Cl SO4 Research Articles

Shallow and deep aquifer hydrochemistry and chemical variability of water quality at Shariatpur district, Bangladesh

Water quality and hydrochemistry of Shariatpur district were evaluated in terms of hydrochemical composition and some important physico-chemical parameters. The groundwater of the study area is good for drinking, domestic as well as for irrigation purposes. Among the major ions, shallow tube well waters give higher concentration of Ca2+ which ranges from 24 to 260 mg/L. The deep tubewell waters show higher concentration of Na+ which varies from 74 to 582 mg/L during dry season. Among the trace elements most of the shallow aquifer samples show higher concentration of Fe2+, Mn2+ and As. Concentration of Fe2+ varies from 0.655 to 18.8 mg/L, and Mn2+ from trace to 0.868 mg/L during dry period. Hydrochemical analyses reveal significant seasonal variation in water quality of shallow aquifer. Both the shallow aquifer and the surface water of the study area are predominantly of Ca–Mg–HCO3 type, while the deep aquifer water is mainly of Na–K–Cl–SO4 type with slight inclination to Ca–Mg–HCO3 type. The study area is suitable for groundwater development if comprehensive and holistic approaches towards water resource management are taken into consideration.

Sedimentology and origin of Meyghan lake/playa deposits in Sanandaj–Sirjan zone, Iran

The inland lake/playa of Meyghan is a closed basin in the Sanandaj–Sirjan tectonic zone of Iran, containing largest mineable sodium sulfate deposit of the Middle East. The Late Pleistocene–Holocene sediments of this lake/playa were studied for sedimentological and mineralogical characteristics, brine evolution path and the record of climatic variations in the area. Minerals of this lake/playa are mainly evaporites along with some fine-grained clastic components. Evaporite minerals include calcite, gypsum, halite, glauberite, thernadite, polyhalite, natron along with minor volumes of bassanite and sylvite. Gypsum and halite abundances show increasing trend from margin to the center forming a Bull’s-eye pattern. Main clastic components are fine-grained quartz, muscovite, clay minerals, calcite and organic matters. Clay minerals are the most dominant clastic minerals in these sediments. In the surface sediments of Meyghan lake/playa, illite and chlorite abundances reduce from the margin to the center which is contrary to the general patterns of clay distribution in other playas. Inflowing water with low percentage of ions and Na–(Ca)–(Mg)–SO4–Cl–(CO3) type evolve into a brine with high ions concentration during the geochemical evolution and mineral deposition. Finally, geochemical evolution resulted in a Na–SO4–Cl type brine. While clastic components (calcite, quartz and so on) show increasing trend with depth, evaporites abundances show opposite trends. Furthermore, the average grain size of sediments increases with depth indicating higher energy in the past. Typical sedimentary structures of lake/playa environment that are present include efflorescent salt crusts composed of halite, tepees, polygonal structures as the most common sedimentological features of this basin as well as Puffy Grounds. In conclusion, sedimentary records of this lake/playa represent important climatic changes in the area from relatively more humid cold climate in the Late Pleistocene to semi-arid warm climate in the Holocene, with higher evaporation rate.

Hydrochemical and isotopic study of groundwater in the Yinchuan plain, China

The Yinchuan plain is located in the arid climate zone of NW China. The western margin of the plain is the Helan mountain connecting a series of normal slip faults. The eastern margin of the plain connects with the Yellow River and adjacents with the Ordos platform. The south of the plain is bordered by the EN fault of the Niushou mountain. The bottom of the plain is the Carboniferous, Permian, or Ordovician rocks. Based on the analysis of groundwater hydrochemical and isotopic indicators, this study aims to identify the groundwater recharge and discharge in the Yinchuan plain, China. The hydrochemical types of the groundwater are HCO3–SO4 in the west, HCO3–Cl in the middle, and Cl–SO4 in the east. The hydrochemical types are HCO3–SO4 in the south, HCO3–Cl and SO4–HCO3 in the middle. The hydrochemical types are complex in the north, mainly SO4–HCO3 and Cl–SO4. Deuterium, 18O, and tritium values of groundwater indicate that groundwater recharge sources include precipitation, bedrock fissure water, and irrigation return water. Groundwater discharges include evaporation, abstraction, and discharge to surface water. According to the EW isotopic profile, the groundwater flow system (GFS) in the Yinchuan plain can be divided into local flow systems (LFS) and regional flow systems (RFS). Groundwater has lower TDS and higher tritium in the southern Yellow River alluvial plain and groundwater age ranges from 6 to 25 years. The range of groundwater renewal rates is from 11 to 15 % a−1. The depth of the water cycle is small, and groundwater circulates fast and has high renewal rates. Groundwater has higher TDS and lower tritium in the northern Yellow River alluvial plain. The range of groundwater age is from 45 to 57 years, and renewal rate is from 6 to 0.1 % a−1. The depth of the water cycle is larger. Groundwater circulates slowly and has low renewal rates.

