Investigation of groundwater mineralization in the Hammamet–Nabeul unconfined aquifer,north-eastern Tunisia: geochemical and isotopic approach

Detailed hydrogeochemical and isotopic data of groundwaters from the Hammamet–Nabeul unconfined aquifer are used to provide a better understanding of the natural and anthropogenic processes that control the groundwater mineralization as well as the sources of different groundwater bodies. It has been demonstrated that groundwaters, which show Na–Cl and Ca–SO₄–Cl water facies, are mainly influenced by the dissolution of evaporates, the dedolomitization and the cation-exchange process; and supplementary by anthropogenic process in relation with return flow of irrigation waters. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted δ ¹⁸O and δ ²H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. Carbon-14 activities in shallow wells that provide evidence to the large contamination by organic ¹⁴C corroborate the recent origin of the groundwaters in the study area.     

Salinization properties of a shallow groundwater in a coastal reclaimed area,Yeonggwang, Korea

The need for more agricultural or residential land has encouraged reclamation at the coastal areas of Korea since 1200 ad (approximately). The groundwaters of these reclaimed areas could be expected to reveal hydrogeochemical properties different from those of areas directly affected by seawater intrusion. The purpose of this study, therefore, was to examine the salinization of shallow groundwater in a coastal reclaimed area and to identify the effect of land reclamation on groundwater quality. Major cations and anions, iodide, total organic carbon, δD, δ ¹⁸O and δ ¹³C were measured to assist the hydrogeochemical analysis. Chloride, δD and δ ¹⁸O data clearly show that the Na–Cl type water results from mixing of groundwater with seawater. In particular, the δD and δ ¹⁸O of Ca+Mg–Cl+NO₃ type groundwaters are close to the meteoric water line, but Na–Cl type waters enriched in chloride are ¹⁸O-enriched with respect to the meteoric water line. Meanwhile, carbon isotopic data and I/Cl ratios strongly suggest that there are various sources of salinity. The δ ¹³C values of Na–Cl type groundwaters are generally similar to those of Ca+Mg–Cl+NO₃ type waters, which are depleted in ¹³C with respect to seawater. I/Cl ratios of Na–Cl type groundwater are 10–100 times higher than that of seawater. Because the reclamation has incorporated a large amount of organic matter, it provides optimum conditions for the occurrence of redox processes in the groundwater system. Therefore, the salinization of groundwater in the study area seems to be controlled not only by saltwater intrusion but also by other effects, such as those caused by residual salts and organic matter in the reclaimed sediments.     

Groundwater flow system in Bengal Delta,Bangladesh revealed by environmental isotopes

A total number of 328 groundwater samples are analysed to evaluate the groundwater flow systems in Bengal Delta aquifers, Bangladesh using environmental isotope (²H, ¹⁸O, ¹³C, ³H, and ¹⁴C) techniques. A well-defined Local Meteoric Water Line (LMWL) δ²H = 7.7 δ¹⁸O + 10.7 ‰ is constructed applying linear correlation analyses to the monthly weighted rainfall isotopic compositions (δ¹⁸O and δ²H). The δ¹⁸O and δ²H concentrations of all groundwater samples in the study area are plotted more or less over the LMWL, which provides compelling evidence that all groundwaters are derived from rainfall and floodwater with a minor localized evaporation effects for the shallow groundwaters. Tritium concentration is observed in 40 samples out of 41 with values varying between 0.3 and 5.0 TU, which represents an evidence of young water recharge to the shallow and intermediate aquifers. A decreasing trend of ¹⁴C activity is associated with the heavier δ¹³C values, which indicates the presence of geochemical reactions affecting the ¹⁴C concentration along the groundwater flow system. Both vertical and lateral decrease of ¹⁴C activity toward down gradient show the presence of regional groundwater flow commencing from the unconfined aquifers, which discharges along the coastal regions. Finally, shallow, intermediate, and deep groundwater flow dynamics has revealed in the Bengal Delta aquifers, Bangladesh.     

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ ¹⁸O and δ ¹³C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ ¹⁸O and 0.5‰ δ ¹³C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ ¹⁸O and −18‰ δ ¹³C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ ¹⁸O and −7‰ δ ¹³C, with shifts towards higher δ ¹⁸O values and higher and lower δ ¹³C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ ¹³C signatures of CO₂-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO₂-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ ¹⁸O and −7‰ δ ¹³C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ ¹⁸O and δ ¹³C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ ¹⁸O (<10‰ calcite) and CO₂-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation.     

Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district,South Africa

Stratiform and stratabound barite ± magnetite beds are intimately associated with the polymetallic Broken Hill-type (BHT) massive sulfide deposits of the Aggeneys-Gamsberg Pb–Zn–Cu ± Ag–Ba district in the Northern Cape Province, South Africa. Barite samples were collected and studied from four localities in the district. Although metamorphic water–rock interaction processes have partially altered the chemical and to a lesser degree the isotopic composition of barite, samples identified as being the least altered display distinctly different isotopic compositions that are thought to reflect different modes of origin. All barite samples are marked by low concentrations of SrO (0.5 ± 0.2 wt%), highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios, elevated δ ³⁴S and δ ¹⁸O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic ⁸⁷Sr/⁸⁶Sr signatures (0.7164 ± 0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ ³⁴S values (27.3 ± 4.9‰) indicate that contemporaneous seawater sulfate, modified by bacterial sulfate reduction, was the single most important sulfur reservoir for barite deposition. Most importantly, δ ¹⁸O values suggest a lower temperature of formation for the Gamsberg deposit compared with the occurrences in the Aggeneys area, i.e. Swartberg-Tank Hill and Big Syncline. The obvious differences in temperature of formation are in good agreement with the Cu-rich, Ba-poor nature of the sulfide mineralization of the Aggeneys deposits vs the Cu-poor, Ba-rich character of the Gamsberg deposit. In conjunction with this, isotopic and petrographic arguments favor a sub-seafloor replacement model for the stratabound barite occurrences of the Aggeneys deposits, while at Gamsberg, deposition at the sediment–water interface as a true sedimentary exhalite appears more likely.     

Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

Sulfur isotopic composition of hydrothermal precipitates from the Lau back-arc: implications for magmatic contributions to seafloor hydrothermal systems

The sulfur isotopic composition of sulfides and barite from hydrothermal deposits at the Valu Fa Ridge back-arc spreading center in the southern Lau Basin has been investigated. Sulfide samples from the White Church area at the northern Valu Fa Ridge have δ³⁴S values averaging +3.8‰ (n= 10) for bulk sphalerite-chalcopyrite mineralization and +4.8‰ for pyrite (n= 10). Barite associated with the massive sulfides exhibits an average of +20.7‰ (n= 10). Massive sulfides from the active Vai Lili hydrothermal field at the central Valu Fa Ridge have much higher δ³⁴S ratios averaging +8.0‰ for bulk sphalerite-chalcopyrite mineralization (n= 5), +9.3‰ for pyrite samples (n= 5), and +8.0‰ and +10.9‰ for a chalcopyrite and a sphalerite separate, respectively. The isotopic composition of barite from the Vai Lili field is similar to that of barite from the White Church area and averages +21.0‰ (n= 8). Sulfide and barite samples from the Hine Hina area at the southern Valu Fa Ridge have δ³⁴S values that are considerably lighter than those observed for samples from the other areas and average −4.9‰ for pyrite (n= 9), −4.0 and −5.7‰ for two samples of sphalerite-chalcopyrite intergrowth, and −3.4‰ for a single chalcopyrite separate. The total spread in the isotopic composition of sulfides from Vai Lili and Hine Hina is more than 20‰ over a distance of less than 30 km. The δ³⁴S values of sulfides at Hine Hina are the lowest values so far reported for volcanic-hosted polymetallic massive sulfides from the modern seafloor. Barite from the Hine Hina field also has unusually light sulfur with δ³⁴S values of +16.1 to +16.7‰ (n= 5). Isotopic compositions of the sulfides at Hine Hina indicate a dramatic decrease in δ³⁴S from ordinary magmatic values and, in the absence of biogenic sulfur and/or boiling, imply a unique ³⁴S-depleted source of probable magmatic origin. Sulfide-barite mineralization in the Hine Hina area is associated with a distinctive alteration assemblage consisting of cristobalite, pyrophyllite, kaolinite, opal-CT, talc, pyrite, native sulfur, and alunite. Similar styles of alteration are typically known from high-sulfidation epithermal systems on land. Alunite-bearing, advanced argillic alteration in the Hine Hina field confirms the role of acidic, volatile-rich fluids, and a δ³⁴S value of +10.4‰ for the sulfur in the alunite is consistent with established kinetic isotope effects which accompany the disproportionation of magmatic SO₂ into H₂S and H₂SO₄. The Hine Hina field occurs near the propagating tip of the Valu Fa back-arc spreading center (i.e., dominated by dike injections and seafloor eruptions) and therefore may have experienced the largest contribution of magmatic volatiles of the three fields. The sulfur isotopic ratios of the hydrothermal precipitates and the presence of a distinctive epithermal-like argillic alteration in the Hine Hina field suggest a direct contribution of magmatic vapor to the hydrothermal system and support the concept that magmatic volatiles may be an important component of some volcanogenic massive sulfide-forming hydrothermal systems. Received: 16 January 1997 / Accepted: 28 October 1997     

