The Tunisian Jurassic aquifer in the North African Sahara aquifer system: information derived from two-dimensional seismic reflection and well logs

Southern Tunisia is an arid area where socio-economic activities are dependent on groundwater resources. The presented study aims to better characterize the Jurassic aquifer based on geological and geophysical data, with a view to develop a rational exploitation program. Well logs are used to precisely determine the position and composition of the known Jurassic aquifer layers and to identify others able to produce good quality water. The logs show that limestones, sandstones and dolomites of the Krachoua, Techout and Foum Tataouine formations are the main Jurassic aquifers. Sixty-eight seismic-reflection sections are integrated within this study. The interpolation between the interpreted sections leads to the construction of isochronous isopach maps and geoseismic sections, and their analysis finds that compressive and extensive tectonic deformations have influenced the Jurassic aquifer geometry. The Hercynian orogeny phase manifestation is remarkable in that there are several stratigraphic gaps in the Jurassic sequence. The E–W, NW–SE, and NNW–SSE accidents, reactivated in normal faults since the Permian to Lower Cretaceous epochs, have generated the structures found in the Jurassic series, such as subsided and raised blocks. Their syn-sedimentary activity has controlled the thickness and facies of these series. The Cretaceous, Tortonian and Post-Villafranchian compressions are responsible for the Jurassic-deposits folding in some localities. The highlighted tectonic and sedimentary events have an important impact on the Jurassic aquifer function by favoring the Jurassic aquifer interconnections and their connections with the Triassic and Cretaceous permeable series.     

Variographic analysis of water table data from the Oued-Souf phreatic aquifer, northeastern part of the Algerian Sahara

Phreatic aquifers present many serious problems for local public authorities, and they have done so especially for the water resources managers in Oued-Souf, Algeria, since 1980. Recently, a perturbation in piezometric level was observed in Oued-Souf. The spatial distribution of piezometric level data was analyzed by applying geostatistical methods, which provide an indication of the uncertainty of the estimation using the computer software VARIOWIN 2.2. In this paper, the evolution of the piezometric level by referring to other research campaigns was studied. Campaigns were conducted from March 1993 to April 2002. In the beginning, elementary statistics has been carried out to understand the statistical distribution and performed an analysis of variance so that two campaigns could be chosen for comparison. Geostatistics for modeling and cartography were applied. The results show that piezometric levels admit power model that the direction of groundwater flow is from south to north and that, due to the return of irrigation water (there is no drainage system), water levels increase in places like Foulia (1.35 m), northwest of Oued-Souf (1.43 m), Kouinine (4.58 m), and Ouzitene (3.01 m). The water levels also decrease in places like Guemar, Dmitha, Rhamra, and Djeldida, from between −2 and −6 m, due to excessive pumping of groundwater.     

Hydrogeochemical and isotopic evolution of water in the Complexe Terminal aquifer in the Algerian Sahara

The hydrogeochemical and isotopic evolution of groundwaters in the Mio–Pliocene sands of the Complexe Terminal (CT) aquifer in central Algeria are described. The CT aquifer is located in the large sedimentary basin of the Great Oriental Erg. Down-gradient groundwater evolution is considered along the main representative aquifer cross section (south–north), from the southern recharge area (Tinrhert Plateau and Great Oriental Erg) over about 700 km. Groundwater mineralisation increases along the flow line, from 1.5 to 8 g l^?1, primarily as a result of dissolution of evaporite minerals, as shown by Br/Cl and strontium isotope ratios. Trends in both major and trace elements demonstrate a progressive evolution along the flow path. Redox reactions are important and the persistence of oxidising conditions favours the increase in some trace elements (e.g. Cr) and also NO₃ ^?, which reaches concentrations of 16.8 mg l^?1 NO₃-N. The range in ¹⁴C, 0–8.4 pmc in the deeper groundwaters, corresponds with late Pleistocene recharge, although there then follows a hiatus in the data with no results in the range 10–20 pmc, interpreted as a gap in recharge coincident with hyper-arid but cool conditions across the Sahara; groundwater in the range 24.7–38.9 pmc signifies a distinct period of Holocene recharge. All δ¹⁸O compositions are enriched relative to deuterium and are considered to be derived by evaporative enrichment from a parent rainfall around ?11‰ δ¹⁸O, signifying cooler conditions in the late Pleistocene and possibly heavy monsoon rains during the Holocene.     

