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Determination of Recharge Sources of Sfax Aquifer System (Southeastern Tunisia) Using Environmental Isotope Tracers
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A hydrogeochemical approach has been carried out in the Mio-Plio-Quaternary aquifer system of northern Sfax to investigate the geochemical evolution, the origin of groundwaters and their circulation patterns. The groundwater samples collected from different wells seem to be dominated by sodium chloride type to sulphate chloride type. Detail analysis of chemical data including the thermodynamic calculations was used to assess that the chemical evolution of groundwater is primarily controlled by water–rock interactions. The values of sodium absorption ratio and electrical conductivity of the groundwater were plotted in the US Salinity Laboratory diagram for irrigation water. Most of the water samples in northern Sfax fall in the fields of C4S1, C4S2 and C4S3 indicating very high salinity and medium to high sodium alkalinity hazard. Thus, groundwater quality is ranging between doubtful to unsuitable for irrigation uses under normal condition, and further action for salinity control is required in remediating such problem. Principal component analysis of geochemical data used in conjunction with bivariate diagrams of major elements indicates that groundwater mineralization is mainly controlled by (1) water–rock interaction processes, (2) anthropogenic process in relation with return flow of NO3-rich irrigation waters and (3) domestic discharges.
The International Atomic Energy Agency (IAEA), in cooperation with the
World Meteorological Organization (WMO), has been conducting a
world-wide survey of hydrogen (2H/1H) and oxygen
(18O/16O) isotope composition of monthly
precipitation since 1961. At present, 72 IAEA/WMO network stations are
in operation. Another 82 stations belonging to national organizations
continue to send their results to the IAEA for publication. The paper
focuses on basic features of spatial and temporal distribution of
deuterium and 18O in global precipitation, as derived from
the IAEA/WMO isotope database. The internal structure and basic
characteristics of this database are discussed in some detail. The
existing phenomenological relationships between observed stable isotope
composition of precipitation and various climate-related parameters such
as local surface air temperature and amount of precipitation are
reviewed and critically assessed. Attempts are presented towards
revealing interannual fluctuations in the accumulated isotope records
and relating them to changes of precipitation amount and the surface air
temperature over the past 30 years.
The intensive agricultural and economic activities induce the increase of the risk of groundwater quality degradation through high groundwater pumping rates. The salinization and contamination are the main sources of this pollution, especially in coastal aquifers. The explanation of the origin of salinity for the shallow aquifer of Northern Sahel of Sfax was analysed by a chemical study of the groundwater main compounds. The partitioning of groundwaters into homogenous groups is undertaken by graphical techniques, including a Stiff pattern diagram, an expanded Durov diagram and several binary diagrams. The study indicates the presence of various salinization processes. In the recharge area, salinization is the result of dissolution/precipitation of the aquifer formation material (group I). The irrigation water return and the intensive pumping have been identified as major sources of salinization in the south by direct cation exchange and mixing reactions (groups II and III). The anomaly of high groundwater salinity observed near the Hazeg zone was explained by the presence of a seawater intrusion in this area. This hypothesis is related to the high chloride concentration, to the presence of inverse cation exchange reactions (group IV), and to the piezometric level inferior to sea level. To cite this article: R. Trabelsi et al., C. R. Geoscience 337 (2005).
