Continental Intercalaire Research Articles

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.

Hydrodynamic Characteristics of the Continental Intercalaire and Continental Hamadien Aquifer Systems in the Iullemeden Basin of Tahoua Region Republic of Niger

This study was conducted in the region of Tahoua the northeastern part of the Southern Illulemeden sedimentary basin. In this region the transboundary aquifer system of Continental Intercalaire (CI) and the Continental Hamadien (CH) are potentially rich in underground water especially in free and captive aquifers. More, this water is the main source of drinking water for the populations, the animals and for agriculture and extractive industries. However, the population growth, the agricultural development, and the climate change have a real impact on the dynamics of groundwater and the exploitation of these resources. Hence, this study aims at analyzing the behavior of aquifers and assessing the impact of various natural and anthropogenic factors. Especially it matters to analyze the drilling parameters and cuttings, logging data and pumping test of nine new boreholes. The results reveal that the depths equipped with boreholes catching the CI/CH complex vary from 70 to 800 meters from East to West while the average thickness of the reservoir CH is 50 meters and it mainly consists of coarse sandstone. Drilling penetrated more than 100 meters into the CI without reaching the base. The CI tank is composed of several layers of gravel and clay whilst the CI/CH aquifers flow rates range from 0.2 to 22.8 m3/h/m. So this is cause of great variability of the water potential within this aquifer. Lastly, the transmissivity coefficients are more important in the CH than in the CI, and the water flow average rates range respectively from 29 m/year to 7 m/year.

The age and origin of groundwater in the Great Western Erg sub-basin of the North-Western Sahara aquifer system: Insights from Krechba, central Algeria

The North-Western Sahara aquifer system (NWSAS) forms an important transboundary groundwater resource whose properties remain to be fully understood across its whole extent. For example, groundwater flow in the Cretaceous Continental Intercalaire (CI) unit of the NWSAS is well-characterised in the northern part of its range around the Algeria/Tunisia/Libya borders and in the Great Eastern Erg sub-basin immediately to the south. To the southwest, however, the CI of the Great Western Erg sub-basin has been much less studied. The present paper reports hydrogeochemical data from a wellfield in central Algeria which will contribute to a better understanding of this sector of the NWSAS in terms of the age and origin of groundwater within it.Groundwater pumped from five deep boreholes in the CI aquifer overlying the Krechba gas field has been studied using a variety of environmental tracers including hydrochemistry, environmental isotopes, and reactive and noble gases (the latter being reported for the first time for this sub-basin). All the waters were dilute (SEC 460–600 μS/cm), contained detectable O2 (6.3–7.5 mg/L), showed evidence of evaporation (relative enrichment in δ18O), gave late-Pleistocene 14C model ages (13.5–19.3 ka), and yielded lower than present-day noble gas recharge temperatures (14.3–17.6 °C). Various lines of evidence suggest that these waters are the product of mixing between water recharged direct to the CI and leakage from the Neogene–Quaternary Erg aquifer. The results support the long-held concept of regional flow from a palaeo-recharge area to the northwest. Finally, while the Krechba gas field (Carboniferous) has been since 2004 the site of a pilot carbon capture and storage (CCS) project, the data revealed no evidence for leakage of fluids (gas or brines) into the overlying CI aquifer at the time of sampling (October 2014).

RETRACTED: Hydrogeochemistry and strontium isotopes from the Continental Intercalaire and Complex Terminal aquifers in southern Tunisia

RETRACTED: Hydrogeochemistry and strontium isotopes from the Continental Intercalaire and Complex Terminal aquifers in southern Tunisia

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.

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.

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.

Groundwater mixing and geochemical assessment of low-enthalpy resources in the geothermal field of southwestern Tunisia

In southern Tunisia, the lack of permanent surface water, associated with drought conditions, leads to an endless abstraction from the limited low renewable multi-layered aquifer system of Système Aquifère du Sahara Septentrional containing two principal aquifers, the complex terminal and the Continental Intercalaire (CI) multi-layered systems. The deepest aquifer embodies the low-enthalpy thermal resources of Southern Tunisia. The thermo-mineral surface manifestations of the conductively heated waters reveal the complexity of the geotectonic history of the region and create oases systems in net contrast to the desertic landscape of the Chotts region. As agriculture constitutes the principal economic activity in the studied area and as it is currently threatened by thermal water quality degradation, the present hydrogeological investigation aims for a characterization of the CI thermal resources on the basis of a critical quality evaluation. Thus, different geothermometry approaches have been applied to characterize the low-enthalpy thermal field showing a temperature ranging from 27 to 72 °C. The application of cations geothermometers owns to different temperature estimations ranging from 28 to 202 °C. Based on Na+–Mg2+–K+, K+–Mg2+–Ca2+ and HCO3−–Cl−–SO42− geoindicators, the CI thermal resources refer to immature waters characterized by MI varying from − 0.5 to 0.19. The assessment of groundwater chemical composition based on a Durov diagram proves that CI thermal water is governed by mixing between different water bodies with relative abundance of Na+, Cl−, and SO42− elements. According to D’Amore genetic diagram and meteoric genesis indices, the analyzed groundwater derived from deep circulation in sandy formations. The water quality can change along fluid migration pathways expressed by scattered isotopic and chemical data and a number of mixing ratios. Most changes in the background CI composition result from continuous pumping that modifies natural flows and increase quality degradation.