The copper-silver occurrences of Rahmani,Western Sahara,Algeria

The copper-silver occurrences of Rahmani (Western Sahara, Algeria) are located in paleochannel facies of Cambrian sandstones deposited onPan-African volcanics and intrusives. Sulfur isotope analyses were performed on pyrite and copper sulfides in order to trace the origin of the copper-silver mineralization. S isotopic data preclude that bacterial reduction of Cambrian sulfate could have induced the formation of the sulfides. Non-bacterial reduction of sulfate during burial diagenesis is the most valuable explanation for disseminated pyrite. Isotopic ratios on copper sulfides indicate that they result from the reaction of actual or subactual sulfate-bearing surface water with the disseminated pyrite. The origin of copper and silver remains unclear. They are thought to be brought by the downward migrating surface water but their origin could be either the leaching of the Cambrian sandstones or of the weathered volcanics.     

Cenozoic stratigraphy of the Sahara,Northern Africa

This paper presents an overview of the Cenozoic stratigraphic record in the Sahara, and shows that the strata display some remarkably similar characteristics across much of the region. In fact, some lithologies of certain ages are exceptionally widespread and persistent, and many of the changes from one lithology to another appear to have been relatively synchronous across the Sahara. The general stratigraphic succession is that of a transition from early Cenozoic carbonate strata to late Cenozoic siliciclastic strata. This transition in lithology coincides with a long-term eustatic fall in sea level since the middle Cretaceous and with a global climate transition from a Late Cretaceous–Early Eocene “warm mode” to a Late Eocene–Quaternary “cool mode”. Much of the shorter-term stratigraphic variability in the Sahara (and even the regional unconformities) also can be correlated with specific changes in sea level, climate, and tectonic activity during the Cenozoic. Specifically, Paleocene and Eocene carbonate strata and phosphate are suggestive of a warm and humid climate, whereas latest Eocene evaporitic strata (and an end-Eocene regional unconformity) are correlated with a eustatic fall in sea level, the build-up of ice in Antarctica, and the appearance of relatively arid climates in the Sahara. The absence of Oligocene strata throughout much of the Sahara is attributed to the effects of generally low eustatic sea level during the Oligocene and tectonic uplift in certain areas during the Late Eocene and Oligocene. Miocene sandstone and conglomerate are attributed to the effects of continued tectonic uplift around the Sahara, generally low eustatic sea level, and enough rainfall to support the development of extensive fluvial systems. Middle–Upper Miocene carbonate strata accumulated in northern Libya in response to a eustatic rise in sea level, whereas Upper Miocene mudstone accumulated along the south side of the Atlas Mountains because uplift of the mountains blocked fluvial access to the Mediterranean Sea. Uppermost Miocene evaporites (and an end-Miocene regional unconformity) in the northern Sahara are correlated with the Messinian desiccation of the Mediterranean Sea. Abundant and widespread Pliocene paleosols are attributed to the onset of relatively arid climate conditions and (or) greater variability of climate conditions, and the appearance of persistent and widespread eolian sediments in the Sahara is coincident with the major glaciation in the northern hemisphere during the Pliocene.     