The geochemical processes taking place along an 800 km flow line in the non-carbonate Continental Intercalaire aquifer (CI) aquifer 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 highest discharge temperature is 73 °C but silica geothermometry indicates a maximum temperature of 94 °C at depth. Chloride concentrations increase from 200 to 800 mg l−1 and the Br/Cl ratios confirm the dissolution of non-marine evaporites or interstitial waters as the main source of salinity. Fluoride concentrations are low and are likely to be derived from rainfall, recording oscillations in source. Radiocarbon ages, except near outcrop, are at or near detection limits and the δ18O and δ2H values indicate a cooler recharge regime with rainfall having lower primary evaporation than today. This is shown by the fact that mean isotope ratios of CI waters are around 3‰ lighter than the present-day weighted mean value for rain. Major ion ratios and most trace elements indicate that despite the complex structure and stratigraphy, uniform evolution with continuous water-rock interaction takes place along the flow lines, which are only disturbed near the Tunisian Chotts by groundwater converging from additional flow lines. The ageing of the water can also be followed by the smooth increase in several indicator elements such as Li, K and Mn which are least affected by solubility controls. Similarly the influence of marine facies in the Tunisian sector may be recognised by the changing Mg/Ca and higher Br/Cl as well as trace element indicators. The groundwaters are oxidising up to 300 km from outcrop (dissolved O2 has persisted for at least 20 ka) and within this zone the concentrations of several elements forming oxy-anions, such as U and Cr, increase and NO3 remains conservative. Beyond 300 km from outcrop, the groundwaters are reducing and contain high Fe concentrations. The basin contains huge reserves of fresh/brackish waters but these need careful development due to the limiting high salinity and scaling tendency resulting from the high temperatures and mineral super-saturation caused during abstraction as well as high concentrations of some harmful elements such as Cr in the oxidising section.
This paper provides a summary overview of the current state-of-art in the radiocarbon dating of groundwater. While the use of natural 14 C measurements in applied hydrogeology still presents a difficult challenge, meaningful dates can be achieved if they are determined and interpreted in conjunction with the analyses of other isotopic species that occur in the natural environment. Although 14 C dating of groundwater can be, and often is, carried out as a matter of routine, any specific case study requires its own scientific design and effort. As is widely recognized, and discussed in considerable detail through-out the scientific literature, there are many hydrogeochemical reactions and/or physical processes that can alter the natural 14 C enrichment measured in environmental materials. Fortunately, for fresh groundwater resources such effects are in general well defined and therefore of limited significance. The primary challenge in applied groundwater dating is with the develop-ment of the appropriate theoretical background against which 14 C dates can be used to calibrate numerical analogues of the groundwater system. The hydraulic properties of each of the widely used finite-element models can be well estimated from numerous piezometric data and extrapolations. In contrast, only a few groundwater ages can be provided for the calibration of those models that are complex functions of aging mixture and sometimes also hydrochemical reactions.
Global synthesis of the findings from ∼140 recharge study areas in semiarid and arid regions provides important information on recharge rates, controls, and processes, which are critical for sustainable water development. Water resource evaluation, dryland salinity assessment (Australia), and radioactive waste disposal (US) are among the primary goals of many of these recharge studies. The chloride mass balance (CMB) technique is widely used to estimate recharge. Average recharge rates estimated over large areas (40–374 000 km2) range from 0·2 to 35 mm year−1, representing 0·1–5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ∼720 m year−1, results from focussed recharge beneath ephemeral streams and lakes and preferential flow mostly in fractured systems. System response to climate variability and land use/land cover (LU/LC) changes is archived in unsaturated zone tracer profiles and in groundwater level fluctuations. Inter-annual climate variability related to El Niño Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW US during periods of frequent El Niños (1977–1998) relative to periods dominated by La Niñas (1941–1957). Enhanced recharge related to ENSO is also documented in Argentina. Climate variability at decadal to century scales recorded in chloride profiles in Africa results in recharge rates of 30 mm year−1 during the Sahel drought (1970–1986) to 150 mm year−1 during non-drought periods. Variations in climate at millennial scales in the SW US changed systems from recharge during the Pleistocene glacial period (≥10 000 years ago) to discharge during the Holocene semiarid period. LU/LC changes such as deforestation in Australia increased recharge up to about 2 orders of magnitude. Changes from natural grassland and shrublands to dryland (rain-fed) agriculture altered systems from discharge (evapotranspiration, ET) to recharge in the SW US. The impact of LU change was much greater than climate variability in Niger (Africa), where replacement of savanna by crops increased recharge by about an order of magnitude even during severe droughts. Sensitivity of recharge to LU/LC changes suggests that recharge may be controlled through management of LU. In irrigated areas, recharge varies from 10 to 485 mm year−1, representing 1–25% of irrigation plus precipitation. However, irrigation pumpage in groundwater-fed irrigated areas greatly exceeds recharge rates, resulting in groundwater mining. Increased recharge related to cultivation has mobilized salts that accumulated in the unsaturated zone over millennia, resulting in widespread groundwater and surface water contamination, particularly in Australia. The synthesis of recharge rates provided in this study contains valuable information for developing sustainable groundwater resource programmes within the context of climate variability and LU/LC change. Copyright © 2006 John Wiley & Sons, Ltd.
