness and Langelier Saturation Index (LSI). It has been demonstrated that scaling potential is the most important in the basin axis, around El Oued region (southern Algeria) and in the south-east of Tunisia and the Chott Djerid borders (El Hamma of Gabes, Tozeur and Kebili) corresponding to the discharge zones. However, the scaling potential decreases towards Algerian reliefs (Tademait and Tinrhert) representing aquifer recharge zones.

Geothermometry application to the Continental Intercalaire geothermal aquifer: a case study of Ouargla region

The aim of this study is to estimate reservoir temperature of the Continental intercalaire aquifer (CI), for possible geothermal exploitation. As CI depth increases, water increases its temperature, under the effect of geothermal gradient.The approach is based on sampling and analysis of water physico-chemical parameters, then hydrochemistry and geothermometry were performed.The results show that CI waters have got a high mineralization, high electrical conductivity values, discharge temperatures of 43 to 52°C and a SO4-Cl-Na-Ca water type that refers to the evaporate albian aquifer. After that, different geothermometers were used that allowed the estimation of the reservoir temperatures (60-80°C).

Analyzing the effect of transmissivity uncertainty on the reliability of a model of the northwestern Sahara aquifer system

Groundwater flow model of the transboundary Saharan aquifer system is developed in 2003 and used for management and decision-making by Algeria, Tunisia and Libya. In decision-making processes, reliability plays a decisive role. This paper looks into the reliability assessment of the Saharan aquifers model. It aims to detect the shortcomings of the model considered properly calibrated. After presenting the calibration results of the effort modelling in 2003, the uncertainty in the model which arising from the lack of the groundwater level and the transmissivity data is analyzed using kriging technique and stochastic approach. The structural analysis of piezometry in steady state and logarithms of transmissivity were carried out for the Continental Intercalaire (CI) and the Complexe Terminal (CT) aquifers. The available data (piezometry and transmissivity) were compared to the calculated values, using geostatistics approach. Using a stochastic approach, 2500 realizations of a log-normal random transmissivity field of the CI aquifer has been performed to assess the errors of the model output, due to the uncertainty in transmissivity. Two types of bad calibration are shown. In some regions, calibration should be improved using the available data. In others areas, undertaking the model refinement requires gathering new data to enhance the aquifer system knowledge. Stochastic simulations’ results showed that the calculated drawdowns in 2050 could be higher than the values predicted by the calibrated model.

Application of environmental isotope to assess the renewability of groundwater of continental intercalaire aquifer of Sokoto Basin in Northwestern Nigeria

The southeast sector of Iullemmeden Basin is located in Nigeria and referred to as the Sokoto Basin. The aquifer system of Sokoto Basin is multilayered with Continental Intercalaire (CI) aquifer also known as Gundumi-Illo Formation at the bottom, overlain by the Rima Group, Sokoto Group and Continental Terminal (Gwandu Formation). This study is aimed at determining the validity of the statement that the Continental Intercalaire of the basin receives no recharge, through application of environmental isotopes of water molecules (3H, 2H, and 18O) and characteristics deuterium excess (d-excess). The isotope result of oxygen-18 (δ18O) content ranges between −7.72‰ and 3.69‰, and deuterium (δ2H) content from −51‰ to 9.42‰. These results indicates presence of three categories of water, the waters depleted slightly in isotope signature, the moderately depleted waters and waters highly depleted in δ18O and δ2H. These results suggests that the waters depleted slightly in isotope signature may be evaporated waters, while moderately depleted values of 18O and 2H of water content from the Continental Intercalaire designates the modern waters and waters highly depleted in isotope signature may be regarded as paleowaters. The tritium (3H) value recorded falls within the range of 0.5–4 TU represents a mixture of pre-1952 and post-1952 water origin. It is thus concluded from the results that Continental Intercalaire aquifer of Sokoto Basin receives modern water as recharge.

Hydrochemical and isotopic investigations as indicators of recharge processes of the Continental Intercalaire aquifer (eastern piedmont of Dahar, southern Tunisia)

Groundwaters of the Continental Intercalaire sandstone formations at the eastern piedmont of Dahar (southern Tunisia), where they outcrop, were analysed to study the mineralization, the water origin and the current recharge process. Hydrochemical study shows that these waters present a spatial variability of chemical facies which is linked to the lithology of the host rock. The factors contributing to the mineralization are mainly the water–rock interaction in particular the dissolution of evaporitic minerals and the cation exchange reactions. Stable isotopes show an enrichment in δ18O and δ2H contents principally for piezometric levels closest to the surface, reflecting measures found in the current rainfall subject to a semi-arid to arid context. These results are coherent with those of 14C and 3H that allow the qualitative identification of a current recharge which could probably be supplied by the recent infiltration of waters in Wadis and by the direct infiltration of meteoric water through the local sandstone outcrops of the basin. This concerns in particular the well of Salem ben Salem Dabbabi, showing a content of 1.03 TU, and its static level is controlled by the Tataouine Wadi, indicating therefore an evident sign of recent recharge.