Recharge rate estimation in the Mountain karst aquifer system of Figeh spring, Syria

Figeh watershed spring is one of the important groundwater aquifer, which is considered a major source for drinking waters of Damascus city and countryside. The origin identification and recharge estimates of groundwater are significant components of sustainable groundwater development in this Mountain karst aquifer of Figeh spring. During the period 2001–2009, monthly groundwater and precipitation samples were taken and the isotopic compositions of δ¹⁸O, δ²H, and chloride contents were analyzed to identify groundwater origins and to estimate recharge rates. The δ¹⁸O, δ²H of the groundwater show that the groundwater recharge is of meteoric origin. The chloride mass balance (CMB) method was used to quantify recharge rates of groundwater in the Mountain karst aquifer of Figeh spring. The recharge rate varies from 192 to 826 mm/year, which corresponds to 43 and 67% of the total annual rainfall. Recharge rates estimated by CMB were compared with values obtained from other methods and were found to be in good agreement. This study can be used to develop effective programs for groundwater management and development.     

Hydrogeological characterization of carbonated Jurassic aquifers in the Gijón-Villaviciosa basin (Asturias,N Spain) by means of geochemical and isotopic techniques

Chemical and isotopic analyses of groundwater from the carbonated Jurassic aquifers in the Gijón-Villaviciosa basin (Asturias, northern Spain) were carried out. Nine springs were sampled to determine major cations and anions, as well as the stable isotopes of the water molecule (δ²H and δ¹⁸O) and sulphate (δ³⁴S) values. Also, δ³⁴S values from gypsum coming both from Triassic rocks and bottom of Jurassic sequence were also determined. The results obtained were used to classify the waters with a genetic criteria in three groups: (1) waters with a high gypsum influence, with sulphate coming from Jurassic gypsum, (2) waters without gypsum influence, where sulphate source could be atmospheric deposition from industrial processes and marine aerosol, and (3) waters with some gypsum influence, in which sulphate origin could be a combination of different sources. In relation to recharge, δ²H and δ¹⁸O values were close to those of Global Meteoric Water Line and fit a local line that suggests a meteoric origin. The estimated elevations for spring recharge are in agreement with those obtained from hydrogeological maps.     

A comparative study on isotopic composition of precipitation in wet tropic and semi-arid stations across southern India

Isotopic composition of monthly composite precipitation samples from Kozhikode (n = 31), a wet tropic station and Hyderabad (n = 25), a semi-arid station across southern India were studied for a period of four years from 2005 to 2008. During the study period, the Kozhikode station recorded an average rainfall of 3500 mm while the Hyderabad station showed an average rainfall of 790 mm. The average stable isotope values in precipitation at the Kozhikode station were δ ¹⁸O = −3.52‰, d-excess = 13.72‰; δ ¹⁸O = −2.94‰, d-excess = 10.57‰; and δ ¹⁸O = −7.53‰, d-excess = 13.79‰, respectively during the pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–February) seasons. For the Hyderabad station, the average stable isotope values were δ ¹⁸O = −5.88‰, d-excess = 2.34‰; δ ¹⁸O = −4.39‰, d-excess = 9.21‰; and δ ¹⁸O = −8.69‰, d-excess = 14.29‰, respectively for the three seasons. The precipitation at the two stations showed distinctive isotopic signatures. The stable isotopic composition of precipitation at the Hyderabad station showed significant variations from the global trend while the Kozhikode station almost followed the global value. These differences are mainly attributed to the latitudinal differences of the two stations coupled with the differences in climatic conditions.     