Tunisian Chott’s region is one of the most productive artesian basins in Tunisia. It is located in the southwestern part of
the country, and its groundwater resources are developed for water supply and irrigation. The chemical composition of the
water is strongly influenced by the interaction with the basinal sediments and by hydrologic characteristics such as the flow
pattern and time of residence. The system is composed of an upper unconfined “Plio-Quaternary” aquifer with a varying thickness
of 20–200m, an intermediate confined/unconfined “Complex Terminal” aquifer about 100m in thickness and a deeper “Continental
Intercalaire” aquifer about 150m in thickness separated by thick clay and marl layers. The dissolution of evaporites and
carbonates explains part of the contained Na+, Ca2+, Mg2+, K+, SO42− and Cl-, but other processes, such carbonate precipitation, also contributes to the water composition. The stable isotope composition
of waters establishes that the deep groundwater (depleted as compared to present corresponding local rainfall) is ancient
water recharged probably during the late Pleistocene and the early Holocene periods. The relatively recent water in the Plio-Quaternary
aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater.
The hydrodynamic groundwater data and stable isotopes of water have been used jointly for better understanding of upward leakage
and mixing processes in the Djerid aquifer system (southwestern Tunisia). The aquifer system is composed of the upper unconfined
Plio-Quaternary (PQ) aquifer, the intermediate (semi-)confined Complex Terminal (CT) aquifer and the deeper confined Continental
Intercalaire (CI) aquifer. A total of 41 groundwater samples from the CT and PQ aquifers were collected during June 2001.
The stable isotope composition of waters establishes that the CT deep groundwater (depleted as compared to present Nefta local
rainfall) is ancient water recharged during late Quaternary time. The relatively recent water in the shallow PQ aquifer is
composed of mixed water resulting from upward leakage and sporadic meteoric recharge. In order to characterize the meteoric
input signal for PQ in the study area, rainfall water samples were collected during 4years (2000–2003) at the Nefta meteorological
station. Weighted mean values of isotopic contents with respect to rainfall amounts have been computed. Despite the short
collection period in the study area, results agree with those found in Beni Abbes (southwestern Algerian Sahara) by Fontes
on 9years of rainfall surveillance. Stable isotopic relationships provide clear evidence of shallow PQ aquifer replenishment
by deep CT groundwater. The 18O/upward leakage rate allowed the identification of distinctive PQ waters related to CT aquifer configuration (confined in
the western part of the study area, semi-permeable in the eastern part). These trends were confirmed by the relation 18O/TDS. The isotope balance model indicated a contribution of up to 75% of the deep CT groundwater to the upper PQ aquifer
in the western study area, between Nefta and Hazoua.
