Dissolution Of Evaporite Minerals Research Articles

An assessment of the physico-chemical parameters of Oran sebkha basin

Growing populations and increasing industrialization cause increase in living standard, which result in decrease in the quality of water and may put stresses on natural waters by impairing both the quality of the water and the hydrological budget. This research aims at determining the origin of the chemical elements of groundwater from the Oran sebkha basin. It applies the inverse geochemical modeling to derive the sources of variation in the hydrochemistry by Belkhiri et al. (doi: 10.1016/j.geoderma.2010.08.016 , 2010). Fifty-five water samples were selected from different point in Oran sebkha basin for sampling purpose in July 2011. Physico-chemical parameters such as pH and electric conductivity were measured in situ. Moreover, chloride, sulfate, alkalinity, calcium, magnesium, sodium and potassium were measured in the laboratory. Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) weathering reactions of silicate minerals by Belkhiri et al. (doi: 10.1016/j.geoderma.2010.08.016 , 2010). The high values of the physico-chemical parameters of water obtained in the present study sites indicate a variation in the physico-chemical parameters demonstrated that relatively few phases are required to derive water chemistry in the area. Range of values was found as pH (5.1–7.6), conductivity (720–15,820 μS cm−1), chloride (994–7,810 mg l−1), sulfate (6.1–112.4 mg l−1), alkalinity (421–19,962 mg l−1), calcium (80–680 mg l−1), magnesium (212.4–4,525 mg l−1), sodium (124.2–4,687.4 mg l−1) and potassium (0.9–42.5 mg l−1).

English

Growing populations and increasing industrialization causes increase in living standard, which result in decrease in the quality of water and may put stresses on natural waters by impairing both the quality of the water and the hydrological budget. This research aimed at determining the origin of the chemical elements of groundwater from the Oran Sebkha basin. It applied the inverse geochemical modeling to derive the sources of variation in the hydrochemistry. Fifty five (55) water samples were selected from different point in Oran Sebkha basin for sampling purpose in July 2011. Physico-chemical parameters such as pH and electric conductivity were measured in situ . Moreover, chloride, sulfate, alkalinity, calcium, magnesium, sodium, potassium, were measured in the laboratory. Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) weathering reactions of silicate minerals. The high values of the physico-chemical parameters of water obtained in the present study sites indicate a variation in the physico-chemical parameters and demonstrated that relatively few phases are required to derive water chemistry in the area. Range of values were found as pH (5.1-7.6), conductivity (720-15820 μS cm -1 ), chloride (994-7810 mg l -1 ), sulfate (6.1-112.4 mg l -1 ), alkalinity (421-19962 mg l -1 ), calcium (80-680 mg l -1 ), magnesium (212.4-4525 mg l -1 ), sodium (124.2-4687.4 mg l -1 ) and potassium (0.9-42.5 mg l -1 ). Key words : Physico-chemical parameters, water, Sebkha.

Geochemical modeling of groundwater in the El Eulma area, Algeria

Multivariate statistical and geochemical modeling techniques were used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the El Eulma area. Three major water groups resulted from the Q-mode cluster analysis. The samples from the area were classified as low salinity (Group 1), moderate salinity (Group 2), and high salinity waters (Group 3). Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals, quartz, kaolinite, and Ca-smectite; and (3) ion exchange.

Recharge and mineralization of groundwater of the Upper Cretaceous aquifer in Orontes basin, Syria