Origin and recharge estimates of groundwater in the ordos plateau,People’s Republic of China

Groundwater is a valuable resource in the semiarid Ordos Plateau region where abundant mineral resources, such as coal, natural gas, and halite, are present. With resources development, groundwater demand will increase dramatically. The origin identification and recharge estimates of groundwater are significant components of sustainable groundwater development in the Ordos Plateau. Groundwater and precipitation samples were taken and the isotopic compositions δ²H, δ¹⁸O, and chloride were analyzed to identify groundwater origins and to estimate recharge rates. The δ²H and δ¹⁸O of the groundwater show that the groundwater recharge is of meteoric origin. The chloride mass balance (CMB) method was used to quantify recharge rates of groundwater in the Ordos Plateau, which varies from 2.93 to 22.11% of the effective annual rainfall. Recharge rates estimated by CMB were compared with values obtained from other methods and were found to be in good agreement. This study can be used to develop effective programs for groundwater management and development.     

Isotopic and molecular distributions of biochemicals from fresh and buried <Emphasis Type="Italic">Rhizophora mangle</Emphasis> leaves†

Rhizophora mangle L. (red mangrove) is the dominant species of mangrove in the Americas. At Twin Cays, Belize (BZ) red mangroves are present in a variety of stand structures (tall >5 m in height, transition ~2–4 m and dwarf ~1–1.5 m). These height differences are coupled with very different stable carbon and nitrogen isotopic values[1] (mean tall δ ¹³C = -28.3‰, δ ¹⁵N = 0‰; mean tall δ ¹³C = -25.3‰, δ ¹⁵N = -10‰). To determine the utility of using these distinct isotopic compositions as 'biomarkers' for paleoenvironmental reconstruction of mangrove ecosystems and nutrient availability, we investigated the distribution and isotopic (δ ¹³C and δ ¹⁵N) composition of different biochemical fractions (water soluble compounds, free lipids, acid hydrolysable compounds, individual amino acids, and the residual un-extractable compounds) in fresh and preserved red mangrove leaves from dwarf and tall trees. The distribution of biochemicals are similar in dwarf and tall red mangrove leaves, suggesting that, regardless of stand structure, red mangroves use nutrients for biosynthesis and metabolism in a similar manner. However, the δ ¹³C and δ ¹⁵N of the bulk leaf, the biochemical fractions, and seven amino acids can be used to distinguish dwarf and tall trees at Twin Cays, BZ. The data support the theory that the fractionation of carbon and nitrogen occurs prior to or during uptake in dwarf and tall red mangrove trees. Stable carbon and nitrogen isotopes could, therefore, be powerful tools for predicting levels of nutrient limitation at Twin Cays. The δ ¹³C and δ ¹⁵N of biochemical fractions within preserved leaves, reflect sedimentary cycling and nitrogen immobilization. The δ ¹⁵N of the immobilized fraction reveals the overlying stand structure at the time of leaf deposition. The isotopic composition of preserved mangrove leaves could yield significant information about changes in ecosystem dynamics, nutrient limitation and past stand structure in mangrove paleoecosystems.     

Tracing the sources of nitrate in karstic groundwater in Zunyi,Southwest China: a combined nitrogen isotope and water chemistry approach

Nitrate (NO₃ ⁻) is major pollutant in groundwater worldwide. Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources due to the rapid movement of water in their conduit networks. In this study, the isotopic compositions (δ¹⁵N–NO₃ ⁻, δ¹⁵N–NH₄ ⁺) and chemical compositions(e.g., NO₃ ⁻, NH₄ ⁺, NO₂ ⁻, K⁺) were measured in groundwater in the Zunyi area of Southwest China during summer and winter to identify the primary sources of contamination and characterize the processes affecting nitrate in the groundwater. It was found that nitrate was the dominant species of nitrogen in most of the water samples. In addition, the δ¹⁵N–NO₃ ⁻ values of water samples collected in summer were lower than those collected in winter, suggesting that the groundwater received a significant contribution of NO₃ ⁻ from agricultural fertilizer during the summer. Furthermore, the spatial variation in the concentration of nitrate and the δ¹⁵N–NO₃ ⁻ value indicated that some of the urban groundwater was contaminated with pollution from point sources. In addition, the distribution of δ¹⁵N–NO₃ ⁻ values and the relationship between ions in the groundwater indicated that synthetic and organic fertilizers (cattle manure) were the two primary sources of nitrate in the study area, except in a few cases where the water had been contaminated by urban anthropogenic inputs. Finally, the temporal and spatial variation of the water chemistry and isotopic data indicated that denitrification has no significant effect on the nitrogen isotopic values in Zunyi groundwater.     