KeywordsTunisia–Oxygen-18–Deuterium–Groundwater–Leakage
The Swiss National Network for the Observation of Isotopes in the Water Cycle (NISOT) includes eleven precipitation, seven surface water (river) and three groundwater stations, where tritium, deuterium and oxygen-18 are monthly measured in composite samples. The network provides a good overview of the characteristic isotope signatures in recharge waters in Switzerland and of the relations between isotopes and altitude, orography and the amount of precipitation. Mixing of air water vapour and surface waters can be observed along a NW/SE cross section through the Alps. With increasing length of the data series, the network provides a valuable contribution for national and international scientific and practical applications in surface and subsurface hydrology, climatology and biology. The Swiss Geological Survey at the Federal Office for Water and Geology operates the isotope network within the legal framework of the Federal Law on the Protection of Waters and guarantees quality, access and distribution of the isotope data.
Groundwater and surface water in Souss–Massa basin in the west-southern part of Morocco is characterized by a large variation in salinity, up to levels of 37 g L−1. 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 using several chemical and isotopic tracers such as Br/Cl, δ18O, δ2H, 3H, 87Sr/86Sr, δ11B, and 14C was carried out in order to determine the sources of water recharge to the aquifer, the origin of salinity, and the residence time of water. Stable isotope, 3H and 14C data indicate that the high Atlas mountains in the northern margin of the Souss–Massa basin with high rainfall and low δ18O and δ2H values (−6 to −8‰ and −36 to −50‰) is currently constitute the major source of recharge to the Souss–Massa shallow aquifer, particularly along the eastern part of the basin. Localized stable isotope enrichments offset meteoric isotopic signature and are associated with high nitrate concentrations, which infer water recycling via water agricultural return flows. The 3H and 14C data suggest that the residence time of water in the western part of the basin is in the order of several thousands of years; hence old water is mined, particularly in the coastal areas. The multiple isotope analyses and chemical tracing of groundwater from the basin reveal that seawater intrusion is just one of multiple salinity sources that affect the quality of groundwater in the Souss–Massa aquifer. We differentiate between modern seawater intrusion, salinization by remnants of seawater entrapped in the middle Souss plains, recharge of nitrate-rich agricultural return flow, and dissolution of evaporate rocks (gypsum and halite minerals) along the outcrops of the high Atlas mountains. The data generated in this study provide the framework for a comprehensive management plan in which water exploitation should shift toward the eastern part of the basin where current recharge occurs with young and high quality groundwater. In contrast, we argued that the heavily exploited aquifer along the coastal areas is more vulnerable given the relatively longer residence time of the water and salinization processes in this part of the aquifer.
Groundwater is the most important source of water supply in Sidi Bouzid plain located in central Tunisia. Proper understanding of the geochemical evolution of groundwater is important for sustainable development of water resources in this region. A hydrogeochemical survey was conducted on the Mio–Plio–Quaternary aquifer system using stable isotopes, radiocarbon, tritium and major elements, in order to evaluate the groundwater chemistry patterns and the main mineralization processes occurring in this system. The chemical data indicate that dissolution of evaporate minerals and evaporation are the main processes controlling groundwater mineralization. The isotopic data show that groundwater in the study area is a mixture of recent shallow waters located upstream and along Wadi Al Fakka bed and paleowaters located towards plain limits and discharge areas. Low 3H and 14C contents are observed in major part of the plain indicating that recharge of the aquifer occurs mainly through direct infiltration at Wadi Al Fakka while there is no evidence of significant recharge in major part of the plain and mountains piedmonts.
stable isotope analysis
deuterium
hydrogen
groundwater
surface water
source
The major ion hydrochemistry and environmental isotope composition (18O, 2H) of the Tazoghrane shallow groundwaters were investigated to identify the sources and processes that affect the groundwater composition. The Quaternary shallow aquifer of Tazoghrane represents one of the most important aquifers in the Cap Bon peninsula. Groundwater from this aquifer is classified into two water types: Ca–SO4–Cl and Na–Cl. The groundwater composition is largely controlled by the water–rock interaction, particularly the dissolution of evaporate minerals and the ion exchange process. Elevated content of nitrates indicates that agricultural activities are probably the most significant anthropogenic sources of nitrogen contamination. The stable isotopic signatures reveal two water groups. Non-evaporated waters, which are distinguished by depleted d18O and d2H contents, indicate the significant recharge by modern rainfall from Mediterranean air masses. Evaporated waters that are characterized by relatively enriched d18O and d2H contents indicate an important component of recharge by return flow of irrigation waters.