Chemical and isotopic data of groundwater of the Upper Cretaceous aquifer in the Orontes basin, Syria, have been used to assess the groundwater geochemistry, the origin of groundwater recharge and groundwater residence time. The chemical data indicate that dissolution of evaporite minerals is the main process controlling groundwater mineralization. The composition of stable isotopes δ18O and δ2H, together with 14C activity, reflect the existence of three different groups: (a) groundwater in the Coastal Mountains with δ18O of –6.65‰, quite similar to modern-day precipitation, and high 14C (>50 pmC); (b) groundwater in the unconfined aquifer of the Hama Uplift, which has δ18O of –5.52‰ and 14C near 20 pmC, and is recharged locally; and (c) groundwater from the confined aquifer of the Homs Depression, which is characterized by more depleted δ18O,, –8.01‰, and low 14C (<7 pmC), and might be recharged in the northern piedmont of the Anti-Lebanon Mountains. The distinctive isotope signatures of the latter two groups indicate different recharge elevations and palaeoclimatic effects. The low recharge rate of the groundwater in the Hama Uplift aquifer, and the even slower recharge rate in the Homs Depression aquifer, are reflected by groundwater 14C residence times of 5 and over 22 Ka BP, respectively. Editor D. Koutsoyiannis Citation Al-Charideh, A., 2013. Recharge and mineralization of groundwater of the Upper Cretaceous aquifer in Orontes basin (Syria). Hydrological Sciences Journal, 58 (2), 452–467.

Isotopic and hydrochemical investigation of the Grombalia deep aquifer system, northeastern Tunisia

Hydrochemical and environmental isotope tracers have been used to identify the geochemical processes that affect groundwater masses in the aquifer system of Grombalia (northeastern Tunisia). The study area presents a multilayer aquifer system logged in the Quaternary and Miocene sandstones. Multi-tracer results show that the dissolution of evaporite minerals as well as the evaporation and the irrigation return flow are the main processes controlling groundwater quality. The isotopic data proves the origin of different groundwater masses as well as the natural and anthropogenic processes that control their mineralization.

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.

Geochemical evolution of groundwater in an alluvial aquifer: Case of El Eulma aquifer, East Algeria

Hydrochemical, multivariate statistical and inverse hydrogeochemical modeling techniques were used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the El Eulma Mio-Plio-Quaternary aquifer, East Algeria. Cluster analysis based on major ion contents defined three main chemical water types, reflecting different hydrochemical processes. The first, group 1, has low salinity (mean EC=937μS/cm) and abundance orders Ca2+>Na+≈Mg2+>K+ and HCO3+>Cl->SO42->NO3-. With increased water–rock interaction, waters in groups 2 and 3 become more saline, changing composition towards Cl–HCO3–Ca and Cl–Ca–Na types. The PHREEQC geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals, quartz, kaolinite and Ca-smectite; (3) ion exchange.

Hydrochemical and isotopic study of groundwater in Wadi El Hechim–Garaa Hamra basin, Central Tunisia

Hydrochemical and isotopic study of Miocene and Mio-Plio-Quaternary (M-P-Q) aquifers in Wadi El Hechim–Garaa Hamra basin, Central Tunisia was undertaken in order to investigate recharge mode and processes leading to mineralization of groundwater as well as interaction between both systems. The results revealed striking differences between the two aquifer systems. While the Miocene aquifer contains recently recharged waters with generally low mineralization (around 0.5 g L−1), stemming mainly from dissolution of carbonate minerals, the M-P-Q aquifer reveals TDS values reaching 3 g L−1, controlled mainly by dissolution of evaporitic minerals. Isotopic data indicate that the Miocene aquifer contains water recharged in past several decades (bomb tritium and bomb radiocarbon detected). The M-P-Q system appears to be much slower, with time scales of groundwater flow possibly reaching some thousands of years. Sharp discontinuity of hydrochemical and isotope characteristic of groundwater observed across the major tectonic fault separating the Miocene and M-P-Q aquifers supports the idea of very limited (if any) hydraulic interconnection between both studied systems. This in turn calls for revision of existing conceptual models of groundwater flow in the region postulating significant groundwater fluxes crossing the fault in the direction of M-P-Q aquifer and adjacent aquifers in the Wadi al Fakka plain.

