Water-rock interaction and chemistry of groundwaters from the Canadian Shield

The chemical and isotopic compositions of groundwaters in the crystalline rocks of the Canadian Shield reflect different degrees of rock-water interactions. The chemistry of the shallow, geochemically immature groundwaters and especially of the major cations is controlled by local rock compositions, whereby dissolution reactions dominate. Conservative constituents, such as chloride and bromide, however, are not entirely a result of such reactions but appear to be readily added from leachable salts during the initial stages of the geochemical evolution of these waters. Their concentration changes little as major cations increase, until concentrations of Total Dissolved Solids (TDS) reach 3000 to 5000 mg 1^?1. The isotopic composition of these shallow waters reflects local, present day precipitations.In contrast to the shallow groundwaters, the isotopic and chemical compositions of the deep, saline waters and brines are determined by extensive, low-temperature rock-water interactions. This is documented in major ion chemistries, ¹⁸O contents and strontium isotopic compositions. These data indicate that the deep brines have been contained in hydrologically isolated “pockets”. The almost total loss of primary compositions make discussions on the origin of these brines very speculative. However, all brines from across the Canadian Shield have a very similar chemical composition, which probably reflects a common geochemical history. The concentrations of some major and most minor elements in these fluids appear to be governed by reactions with secondary mineral assemblages.     

Mixing processes in hydrothermal spring systems and implications for interpreting geochemical data: a case study in the Cappadocia region of Turkey

Mixing is a dominant hydrogeological process in the hydrothermal spring system in the Cappadocia region of Turkey. All springs emerge along faults, which have the potential to transmit waters rapidly from great depths. However, mixing with shallow meteoric waters within the flow system results in uncertainty in the interpretation of geochemical results. The chemical compositions of cold and warm springs and geothermal waters are varied, but overall there is a trend from Ca–HCO₃ dominated to Na–Cl dominated. There is little difference in the seasonal ionic compositions of the hot springs, suggesting the waters are sourced from a well-mixed reservoir. Based on δ¹⁸O and δ²H concentrations, all waters are of meteoric origin with evidence of temperature equilibration with carbonate rocks and evaporation. Seasonal isotopic variability indicates that only a small proportion of late spring and summer precipitation forms recharge and that fresh meteoric waters move rapidly into the flow system and mix with thermal waters at depth. ³H and percent modern carbon (pmC) values reflect progressively longer groundwater pathways from cold to geothermal waters; however, mixing processes and the very high dissolved inorganic carbon (DIC) of the water samples preclude the use of either isotope to gain any insight on actual groundwater ages.     

Groundwater Recharge and Mixing in Arid and Semiarid Regions: Heihe River Basin, Northwest China

A sound understanding of groundwater recharged from various sources occurring at different time scales is crucial for water management in arid and semi-arid river basins. Groundwater recharge sources and their geochemical evolution are investigated for the Heihe River Basin(HRB) in northwest China on the basis of a comprehensive compilation of geochemical and isotopic data. Geochemical massbalance modeling indicates that mountain-block recharge accounts for a small fraction(generally less than 5%) of the shallow and deep groundwater sustaining the oasis, whereas infiltration of rivers and irrigation water contribute most of the groundwater recharge. Dedolomitization is the primary process responsible for the changes in groundwater chemical and carbon isotope compositions from the piedmont to the groundwater discharge zone, where the dedolomitization is very likely enhanced by modern agricultural activities affecting the shallow groundwater quality. Analysis of radioactive isotopes suggests that these primary recharge sources occur at two different time scales. Radiocarbon-derived groundwater age profiles indicate a recharge rate of approximately 12 mm/year, which probably occurred during 2000–7000 years B.P., corresponding to the mid-Holocene humid period. The recharge of young groundwater on the tritium-dated time scale is much higher, about 360 mm/year in the oasis region. Infiltration from irrigation canals and irrigation return flow are the primary contributors to the increased young groundwater recharge. This study suggests that groundwater chemistry in the HRB has been influenced by the complex interaction between natural and human-induced geochemical processes and that anthropogenic effects have played a more significant role in terms of both groundwater quantity and quality.     