GIS-based evaluation of groundwater quality and estimation of soil salinization and land degradation risks in an arid Mediterranean site (SW Tunisia)

Southwestern Tunisia is known for its high-quality date palm production. The agricultural activity supports the social and economic pressure. Considering prolonged drought conditions and surface water insufficiency, increasing supplies rely on deep aquifers in spite of the critical status of the water quality used. Intense agriculture development has placed oases durability and crop quality to high risks of soil fertility loss, land degradation, and water availability issues, due to severe climate conditions, overexploitation of groundwater resources, intense desertification, overuse of soil, and poor land management. The recent contamination of the largest aquifer in southern Tunisia (Continental Intercalaire) by petroleum flows adds another dimension to the problem of water crisis and soil salinization in the region of interest. In this study, 41 water samples from CI wells were correspondingly collected for hydrochemical analyses. Major and minor element concentrations were evaluated by different standard methods. The obtained dataset indicates that the western side possesses low salinity hazard, whereas Nefzaoua and El Fedjej areas reveal highly mineralized water unsuitable for irrigation purposes. A number of assessment ratios (KR, TH, SAR, ESP, PS, etc.) and graphical methods (Riverside and Wilcox) confirm this classification. The combination of geochemical indicators with geographic information system permits the assessment of water quality on the basis of two different indexes. The resulted maps show, in turn, scattered classification between the western field with acceptable water quality and El Fedjej-Nefzaoua areas where the CI water is unsuitable for agricultural activities. Besides water chemistry evaluation, the assessment of soil composition, particularly toxic element concentrations, constitutes reliable criteria for monitoring the effectiveness of agricultural practices and the suitability of irrigation water. Hence, the concentrations of four heavy metals have been monitored along three soil profiles ranging from the surface to 80 cm of depth. The results are within permissible limits. The vertical distribution of Ni, Pb, Cu, and Zn is related mainly to physiochemical soil parameters, difference in moisture, and fine fraction distribution.

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).

Identification of dominant hydrogeochemical processes for groundwaters in the Algerian Sahara supported by inverse modeling of chemical and isotopic data

Abstract. Unpublished chemical and isotopic data taken in November 1992 from the three major Saharan aquifers, namely the Continental Intercalaire (CI), the Complexe Terminal (CT) and the phreatic aquifer (Phr), were integrated with original samples in order to chemically and isotopically characterize the largest Saharan aquifer system and investigate the processes through which groundwaters acquire their mineralization. Instead of classical Debye–Hückel extended law, a specific interaction theory (SIT) model, recently incorporated in PHREEQC 3.0, was used. Inverse modeling of hydrochemical data constrained by isotopic data was used here to quantitatively assess the influence of geochemical processes: at depth, the dissolution of salts from the geological formations during upward leakage without evaporation explains the transitions from CI to CT and to a first end member, a cluster of Phr (cluster I); near the surface, the dissolution of salts from sabkhas by rainwater explains another cluster of Phr (cluster II). In every case, secondary precipitation of calcite occurs during dissolution. All Phr waters result from the mixing of these two clusters together with calcite precipitation and ion exchange processes. These processes are quantitatively assessed by the PHREEQC model. Globally, gypsum dissolution and calcite precipitation were found to act as a carbon sink.

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.

Qualité Hydro Chimique des Eaux de la Nappe Superficielle dans la Région de l'Oued Righ et Évaluation de sa Vulnérabilité à la Pollution = Hydrochemical Quality of the Groundwater of the Unconfined Aquifer in Oued Righ Region and Assessement of Its Vulnerability to Pollution

In the desert region of the Wadi Righ, the significant water demand, because of the population growth and the development of the agriculture, is satisfied by the intensive exploitation of the deep aquifers of “Continental Intercalaire” and “Complex Terminal”. The surplus of extracted water has caused a rise of the groundwater table of the upper unconfined aquifer, which has flooded some places, particularly low-lying zones, in the regions of Meghaier and Djemaa. This hydrodynamic situation influenced the quality of waters. The objective of this work is to define the hydrogeologic, hydrodynamic and physicochemical aspects of the upper aquifer in the first step and in the second to evaluate its vulnerability to anthropogenic and agricultural pollution. The use of the GOD method shows that the ground water of Righ Wadi is classified from high to moderate vulnerability. The agricultural areas, chotts and urban zones are characterized by a high vulnerability. Key-words: aquifershydrochemistryvulnerabilityGODWadi Righ Algeria  Auteur correspondant : mouhbelksier@yahoo.fr Rev. Sci. Technol., Synthese 32: 42 -57 (2016) M.S. Belksier & al ©UBMA 2016 43