Deciphering interaction of regional aquifers in Southern Tunisia using hydrochemistry and isotopic tools

► We examine the origin and chemical evolution of groundwater in southern Tunisia. ► Contribution of water from the deep CI aquifer to Turonian formation was demonstrated. ► Influence of geogenic carbon dioxide in the studied groundwater system was shown. ► The work aids management strategies of scarce groundwater resources in Tunisia. Groundwater is the most important source of water supply in southern Tunisia. Previous hydrogeologic and isotopic studies carried out in this region revealed the existence of two major aquifer systems: the “Complex Terminal” (CT) and the “Continental Intercalaire” (CI). Turonian carbonates constitute one of the major aquifer levels of the CT multilayered aquifer. It extends over most of southern Tunisia, and its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Forty-eight groundwater samples from the CI and Turonian aquifers were collected between January and April 2004 for chemical and isotopic analyses. Hydrochemistry and isotopic tools were combined to get an insight into the processes controlling chemical composition of groundwater and wide-scale interaction of these two aquifer systems. Analysis of the dissolved constituents revealed that several processes control the observed chemical composition: (i) incongruent dissolution of carbonate minerals, (ii) dissolution of evaporitic minerals, and (iii) cation exchange. Dissolution alone cannot account for the observed high supersaturation states of groundwater with respect to calcite and dolomite. The observed supersaturation is most probably linked to geogenic CO 2 entering water-bearing horizons of the CT and CI aquifers via deep tectonic faults and discontinuities and subsequent degassing in the exploitation wells. Presence of geogenic CO 2 in the investigated region was confirmed by C isotope data of the DIC reservoir. The radiocarbon content of the Turonian samples varied between 9.5 and 43 pmc. For CI samples generally lower values were recorded, between 3.8 and 22.5 pmc. Stable isotope composition of Turonian groundwater samples varied from −8.3 to −5.3‰ for δ 18 O and from −60 to −25‰ for δ 2 H. The corresponding ranges of δ values for the Continental Intercalaire samples were from −8.9‰ to −6.9‰ for δ 18 O and from −68.2‰ to −45.7‰ for δ 2 H. Stable isotope composition of groundwater representing CT and CI aquifers provide strong evidence for regional interaction between both systems.

Trace element mobility in dolomitic argillites of the Mesoproterozoic Belt-Purcell Supergroup, Western North America

The Belt-Purcell Supergroup comprises dolomite-rich stratigraphic units in a dominantly siliciclastic succession, where sedimentation spans 1400–1470 Ma. Dolomitic units are variable mixtures of co-sedimented argillite and primary carbonate post-depositionally converted to secondary dolomite. Based on rare earth element (REE) relationships three distinct REE patterns are identified in the dolomite-rich units: Type 1 (T1d; d = dolomitic sample) with REE patterns parallel to post-Archean Upper Continental Crust (PA-UCC), albeit at lower absolute abundances due to dilution by carbonate content; Type 2 (T2d) with Heavy REE (HREE) enrichment but Light REE (LREE) depletion relative to T1d; and Type 3 (T3d) with enrichment in LREE and HREE relative to T1d, but erratic Middle REE (MREE) patterns. There is a progressive increase of ΣREE from T1d through T2d to T3d, whereas for ΣLREE/ΣHREE T2d < T1d < T3d. T1d–T2d and T3d represent three different “snapshots” of a continuous process. In terms of timing, dolomitization of calcite primary sediment in all samples likely took place broadly during burial diagenesis, as inferred for most Proterozoic dolomites. T1d is easily explained by provenance: however, T2d and T3d cannot be related to provenance, weathering or sedimentary sorting processes to explain higher concentrations of HREE referenced to PA-UCC and consequently developed in the sediment from a T1d precursor. The same three REE signatures have been described in previous studies in counterpart siliciclastic counterparts throughout the Belt-Purcell Supergroup at three different locations. Mobility of normally stable REE is accompanied by mobility of normally isochemical high field strength elements (HFSE) in T2d and T3d to give REE/REE, HFSE/HFSE, REE/HFSE and Y/HREE fractionations. No specific REE–HFSE signatures are apparent in the carbonate-rich units as compared to their non-dolomitic siliciclastic counterparts. This unusual mobility of REE and HFSE reflected in T2d and T3d is attributed to alkaline oxidizing post-depositional brines. Salinity was derived from seawater–sediment reactions, dissolution of evaporite minerals, and the smectite–illite transformation, whereas alkaline oxidizing conditions were promoted by groundwater interaction with mafic units in the basin, CO 2 introduced into the system during episodic rifting with mantle degassing, and interaction of syn-sedimentary mafic intrusions with carbonate units at early stages of BPS deposition. Intermittent brine activity, inducing T2d and T3d patterns, spanned >1 Ga as recorded by secondary monazite grains with age distributions that correspond to large scale tectono-thermal events in Laurentia. Post-depositional processes and redistribution of carbonate can have an impact on transitional stratigraphic contacts between dolomitic and siliciclastic units which may have been incorrectly described as primary due to sedimentary environment changes.