Distribution of terminal electron-accepting processes in an aquifer having multiple contaminant sources

Concentrations of electron acceptors, electron donors, and H₂ in groundwater were measured to determine the distribution of terminal electron-accepting processes (TEAPs) in an alluvial aquifer having multiple contaminant sources. Upgradient contaminant sources included two separate hydrocarbon point sources, one of which contained the fuel oxygenate methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO₃, SO₄ and organic carbon (DOC) to the downgradient part of the aquifer. Groundwater downgradient from the MTBE source had larger concentrations of electron acceptors (dissolved O₂ and SO₄) and smaller concentrations of TEAP end products (dissolved inorganic C, Fe²⁺ and CH₄) than groundwater downgradient from the other hydrocarbon source, suggesting that MTBE was not as suitable for supporting TEAPs as the other hydrocarbons. Measurements of dissolved H₂ indicated that SO₄ reduction predominated in the aquifer during a period of high water levels in the aquifer and river. The predominant TEAP shifted to Fe³⁺ reduction in upgradient areas after water levels receded but remained SO₄ reducing downgradient near the river. This distribution of TEAPs is the opposite of what is commonly observed in aquifers having a single contaminant point source and probably reflects the input of DOC and SO₄ to the aquifer from the river. Results of this study indicate that the distribution of TEAPs in aquifers having multiple contaminant sources depends on the composition and location of the contaminants and on the availability of electron acceptors.     

A very large scale GIS-based groundwater flow model for the Nubian sandstone aquifer in Eastern Sahara (Egypt,northern Sudan and eastern Libya)

A three-dimensional GIS-based groundwater flow model for the Nubian Sandstone Aquifer in the eastern Sahara was developed and calibrated under steady-state and transient conditions. The model was used to simulate the response of the aquifer to climatic changes that occurred during the last 25,000 years. The simulation results indicated that the groundwater in this aquifer was formed by infiltration during the wet periods 20,000 and 5,000 years b.p. The recharge of groundwater due to regional groundwater flow from more humid areas in the south was excluded. It also indicates that the Nubian Aquifer System is a fossil aquifer, which had been in an unsteady state condition for the last 3,000 years.

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Resumen Fue desarrollado un modelo de flujo de agua subterránea en tres dimensiones, basado en un SIG, para el Acuífero Arenisca Nubian en el Sahara Oriental, el cual fue calibrado para condiciones de estado estacionario y transitorio. El modelo se usó para simular la respuesta del acuífero a los cambios climáticos que ocurrieron durante los últimos 25000 años. Los resultados de esta simulación indicaron que el agua subterránea en este acuífero, se formó por infiltración, durante los períodos húmedos que hubo hace 20000 y 5000 años, antes del presente. Fue excluida la recarga del acuífero debida a un flujo regional de agua subterránea proveniente de áreas con un clima más húmedo en el sur. El modelo también muestra, que el Sistema Acuífero Nubian es un acuífero fósil, el cual ha permanecido en una condición de estado no estacionario, durante los últimos tres mil años.

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Résumé Pour laquifère gréseux Nubien de Sahara -Est on a mis au points un modèle tridimensionnel, basé sur GIS. Le modèle a été calibré tant pour lécoulement stationnaire que pour lécoulement transitoire. On a simulé après la réponse de laquifère aux changements climatiques des derniers 25000 ans. Les résultats des simulations indiquent que la nappe a été rechargée par des infiltrations pendant une période humide qui sétend 5000 et 20000 ans, dès temps actuel. On na pas pris en compte la recharge de laquifère par la zone plus humide située dans sa partie sud. Le modèle indique aussi que leau de laquifère Nubien est une eau fossile qui a eu un écoulement transitoire pendant les derniers 3000 ans.