Palaeohydrodynamics of fluids in the Brent Group (Oseberg Field,Norwegian North Sea) from chemical and isotopic compositions of formation waters

Generally, the history of past sub-surface fluid movements is difficult to reconstruct. However, the composition of oil-field waters characterizes the origins and mixing processes that allow such a reconstruction. We have investigated present-day formation waters from Brent Group sedimentary rocks of the Oseberg Field in order to assess both their geochemical variations, and their origin(s). Water samples (sampled at the separator) produced from immediately above the oil–water contact and from the aquifer (water-saturated zone below the oil–water contact) were taken from 11 wells across the field. In addition, 3 trace water samples were extracted from oil produced from higher up in the oil column. The water samples were analysed for their chemical components and isotopic compositions. Conservative tracers such as Cl, Br, δD, and δ¹⁸O were used to evaluate the origin of the waters. All formation waters can be characterised as Na–Cl-brines. The separator samples are of aquifer origin, indicating that aquifer water, drawn up by the pressure reduction near the well, is produced from the lower few tens of metres of the oil-zone. By defining plausible endmembers, the waters can be described as mixtures of seawater (60–90%), meteoric water (10–30%), evaporated seawater (primary brines) (3–5%), and possibly waters which have dissolved evaporites (secondary brines). Alternatively, using multidimensional scaling, the waters can be described as mixtures of only 3 endmembers without presupposing their compositions. In fact, they are seawater, very dilute brine, and a secondary brine (confirming the power of this approach). Meteoric water was introduced into the reservoir during the end-Brent and early-Cretaceous periods of emergence and erosion, and partially replaced the marine pore fluids. Lateral chemical variations across the Oseberg Field are extremely small. The waters from closer to the erosion surfaces show slightly stronger meteoric water isotopic signatures. The primary and secondary brines are believed to come from Permian and Triassic evaporitic rocks in the deeply buried Viking Graben to the west, and to have been modified by water–rock interactions along their migration path. These primary basinal brines have not been detected in the oil–zone waters, suggesting that the brines entered the reservoir after the main phase of oil-migration. There are indications that these external fluids were introduced into the reservoir along faults. Present-day aquifer waters are mixtures of waters from different origins and hardly vary at a field-scale. They are different in composition to the water trapped in the present oil-zone. One of the oil-zone samples is a very dilute brine. It is thought to represent a simple mixture of seawater and meteoric water. Due to oil-emplacement, this geochemical signature was preserved in the waters trapped within the oil-zone. Another oil-zone water shows a very similar chemical signature to the aquifer waters, but the chlorine isotopic signature is similar to that of the dilute oil-zone water. This water is interpreted to represent a palaeo-aquifer water. That is, it was within the aquifer zone in the past, but was trapped by subsequent emplacement of more oil. These vertical differences can be explained by two features: (i) emergence of the Brent Group sedimentary rocks in the Early Cretaceous allowed ingress of meteoric water; (ii) subsequent rapid burial of Viking Graben rocks caused migration of petroleum and aqueous fluids into the adjacent, less deeply buried Oseberg Field.     

Comprehensive Chemical and Isotopic Analyses of Basalt and Sediment Reference Materials