Application of multivariate statistical methods and inverse geochemical modeling for characterization of groundwater — A case study: Ain Azel plain (Algeria)

Multivariate statistical methods and inverse geochemical modeling were jointly used to define the variation and the genetic origin of chemical parameters of groundwater in the Ain Azel plain, Algeria. Interpretation of analytical data shows that the abundance of the major ions is as follows: Ca ≥ Mg > Na > K and HCO 3 ≥ Cl > SO 4. Q-mode hierarchical cluster analysis (HCA) was employed for partitioning the water samples into hydrochemical facies, also known as water groups or water types. Three major water groups resulted from the HCA analysis. The samples from the area were classified as recharge area waters (Group 1: Ca–Mg–HCO 3 water), transition zone waters (Group 2: Ca–Mg–Cl–HCO 3 water), and discharge area waters (Group 3: Mg–Ca–HCO 3–Cl water). Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) weathering reactions of silicate minerals.

Presence and Origin of Fluoride in the Complex Terminal Water of Ouargla Basin (Northern Sahara of Algeria)

Problem statement: The underground waters in the oriental regions of the Algerian Sahara, present real chemical quality problems. Their content in fluorides always exceeds the limit of the recommended levels. That is 0.8 mg L -1 , according to the maximum temperature in the region. Combined to a high salinity, it affects the health of the population living around the region. The present work, deals with the presence of fluoride and the geochemical origin in the Complex Terminal aquifer of Ouargla, rarely examined in the Algerian Sahara. Approach: Is based on the following aspects: Sampling and physico-chemicals analysis of water, statistical treatment of the physico- chemical water parameters and simulation to natural and isotherm 25°C evaporation of water parameters. Results: The results show the presence of fluoride in the studied water. The rates vary between 1 and 2 mg L -1 . The calculation of water saturation index in relation with the preponderant minerals, using the thermodynamic model ‹‹phreeqci›› reveals a sensitivity of carbonate minerals towards precipitation and dissolution of evaporitic minerals and clayey fluorides as well. Conclusions/Recommendations: The increasing alkalinity of water in contact with the aquifer during long periods of stay decreases the chemical activity of calcium and helps with alteration of clayey minerals and fluoride as a necessary condition for a possible fluorite mineralization. Knowing the origin of that fluoridation leads to possible solutions, through the optimization of a water treatment meets the standards.

Geochemical and isotopic study of the multilayer aquifer system in the Moulares-Redayef basin, southern Tunisia / Etude géochimique et isotopique du système aquifère multicouche du bassin de Moulares-Redayef, sud tunisien

Major ion geochemistry, and water molecule isotopes (18O, 2H) and radiogenic carbon (14C) of dissolved inorganic carbon (DIC) were used to investigate the hydrodynamic functioning of the multilayer aquifer system in the Moulares-Redayef basin, southern Tunisia. The groundwater of different aquifer levels is characterized by sulphate to calcium sulphate water type. The major geochemical processes in the aquifer system are evaporite mineral dissolution and mixing. The isotopic study allows two groundwater types to be identified: an old palaeoclimatic groundwater, marked by low 14C activity and relatively depleted stable isotope (18O and 2H) content characterizes the shallowest aquifers of the Plio-Quaternary and Miocene formations; however, a recent groundwater, distinguished by relatively high 14C activity and slightly enriched 18O and 2H content, characterizes the deep Upper Cretaceous artesian aquifer. In addition to these two water groups, other groundwaters are identified, indicating a mixing effect.