The trophic structure of the pipefish community (Pisces: Syngnathidae) from a western Mediterranean seagrass meadow based on stable isotope analysis

Syngnathus abaster andSyngnathus typhle (Pisces: Syngnathidae) from a dense Cymodocea nodesa meadow in the western Mediterranean Stagnone di Marsala (Italy) were studied using δ¹³N and δ¹⁵N analysis. Because of the presence of these two species in the same habitat and the specialized parental care by the male, the effect of species and sex on the isotopic composition was also studied to investigate the different feeding strategies between and within species.S. abaster andS. typhle exhibited enriched¹³C and¹⁵N values throughout the sampling period (mean ±SE, δ¹³C =−10.5±1.8‰ and−10.8±2.0‰, δ¹⁵N=11.9±0.7‰ and 10.6±1.0%., respectively), suggesting that these species receive their energy mainly from mixed sources, particularly from sedimentary and particulate organic matter and the seagrassC. nodosa as ultimate organic matter sources. ANOVA results demonstrated that the interaction between season, species, and sex significantly affected the isotopic composition of the pipefish (p<0.001 for both δ¹³C and δ¹⁵N). Differences between species and sex, although significant, were smaller than the values generally reported for trophic level differences (≈1‰ and ≈3.5‰ for δ¹⁵N, respectively) and change in foraging habitat, Slight isotopic differences may mirror small differences in resource exploitation and resource partitioning. Evidence from stomach content analysis from the literature coupled with stable isotope measurements, while disagreeing somewhat, provide additional knowledge of pipefish feeding strategies.     

Recharge source and hydrogeochemical evolution of shallow groundwater in a complex alluvial fan system,southwest of North China Plain

Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population, quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess these groundwater resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater was recharged by precipitation and was characterized by Ca–HCO₃ type water with depleted δ¹⁸O and δD (mean value of −8.8‰ δ¹⁸O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex (Ca–Na–Mg–HCO₃–Cl–SO₄ type), and heavier δ¹⁸O and δD were observed (around −8‰ δ¹⁸O). Before the surface water with mean δ¹⁸O of −8.7‰ recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study area, and stable isotopic compositions of the groundwater (mean value of −8.8‰ δ¹⁸O) were similar to those of transferred water (−8.9‰), increasing from the southern boundary of the study area to the distal end of the fan. The groundwater underwent chemical evolution from Ca–HCO₃, Na–HCO₃, to Na–SO₄. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical evolution, and groundwater flow paths in the complex alluvial fan aquifer system.     

Reconstructing the History of eastern and Central Florida Bay using mollusk-shell isotope records

Stable isotopic ratios of carbon and oxygen (δ¹³C and δ¹⁸O) from mollusk shells reflect the water quality characteristics of Florida Bay and can be used to characterize the great temporal variability of the bay. Values of δ¹⁸O are directly influenced by temperature and evaporation and may be related to salinity, δ¹³C values of δ¹³C are sensitive to organic and inorganic sources of carbon and are influenced by productivity. Analyses of eight mollusk species from five short-core localities across Florida Bay show large ranges in the values of δ¹³C and δ¹⁸O, and reflect the variation of the bay over decades. Samples from southwester Florida Bay have distinct δ¹³C values relative to samples collected in northeastern Florida Bay, and intermediate localities have intermediate values.¹³C values of δ¹³C grade from marine in the southwest bay to more estuarine in the northeast. Long cores (>1m), with excellent chronologies were analyzed from central and eastern Florida Bay. Preliminary analyses ofBrachiodontes exustus andTransenella spp. from the cores showed that both δ¹³C and δ¹⁸O changed during the first part of the twentieth century. After a century of relative stability during the 1800s, δ¹³C decreased between about 1910 and 1940, then stabilized at these new values for the next five decades. The magnitude of the reduction in δ¹³C values increased toward the northeast. Using a carbon budget model, reduced δ¹³C values are interpreted as resulting from decreased circulation in the bay, probably associated with decreased freshwater flow into the Bay. Mollusk shell δ¹⁸O values display several negative excursions during the 1800s, suggesting that the bay was less evaporitic than during the twentieth century. The isotope records indicate a fundamental change took place in Florida Bay circulation early in the twentieth century. The timing of the change links it to railroad building and early drainage efforts in South Florida rather than to flood control and water management measures initiated after World War II.     