Geochemical studies of geological samples require the precise determination of their major and trace element contents and, when measured, of their isotopic compositions. It is now commonly accepted that the accuracy and precision of geochemical analyses are best estimated by the concomitant analysis of international reference materials run as unknown samples. Although the composition of a wide selection of basalts is relatively well constrained, this is far from being the case for sedimentary materials. We present here a comprehensive set of major and trace element data as well as Nd, Hf, Sr and Pb isotopic compositions for thirteen commonly used international reference materials – eight magmatic rocks (BHVO‐2, BR, BE‐N, BR 24, AGV‐1, BIR‐1, UB‐N, RGM‐1) and five sediments (JLk‐1, JSd‐1, JSd‐2, JSd‐3, LKSD‐1). We determined the concentrations of over forty elements in the magmatic rocks together with Sr, Nd, Hf and Pb isotopic compositions. Our trace element results were both accurate (difference ≤ 3%) and precise (reproducibility at 1s ≤ 3%) and the isotopic results were very similar to other published values. In contrast, we observed a significant chemical and isotopic variability in the sedimentary materials, which we attribute to mineral heterogeneities in the powders. Despite the limitation imposed by this heterogeneity, our work presents a complete set of data determined with a precision not yet achieved in the literature for sedimentary material. We also provide the first Nd, Hf and Pb isotopic measurements for the five sediments, which are commonly used by the geochemical community. Our study of both basalt and sediment reference materials represents a comprehensive and self‐consistent set of geochemical data and can therefore be considered as a reference database for the community.     

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

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

Formation of the chemical composition of brackish and brine groundwater in the Tuva depression and surrounding areas

Groundwater with high salinity is widespread in different climatic and geologic environments of the world. The formation of its chemical composition, however, is still debatable. The chemical composition of groundwater has been studied in 19 springs of the Tuva depression. In this area, hydrocarbonate, sulfate, and chloride waters with different cation compositions discharge. Their TDS value varies mainly from 1 to 6 g/L, reaching 315 g/L at only one locality. The chemical composition of the studied waters is reflective of the geostructural, hydrogeologic, landscape, and geochemical conditions. The main processes determining the chemical composition of the waters are their interaction with aluminosilicate minerals, dissolution of gypsum and halite, evaporation, and oxidation of sulfide minerals.     

Primordial compositions of refractory inclusions

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10⁻⁶ to 10⁻¹ bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al₂O₃ relative to more volatile MgO + SiO₂ is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.     

贵阳市区地表/地下水化学与锶同位素研究

贵阳市及邻近地区地表和地下水的化学与Sr同位素组成变化反映了典型喀斯特地区地表/地下水文系统的水-岩反应和城市污染特征:水体中的化学溶解物质主要来源于碳酸盐岩(石灰岩和白云岩)的风化作用和膏岩层的溶解,其次为人为污染物的输入;污染物以K⁺,Na⁺,Cl^-,SO^2-₄,NO^-₃为主,枯水期因大气降水补给小而受人为活动影响较大;丰水期和枯水期地表/地下水的化学组成变化说明地表/地下水交换活跃,地下水环境容易受到人为活动影响。     

Origin of salinity in produced waters from the Palm Valley gas field,Northern Territory,Australia

The chemical composition and evolution of produced waters associated with gas production in the Palm Valley gas field, Northern Territory, has important implications for issues such as gas reserve calculations, reservoir management and saline water disposal. The occurrence of saline formation water in the Palm Valley field has been the subject of considerable debate. There were no occurrences of mobile water early in the development of the field and only after gas production had reduced the reservoir pressure, was saline formation water produced. Initially this was in small quantities but has increased dramatically with time, particularly after the initiation of compression in November 1996.The produced waters range from highly saline (up to 300,000 mg/L TDS), with unusual enrichments in Ca, Ba and Sr, to low salinity fluids that may represent condensate waters. The Sr isotopic compositions of the waters (⁸⁷Sr/⁸⁶Sr = 0.7041–0.7172) are also variable but do not correlate closely with major and trace element abundances. Although the extreme salinity suggests possible involvement of evaporite deposits lower in the stratigraphic sequence, the Sr isotopic composition of the high salinity waters suggests a more complex evolutionary history.The formation waters are chemically and isotopically heterogeneous and are not well mixed. The high salinity brines have Sr isotopic compositions and other geochemical characteristics more consistent with long-term residence within the reservoir rocks than with present-day derivation from a more distal pool of brines associated with evaporites. If the high salinity brines entered the reservoir during the Devonian uplift and were displaced by the reservoir gas into a stagnant pool, which has remained near the reservoir for the last 300–400 Ma, then the size of the brine pool is limited. At a minimum, it might be equivalent to the volume displaced by the reservoired gas.     