Isotopic composition of sulfate as a tracer of natural and anthropogenic influences on groundwater geochemistry in an urban sandstone aquifer, Birmingham, UK

Abstract Development and management of urban groundwater resources is limited in practice by a lack of knowledge of the nature, distribution and sources of groundwater contamination in an urban aquifer that may have had complex land use and abstraction history. Sulfate is associated with many of the solute sources to urban groundwater and in this study the use of dual isotope (δ18O and δ34S) “fingerprinting” of SO 4 2 - in groundwater to assess the contribution of different sources to urban groundwater is investigated. Groundwater (70 locations) and surface water samples from the city of Birmingham on the Triassic Sherwood sandstone aquifer have been analyzed for inorganic chemical and SO 4 2 - isotopic composition. Isotopic compositions of SO 4 2 - associated with various solute sources have also been determined. Sulfate derived from pyrite oxidation during recharge through Quaternary Drift deposits is characteristically depleted in 18O and 34S compared to other sources and is ubiquitous in unconfined zone groundwater, though the contribution from this source has increased markedly following draw down induced by abstraction from the aquifer. In unpolluted groundwaters other SO 4 2 - sources are dissolution of evaporite minerals (confined zone) and rainfall (unconfined zone). Unfortunately, SO 4 2 - isotopic composition cannot distinguish between made-ground (i.e. artificial man-made ground, usually demolition waste) and sewage sources of SO 4 2 - , which constitute the major contributions to SO 4 2 - in polluted urban groundwater at most sites. Contributions of SO 4 2 - from spilt industrial acids do have a distinctive isotopic composition seen at many metal-working and former metal-working sites. Estimates of solute contributions derived from these sources derived in this way provide a useful check on the “calibration” of aquifer and pollutant flux recharge models for urban groundwater.

CONSIDERATIONS CONCERNING THE ORIGIN OF THE ESTORIL (PORTUGAL) THERMAL WATER

In the urban area of Estoril, a Portuguese tourist village 20 km westward from Lisbon, hot mineral waters (thermal waters) spout out from natural springs which have been known since the 19th century. The thermal waters are of the sodium chloride type, with total dissolved solids and temperatures higher than the ones of the regional waters. Isotopic data are consistent with a meteoric origin for both the regional and thermal waters of the Estoril area: δ180 (‰), -4.16 to -3.52 and δ2H (‰), -25.5 to -18.7. The thermal water composition can be derived from the regional water composition assuming the dissolution of evaporite minerals, cation exchange and precipitation of calcite. The thermal water flow system has probably the recharge area somewhere in between Estoril and the Sintra mountain. The elevation difference between the recharge area and the sea provides the driving force for groundwater movement to the Estoril area where the upward movement of the mineralised and warm water is controlled by an impermeable barrier of dykes and open fractures in the pre-existing rocks.

Definition of an equilibration protocol for batch experiments on Callovo-Oxfordian argillite

In order to elude difficulties concerning reproducing controlled conditions and avoiding chemical perturbation from sampling to laboratory experiments, an equilibration protocol of crushed Callovo-Oxfordian argillite in contact with synthetic pore water is proposed. It consists of a rinse-cycle with chosen synthetic waters. Hence, artifacts such as porosity variability, secondary paragenesis of evaporitic feature and a consecutive changing of water chemistry may be corrected. Equilibrium conditions are defined when both rinsing and leaching solutions are identical. In the first phase of this work, an input grid concerning reaction duration, number of rinsing and water/rock ratio associated to a specify methodology (anoxic and P CO 2 controlled atmosphere glove box) are given. We propose a water/rock ratio equal to 5, a first 72 h rinse followed by three others of 24 h. The protocol was tested among nine synthetic waters and validated regarding major aqueous elements. A geochemical modelling allowed us to identify the reactional mechanisms occurring during the whole procedure in terms of (1) modelling reactions taking place during the various rinses; (2) quantification of the dissolution/precipitation reactions and (3) modelling the condition of the sample. According to the rinsing water composition and P CO 2 , dissolution of carbonate minerals may occur and is accompanied by a redistribution of sorbed species at argillaceous mineral surface. Solution analysis coupled with geochemical modelling show an increasing of aqueous K +, consequent upon cationic exchange reactions K +/Na +. Besides, the dissolution of carbonate minerals produced Ca 2+ and Mg 2+ which in turn are exchanged with K + and H +. Silica is controlled by the solubility of chalcedony. However, pH seems to be hardly imposed from the rinsing solution, since it is principally buffered by calco-magnesio-carbonate system. Moreover, anoxic conditions are not sufficient to restore natural reducing conditions. Redox potentials measured are all in the region of 200 mV/ESH. There is every indication that such values are either potential out of equilibrium or values imposed by pyrite oxidation products. At least, high increases of cations, SO 4 2− and in a less extent Cl − contents show typically the dissolution of evaporitic minerals formed by desiccation and inherited from samples storage historic. However, it is not out of the question that reducing potentials are thought to be expected if samples were perfectly stored under anoxic conditions.

