Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina)

We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ¹¹B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ¹¹B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ¹¹B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ¹¹B≤+8‰) provide a potential third boron source.     

Chemical and isotopic (B, Sr) composition of alluvial sediments as archive of a past hydrothermal outflow

The geochemical and isotopic signature of Quaternary alluvial sediments filling a post-orogenic basin along the Tyrrhenian coasts of Italy (Cornia Plain, Tuscany) was investigated to unravel possible interactions with geothermal fluids from the Larderello geothermal field. Two cores located in the upper (UCP) and lower (LCP) sector of the plain were sampled to depths of up to 80 m. A third core in a neighbouring area not affected by geothermal activity was also sampled (Arno plain at Pisa), and its sediment composition was used as reference. The Cornia sediments (fraction < 65 μm) show high B, Cs and Sb concentrations related to a peculiar chemical enrichment of the clay fraction. They also show remarkable enrichments in As (up to 1000 μg g^− 1) reflecting a contribution from local ore deposits.⁸⁷Sr/⁸⁶Sr ratios, ranging from 0.71022 to 0.71698, reveal the nature of the weathered mother rocks of the alluvial sediments, whereas the boron isotopic composition, varying from − 20‰ to − 10‰, suggests an interaction between the clay fraction and boron-rich fluids at temperatures greater than 50 °C. This implies that hydrothermal fluids widely circulated within the Cornia basin in the past, ultimately leading to the geochemical anomalies currently recorded in local sediments.Although natural (geogenic) in origin, these anomalies cause severe problems to the regional water management (groundwater exploitation) through leaching of trace elements into circulating groundwater, a phenomenon which has to be carefully studied and monitored.     

Hydrochemical and isotopic characteristics of groundwater in the Souss Upstream Basin, southwestern Morocco

Heterogeneous shallow Plio-Quaternary formations of the Souss Plain represent the most important aquifer in southern High Atlas Mountains in Morocco. The present work was conducted in the Souss Upstream Basin to identify the chemical characteristics and the origin of groundwater in an aquifer under semi-arid climate. Isotopic and hydrochemical compositions combined with geological and hydrogeological data were used for this purpose. The total dissolved solids vary from 239 to 997 mg l⁻¹, and the following groundwater types are recognized: Ca²⁺–Mg²⁺–HCO₃⁻, Ca²⁺–Mg²⁺–SO₄²⁻ and Ca²⁺–Mg²⁺–Cl⁻. The groundwater is saturated and slightly supersaturated with respect to carbonate minerals and undersaturated with respect to evaporite minerals, which means that the groundwater composition is largely controlled by the dissolution of carbonate rocks known in the basin. The isotopic contents of groundwaters ranged from −8‰ to −5.2‰ for δ¹⁸O, from −52‰ to −34‰ for δD, and from 0 to 5.5 TU for tritium. The hydrogen (δD) and oxygen (δ¹⁸O) isotope signatures reveal a significant infiltration before evaporation takes place, indicating a major recharge directly from fractures in the crystalline and limestone formations of Atlas Mountains (above 800 m a.s.l.) and infiltration of surface water in the alluvial cones at the border of the Atlas basins. The very low tritium values suggest that the groundwater recharge follows a long flow path and a mixing between old and modern water is shown. However, a slight evaporation effect is noted in the southern part of the basin close to the Anti-Atlas Mountains.     

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit,NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ¹¹B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852 ppm) and Sr (208–1191 ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ¹¹B = +1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ¹¹B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.     

Origin of groundwater salinity and hydrogeochemical processes in a confined coastal aquifer: Case of the Rhône delta (Southern France)

The Rhône delta, South of France (Camargue, 750 km²) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses (¹⁸O, ²H, ¹³C_TDIC). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na⁺ in groundwater leading to Ca²⁺/Na⁺ exchange processes. The δ¹³C_TDIC analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO₄ reduction which is confirmed by highly depleted δ¹³C_TDIC values (up to −19‰). The δ¹³C_TDIC also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO₄ reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater.     