Influence of various rootstocks on the yield and grape composition of Sauvignon Blanc

This influence of various rootstocks on the yield and grape composition of Sauvignon Blanc was examined. The yield on rootstock 41B/72 was 1 kg/vine or more higher than on rootstocks SO4 cl. 31, Riparia cl. 1 and Kober 5BB. The vines grafted on the Börner rootstock gave the highest weight of 100 berries (241 g). The highest seed weight of 100 berries (6.9 g) was found on rootstock 196/17 Cl, while the lowest one was on the Riparia cl. 1 (5.6 g). There was a strong, positive correlation (r = 0.91) between the berry and seed weights of vines on rootstock 41B/72 (2003). The total acidity content and the relationship between tartaric and malic acids were affected more by the weather conditions than by the rootstock. Relatively strong correlations between the grape yield and total soluble solids (r = –0.89), and between grape yield and total acidity (r = 0.76) were found in the grape juice on rootstock 41B/72 (in 2002).  

Hydrogeochemical processes affecting groundwater in an irrigated land in Central Tunisia

Detailed hydrogeochemical investigation has provided new information concerning the major factors and mechanisms controlling the groundwater chemistry of Chougafiya basin. The hydrogeochemical characteristics of groundwaters comprise three main types: Cl–SO4–Ca, Cl–SO4–Na and Cl–Na. Hydrochemical characteristics based on the bivariate diagrams of major (Cl−, SO42−, NO3−, HCO3−, Na+, Mg2+, K+ and Ca2+) and some trace (Br− and Sr2+) ions, mineral saturation indices and hierarchical cluster analysis indicate different origins of groundwater mineralization. The water–rock interaction (dissolution of evaporitic minerals), followed by cation exchange reactions with clay minerals, constitute the main processes that control groundwater salinization. However, the chemical composition of brackish groundwater in the central and southern parts of the study area is influenced by a mixing process with Sabkhas salt groundwater. The mixing proportions inferred from chloride mass balance prove that the contribution of Sabkhas groundwater to Quaternary aquifer ranges between 2.7 and 9.1 %. These intrusion rates reflect the progress of the saltwater–freshwater interface, which is mainly controlled by the piezometric level variation and the distance to the Sabkhas.

Hydrochemical characterization and groundwater quality in Delta Tokar alluvial plain, Red Sea coast—Sudan

The leaching processes along the flow path and over abstraction of the alluvial aquifer, the principal aquifer in delta Tokar, by the agricultural and domestic sectors and natural factors, have led to its salinization which may be due to interaction between geological formations and adjacent brackish and saline water bodies as well as seawater transgression. The main objectives of this study are to assess the hydrochemical characteristics of the groundwater and to delineate the locations and the sources of aquifer salinization. Water samples in the project area were chemically analyzed for major cations and anions at the laboratory by the standard analytical procedures. Chemical data and water level measurements were manipulated using GIS techniques for hydro chemical and flow direction maps and piper diagram for chemical facies and SPSS software for statistical analyses such as basic statistics (mean and standard deviation) and Spearman’s correlation matrix. The general flow direction of the groundwater is from Southwest towards East and Northeast. The hydraulic gradient is relatively steeper at the apex of the delta (0.06) and amounts to 0.005 at the distal part of the delta. The average transmissivity value of the water bearing formations was found to be about 4.5 × 103 m2/s, whereas, the storage coefficient was about 0.28. A hydrochemical study identified the locations and the sources of aquifer salinization and delineated their areas of influence. The investigation indicates that the aquifer water quality is significantly modified as groundwater flows from the southwestern parts of the study area, where the aquifer receives its water by lateral underflow from Khor Baraka flood plain, to the central and northeastern parts, with few exceptions of scattered anomalous concentration pockets in the deltaic plain. Significant correlation between TDS and/or EC with the major components of Na+, Cl−, and SO4−2 ions is an indication of seawater influence on the groundwater salinity. Moreover, Cl−, SO42−, and Na+ are predominant ions followed by Ca2+ and HCO3−. Hence, four types of groundwater can be chemically distinguished: Na–Ca–SO4–Cl– facies, Na–Cl–SO4–HCO3– facies, Na–Ca–Mg–SO4–Cl–HCO3 facies, and Na–Ca–Mg–Cl–SO4 facies. The processes that govern changes in groundwater composition as revealed by chemical and statistical analyses are mainly associated with over-abstraction, biodegradation, marine intrusions, and carbonate saturation.