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 8gl �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 reac- tions 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 14 C, 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

Significance of early-diagenetic water-rock interactions in a modern marine siliciclastic/evaporite environment: Salina Ometepec, Baja California

AbstractAlthough marine brines are a significant component of pore waters in sedimentary basins, there are few geochemical studies of modern analogues of such systems, especially in siliciclastic settings. For these reasons, we chose the evaporite-associated siliciclastic sediments deposited in the salt flats of the Salina Ometepec, Baja California, for an integrated investigation of sediment, pore-water, and overlying brine geochemistry. Here, the detrital components include quartz, K-feldspar, plagioclase, chlorite, biotite, and smectite, and authigenic minerals are dominated by halite, gypsum, and K-rich magnesium smectite.Thermal and saline stresses on the sediments of the Salina Ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments. Sediment pore waters exhibit little microbial sulfate reduction, and dissolved inorganic C contents are also very low. As a result, we did not observe carbonate and sulfide mineral authigenesis in the Salina Ometepec sediments. Instead, pore-water geochemical evolution is largely controlled by evaporative concentration of seawater, evaporite-mineral dissolution and recrystallization, and diagenetic alteration of detrital aluminosilicates.Evaporite-mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment. Specifically, Na+ and Cl− concentrations are increased owing to halite dissolution. We see significant Br− enrichment relative to expected seawater evaporation trends in near-surface pore water, secondary to dissolution of K- and Mg salts. Because bacterial sulfate reduction is inhibited in the Salina Ometepec sediments, sulfate concentrations are more accurate indicators of the degree of evaporation than Br−, a usually conservative element during geochemical reactions.Pore waters exhibit down-core increases in dissolved Mg2+, K+, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(H_{4}SiO_{4}^{0}\) \end{document} over the upper 1 m. Authigenic K-rich Mg-smectite formation is promoted by the concurrent processes of brine concentration, selective dissolution of K- and Mg-bearing salts, and dissolution of detrital aluminosilicates. Pore waters at a depth of 1 m have 87Sr/86Sr ratios that require input of Sr that is less radiogenic than that of Gulf of California seawater. This Sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources. These results show that significant chemical interactions among marine brines, evaporite minerals, and detrital aluminosilicates can occur relatively soon after sedimentation.