Groundwaters from the intermediate aquifer of Sfax basin have been investigated using chemical tracers and environmental isotopes to reveal the hydrogeological features of this system and characterize the dynamics of groundwater salinization in this sector. The origin of salinity in the studied aquifer was investigated based on the chemical analyses of 30 groundwater samples. Groundwater is characterized by Na–Cl and Na–SO4 water types. The saturation indices for calcite and gypsum, and binary diagrams of different ions showed that the main hydrogeochemical processes were the dissolution of carbonates (mainly calcite scattered through the reservoir rocks), the dissolution of evaporites (halite, gypsum and anhydrite) and the cation exchange processes. The isotopic composition investigation allowed the definition of two groups. The first group is represented by groundwater with the highest oxygen-18 content (δ18O ranges between −5.36 and −4.22 ‰, and δ2H ranges between −39.2 and −36 ‰). The high δ18O values in this group can be attributed to the evaporation effect. The second group includes the most depleted waters (δ18O varies between −6.84 and −5.79 ‰, and δ2H varies between −42.4 and −36.5 ‰). The combined analyses of stable isotopes and major ions (Cl−) showed that mixing between old and recent water and evaporation were the main processes explaining the variation of salinity of the intermediate aquifer of the Sfax basin.
This book examines environmental isotope geochemistry in relation to the terrestrial environment. Topics considered include mathematical models for the interpretation of environmental radioisotopes in groundwater systems; isotopes in cloud physics: multiphase and multistage condensation processes; environmental isotopes in lake studies; environmental isotope and anthropogenic tracers of recent lake sedimentation; stable isotope geochemistry of travertines; isotope geochemistry of carbonates in the weathering zone; geochronology and isotopic geochemistry of speleothems; oxygen and hydrogen isotope geochemistry of deep basin brines; isotope effects of nitrogen in the soil and biosphere; chlorine-36 in the terrestrial environment; radioactive noble gases in the terrestrial environment and isotopes and food.
This paper addresses the problem of 14C age dating of groundwaters in a confined regional aquifer affected by methanogenesis. Increasing CH4 concentrations along the groundwater flow system and 13C and 14C isotopic data for dissolved inorganic carbon, dissolved organic carbon, and CH4 clearly show the effect of methanogenesis on groundwater chemistry. Inverse reaction path modeling using NETPATH indicates the predominant geochemical reactions controlling the chemical evolution of groundwater in the aquifer are incongruent dissolution of dolomite, ion exchange, methanogenesis, and oxidation of sedimentary organic matter. Modeling of groundwater 14C ages using NETPATH indicates that a significant part of groundwater in the Alliston aquifer is less than 13,000 years old; however, older groundwater in the range of 15,000–23,000 years is also present in the aquifer. This paper demonstrates that 14C ages calculated using NETPATH, incorporating the effects of methanogenesis on the carbon pools, provide reasonable groundwater ages that were not possible by other isotopic methods.
Carbon 14 age calculations are based upon the assumption that the
initial activity of the material to be dated was 100% of modern
CO2 activity (100 pmc). This assumption is generally good for
material of organic origin (wood and, to some extent, shells). Dissolved
carbon in groundwater has two main sources: active carbon from the soil
zone and less active (or even `dead') carbon of inorganic origin.