A study on groundwater geochemistry and water quality in layered aquifers system of Pondicherry region, southeast India

Geochemical signatures of groundwater in the Pondicherry region, south India, were determined. The coastal aquifers are fragile and this situation becomes more intense in layered aquifer systems like that of the Pondicherry region. In this region, groundwater occurs in alluvium, Lower Cuddalore, Upper Cuddalore, Tertiary, Cretaceous and mixed aquifers. The geochemical signature of groundwater in these formations was studied by collecting 93 groundwater samples. The collected samples from specific formations were analysed for physical parameters, such as electrical conductivity (EC), pH and major ion concentrations, such as Ca, Mg, Na, K, Cl, HCO3, PO4 and SO4. The results of the analysis were interpreted with geology; the ionic concentrations in the groundwater vary spatially and temporally. The abundance of these ions are in the following order: Na > Ca > Mg > K = Cl > HCO3 > SO4. Interpretation of analytical data shows that Ca–Na and Cl–SO4–HCO3 is the dominant facies in all the formations. Groundwater in the area is generally hard and fresh-brackish in most of the formations and brackish in nature in alluvium formation. The sodium absorption ratio shows that most of the samples are grouped under excellent category to good category in all the formations. The residual sodium carbonate also are in good category in all the formations. Chloro-alkaline indices reveal that the majority of samples show negative values in all the litho units indicating the exchange of Na and K in groundwater with Mg or Ca in rock. Scholler classification of water indicates that longer residence time of water with more prominent base exchange. High EC and TDS values in certain locations of alluvial, Upper Cuddalore and Cretaceous formations prove to be unsuitable for drinking and irrigation purposes.

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.

Activity Coefficients of Fission Products in Highly Salinary Solutions of Na+, K+, Mg2+, Ca2+, Cl–, and SO42–: Cs+

Binary and ternary interaction parameters of the Pitzer formalism for the thermodynamic modeling of the fission product Cs+ in concentrated salt solutions of the hexary oceanic system Na–K–Mg–Ca–Cl–SO4–H2O at 25 °C were calculated from experimental isopiestic, potentiometric, and solubility data available in the literature. The obtained parameters were checked by comparing measured and predicted water activities and solubilities for binary, ternary, and quaternary systems.

Modeling Thermal Conductivity of Electrolyte Mixtures in Wide Temperature and Pressure Ranges: Seawater and Its Main Components

A model has been established for calculating the thermal conductivity of aqueous electrolyte solutions containing the Na+, K+, Mg2+, Ca2+, Cl−, SO42−, CO32−, HCO3−, and Br− ions. The model is based on a previously developed computational framework for the thermal conductivity of mixed-solvent electrolyte systems, which has been expanded by explicitly accounting for pressure effects in addition to temperature and electrolyte composition effects. The model consists of a contribution of the solvent, a contribution of individual species expressed using modified Riedel coefficients, and an ionic strength-dependent term that is due to interactions between species. The model accurately represents the thermal conductivity of solutions containing single and multiple salts at temperatures ranging from 273 K to 573 K, pressures up to at least 1400 bar, and concentrations up to the limit of solid saturation. Further, the model has been applied to seawater and used to elucidate the discrepancies between the experimental data for seawater and those for Na–K–Mg–Ca–Cl–SO4 salt solutions. With parameters evaluated on the basis of data for binary and multicomponent salt solutions, the model provides reliable predictions of the thermal conductivity of seawater.