Editor’s Message Isotope techniques for monitoring groundwater salinization

Steady increase in the salinity of most of the major aquifers being used for water supply in the arid and semi-arid regions of Africa and Asia provides evidence of waterquality deterioration. This salinization is often due to inflow of dense saline water during heavy withdrawals of fresh water from coastal aquifers and/or mobilisation of saline groundwaters by the over-exploitation of inland aquifer systems. Water pollution (salinization) due to extensive irrigation and uUse of fertilisers and other pesticides is also growing. Since there are several sources for the groundwater-quality deterioration, it is necessary to characterise the specific processes involved. Isotope techniques are particularly effective for identifying the sources of salinity and the inflow of fresh groundwater. The fundamental relationships between δ 18 O and δ 2 H and between δ18O and salinity have been used in previous studies to identify different salinization pathways. These include flushing of airborne salts by precipitation or by dissolution of evaporitic minerals from the surface, the soil, or aquifer materials; and the encroachment of seawater, or the induced flow from pockets of connate brine. Although this basic approach is still valid, in the past decade new isotope tools for investigating the dissolved component (such as δ 37 Cl and δ 11 B) have appeared mainly as the results of methodological investigations or in isolated case studies. The International Atomic Energy Agency (IAEA) has a long tradition and wide experience in the use of isotope techniques for water-resources studies and is acting as the international focal point in promoting the practical application of isotope methods on a wider scale. Over the past decades, the Isotope Hydrology Section of the IAEA has monitored more than 40 research contracts and technical cooperative studies in this field, including determination of the origins of groundwater salinization, assessment of groundwater resources in coastal areas and islands, and evaluation of the effect of irrigation activities on groundwater quality (IAEA 1985, 1996, 1998, 2001). In these studies, both naturally occurring stable and radioactive isotopes were used. In 2000, the Agency launched a new initiative aimed at investigating the origins of salinity (salinization processes) and their impacts on coastal fresh-water resources using a multi-isotope approach combining various stable and radioactive isotopes (deuterium, tritium, oxygen18, carbon-14, carbon-13, sulphur-34, boron-11, strontium-87, chlorine-36, and iodine-129. The studies will be conducted within the framework of a Coordinated Research Programme (CRP) entitled, “Origins of salinity and impacts on fresh groundwater resources. Optimisation of isotope techniques.” Several proposals from contract holders in China, Israel, Jordan, Tunisia, Morocco, Korea, and Pakistan were evaluated in terms of how they meet several pre-defined criteria which were found to be the most important for the selection of a “flagship” site where an international team will conduct a study using a multiple-isotope approach.

Factors which determine the hydrogeochemical behaviour of karstic springs. A case study from the Betic Cordilleras, Spain

The Cabra-Alcaide karstic massif situated in the south of Spain constitutes an important part of the so-called Natural Park of the Subbetic Sierras. This aquifer system is drained by various springs which supply a population of some 100,000 inhabitants. The feed areas of these springs show very different characteristics with respect to their geological structure, size of the drainage area, thickness of the vadose zone, elevation and degree of karstification. In addition, the carbonate rocks lie over a clayey substratum which contains large masses of intercalated evaporites. Due to these conditions, the hydrochemical composition of the springs is relatively variable. In this study a hydrogeochemical characterization of the aquifer in both space and time is undertaken and the factors that determine it are discussed. To achieve this, 19 monthly samples were taken from the 6 most significant springs of the hydrogeological system. The commencement of sampling coincided with the transition from a period of several years of severe drought and a very wet two-year period, which amplified considerably the hydrochemical and hydrodynamic response of the springs to the recharge. Identification of hydrogeochemical processes was performed by studying hydrographs, the temporal evolution of physico-chemical parameters, ionic ratios (mainly Mg/Ca) and by means of simple and multivariate statistical analyses. The saturation status was established using the WATEQF program and the mass transfer was quantified using PHREEQC. With the exception of the epikarstic subsystem (i.e. the Zarza spring), the majority of the results indicate that the aquifer exhibits a diffuse flow model, in which piston flow phenomena are seen, closely linked to the most intense precipitation. Along the direction of flow hydrochemical trends are seen as the water type changes from Ca–HCO 3 to Ca–Mg–HCO 3; at the same time enrichment in some ions, derived from the dissolution of evaporitic minerals of the impermeable substratum, and dedolomitization processes occur. In this way, almost 90% of the water samples were oversaturated in calcite, the majority of cases being a consequence of the dissolution of the gypsum of the substratum. Only in the epikarstic springs can it be considered that the oversaturation in calcite is due to loss of CO 2 from the emerging groundwater. It is concluded that hydrodynamic aspects together with hydrogeochemical characteristics need to be taken into account to correctly explain the hydrochemical evolution of the karstic springs. Moreover, the use of both approaches permits a more accurate establishment of the degree of aquifer karstification, which in turn needs to be known in order to assess its vulnerability to contamination and to protect recharge zones.