     

The U-Pb systematics of angrite Sahara 99555

Angrite Sahara 99555 (hereafter SAH), precisely dated by Baker et al. (Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature436, 1127-1131), has been proposed as a new reference point for the early Solar System timescale and for calculation of the revised minimum age of our Solar System. The Pb-Pb age of SAH of 4566.18 ± 0.14 Ma, reported by Baker et al., differs from the Pb-Pb age of D’Orbigny, another basaltic angrite, of 4564.42 ± 0.12 Ma (Amelin Y. (2008) U-Pb ages of angrites. Geochim. Cosmochim. Acta72, 221-232), despite the fact that the relative ⁵³Mn-⁵³Cr and ¹⁸²Hf-¹⁸²W ages of these meteorites are identical. Here I report U-Pb data for 21 whole rock and pyroxene fractions from SAH, analyzed using the same approach as D’Orbigny (Amelin, 2008). These fractions contain between 1.3 and 8.9 pg of total common Pb, slightly more than analytical blank. Measured ²⁰⁶Pb/²⁰⁴Pb ratios are between 625 and 2817 for D’Orbigny, blank-corrected ²⁰⁶Pb/²⁰⁴Pb ratios are between 1173 and 6675. Eight acid-washed whole rock fractions yielded an isochron age of 4564.86 ± 0.38 Ma, MSWD = 1.5. Data for pyroxene fractions plot mostly above the whole rock isochron, and do not form a linear array in ²⁰⁷Pb/²⁰⁶Pb vs. ²⁰⁴Pb/²⁰⁶Pb isochron coordinates. The ²⁰⁷Pb^∗/²⁰⁶Pb^∗ model dates of the pyroxene fractions vary from 4563.8 ± 0.3 to 4567.1 ± 0.5 Ma. The difference between whole rock and pyroxene U-Pb systematics may be a result of re-distribution of radiogenic Pb at a mineral grain scale several million years after crystallization. Complexities of Sm-Nd, Lu-Hf, and possibly ²⁶Al-²⁶Mg mineral systematics of SAH, described previously, may be related to the same process that caused the re-distribution of radiogenic Pb. Disturbance of isotopic chronometers renders SAH an imperfect anchor for the early Solar System timescale. The problems with age determination revealed by the studies of SAH call for greater attention in Pb-isotopic dating of angrites and other achondrites.     

Uranium isotopic disequilibrium for groundwater classification: first results on complexe terminal and continental intercalaire aquifers in Southern Tunisia

A fractionation of uranium (U) series into parent–daughter pairs – ²³⁴U and ²³⁸U – always occurs in natural waters, and the disequilibria between these is commonly used as a tracer of groundwater flow. We report here an interpretation of the U-content and U isotope disequilibria in groundwater sampled from the deep Complexe Terminal and Continental Intercalaire aquifers of southern Tunisia. Variations in both the contents of these isotopes (0.006–2.4 ppb) and ²³⁴U/²³⁸U activity ratios (ARs) (1.7–15.4) were observed. The data could be plotted in two distinct fields of reciprocal U concentration versus ²³⁴U/²³⁸U AR according to groundwater flow and regional bedrock differences. An initial assessment aimed at verifying whether the results of this investigation support those of previous hydrogeological and isotope studies, thereby suggesting that the disequilibrium between U isotopes in groundwater may represent a useful tool for hydrogeological investigations of deep and fossil groundwater. In addition, the disequilibrium can be used for quantifying the recharge or mixing rates between different formations with the aim of delineating the preferential outflow pattern or determining residence times of waters.     

The groundwater age in the Middle-Upper Devonian aquifer system, Lithuania

³H, δ¹³C and hydrochemical data were used to estimate the corrected groundwater age derived from conventional ¹⁴C age of dissolved inorganic carbon (DIC). The Middle-Upper Devonian aquifer system from the Baltic upland recharge area in eastern Lithuania towards the discharge area on the Baltic Sea coast in the west was considered. The concentration of total dissolved solids (TDS) in groundwater changes from 300 to 24,000  mg/L and increases downgradient towards the coast. The other major constituents have the same trend as the TDS. The hydrochemical facies of groundwater vary from an alkali-earth carbonates facies at the eastern upland area to an alkali-earth carbonate-sulfate and chloride facies at transit and discharge areas. Meteoric water percolating through the Quaternary and Devonian aquifers regulate the initial ¹⁴C activities of groundwater involving two main members of DIC: soil CO₂ with modern ¹⁴C activity uptake and dissolution of ¹⁴C-free aquifer carbonates. Other sources of DIC are less common. ¹⁴C activity of DIC in the groundwater ranged from 60 to 108 pMC at the shallow depths. With an increase of the aquifers depth the dolomitization of aqueous solution and leakage of the “old” groundwater from lower aquifers take place, traced by lower activities (7–30 pMC).     