Groundwater recharge history and hydrogeochemical evolution in the Minqin Basin, North West China

The Minqin Basin is a type area for examining stress on groundwater resources in the Gobi Desert, and has been investigated here using a combination of isotopic, noble gas and chemical indicators. The basin is composed of clastic sediments of widely differing grain size and during the past half century over 10 000 boreholes have been drilled with a groundwater decline of around 1 m a⁻¹. Modern diffuse recharge is unlikely to exceed 3 mm a⁻¹, as determined using unsaturated zone profiles and Cl⁻ mass balance. A small component of modern (<50 a) groundwater is identified in parts of the basin from ³H–³He data, probably from irrigation returns. A clear distinction is found between modern waters with median δ¹⁸O values of 6.5 ± 0.5‰ and most groundwaters in the basin with more depleted isotopic signatures. Radiocarbon values as pmc range from 0.6% to 85% modern, but it is difficult to assign absolute ages to these, although a value of 20% modern C probably represents the late Pleistocene to Holocene transition. The δ¹³C compositions remain near-constant throughout the basin (median value of −8.1‰ δ¹³C) and indicate that carbonate reactions are unimportant and also that little reaction takes place. There is a smooth decrease in ¹⁴C activity accompanied by a parallel increase in ⁴He accumulations from S–N across the basin, which define the occurrence of a regional flow system. Noble gas temperatures indicate recharge temperatures of about 5.6 °C for late Pleistocene samples, which is some 2–3 °C cooler than the modern mean annual air temperature and the recharge temperature obtained from several Holocene samples. Groundwaters in the Minqin Basin have salinities generally below 1 g/L and are aerobic, containing low Fe but elevated concentrations of U, Cr and Se (mean values of 27.5, 5.8 and 5.3 μg L⁻¹, respectively). Nitrate is present at baseline concentrations of around 2 mg L⁻¹ but there is little evidence of impact of high NO₃ from irrigation returns. Strontium isotope and major ion ratios suggest that silicate reactions predominate in the aquifer. The results have important implications for groundwater management in the Minqin and other water-stressed basins in NW China – a region so far destined for rapid development. The large proportion of the water being used at present is in effect being mined and significant changes are urgently needed in water use strategy.     

Behaviour of boron and strontium isotopes in groundwater–aquifer interactions in the Cornia Plain (Tuscany,Italy)

The Cornia Plain alluvial aquifer, in Tuscany, is exploited intensely to meet the demand for domestic, irrigation and industrial water supplies. The B concentration of groundwater, however, is often above the European limit of 1 mg L⁻¹, with the result that exploitation of these water resources requires careful management. Boron and Sr isotopes have been used as part of a study on the origin and distribution of B dissolved in groundwater, and indirectly as a contribution to the development of appropriate water management strategies.The geochemistry of the Cornia Plain groundwater changes from a HCO₃ facies in the inland areas to a Cl facies along the coastal belt, where seawater intrusion takes place. The B concentration of groundwater increases towards the coastal areas, while the ¹¹B/¹⁰B ratio decreases. This indicates that there is an increasing interaction between dissolved B and the sediments forming the aquifer matrix, whose B content is in the order of 100 mg kg⁻¹. Adsorption–desorption exchanges take place between water and the sediment fine fraction rich in clay minerals, with a net release of B from the matrix into the groundwater, and a consequent δ¹¹B shift from positive to negative values. The aquifer matrix sediments therefore seem to be the major source of B dissolved in the groundwater.The groundwater–matrix interactions triggered by the ionic strength increase caused by seawater intrusion can also be detected in the Ca–Na ion exchanges. Dissolved Sr follows a trend similar to that of Ca, while the ⁸⁷Sr/⁸⁶Sr ratio is equal to that of the exchangeable Sr of the aquifer matrix and therefore does not change significantly.These results have helped to define a new strategy for groundwater exploitation, with the final objective of reducing B concentration in the water extracted from the aquifer.     

The utilization of boron and strontium isotopes for the assessment of boron contamination of the Cecina River alluvial aquifer (central-western Tuscany,Italy)

The groundwater B concentration in the alluvial aquifer of the upper Cecina River basin in Tuscany, Italy, often exceeds the limit of 1 mg L⁻¹ set by the European Union for drinking water. On the basis of hydrogeological and geochemical observations, the main source of the B contamination of groundwater has been attributed to past releases into streams of exhausted, B-rich geothermal waters and/or mud derived from boric acid manufacturing in Larderello. The releases were discontinued 25–30 years ago.This study confirms that the B dissolved in groundwater is anthropogenic. In fact, the δ¹¹B values of groundwater B match the range −12.2‰ to −13.3‰ of the Turkish B mineral (colemanite) processed in boric acid manufacturing, in the course of which no significant isotopic effects have been observed. This isotopic tracing of the Cecina alluvial aquifer occurs just below the confluence of the Possera Creek, which carries the B releases from Larderello. Strontium isotope ratios support this conclusion.At about 18 km from the Possera Creek confluence, the groundwater δ¹¹B drops to much more negative values (−22‰ to −27‰), which are believed to be produced by adsorption–desorption interactions between dissolved B and the aquifer matrix. The δ¹¹B of B fixed in well bottom sediments shows a similar variation. At present, desorption is prevailing over adsorption because the releases of B-rich water have ceased. A theoretical model is suggested to explain the isotopic trends observed.Thus, B isotopes appear to be a powerful tool for identifying the origin of B contamination in natural waters, although isotopic effects associated with adsorption–desorption processes may complicate the picture, to some extent.     