Furthermore, the existence of a three-phase system (gaseous
CO2 and aqueous and solid carbon) allows significant isotopic
exchange. The estimation of the initial activity of the total dissolved
carbon requires the use of a model. Existing models for the
determination of the initial 14C activity (A0) of
total dissolved carbon are (1) Vogel's model (A0 = 85
± 5% of modern carbon), (2) Tamers' model (A0 is the
activity of dissolved carbon of organic origin diluted by dissolved
carbon of inorganic origin), (3) Pearson's model (A0 is the
mixing of these same sources of carbon, evidenced by their stable
isotope content), and (4) Mook's model (same as Tamers' model plus a
correcting term accounting for an isotopic exchange in closed system
with aqueous CO2). A new model is presented which assumes the
simple mixing of the end-members of the system (gaseous CO2
and solid carbonate). Their relative proportions are given by comparison
of the molality of total dissolved carbon (measured bicarbonates and
calculated H2CO3, assuming equilibrium at the
temperature of the aquifer) to the molality of dissolved carbon of
inorganic origin (DIC). The DIC is determined on the basis of the
alkaline-earth molal concentration corrected for gypsum dissolution and
base exchange. This treatment can be more simply substituted for by the
measurement of field alkalinity, whose value, expressed in molal
concentration, is identical to twice the molal concentration of DIC. In
addition to the mixing, which also leads to Tamers' expression, an
isotope exchange term is calculated, assuming that a part of the
contribution of either soil CO2 or solid carbonate is in
isotopic equilibrium, in an open system, with the other carbon
reservoir. The different models are applied to the results of the
confined aquifer of the `calcaires carbonifères' in northern
France and Belgium. A value of -24‰ (equivalent to about
-22.8‰ for gaseous CO2) is adopted for the
13C content of aqueous carbon dioxide. Values for the isotope
fractionation which occurs between carbon-bearing compounds at aquifer
temperatures are adopted from values of Mook et al. (1974) and Deines et
al. (1974). It is assumed that ∈14C%
≃0.2∈13C‰. Radiometric ages obtained from
the Vogel and Tamers approaches are undercorrected. The model of Mook
appears very sensitive to the variations of δ 13C of
the total dissolved carbon and can lead to overcorrected values. The
model of Pearson is generally in rather good agreement with our
treatment in the range of average values of A0. For extremely
high and low values of A0 our model provides larger
corrections. The ages obtained from the different models are also
discussed on the basis of the 18O content of the waters,
which appear too high to be attributed to glacial epochs. Average flow
velocities calculated from our model range from 1.70 m yr-1
to 0.73 m yr-1.
The study of the isotopic composition (18O and 2H) of groundwater collected in the Sfax basin (Tunisia), helped to understand the behaviour of the different aquifers. It showed that the groundwater in the deep aquifer is old, probably slow moving and recharged under a colder climate than at present. The increasing exploitation of the shallow aquifers probably favoured upward leakage from the deep aquifer. Isotope balance equations allowed us to estimate the contribution of the deep aquifer to the shallow aquifer recharge.
Large-scale interaction between the Continental Intercalaire and the Djeffara aquifer systems in the southeast of Tunisia
has been investigated with the aid of chemical and isotopic tracers. Two distinct groundwater types have been identified:
(1) the Continental Intercalaire groundwater characterized by elevated temperatures (50–61.4°C), low δ18O (−8.4 to−7.87) and δ2H (−67.2 to−59) values and negligible radiocarbon content, both testifying its great age dating from the late Pleistocene
period, and (2) the Djeffara groundwater with distinctly heavier isotopic composition (δ18O=−8.31 to −5.80, δ2H=−65.9 to −31.9). The Djeffara groundwaters reveal a distinct changes of physico-chemical and isotopic parameters near
El Hamma Faults in the northwestern part of the Djeffara basin. These changes could possibly be explained by a vertical leakage
from the Continental Intercalaire aquifer through El Hamma Faults. The mixing proportions inferred from stable isotope mass
balance prove that the contribution of the Continental Intercalaire to the recharge of Djeffara aquifer is very significant
and may reach 100% in the El Hamma region and in the northern part of Gabes. Isotope tracers strongly suggest that recent
recharge to the Djeffara aquifer system is very limited. Its current yield, particularly in its central and northern parts
can be maintained only thanks to large-scale underground inflow from the Continental Intercalaire aquifer system, which carries
late Pleistocene palaeowater. Consequently, current exploitation of groundwater resources of the Djeffara aquifer has non-sustainable
character.