The salinity of Djibouti''s aquifer

Fresh water supplied to Djibouti town is essentially groundwater located inthe fractured Gulf and Somali basalt aquifers. About 30 wells (3 to 6 km inland from the sea) are exploited and provide water of rather poor quality (TDS between 1000 mg 1⁻¹ and 2800 mg 1⁻¹). A sea water interface has been recognized locally some 3.8 km from the sea at 35 m below sea level. However, a well at Hidka Gisiyed, some 11 km from the sea, also contains water with high salinity (TDS=14,000 mg 1⁻¹) at depth. The over-exploitation of the aquifer and the high pumping rate are contributing to an increase in the salinity due to the intrusion of sea water, as shown by the chemical results. The Hidka Gisiyed saline water is another possible source of saline water at depth.The isotopic results from part of the Djibouti aquifer have shown that current recharge from local rain or surface runoff is occurring. An understanding of factors influencing the evolution of the salinity will allow the better management of the aquifer.     

The hydrogeochemical characterization of Morsott-El Aouinet aquifer, Northeastern Algeria

A study of the hydrogeochemical processes in the Morsott-El Aouinet aquifer was carried out with the objective of identifying the geochemical processes and their relation with groundwater quality as well as to get an insight into the hydrochemical evaluation of groundwater. The high salinity coupled with groundwater level decline pose serious problems for current irrigation and domestic water supplies as well as future exploitation. A combined hydrogeologic and isotopic investigation have been carried out using chemical and isotopic data to deduce a hydrochemical evaluation of the aquifer system based on the ionic constituents, water types, hydrochemical facies and factors controlling groundwater quality. The ionic speciation and mineral dissolution/precipitation was calculated by WATEQF package software. The increase in salinity is related to the dissolution and/or precipitation processes during the water–rock interaction and to the cationic exchange reactions between groundwater and clay minerals. The isotopic analysis of some groundwater samples shows a similarity with the meteoric waters reflect their short residence time and a lowest evaporation phenomenon of infiltrated groundwater.     

Variations in groundwater levels and quality due to agricultural over-exploitation in an arid environment: the phreatic aquifer of the Souf oasis (Algerian Sahara)

Groundwater is a major source of supply for domestic and agricultural purposes, especially in arid and semi-arid regions. In this study, we followed the variations in water levels in the Souf oasis in the Algerian Sahara by measuring depths to groundwater across 65 points during the period from 2010 to 2015. Additionally, electrical conductivity (EC) was measured for assessing variations in groundwater salinity in the same groundwater monitoring network over the same time interval. The results from these investigations indicated that there are significant and continuous declines in the groundwater level across all study areas throughout the period of investigation. This is especially the case in the northern part of the study area where the water table declined by up to 18.2 m in Ghamra in 2015. Additionally, this study has indicated that the rate of decline of groundwater levels has increased from 0.29 m/year as an average in 2011 to 2.37 m/year in 2015, where the situation has become alarming. As a consequence of this, the depth to groundwater now exceeds 2 m over more than 77% of the study area, and only about 17% of the study area now has a water table depth that lies within the optimal depth interval for extractive uses (between 1 and 2 m). This decline in groundwater levels has been accompanied by a significant increase in the electrical conductivity values (salinity) of this water, and there is a strong correlation between these variables (R > 0.99). This alarming situation has been caused by the continuous over-exploitation and unsustainable management of this limited resource, especially by the agricultural sector. For a long time, this critical situation led to the demise of the agricultural world heritage cultivation system (Ghout) due to the increasing salinity of groundwater. Two solutions are proposed to manage the effects of groundwater depletion in the area: firstly, rationalizing groundwater use through effective groundwater allocation management measures, and secondly by implementing the reuse of treated wastewater as an alternative water source for agricultural use. This latter measure could be in two ways: either by direct use in irrigation to relieve pressure on the phreatic aquifer, or by artificial recharge of the phreatic aquifer.     