The salinity of Djibouti''s aquifer

Fresh water supplied to Djibouti town is essentially groundwater located inthe fractured Gulf and Somali basalt aquifers. About 30 wells (3 to 6 km inland from the sea) are exploited and provide water of rather poor quality (TDS between 1000 mg 1⁻¹ and 2800 mg 1⁻¹). A sea water interface has been recognized locally some 3.8 km from the sea at 35 m below sea level. However, a well at Hidka Gisiyed, some 11 km from the sea, also contains water with high salinity (TDS=14,000 mg 1⁻¹) at depth. The over-exploitation of the aquifer and the high pumping rate are contributing to an increase in the salinity due to the intrusion of sea water, as shown by the chemical results. The Hidka Gisiyed saline water is another possible source of saline water at depth.The isotopic results from part of the Djibouti aquifer have shown that current recharge from local rain or surface runoff is occurring. An understanding of factors influencing the evolution of the salinity will allow the better management of the aquifer.     

Electron accepting capacity of dissolved organic matter as determined by reaction with metallic zinc

Information about the chemical electron accepting capacity (EAC) of dissolved organic matter (DOM) is scarce owing to a lack of applicable methods. We quantified the electron transfer from metallic Zn to natural DOM in batch experiments at DOC concentrations of 10–100 mg-C L^− 1 and related it to spectroscopic information obtained from UV-, synchronous fluorescence, and FTIR- spectroscopy. The electron donating capacity of DOM and pre-reduced DOM was investigated using Fe(CN)₆³⁻ as electron acceptor. Presence of DOM resulted in release of dissolved Zn, consumption of protons, and slower release of hydrogen compared to reaction of metallic Zn with water at pH 6.5. Comparison with reaction stoichiometry confirmed that DOM accepted electrons from metallic Zn. The release of dissolved Zn was dependent on pH, DOC concentration, ionic strength, and organic matter properties. The reaction appeared to be completed within about 24 h and was characterized by pseudo first order kinetics with rate constants of 0.5 to 0.8 h^− 1. EAC per mass unit of carbon ranged from 0.22 mmol g^− 1 C to 12.6 mmol g^− 1 C. Depending on the DOM, a calculated 28–127% of the electrons transferred from metallic Zn to DOM could be subsequently donated to Fe(CN)₆³⁻. EAC decreased with DOC concentration, and increased with aromaticity, carboxyl, and phenolic content of the DOM. The results indicate that an operationally defined EAC of natural DOM can be quantified by reaction with metallic Zn and that DOM properties control the electron transfer. Shortcomings of the method are the coagulation and precipitation of DOM during the experiment and the production of hydrogen and dissolved Zn by reaction of metallic Zn with water, which may influence the determined EAC.     

Carbon isotopes of Middle–Lower Jurassic coal-derived alkane gases from the major basins of northwestern China

Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ¹³C values are found in the Tarim and Qaidam basins (δ¹³C₁: − 19.0 to − 29.9‰; δ¹³C₂: − 18.8 to − 27.1‰), and those with lowest δ¹³C values occur in the Turpan-Harmi and Junggar basins (δ¹³C₁: − 40.1 to − 44.0‰; δ¹³C₂: − 24.7 to − 27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C_5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ¹³C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world.     

Isopropylxanthate ions uptake by modified natural zeolite and removal by dissolved air flotation

This work describes a laboratory study concerning the adsorption of isopropylxanthate ions onto modified zeolites particles. The separation of the loaded carrier and their removal, from aqueous solutions, was conducted by flocculation followed by dissolved air flotation, DAF. The zeolite employed was a natural sample (approximately 48% clinoptilolite and 30% mordenite) which was previously treated with sodium ions (activation) and modified with copper ions (Cu–Z) before the xanthate ions uptake. Adsorption capacities (q_m) for Cu–Z were 0.34 meq g^− 1 for the powdered form, and 1.12 meq g^− 1 for the floc form. The adsorption capacity for the floc form appears to involve an enhanced electrostatic adsorption due to the positive sites on the floc surface. In all cases, the isopropylxanthate concentration in the treated water was found to be negligible (< 0.04 mg L^{− 1}). The flotation technique showed to be a fast process, requires a low recycle ratio (20%) in air saturated water, and the treated water ended up with a very low residual turbidity (6.8 NTU). It is believed that this adsorption–flotation technique, here named adsorptive particulate flotation, using activated and modified natural zeolite has a high potential as an alternative for pollutants removal (copper and isopropylxanthate ions) from waste mining effluents.     