Natural chemical and isotopic tracers contained in unsaturated-zone moisture profiles are being developed as potential new archives for reconstructing recharge history, as well as palaeoclimatic or palaeobotanical conditions over time scales ranging from 20–120,000 years. Results worldwide to date are reviewed, and examples from northern Africa and the western USA are discussed in detail. Encouraging results are obtained from relatively homogeneous deposits such as Quaternary dune sands, where Cl profiles are compared both with the instrumental record, such as rainfall and river-gauging records, and 3H profiles. Model studies have helped to define the persistence time of unsaturated-zone signals, where evidence of a 20-year event such as the Sahel drought may persist for 1,000 years.
Significant questions remain regarding the assumptions used in interpreting profiles, particularly the extent to which preferential flow may occur, transient flow phenomena, and stability of tracer-input function. Unsaturated zones that exhibit strong preferential flow are probably unsuitable as archives. Questions remain also on the assumption that flow remains downward, especially in deep unsaturated zones where non-isothermal vapour transport may occur. Estimation of depositional flux for Cl and other parameters is probably the greatest source of uncertainty, both at the spatial scale and also in the long term. Advances are being made in all areas, however, and the use of multiple tracers (chemical, especially Cl and NO3) and isotopic signals (δ18O, δ2H, 36Cl), together with soil hydraulic properties, is promising for palaeohydrological reconstruction.
As in many other semi-arid regions, the Plio-quaternary aquifer of the eastern coast of Cap Bon peninsula (NE Tunisia) shows
a parallel increase in overexploitation and mineralization of groundwater resources and so the water quality is deteriorating.
Different methods using geochemistry (ions Na+, Cl−, Ca2+, Mg2+, Br−) and stable isotopes (18O, 2H) are compared with the hydrodynamic information for identifying the main processes involved in the increase of salinization.
Along the coast, intrusion of seawater resulting from groundwater overexploitation is identified, but is not the only cause
of qualitative degradation: the development of irrigation that induces soil leaching and transfer of fertilizers to groundwater
over the whole aquifer extent is another major reason for the increase in salinization. A total of 48 groundwater wells were
sampled to obtain additional information on the hydrochemical characteristics of the groundwater defined in previous studies.
KeywordsHydrogeology–Geochemistry–Isotopes–Coastal aquifer–Tunisia
Multiple isotopic and hydrogeochemical tracers were utilized to understand the limits to recharge from Tibetan plateau to Gobi desert. The average direct recharge rate according to chloride mass balance is between 0.9–2.5 mm yr−1 based on an annual rainfall of 120 mm yr−1, indicating that this has negligible impact on groundwater. The groundwater shows markedly depleted stable isotopic composition compared to modern rainfall. The signature of groundwaters (around −10.4‰ to −9.6‰ δ18O in several wells) from Dajing and South Lake playa differ clearly from that of the alluvial fan and modern rainfall, and has lower or undetected tritium activity, implying that the aquifer in the desert is maintained by palaeowater. This groundwater has an estimated 14C age of ∼4 kyr, with a recharge temperature of 7.3 °C. At current hydraulic head gradients, transit time from the recharge zone (Wuwei-Gulang alluvial fan) to South lake is approximately 2200–3500 yrs, indicating that groundwater flow is relatively slow and that the water resources are non-renewable.The geochemical information is comparable to the isotopic results of limit recharge sources of groundwater from Tibetan plateau to the Gobi desert. The buildup of dissolved solids through evaporation is a major control on groundwater composition, and the dominant anion species change systematically from to Cl−, but the dissolved ions from albite, calcite, dolomite and gypsum also have a significant contribution. It is advised to re-consider seriously the immigration policy of South lake to meet the groundwater management in the mountain front to Gobi desert.