Comprehension and hydrogeological conceptualization of aquifer in arid and semi-arid regions using integrated hydrogeological information system: case of the deep aquifer of Zéramdine-Béni Hassen (east-central Tunisia)

In arid and semi-arid regions, the groundwater overexploitation caused drawdown in piezometric levels and a degradation of chemical water quality. That is why the groundwater monitoring needs a good comprehension of the hydrogeological aquifer properties. This is specially the case of Zeramdine–Beni Hassen deep aquifer (east-central Tunisia). Seismic profiles interpretation highlights the existence of the Zeramdine fault corridor, the Boumerdes anticline, the Moknine and Mahdia grabens that represent lateral boundaries for the study aquifer. The outcrop of the aquifer is located in the Zeramdine, Beni Hassen and Ain Ben Jannet regions, where two lithostratigraphic sections were realized. The piezometric study shows that the principal groundwater flow is from west to east. A secondary flow is from NW to SE. The hydrochemical study of 22 sample shows that the aquifer is characterized by freshwater, dominated by Na–Ca–Cl–SO4 facies. The salinity increase is from the west to the east, which coincides with the principal water flow direction. The integration of all results deduced from the hydrogeophysic, hydrodynamic and hydrochemical studies is developed to investigate hydrological processes of Zeramdine–Beni Hassen aquifer and consequently to propose a conceptual model, which will help to propose a rescue plan for the studied aquifer and to implement a spatial hydrogeological database using the global information system and then to characterize the complex aquifer system.

The origin of thermal waters in the northeastern part of the Eger Rift, Czech Republic

An investigation of the thermal waters in the Ústí nad Labem area in the northeastern part of the Eger Rift has been carried out, with the principal objective of determining their origin. Waters from geothermal reservoirs in the aquifers of the Bohemian Cretaceous Basin (BCB) from depths of 240 to 616 m are exploited here. For comparison, thermal waters of the adjacent Teplice Spa area were also incorporated into the study. Results based on water chemistry and isotopes indicate mixing of groundwater from aquifers of the BCB with groundwater derived from underlying crystalline rocks of the Erzgebirge Mts. Unlike thermal waters in Děčín, which are of Ca–HCO3 type, there are two types of thermal waters in Ústí nad Labem, Na–HCO3–Cl–SO4 type with high TDS values and Na–Ca–HCO3–SO4 type with low TDS values. Carbon isotope data, speciation calculations, and inverse geochemical modeling suggest a significant input of endogenous CO2 at Ústí nad Labem in the case of high TDS groundwaters. Besides CO2 input, both silicate dissolution and cation exchange coupled with dissolution of carbonates may explain the origin of high TDS thermal waters equally well. This is a consequence of similar δ13C and 14C values in endogenous CO2 and carbonates (both sources have 14C of 0 pmc, endogenous CO2 δ13C around −3‰, carbonates in the range from −5‰ to +3‰ V-PDB). The source of Cl− seems to be relict brine formed in Tertiary lakes, which infiltrated into the deep rift zone and is being flushed out. The difference between high and low TDS groundwaters in Ústí nad Labem is caused by location of the high mineralization groundwater wells in CO2 emanation centers linked to channel-like conduits. This results in high dissolution rates of minerals and in different δ13C(DIC) and 14C(DIC) fingerprints. A combined δ34S and δ18O study of dissolved SO4 indicates multiple SO4 sources, involving SO4 from relict brines and oxidation of H2S. The study clearly demonstrates potential problems encountered at sites with multiple sources of C, where several evolutionary groundwater scenarios are possible.

The hydrogeology of the deep groundwater system in the Hammamet–Nabeul regional basin, north-eastern Tunisia: a hydrochemical and isotopic approach

The scarcity of surface water in the Hammamet–Nabeul regional basin has imposed an over-abstraction of groundwater resources to meet the population demand for touristic and agricultural sectors. The impact of such a groundwater management on the aquifer system hydrodynamics and water quality has been investigated using a set of hydrochemical and isotopic data. Furthermore, a better understanding of the mineralization processes, the groundwater origins and the sources of recharge have been provided. Four water types were recognized; Na–HCO3, Ca–HCO3, Ca–SO4–Cl and Na–Cl. They are in relation with water–rock interaction processes i.e. the dissolution of evaporates and the cation exchange. Stable isotopes of water molecule (18O and δ2H) permit to identify three different groups. The first is characterized by relatively enriched isotope contents lending support to the important infiltration of recent rainwater in the foot of mountains. The second is distinguished by relatively depleted contents of stable isotopes reflecting a paleoclimatic origin. The third is marked by a significant enrichment of deuterium, which suggests a significant exchange with H2S in relation with the bacterial reduction of sulfate.