The origin of fluids in the salt beds of the Delaware Basin,New Mexico and Texas

Oxygen and hydrogen isotope analyses have been made of (1) brines from several wells in the salt deposits of the Delaware Basin, (2) inclusion fluids in halite crystals from the ERDA No. 9 site, and (3) local ground waters of meteoric origin. The isotopic compositions indicate that the brines are genetically related and that they probably originated from the evaporation of paleo-ocean waters. Although highly variable in solute contents, the brines have rather uniform isotopic compositions. The stable isotope compositions of brine from the ERDA No. 6 site (826.3 m depth) and fluid inclusions from the ERDA No. 9 site are variable but remarkably regular and show that (1) mixing with old or modern meteoric waters has occurred, the extent of mixing apparently decreasing with depth, and (2) water in the ERDA No. 6 brine may have originated from the dehydration of gypsum. Alternatively, the data may reflect simple evaporation of meteoric water on a previously dry marine flat. Stable isotope compositions of all the waters analyzed indicate that there has been fairly extensive mixing with ground water throughout the area, but that no significant circulation has occurred. The conclusions bear importantly on the suitability of these salt beds and others as repositories for nuclear waste.     

Hydrogeochemical and isotopic study of the Kumho River, Korea: implications for anthropogenic and seasonal effects

The geochemical and isotopic compositions of river water are controlled by different factors. The seasonal and spatial variations in the geochemical composition, δD, δ¹⁸O, and δ¹⁵N–NO₃ of the Kumho River were investigated to reveal the geochemical processes occurring at different seasons. The Kumho River, which runs through different geologic terrains with different land use characteristics, is the largest tributary of the Nakdong River, the longest river in South Korea. The data varied significantly according to the land use and the season. Each monitoring station showed the lowest concentrations of various ions during July, the rainy season, due to the increase of precipitation rate. The ionic concentrations gradually increased downstream by the mineral weathering and anthropogenic activity. At the upper regions of the river, Ca and HCO₃, which are closely associated with mineral weathering, were the most dominant cation and anion, respectively. The relatively high Si concentration of the headwater samples, caused by the weathering of volcanic rocks, also showed the importance of weathering in the upper regions mainly composed of volcanic rocks. The downstream regions of the Kumho River are mainly influenced by sedimentary rocks. At the lower reaches of the river, especially near the industrial complexes in Daegu, the third largest city in Korea, Na, Cl, and SO₄ became the dominant ions, indicating that the anthropogenic pollution became more important in regulating the chemical composition of the river. The increasing (Ca + Mg + Na + K)/HCO₃ ratio downstream also indicates that the anthropogenic effects became more important as the river flows downstream. The isotopic compositions of δD and δ¹⁸O indicate that the river waters were significantly affected by evaporation during May and July, but the evaporation effect was relatively low during October. The isotopic composition of δ¹⁵N–NO₃ increased downstream, also confirming that anthropogenic effects became more significant at the lower reach of the river and near Daegu.     

Assessment of water quality index for the groundwater in the upper Cheliff plain,Algeria

Assessment of groundwater suitability for drinking and agricultural purposes was carried out in the plain of upper Cheliff. The study area covers an area of 375 km² and lies in a semiarid climate. Groundwater is the major source for domestic and agricultural activity in this area. Groundwater samples were collected from 19 wells during dry and wet periods in 2012, and they were analyzed for major cations and anions and compared with drinking and irrigation specification standards. The concentration of the majority of chemical constituents exceeds the standards of WHO as a result of various sources of pollution. It indicates the dominance of groundwater types: Ca-Mg-Cl, and Ca-Mg-HCO₃. Suitability of groundwater for drinking was evaluated based on the water quality index; it shows more than 60% of samples have very poor quality for dry and wet periods, which means water is severely contaminated and unsuitable for drinking purpose. In terms of the irrigation usage, generally groundwater is suitable for both periods in the major part of the plain. The Mineralization processes in this area is determined by the lithology of the aquifer (exchange water-rock), by anthropogenic factors (discharges of urban sewage, use of fertilizers) and also by evaporation (semi-arid climate).     