Although there is no proof of the existence of salt-beds in the Benue Trough, outcrops of saline groundwaters are common features associated with the anticlinal structures in this trough. The outcrops in the Ogoja area, referred to here as the "Ogoja brine field", situated at the eastern margin of the Lower Benue Trough, are presented as a case study. The saline groundwaters in this brine field, which occur as ponds and in dug-wells, support an age-old local salt production in this area. The saline waters are characterised as Na-Cl type with Na+ constituting 78-85% of the cations and Cl- accounting for at least 85% of the anions. Ogoja brines display specific straight-line relationships between chloride and alkaline-earth metals, especially with lithium. Barium and strontium appear to be enriched, the former due to an environment free of sulphate. On the other hand, the observed depletion of the underground saline waters with respect to tritium (< 1.5 TU) and stable isotopes of oxygen and hydrogen (<-3.5‰ δ18O and <-10.5‰ δ2H) suggests little or no surface in-put (or near surface mixing) relative to those of the surface saline ponds (-0.15 to -3.3‰ δ18O and -6 to -10.4‰ δ2H). The combination of hydrochemical and isotope data, coupled with the analyses of the stratigraphic setting of the area suggests that the brines are marine in origin, related to palaeo/fossil sea water embedded within the transgressive marine sediments, and/or disseminated (precipitated) salts formed within the regressive interbeds during the sedimentation cycle.
The relationship between deuterium and oxygen-18 concentrations in natural meteoric waters from many parts of the world has
been determined with a mass spectrometer. The isotopic enrichments, relative to ocean water, display a linear correlation
over the entire range for waters which have not undergone excessive evaporation.
Newly available gas analyzers based on off-axis integrated cavity output spectroscopy (OA-ICOS) lasers have been advocated as an alternative to conventional isotope-ratio mass spectrometers (IRMS) for the stable isotopic analysis of water samples. In the case of H2O, OA-ICOS is attractive because it has comparatively low capital and maintenance costs, the instrument is small and field laboratory portable, and provides simultaneous D/H and 16O/18O ratio measurements directly on H2O molecules with no conversion of H2O to H2, CO, or H2/CO2-water equilibration required. Here we present a detailed assessment of the performance of a liquid-water isotope analyzer, including instrument precision, estimates of sample memory and sample mass effects, and instrumental drift. We provide a recommended analysis procedure to achieve optimum results using OA-ICOS. Our results show that, by using a systematic sample analysis and data normalization procedure routine, measurement accuracies of +/-0.8 per thousand for deltaD and +/-0.1 per thousand delta18O are achievable on nanoliter water samples. This is equivalent or better than current IRMS-based methods and at a comparable sample throughput rate.
Etude géologique de la région d'Agareb-Sfax: évolution géomorphologique néotectonique et paléogéographique
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Ben Akacha M (2001) Etude géologique de la région d'Agareb-Sfax:
évolution géomorphologique néotectonique et paléogéographique. DEA, Fac. Sc., Sfax
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urbaine de Sfax. Mémoire de DEA, Fac. Sc., Tunis
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et du deutérium, thèse d'É tat, université Paris-6, p 316
Les plissements quaternaires en Tunisie
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Castany G (1953) Les plissements quaternaires en Tunisie. Comptes
Rendus Sommaires Société Géologique de France, Paris,
pp 155-157
New guidelines for reporting stable hydrogen, carbon, and oxygen isotope-ratio data
- I D Clark
- P Fritz
Clark ID, Fritz P (1997) Environmental isotopes in hydrogeology.
CRC Press/Lewis Publishers, Boca Raton
Coplen TB (1996) New guidelines for reporting stable hydrogen,
carbon, and oxygen isotope-ratio data. Geochim Cosmochim
Acta 60:3359-3360
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