Investigation of uranium geochemistry along groundwater flow path in the Continental Intercalaire aquifer (Southern Tunisia)

Environmental tracers (2H, 18O, isotopes of Uranium) and geochemical processes occurring within groundwaters from the Continental Intercalaire (CI) in Southern Tunisia were used to understand the hydrodynamics and the recharge conditions of this aquifer. This study investigates the chemical and isotopic compositions of the CI groundwater. The water types are dominated by Na+, SO42−, Cl− throughout most of the basin with a general increase in total dissolved solids from the Saharan Platform margins towards the Chotts region. Large scale groundwater flow paths are toward the Chotts region. The stable isotopes composition of the analyzed groundwater ranges from −8.8 to −6‰ vs V-SMOW for δ18O and from −67 to −40‰ vs V-SMOW for δ2H. The relatively enriched stable isotopes contents suggest the contribution of the Dahar sandstones outcrops in the current recharge of the CI aquifer in an arid context. However, the most depleted values in heavy isotopes indicate a paleorecharge of the aquifer under wetter conditions revealing a long residence time of groundwaters. The results from water samples using alpha spectrometry method indicate a range in 238U concentrations and 234U/238U activity ratios (AR) of 0.044–1.285 μg kg−1 and 1.2 to 8.84 respectively. The geochemistry of uranium isotopes in groundwater is controlled by many factors, essentially, the influence of water rock interactions, the preferential dissolution of 234U relative to 238U due to alpha recoil and the mixing processes between different waters with distinct AR as well as 238U concentrations.

Use of geochemical, isotopic, and age tracer data to develop models of groundwater flow: A case study of Gafsa mining basin-Southern Tunisia

Hydro-(major and trace elements: Cd, F and Sr), isotope (18O, 2H, 3H and 13C) geochemistry and radiogenic carbon (14C) of dissolved inorganic carbon (DIC) were used to investigate the sources of groundwater contamination and the hydrodynamic functioning of the multilayer aquifer system in the mining Gafsa basin (Southwestern Tunisia). The groundwater of the study area is subject to intense exploitation to accommodate all the water demands of this arid area. The Gafsa basin contains a multi-layered aquifer with four principal levels: Upper Zebbag (Cenomanian–Turonian), Abiod (Campanien–Maastrichian), Beglia (Miocene) and Segui (Plio–Quaternary) Formations. The hydrogeology of this system is largely affected by tectonics (Gafsa–Tebessa, Sehib, Negrine–Tozeur, Tabeddit and Metlaoui faults…). The groundwater of these aquifers undergoes a significant decline in water level (≈0.5my−1), increasing salinity (TDS increase from 400 to 800–6000mgl−1: generally, TDS increases from the mountainous regions towards the discharge area) due to a long time of aridity, irregular rainfall and overexploitation (irrigation and industrial activities). Groundwater pumped from the semi-confined Complex Terminal (C.T) aquifers (Cretaceous and Mio-Plio-Quaternary: MPQ) and from the confined Continental Intercalaire (C.I) aquifers is an important production factor in irrigated oases agriculture and phosphate washing in Southwestern Tunisia. A rise in the groundwater salinity has been observed as a consequence of increasing abstraction from the aquifer during the last few decades. The salinization phenomena in the region are complex. Several possible causes for salinization exist: (1) the upwelling of saline and “fossil” water from the underlying, confined “C.I” aquifer; (2) as well as the backflow of agricultural drainage water; (3) phosphate and domestic wastewater; (4) brine intrusion from the salt lake (Sebkha/Garaat); (5) evaporate meteoric water dams (El Khangua and El Oudeï); (6) reduced rainfall and (7) land and air alterations. The istopic study of waters establishes that the deep groundwater is “fossil” water (6000–37,000years) recharged probably during the late Pleistocene and the early Holocene periods. The relatively recent water in the MPQ aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater.