An aquifer vulnerability assessment of the Benin Formation aquifer,Calabar, southeastern Nigeria,using DRASTIC and GIS approach

An aquifer vulnerability of the Benin Formation aquifer (Calabar, southern Nigeria) has been assessed using a combination of DRASTIC index and GIS technology. The assessment was necessitated by the fact that uncontrolled disposal of domestic, industrial and agricultural wastes have caused groundwater contamination. Therefore, prevention of contamination, monitoring and management of the aquifer was urgently required to increase the efficient use of the current water supplies. The DRASTIC method uses seven parameters (depth to groundwater table, net recharge, aquifer media, soil media, topography, influence of vadose zone and hydraulic conductivity), which were used to produce vulnerability maps. The drastic vulnerability index ranged between 124 and 170. The vulnerability map shows that the aquifer is highly vulnerable in southeastern parts of the area covering about 22 %. The medium vulnerability area covers about 56.8 % of Calabar extending from the southwest to northern parts. 21.2 % of the area covering the central and northern parts the area lies within the low vulnerability zone. The present industrial and activities are located in the eastern and western parts, which falls within the low-medium vulnerability areas. Documented nitrate concentration in hand-dug wells and boreholes are in agreement with vulnerability zones. Sensitivity analysis was performed to evaluate the sensitivity of each parameter between map layers such that subjectivity can be reduced to an extent and new weights computed for each DRASTIC parameter. As management options sensitive areas, especially in the southern parts of Calabar area, should be protected from future development.     

The hydrogeology and hydrogeochemistry of the Barwon Downs Graben aquifer, southwestern Victoria, Australia

The Barwon Downs Graben lies on the northern flanks of the Otway Ranges and is situated approximately 70 km southwest of Geelong, Victoria, Australia. The major lower Tertiary Barwon Downs Graben aquifer comprises highly permeable sands and gravels interbedded with clays and silts of the hydraulically interconnected Pebble Point, Dilwyn and Mepunga Formations. Groundwater flows east into the Barwon Downs Graben from the Barongarook High, and yields ¹⁴C ages up to ～20 ka implying that recharge rates are low and, consequently, that the resource could be impacted by overabstraction. The presence of three different lithological units has led to the development of localized flow systems that has resulted in a lack of regular spatial variations in groundwater chemistry. Stable isotopic data suggests that groundwater was recharged under similar climatic conditions as of today. The major ion chemistry of the freshest groundwater is dominated by Na and HCO₃ while higher TDS groundwater, from the confining Narrawaturk Marl, is dominated by Na and Cl. Cl/Br ratios are close to rainfall suggesting that halite dissolution is not the principle source of salts. An excess of Na relative to Cl in fresher groundwater suggests that feldspar dissolution has occurred, however, water–rock interaction is limited. The concentrations of Ca, Mg, and SO₄ are controlled by silicate dissolution and ion-exchange reactions with clays.     

Geological exploration history of the Eastern Sahara

The major contributions to our knowledge of the geological history of the East Saharan countries are described. Much work has been published on the history and progress of geographical, socioeconomic and archaeological exploration in North Africa. Little attention, however, has been paid to the question of how our knowledge of the geology of this vast area was developed. Thus, even though progress in geology is vital to the development of Saharan countries, little has been recorded of this progress. Without an understanding of regional geology and without a good knowledge of groundwater, mineral and petroleum potentials, these countries would be in very different situations today. The author has, over the last 35 years, been involved in scientific research and exploration of petroleum, mineral and groundwater resources in Libya, Egypt, Sudan and Chad.     