Hydrogeochemical parameters for assessment of groundwater quality in the upper Gunjanaeru River basin,Cuddapah District,Andhra Pradesh,South India

In the management of water resources, quality of water is just as important as its quantity. In order to know the quality and/or suitability of groundwater for domestic and irrigation in upper Gunjanaeru River basin, 51 water samples in post-monsoon and 46 in pre-monsoon seasons were collected and analyzed for various parameters. Geological units are alluvium, shale and quartzite. Based on the analytical results, chemical indices like percent sodium, sodium adsorption ratio, residual sodium carbonate, permeability index (PI) and chloroalkaline indices were calculated. The pre-monsoon waters have low sodium hazard as compared to post-monsoon season. Residual sodium carbonate values revealed that one sample is not suitable in both the seasons for irrigation purposes due the occurrence of alkaline white patches and low permeability of the soil. PI values of both seasons revealed that the ground waters are generally suitable for irrigation. The positive values of Chloroalkaline indices in post-monsoon (80%) and in pre-monsoon (59%) water samples indicate absence of base-exchange reaction (chloroalkaline disequilibrium), and remaining samples of negative values of the ratios indicate base-exchange reaction (chloroalkaline equilibrium). Chadha rectangular diagram for geochemical classification and hydrochemical processes of groundwater for both seasons indicates that most of waters are Ca–Mg–HCO₃ type. Assessment of water samples from various methods indicated that majority of the water samples in both seasons are suitable for different purposes except at Yanadipalle (sample no. 8) that requires precautionary measures. The overall quality of groundwater in post-monsoon season in all chemical constituents is on the higher side due to dissolution of surface pollutants during the infiltration and percolation of rainwater and at few places due to agricultural and domestic activities.     

Identification of the origin of salinization in groundwater using multivariate statistical analysis and geochemical modeling: a case study of Kaohsiung,Southwest Taiwan

The study of brine aquifers in southern Taiwan is highly complicated by hybrid geochemical reactions, which obscure important geochemical information. Using multivariate analysis on major and minor ion compositions normalized by Cl⁻ content, chemical constituents were combined into two principal components representing brine mixing and mineral precipitation. Comparing to multivariate analysis on the original data, this procedure reveals more geochemical information. It demonstrates that the brine groundwater of the region is primarily composed of highly evaporated seawater. The evaporation ratio is >70%; a point at which calcite, dolomite and gypsum precipitate. Oxygen and hydrogen isotopic compositions confirm this inference; and further, geochemical modeling quantitatively determined the evaporation ratio to be about 85%. Natural boron contamination is a consequence of brine groundwater. Two evolutionary trends in the plotting of the Cl/B ratio versus Cl⁻ can be identified: (1) Cl/B ratio decreases with boron being released from clay minerals when brine aquifers are flushed with freshwater; and (2) Cl/B ratio increases when seawater of a high Cl/B ratio infiltrates coastal aquifers.     