Quantifying paleorecharge in the Continental Intercalaire (CI) aquifer by a Monte-Carlo inversion approach of [formula omitted] data

Past variations of the recharge is estimated in the Atlas Mountains, the main recharge area of the Continental Intercalaire (CI) aquifer, one of the major Saharan aquifers, over the last 775kyr. In this Mediterranean climatic context, continental archives generally record single humid events, grouped in hypothetical time interval and do not offer continuous chronicles of precipitation. We propose to use spatially distributed 36Cl/Cl data as a temporal constraint, to infer the past recharge in the Atlas area. Based on a simplified but robust climatic scenario, assuming a piston model, we apply a Markov Chain Monte Carlo (MCMC) inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods. The interglacial recharge values vary from a few mmyr−1 to more than 60mmyr−1. Glacial recharge is less than 1mmyr−1. These values are then analyzed in terms of intensity and allow questioning some initial hypothesis, especially the generally accepted value of the initial 36Cl/Cl value (around 133×10-15atat-1). Our analysis suggests a higher value, around 175×10-15atat-1. This approach allows us to bring out paleoclimatic information retained in these continental archives. Especially, computed recharges provide reliable evidence that Marine Isotope Stage (MIS) 5 was more humid than the Holocene period in agreement with marine archives that document wet condition in the North Sahara during that period.

Hydrochemistry and geothermometry of an Albian aquifer from Oued Righ region in northeastern Algerian Sahara

The area of Oued Righ is one of the important geothermal areas in northeastern Algerian Sahara. It is characterized by a hot arid climate with intense dryness and very high evaporation rates. These various climatic characteristics are reflected on the hydrography of the study area. The water requirements of the Oued Righ region are provided by groundwater resources contained in the two aquifers: the complex terminal and the deeper confined continental intercalaire aquifer. Twenty-seven samples were taken in November 2010 and April 2012; Water samples were analyzed for major and minor dissolved chemical constituents. Samples collected from the CI aquifer are characterized by high temperatures varying from 40°C to 60°C and conductivities of 2110 to 3370 µS/cm. The total dissolved solids (TDS) of the thermal waters range from 1488 to 5480 mg/l. The waters are generally of sodium and magnesium sulfated and sodium chlorinated types. The results of mineral equilibrium modeling indicate that the thermal waters of Oued Righ are undersaturated with respect to evaporite minerals and oversaturated or nearly in equilibrium with respect to dolomite, calcite, and aragonite. This paper presents ternary diagrams of Na-K-Mg1/2 and Cl-SO4-HCO3 and a calculation model, which allows location of geothermal water with the two diagrams, involving knowledge of their chemical composition. Groundwater in the Albian aquifer of Oued Righ shows a change in its chemical properties between the two sampling periods, resulting from water-rock interactions and mixing processes.

Groundwater evolution of the Continental Intercalaire aquifer of Southern Tunisia and a part of Southern Algeria: use of geochemical and isotopic indicators

The expansion of irrigated agriculture and the overexploitation of groundwater aquifers are leading to saltwater intrusion, severe deterioration of groundwater quality and soil subsidence at arid areas. The geochemical processes taking place along an 800 km flow line in the non-carbonate Continental Intercalaire aquifer (CI) in North Africa are described using chemical (major and trace element) and isotopic indicators. The aquifer is hydraulically continuous from the Atlas Mountains in Algeria to the Chotts of Tunisia and the geochemical evidence corroborates this. The CI aquifer of North Africa is one of the largest confined aquifers in the world. The aquifer is hydraulically continuous from the Atlas Mountains in Algeria (recharge area) to the Chotts of Tunisia (discharge area) and the geochemical evidence corroborates this. The isotopic study (Delta18O, Delta2H) permits classifying groundwater into three groups. The first group is characterized by low 3H concentrations, low 14C activities and depleted stable isotope contents. It corresponds to an old end-member in relation with palaeoclimatic recharge which occurred during the Late Pleistocene and the Early Holocene humid periods. The second group is distinguished by high to moderate 3H concentrations, high 14C activities and enriched heavy isotope signatures. It corresponds to a modern end-member originating from a mixture of post-nuclear and present-day recharge in relation to return flow of irrigation. The third group is characterized by an average composition of stable and radiogenic isotope signatures. It provides evidence for the mixing between the upward moving palaeoclimatic end-member and the downward moving present-day end-member. Rainfall, originating from a mixture of Atlantic and Mediterranean air masses.

Water-Rock Interaction and Residence Time of Groundwater Inferred by 234U/238U Disequilibria in the Tunisian Continental Intercalaire Aquifer System

The uranium content and 234U/238U ratios have been measured in more than thirty groundwater samples over the Tunisian part of the Continental Intercalaire (CI) aquifer. In this large artesian aquifer, uranium behavior is mainly controlled by redox front systems. The uranium concentrations vary by 5 orders of magnitude from 10-4 to 20 ppb and the (234U/238U) activity ratio from 1 to 10. Close to CI outcrops in the Dahar Mountains, the stepwise decrease in U concentrations combined with a significant increase of the (234U/238U) ratio up to 10 (“augmenting regime”) is followed, downflow by a (234U/238U) decrease up to 3 (“decaying regime”), clearly indicating the occurrence of a redox front. We will discuss the significance of 234U-238U disequilibria in terms of water residence times and its potential to trace water-rock interaction mechanisms.