The impact of the geologic setting on the Quaternary aquifer,El-Tur area,Southwest Sinai,Egypt

The groundwater extracted from the unconfined Quaternary aquifer is the main source of water supply in El-Tur area. The area is bounded from the east by the elevated basement complex of Southern Sinai and from the west by El-Qabaliyat Ridge. The wadis dissecting these highlands form effective watersheds of the Quaternary aquifer. These wadis form areas of focused recharge. Recharge also occurs directly via the Quaternary sediments covering El-Qaa Plain. Subsurface lateral groundwater flow from the fractured basement contributes significant recharge to the aquifer as well. The aquifer sediment facies affect the type and quality of groundwater. In the eastern part where the aquifer is composed mainly of gravel and coarse sand with fragments of weathered basement, the Na-Cl-SO₄ water dominates. In the west where the facies change is rapid and complex, many water types arise. The base exchange index (BEX) is positive in this part reflecting the role of clay minerals in changing the water types via cation exchange. In the east where clays are insignificant in the aquifer, the BEX is negative. In the western part next to El-Qabaliyat Ridge, the wells discharging from the calcareous sand zone have low groundwater salinities compared to the wells discharging from the alluvium. In general, the groundwater salinity increases in the direction of groundwater flow from the northeast to the southwest which reflects the dissolution of aquifer sediments. The concentration relationships between the major ions on one hand and chloride on the other reflect the dissolution of calcium carbonates, precipitation of K- and Mg-bearing minerals, and cation exchange of Ca for Na on clay minerals. The hydrochemical models support these reactions. In addition, they show that the effect of evaporation on the recharge water in the western catchment is about four times its effect on the eastern recharge water which reflects the rapid recharge through the wadis draining the fractured basement. Moreover, the contribution from the eastern catchment in sample No. 23 is more than four-folds the contribution from the western recharge area. The stable isotopes (²H and ¹⁸O) show that the Quaternary aquifer is recharging from recent rainfall. However, upward leakage of Paleogene groundwater (depleted in ¹⁸O) also occurs. The groundwater level map shows strong overpumping impact especially in the areas close to El-Tur city.     

The origin of fluoride in the granitic aquifer of Porto Alegre, Southern Brazil

The Porto Alegre region, Southern Brazil, comprises a fractured aquifer system constituted by pre-Cambrian granitic and gneissic rocks, and a porous aquifer system formed by Cenozoic muddy–sandy to sandy sediments. A model is presented for the origin of the fluoride in the groundwater of the fractured aquifer system, based on ionic interrelations, a statistical analysis of physicochemical parameters and geochemical modeling. The fluoride is present at levels of up to 6.13 mg/L in groundwater and it arises due to the dissolution of secondary fluorite filling fractures in the granitic and gneissic rocks. The dissolution of fluorite occurs at the same time as calcite and dolomite solubilization. The statistical analysis identified three chemically distinct groups (named 1, 2 and 3) and two subgroups (1A and 1B) of groundwater in the fractured aquifer. The most significant differences between these groups are the different concentration ranges of fluoride, calcium, bicarbonate, magnesium, total dissolved solids and chloride and pH values, as well as the correlations between them. The compositional evolution of each groundwater group is governed mainly by how much high-salinity groundwater from the porous aquifer system is mixed with them and the different thermodynamic equilibrium conditions of calcite, dolomite and fluorite.     

电测深法在撒哈拉沙漠地区找水应用

撒哈拉沙漠地区气候干燥,水资源缺乏,相应的地质、水文地质工作研究程度低,为水源地勘探工作带来很大困难。结合实例,采用电测深法,遵循从已知到未知原则,经过点、线、面逐步开展工作,总结含水层物性特征,划定主要含水区域,为水文工作的开展提供重要的基础资料。