Hydrogeochemical stratification of the unconfined Samobor aquifer (Zagreb,Croatia)

In the western part of the city of Zagreb, Croatia, hydrogeochemical and isotopic investigations of the Samobor aquifer were carried out with the aim of determining the differences in hydrogeochemical characteristics at increasing aquifer depths. The aquifer comprises 40-m thick gravelly–sandy deposits, with lenses and interlayers of silt and clay. The analyses have proven that with increasing aquifer depth, there are decreases in groundwater temperature and the values of electrical conductivity and increases in the sodium, iron and manganese concentrations. The δ¹³C distribution shows an evident increase in biogenic carbon concentrations with increasing aquifer depth. The measured specific ¹⁴C activities showed that the deeper part of the aquifer is characterized by slow water exchange, while the shallower part is influenced by current recharge, although the pumping wells located on the well-field downstream penetrate the aquifer fully. A direct exchange of water from the Sava River and groundwater occurs in the near vicinity of the river. This exchange weakens further away, while the difference in hydrogeochemical characteristics between the Sava River water and groundwater increases.     

Stable isotope analyses (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) of the trophic relationships of<Emphasis Type="Italic">Callinectes sapidus</Emphasis> in two North Carolina estuaries

The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ¹⁵N and δ¹³C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ¹³C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R² = 0.92, p < 0.01). There was a positive relationship between blue crab δ¹⁵N values and nitrate concentrations for the NRE (R² = 0.48, p = 0.12). This study found that blue crab δ¹³C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ¹³C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.     

Isotopic composition of dissolved boron and its geochemical behavior in a freshwater-seawater mixture at the estuary of the Changjiang （Yangtze） River

The isotopic composition of dissolved boron, in combination with the elemental concentrations of B, Cl and salinities in freshwater-seawater mixed samples taken from the estuary of the Changjiang River, the largest one in China, was investigated in detail in this study. Brackish water and seawater samples from the estuary of the Changjiang River were collected during low water season in November, 1998. Boron isotopic compositions were determined by the Cs2BO^＋2-graphite technique with a analytical uncertainty of 0.2‰ for NIST SRM 951 and an average analytical uncertainty of 0.8‰ for the samples. The isotopic compositions of boron, expressed in δ^11B, and boron concentrations in the Changjiang River at Nanjing and seawater from the open marine East Sea, China, are characterized by δ^11B values of -5.4‰ and 40.0‰, as well as 0.0272 and 4.43 mg B/L, respectively. Well-defined correlations between δ^11B values, B concentrations and Cl concentrations are interpreted in terms of binary mixing between fiver input water and East Sea seawater by a process of straightforward dilution. The offsets of δ^11B values are not related to the contents of clastic sediment and to the addition of boron. These relationships favor a conservative behavior of boron at the estuarine of the Changjiang River.     

Microbial alteration of natural gas in Xinglongtai field of the Bohai Bay Basin, China

Xinglongtai field has been an important petroleum-producing area of Liaohe Depression for 30 years. Oil exploration and production were the focus of this field, but the gas was ignored. This study examined twenty five gas samples with the purpose of determining the gas genetic types and their geochemical characteristics. Molecular components, stable carbon isotopic compositions and light hydrocarbons were also measured, and they proved that microbial activity has attacked some of the gas components which resulted in unusual carbon isotopic distributions. Propane seems to be selectively attacked during the initial stage of microbial alteration, with abnormally lower con-tent compared to that of butane as well as anomalously heavy carbon isotope. As a consequence, the carbon isotopic distribution among the gas components is partially reverse, as δ13C1<δ13C2<δ13C3>δ13C4. Besides, n-alkanes of C3+ gas components are preferentially attacked during the process of microbial alteration. This is manifested that n-alkanes are more enriched in 13C than corresponding iso-alkanes. As a result, the concentrations of n-alkanes be-come very low, which may be misleading in indentifying the gas genetic types. As to four gas samples, light hydro-carbon compositions display evidence for microbial alteration. The sequence of hexane isomers varies obviously with high content of 2,3-DMC4, which indicates that they have been in the fourth level of extensively bacterial al-teration. So the geochemical characteristics can be affected by microbial alteration, and recognition of microbial alteration in gas accumulations is very important for interpreting the natural gas genetic types.