Hydrochemistry and isotope compositions of groundwater from the Shihongtan sandstone-hosted uranium deposit, Xinjiang, NW China

Groundwaters and surface water in the Shihongtan sandstone-hosted U ore district, Xinjiang, NW China, were sampled and analyzed for their major-, and trace element concentrations and oxygen, hydrogen, boron and strontium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The waters in the study district have been grouped into three hydrochemical facies: Facies 1, potable spring-water, is a pH neutral (7.0), Na–Ca–HCO₃ type water with low total dissolved solids (TDS; 0.2 g/l, fresh) and has δ¹⁸O of − 8.3‰, δD of − 48.2‰,δ¹¹B of 1.5‰, and ⁸⁷Sr/⁸⁶Sr of 0.70627. Facies 2 groundwaters are mildly acidic to mildly alkaline (pH of 6.5–8.0, mean 7.3), Na–Ca–Mg–Cl–SO₄ type waters with moderate TDS (8.2 g/l–17.2 g/l, mean 9.3 g/l, brackish) and haveδ¹⁸O values in the − 5.8‰ to − 9.3‰ range (mean − 8.1‰), δD values in the − 20.8‰ to − 85.5‰ range (mean − 47.0‰),δ¹¹B values in the + 9.5‰ to + 39.1‰ range (mean + 17.1‰), and ⁸⁷Sr/⁸⁶Sr values in the 0.70595 to 0.70975 range (mean 0.70826). Facies 3, Aiting Lake water, is a mildly alkaline (pH = 7.4), Na–Ca–Mg–Cl–SO₄ type water with the highest TDS (249.1 g/l, brine) and has δ¹⁸O of − 2.8‰, δD of − 45.8‰,δ¹¹B of 21.2‰, and ⁸⁷Sr/⁸⁶Sr of 0.70840. The waters from the study district show a systematic increase in major, trace element and TDS concentrations and δ¹¹B values along the pathway of groundwater migration which can only be interpreted in terms of water–rock interaction at depth and strong surface evaporation. The hydrochemical and isotopic data presented here confirm that the groundwaters in the Shihongtan ore district are the combined result of migration, water–rock interaction and mixing of meteoric water with connate waters contained in sediments.     

Elevated naturally occurring arsenic in a semiarid oxidizing system, Southern High Plains aquifer, Texas, USA

High groundwater As concentrations in oxidizing systems are generally associated with As adsorption onto hydrous metal (Al, Fe or Mn) oxides and mobilization with increased pH. The objective of this study was to evaluate the distribution, sources and mobilization mechanisms of As in the Southern High Plains (SHP) aquifer, Texas, relative to those in other semiarid, oxidizing systems. Elevated groundwater As levels are widespread in the southern part of the SHP (SHP-S) aquifer, with 47% of wells exceeding the current EPA maximum contaminant level (MCL) of 10 μg/L (range 0.3–164 μg/L), whereas As levels are much lower in the north (SHP-N: 9%  As MCL of 10 μg/L; range 0.2–43 μg/L). The sharp contrast in As levels between the north and south coincides with a change in total dissolved solids (TDS) from 395 mg/L (median north) to 885 mg/L (median south). Arsenic is present as arsenate (As V) in this oxidizing system and is correlated with groundwater TDS (Spearman’s ρ = 0.57). The most likely current source of As is sorbed As onto hydrous metal oxides based on correlations between As and other oxyanion-forming elements (V, ρ = 0.88; Se, ρ = 0.54; B, ρ = 0.51 and Mo, ρ = 0.46). This source is similar to that in other oxidizing systems and constitutes a secondary source; the most likely primary source being volcanic ashes in the SHP aquifer or original source rocks in the Rockies, based on co-occurrence of As and F (ρ = 0.56), oxyanion-forming elements and SiO₂ (ρ = 0.41), which are found in volcanic ashes. High groundwater As concentrations in some semiarid oxidizing systems are related to high evaporation. Although correlation of As with TDS in the SHP aquifer may suggest evaporative concentration, unenriched stable isotopes (δ²H: −65 to −27; δ¹⁸O: −9.1 to −4.2) in the SHP aquifer do not support evaporation. High TDS in the SHP aquifer is most likely related to upward movement of saline water from the underlying Triassic Dockum aquifer. Mobilization of As in other semiarid oxidizing systems is caused by increased pH; however, pH in the SHP aquifer is near neutral (10–90 percentiles, 7.0–7.6). Although many processes, such as competitive desorption with SiO₂, VO₄, or PO₄, could be responsible for local mobilization of As in the SHP aquifer, the most plausible explanation for the regional As distribution and correlation with TDS is the counterion effect caused by a change from Ca- to Na-rich, water as shown by the high correlation between As and Na/(Ca)^0.5 ratios (ρ = 0.57). This change in chemistry is related to mixing with saline water that moves upward from the underlying Dockum aquifer. This counterion effect may mobilize other anions and oxyanion-forming elements that are correlated with As (F, V, Se, B, Mo and SiO₂). Competition among the oxyanions for sorption sites may enhance As mobilization. The SHP case study has similar As sources to those of other semiarid, oxidizing systems (original volcanic ash source followed by sorption onto hydrous metal oxides) but contrasts with these systems by showing lack of evaporative concentration and pH mobilization of As but counterion mobilization of As instead in the SHP-S aquifer.     