Hydrogeochemical and stable isotope data of groundwater of a multi-aquifer system: Northern Gafsa basin – Central Tunisia

The hydrodynamic of the multi-aquifer system (the Continental Intercalaire “ C.I ” and the Complex Terminal “ C.T ”) of the North Gafsa basin is largely determined by tectonics (Tebessa – Gafsa fault). The composition of groundwater is controlled by complex reactions at gas-liquid-solid “mineralogical composition of associated rocks” interfaces, which depend on the natural surrounding and potential anthropogenic impact. The hydrochemical data (major ion geochemistry) indicate that these groundwaters are characterized by the dominance a Ca–Mg–HCO3/SO4 and Na–Cl–NO3 water types. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of carbonate minerals. The pH of these samples range from 6.54 to 8.89, supporting the conclusion that the H2CO3/HCO3 couple control pH buffering. Oxygen-18 (δ18O‰SMOW) and deuterium (dD‰SMOW) isotopic data show the exchange between the groundwater and the rock (water–rock interaction) and the evaporation effect. The isotopic content of the boreholes waters is of mixed Mediterranean - Atlantic origin and is opposite to the quantity of rainwater distribution, both in space and time in the study area. This is due to its geographical situation in the southern and south-western of the Mediterranean Sea and between the Atlas area and the Sahara Platform. The concentrations of the isotopic composition of the groundwater are significantly higher than the rainwater. This is indicative of the dissolution of salts and other processes modifying the rainwater geochemical composition during infiltration into the vadose zone. The hydraulic interconnection of these components of the system has led to the evolution of these interesting groundwater types.

Declining water budget in a deep regional aquifer assessed by geostatistical simulations of stable isotopes: Case study of the Saharan “Continental Intercalaire”

Summary The stable isotopes of water were used to improve the determination of the groundwater budget of the deep Saharan “Continental Intercalaire” (CI), a deep confined aquifer. Mixing processes between the CI and shallower aquifers have been described in several regional studies over the last few decades, based on observed isotopic differences between local water masses. Here, we improve the quantitative determination of the discharge flux of this aquifer in one of its main outlet regions, the Djeffara plain in Tunisia, based on geostatistics and a simple mass balance mixing model, applied before and after the beginning of extensive pumping in the 1970s. First, the average values of δ 18 O and δ 2 H were precisely documented in the mixing zone between CI water and the local recharge, based on conditional simulations using spatially distributed isotopic data. Together with the available estimate of local recharge and conservative hypotheses on the isotopic end-members, we estimate the discharge flux of the CI in the Djeffara plain at 1.78 ± 1.03 m 3 s −1 in 1970, probably near natural steady-state, reduced to 1.02 ± 0.58 m 3 s −1 in 2004 under strong anthropogenic pressure, related to the drastic increase in pumping rates in the deep CI aquifer during this period. Considering the general groundwater budget over the entire CI aquifer, we estimate a recharge value of 5.13 m 3 s −1 , or 6.5 mm yr −1 over the 25,000 km 2 of recharge area in the Saharan Atlas. This value is in line with the evaluation of 2.1 mm yr −1 obtained recently from the GRACE satellite gravity data for the overall outcrops considering that recharge occurs mostly in the Atlas region.

The Cretaceous Continental Intercalaire in central Algeria: Subsurface evidence for a fluvial to aeolian transition and implications for the onset of aridity on the Saharan Platform

Abstract The Lower Cretaceous Continental Intercalaire of North Africa is a terrestrial to shallow marine continental wedge deposited along the southern shoreline of the Neotethys Ocean. Today it has a wide distribution across the northern Sahara where it has enormous socio-economic importance as a major freshwater aquifer. During the Early Cretaceous major north-south trending basement structures were reactivated in response to renewed Atlantic rifting and in Algeria, faults along the El Biod-Hassi Messaourd Ridge appear to have been particularly important in controlling thickness patterns of the Lower Cretaceous Continental Intercalaire. Subsurface data from the Krechba gas field in Central Algeria shows that the Lower Cretaceous stratigraphy is subdivided into two clear parts. The lower part (here termed the In Salah Formation) is a 200 m thick succession of alluvial deposits with large meandering channels, clearly shown in 3D seismic, and waterlogged flood basins indicated by lignites and gleyed, pedogenic mudstones. The overlying Krechba Formation is a 500 m thick succession of quartz-dominated sands and sandstones whose microstructure indicates an aeolian origin, confirming earlier observations from outcrop. These interbed with brick red, highly oxidised mudstones representing deposition in temporary lakes or lagoons under an arid climate. The switch from fluvial to aeolian sedimentation at Krechba on the Saharan Platform occurred in the late Aptian and Albian and is thus synchronous to a comparable change observed by previous authors in Lower Cretaceous non-marine deposits of NE Spain. This was probably driven by a combination of sea-level fall and the northward shift of global arid belts into western Neotethys caused by oceanic rifting between Africa and South America.