Geochemical characterization of the Biga Peninsula thermal waters (NW Turkey)

Thermal water chemistry from the Biga Peninsula (NW Turkey) was investigated in order to discriminate among hydrochemical facies, and isotopic groups and identify the major geochemical processes. A systematic hydrogeochemical survey was carried out, incorporating new data as well as results from the previous studies. Results were used to further develop hydrogeological and geochemical models. Thermal water compositions were classified into four groups and the processes affecting evolution of water compositions were interpreted. Types 1, 2 and 3 are representatives of water corresponding to sulfate dominant fluids (mainly NaSO₄-type), chloride dominant fluids (mainly NaCl-type), and bicarbonate dominant fluids (Na- or CaHCO₃-type), respectively. Group 4 comprises the fluids with compositions that are not dominated by any distinctive anion. Groundwater infiltrates and circulates through the marbles of the Paleozoic basement. The isotopic composition of thermal waters revealed that deep infiltration of meteoric water took place in periods of changed climatic conditions.     

Spatial characteristics of water quality,stable isotopes and tritium associated with groundwater flow in the Hutuo River alluvial fan plain of the North China Plain

The groundwater flow system and the flow velocity in the alluvial fan plain of the Hutuo River, China, have been studied, with an emphasis on relating geochemical characteristics and isotopes factors. Seven stretches of one river, six springs and 31 wells, with depths ranging from 0 m (river waters) to 150 m, were surveyed. The groundwater has a vertical two-layer structure with a boundary at about 80–100 m depth, yielding an upper and a lower groundwater layer. The δ¹⁸O and δD values range from ?10.56 to ?7.05‰ and ?81.83 to ?59‰, respectively. The groundwater has been recharged by precipitation, and has not been subjected to significant evaporation during infiltration into the aquifer in the upper layer. Using a tritium model, the groundwater flow in the alluvial fan plain showed horizontal flow velocity to be greater than vertical velocity. Groundwater in the upper layer is characterized by Ca–HCO₃ type. From the spatial distribution characteristics of the stable isotope and chemical composition of the groundwater, agricultural irrigation was considered to have an influence on the aquifer by causing excessive groundwater abstraction and irrigation return.     

Geochemistry of the thermal waters of Sikhote Alin

This paper reports new geochemical data on the low temperature nitric thermal waters of Sikhote Alin. The studied alkaline waters belong to the HCO₃-Na type with significant trace element variations. The waters demonstrate an increase in temperature and TDS from the south northward of Sikhote Alin. The oxygen and hydrogen isotopic data suggest their infiltration origin. The chemical composition of these waters was formed by water-rock interaction.     

Geochemical variations within a laminated evaporite deposit: evidence for brine composition during formation of the Permian Castile Formation, Texas and New Mexico, USA

The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite.

Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of ¹⁸O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ¹⁸O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes are independent of δ¹⁸O values. The geochemical data do not provide conclusive proof of water depth during deposition of the Castile Formation. The data are interpreted as reflecting small-scale changes in conditions of deposition, despite the fact that water input remained essentially constant in terms of chemical composition.     

Study of exchangeable metal on colloidal humic acids and particulate matter by coupling ultrafiltration and isotopic tracers: Application to natural waters

A new method is proposed to characterize the complexation properties of colloids and metal–colloid interactions in natural waters. Based on the association of ultrafiltration with isotopic tracing, this method could quantify the pool of elements in an exchangeable position and also address the kinetic aspects of these exchanges. Basically, it consists of the comparison of isotopic compositions between the bulk sample and a succession of filtrates through time. Exchanges between colloidal humic acids (HA) and metals were first characterized, before applying such manipulations on natural waters. A few elements, representative of a wide range of complexation properties, were chosen: Cu, Zn, Cd, Pb, Sr, Nd, Ni, Th and U. In the case of humic acids, very small (less than 10% of isotopes), but significant isotopic shifts were observed compared to the isotopic equilibrium. It means that more than 90% of the isotopes were exchanged just after addition of isotopic tracers. Experiments on natural organic-rich waters (Mengong and Nyong streams) indicate isotopic composition variations close to those of humic acids. On the contrary, ultrafiltration performed on the total Sanaga River water (including suspended matter “SM”) shows an important isotopic shift between the filtered and unfiltered solutions. It means that in the case of the Sanaga River, a significant part of the chemical elements did not exchange.