Mercury mobilization by oxidative dissolution of cinnabar (α-HgS) and metacinnabar (β-HgS)

Cinnabar (α-HgS) and metacinnabar (β-HgS) dissolved at environmentally significant rates in oxygenated slurry experiments simulating a low-flow fluvial system. Based on SO₄²⁻ production, cinnabar dissolution rates were 2.64 to 6.16 μmol (SO₄²⁻) m^− 2 day^− 1, and metacinnabar dissolution rates were 1.20 to 1.90 μmol (SO₄²⁻) m^− 2 day^− 1. Monodentate-bound thiosulfate (S₂O₃²⁻) was identified as an oxidation product on the HgS surface by ATR-IR spectroscopy based on strong infrared absorption bands in the 1140–1145 cm^− 1 and 1006–1014 cm^− 1 regions. The presence of sulfide oxidation intermediates on the HgS surface indicates that SO₄²⁻ concentration underestimates α-HgS and β-HgS dissolution in this setting. Mercury release rates during dissolution were more than two orders of magnitude less than SO₄²⁻ production, but were significant: 0.47 mg (Hg) m^− 2 y^− 1 from cinnabar [6.45 nmol (Hg) m^− 2 day^− 1], and 0.17 mg (Hg) m^− 2 y^− 1 from metacinnabar [2.29 nmol (Hg) m^− 2 day^− 1]. The Hg mobilized during α-HgS and β-HgS dissolution is sufficient to form natural Au–Hg amalgam in downstream placer settings. The proportion of mercury that is not remobilized during α-HgS and β-HgS dissolution likely adsorbs to the dissolving mercuric sulfide. Adsorption of Hg²⁺ to cinnabar was detected in situ by anodic stripping voltammetry using a cinnabar-modified carbon paste electrode following accumulation of Hg²⁺ on the electrode at open circuit potential.     

A regional-scale study of chromium and nickel in soils of northern California, USA

A soil geochemical survey was conducted in a 27,000-km² study area of northern California that includes the Sierra Nevada Mountains, the Sacramento Valley, and the northern Coast Range. The results show that soil geochemistry in the Sacramento Valley is controlled primarily by the transport and weathering of parent material from the Coast Range to the west and the Sierra Nevada to the east. Chemically and mineralogically distinctive ultramafic (UM) rocks (e.g. serpentinite) outcrop extensively in the Coast Range and Sierra Nevada. These rocks and the soils derived from them have elevated concentrations of Cr and Ni. Surface soil samples derived from UM rocks of the Sierra Nevada and Coast Range contain 1700–10,000 mg/kg Cr and 1300–3900 mg/kg Ni. Valley soils west of the Sacramento River contain 80–1420 mg/kg Cr and 65–224 mg/kg Ni, reflecting significant contributions from UM sources in the Coast Range. Valley soils on the east side contain 30–370 mg/kg Cr and 16–110 mg/kg Ni. Lower Cr and Ni concentrations on the east side of the valley are the result of greater dilution by granitic sources of the Sierra Nevada.Chromium occurs naturally in the Cr(III) and Cr(VI) oxidation states. Trivalent Cr is a non-toxic micronutrient, but Cr(VI) is a highly soluble toxin and carcinogen. X-ray diffraction and scanning electron microscopy of soils with an UM parent show Cr primarily occurs within chromite and other mixed-composition spinels (Al, Mg, Fe, Cr). Chromite contains Cr(III) and is highly refractory with respect to weathering. Comparison of a 4-acid digestion (HNO₃, HCl, HF, HClO₄), which only partially dissolves chromite, and total digestion by lithium metaborate (LiBO₃) fusion, indicates a lower proportion of chromite-bound Cr in valley soils relative to UM source soils. Groundwater on the west side of the Sacramento Valley has particularly high concentrations of dissolved Cr ranging up to 50 μg L⁻¹ and averaging 16.4 μg L⁻¹. This suggests redistribution of Cr during weathering and oxidation of Cr(III)-bearing minerals. It is concluded that regional-scale transport and weathering of ultramafic-derived constituents have resulted in enrichment of Cr and Ni in the Sacramento Valley and a partial change in the residence of Cr.     