Geochemical and isotopic investigation of groundwater mineralization process in the Zeroud basin, central Tunisia

The determination of the origin of the salinity in the Zeroud aquifer (central of Tunisia) and the understanding of its hydrogeological and geochemical behaviors related to severe natural and anthropogenic constraints were approached by the combined survey of major element concentrations and stable isotopes (18O, 2H). The hydrogeochemical investigations of the Zeroud aquifer show that groundwaters can be classified into two major water facies: Ca–SO4–Cl and Na–Cl. The main hydrogeochemical processes that typically control the groundwater mineralization are dissolution of evaporates (halite, gypsum and anhydrite), dedolomitization, cation exchange and mixing processes related to the return flow of irrigation waters and the artificial recharge operations. The stable isotope signatures reveal that the first significant recharge origin is related to the direct infiltration of actual meteoric water particularly observed in the foot of hills bordering the basin and in the vicinity of the Zeroud Wady. A main part of groundwater is significantly affected by evaporation, giving evidence of a recharge mostly from the dam water during the artificial recharge operations. Moderated evaporation of waters was observed in the southern part of the basin, especially in the agricultural areas.

Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia)

The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 μS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO4–Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to δ18O and δ2H values, all thermal and cold groundwater is of meteoric origin.

Environmental problems at the Nevşehir (Kozakli) geothermal field, central Turkey

The Kozakli–Nevsehir geothermal field extends a long a NW–SE direction at SE of the Centrum of Kozakli. The area is not rugged and average elevation is 1,000 m. The Kozanozu Creek flows towards north of the area. In the Kozakli thermal Spa area, thermal waters are manifested along a valley with a length of 1.5 km and 200 m width. In this resort some hot waters are discharged with no use. The thermal water used in the area comes from wells drilled by MTA. In addition, these waters from wells are also utilized by hotels, baths and motels belonging to City Private Management, Municipality and private sector. The measured temperature of Kozakli waters ranges from 43–51°C in springs and 80–96°C in wells. Waters are issued in a wide swampy area as a small group of springs through buried faults. Electrical conductivity values of thermal spring and well waters are 1,650–3,595 μS/cm and pH values are 6.72–7.36. Kozakli cold water has an electrical conductivity value of 450 μS/cm and pH of 7.56. All thermal waters are dominated by Na+ and Cl–SO4 while cold waters are dominated by Ca+2 and HCO3−. The aim of this study was to investigate the environmental problems around the Kozakli geothermal field and explain the mechanisms of karstic depression which was formed by uncontrolled use of thermal waters in this area and bring up its possible environmental threats. At the Kozakli geothermal field a sinkhole with 30 m diameter and 15 m depth occurred in January, 17th 2007 at the recreation area located 20 m west of the geothermal well which belongs to the government of Nevsehir province. The management of the geothermal wells should be controlled by a single official institution in order to avoid the creation of such karstic structures affecting the environment at the source area.

Co-precipitation of radium in high ionic strength systems: 1. Thermodynamic properties of the Na–Ra–Cl–SO 4–H 2O system – Estimating Pitzer parameters for RaCl 2

The fate of divalent radium (Ra 2+) in high ionic strength systems is an important issue with relevant aspects to environmental concerns. As a consequence, the description of the thermodynamic properties of Ra 2+ in such systems is a critical matter which was not addressed yet. This can be done using the Pitzer formalism, which is regarded as the most accurate approach to describe activity coefficients of aquatic species in high ionic strength solutions. Isopiestic measurements, measurements of the electromotive force or the solubility of RaSO 4,S in NaCl solutions, which are essential for the evaluation of the Pitzer ion interaction parameters, were not experimentally tested, due to the high radioactivity concentration which involves such experiments. Therefore, the present study examined the possibility to extrapolate the RaCl 2 cation–anion interaction parameters ( β (0), β (1) and C ϕ) by linear regression of other MCl 2 cation–anion interaction parameters with their respective hydrated ionic radii (M 2+ = Mg 2+, Ca 2+, Sr 2+ and Ba 2+). γ Ra 2 + calculated with the estimated RaCl 2 parameters revealed that the ratios of activity coefficients γ Mg 2 + / γ Ba 2 + > γ Ca 2 + / γ Ba 2 + > γ Sr 2 + / γ Ba 2 + > 1 > γ Ra 2 + / γ Ba 2 + follow the opposite order of the ratios of hydrated ionic radii R Mg 2 + / R Ba 2 + < R Ca 2 + / R Ba 2 + < R Sr 2 + / R Ba 2 + < 1 < R Ra 2 + / R Ba 2 + . The activity coefficient of Ra 2+, γ Ra 2 + , calculated with these estimated values is used to test the limitation of the common assumption that the activity coefficients ratio, γ Ra 2 + / γ Ba 2 + , is close to unity. It is found that the common assumption of γ Ra 2 + / γ Ba 2 + = 1 may be considered reasonable up to high NaCl concentration. The largest deviation in the γ Ra 2 + / γ Ba 2 + ratio from this assumption was of the order of ∼25% in the range of halite solubility (6 mol kg −1 of NaCl). However, experimental results of co-precipitation of Ra 2+ in barite are described with more accuracy when the estimated RaCl 2 Pitzer parameters are used. These results are described in a consecutive paper of this journal.

Characterizing a complex aquifer system using geophysics, hydrodynamics and geochemistry: A new distribution of Miocene aquifers in the Zéramdine and Mahdia–Jébéniana blocks (east-central Tunisia)

The Zéramdine and Mahdia–Jébéniana blocks are located in the Sahel region in east-central Tunisia. Active tectonics have divided the region into numerous sub-units, as result of multiple phases of distension and compression. The Miocene fluvio-deltaic sediment sandy layers have aquiferous capacities but their hydraulic properties are still unknown, due to the lack of investigation wells. This study proposes a new description of the regional hydrogeology of Miocene deposits. Seismic-reflection and wireline logging of petroleum and water wells were used to understand the structure and the geometry of the Miocene reservoirs. The groundwater flow and its relationship to the sedimentary and tectonic context were then identified by studying piezometry and hydrochemistry. Two Miocene deep aquifer systems were identified: (1) Zéramdine–Béni Hassen to the north and (2) Jébéniana–Ksour Essef to the south. These aquifers are separated by the Mahdia graben. Other major tectonic structures, such as the Zéramdine fault corridor, the Moknine graben, and the El-Jem half-graben represent lateral boundaries for these aquifers. Other deeper sandy and clayey-sandy reservoirs were also identified in the area. Their repartition, thickness and depth vary from one block to other. Hydrodynamics of the deep aquifers seems to be controlled by geological structures. Two independent compartments were identified: in the northern block groundwater flows from West to East and from Northwest to Southeast, while in the southern block it flows from Northwest to Southeast. Geochemical facies are of two types: Na–Ca–Cl–SO 4 for the Zéramdine–Béni Hassen deep aquifer and Na–Cl for the Jébéniana–Ksour Essef deep aquifer. The hydrodynamic and geochemical results confirm the sharing of the Miocene sediments into two aquifers.

From Water Clustering to Osmotic Coefficients

Water activity is an important macroscopic property of aerosol particles and droplets in the atmosphere as well as aqueous solutions in many other fields of physical chemistry. This study focuses on relating water activity, described using osmotic coefficients, to the microscopic water structure in systems of atmospheric relevance, namely, aqueous solutions of each of the four electrolytes: NaCl, (NH(4))(2)SO(4), NH(4)Cl, and Na(2)SO(4). The osmotic coefficients of these compounds, as reported in literature based on thermodynamic measurements, decrease as a function of molality for dilute solutions and increase as a function of molality for concentrated solutions. At an intermediate molality, a minimum value of the osmotic coefficient is observed. We explain this behavior by describing osmotic coefficients as the product of two concentration-dependent effects: incomplete electrolyte dissociation and variations in the microphysical water structure. The degree of dissociation in electrolyte solutions can be obtained directly from literature or derived from reported pK values, and in this work the water structure is quantified using low-wavenumber Raman spectroscopy. We use the band at 180 cm(-1) in Raman spectra of aqueous electrolyte solutions, which has been assigned to the displacement of the central oxygen atom in a tetrahedral hydrogen bonding environment composed of five H(2)O units. The abundance of such translationally restricted water molecules is essential in describing the local microphysical structure of water, and the height of the band is used to estimate the amount of such translationally restricted water molecules in solution. We were able to qualitatively reproduce and explain literature values of osmotic coefficients for the four studied electrolytes. Our results indicate that the effect of electrolyte dissociation, which decreases as a function of molality, dominates in dilute solutions, whereas changes in water structure are more significant at higher concentrations.

A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater, Lake Kitagata, Uganda

Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO4–Cl–HCO3–CO3 chemistry, high pH and relatively small amounts of SiO2. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSi7O13(OH)3·3H2O], burkeite [Na6(CO3)(SO4)2]) and as diagenetic intrasediment growths (mirabilite (Na2SO4·10H2O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO2 concentrations than exist today.