Stable carbon and oxygen isotope investigation in historical lime mortar and plaster – Results from field and experimental study

Lime mortar and plaster were sampled from Roman, medieval and early modern buildings in Styria. The historical lime mortar and plaster consist of calcite formed in the matrix during setting and various aggregates. The stable C and O isotopic composition of the calcite matrix was analyzed to get knowledge about the environmental conditions during calcite formation. The δ¹³C_matrix and δ¹⁸O_matrix values range from −31 to 0‰ and −26 to −3‰(VPDB), respectively. Obviously, such a range of isotope values does not represent the local natural limestone assumed to be used for producing the mortar and plaster. In an ideal case, the calcite matrix in lime mortar and plaster is isotopically lighter in the exterior vs. the interior mortar layer according to the relationship δ¹⁸O_matrix = 0.61 · δ¹³C_matrix − 3.3 (VPDB). Calcite precipitation by uptake of gaseous CO₂ into alkaline Ca(OH)₂ solutions shows a similar relationship, δ¹⁸O_calcite = 0.67 · δ¹³C_calcite − 6.4 (VPDB). Both relationships indicate that the ¹³C/¹²C and ¹⁸O/¹⁶O values of the calcite reflect the setting behaviour of the lime mortar and plaster. Initially, CO₂ from the atmosphere is fixed as calcite, which is accompanied by kinetic isotope fractionation mostly due to the hydroxylation of CO₂ (δ¹³C_matrix ≈  −25‰ and δ¹⁸O_matrix ≈ −20‰). As calcite formation continued the remaining gaseous CO₂ is subsequently enriched in ¹³C and ¹⁸O causing later formed calcite to be isotopically heavier along the setting path in the matrix. Deviations from such an ideal isotopic behaviour may be due to the evolution of H₂O, e.g. evaporation, the source of CO₂, e.g. from biogenic origin, relicts of the natural limestone, and secondary effects, such as recrystallization of calcite. The results of the field and experimental study suggest that isotope values can be used as overall proxies to decipher the origin of carbonate and the formation conditions of calcite in the matrix of ancient and recent lime mortar and plaster. Moreover, these proxies can be used to select calcite matrix from historical lime mortar and plaster for ¹⁴C dating.     

Chromium oxidation by manganese (hydr)oxides in a California aquifer

There are increasing concerns with elevated levels of Cr(VI) in the environment because it is a strong oxidant, corrosive, and carcinogenic. The concerns extend to the presence of Cr(VI) in many aquifers in California and elsewhere, where relatively high levels have been attributed to both industrial pollution and natural processes. The authors have, therefore, determined if natural redox processes contribute to the presence of high Cr(VI) concentrations (6–36 μg L⁻¹) in an aquifer in central California relative to non-detectable concentrations (<0.1 μg L⁻¹) in an adjacent aquifer. Specifically, the distribution and the redox speciation of dissolved (<0.45 μm) Cr have been compared with those of particulate Mn and Fe oxy-hydroxides in sediments, using X-ray absorption spectroscopy at the Mn and Fe L-edges. The analyses show a correlation between the presence of dissolved Cr(VI) and Mn (hydr)oxide minerals, which are the only common, naturally occurring minerals known to oxidize Cr(III) in laboratory experiments. This covariance substantiates the results of those experiments and previous field studies that indicate natural oxidation mechanisms might account for the relatively high levels of Cr(VI) in the study site, as well as for elevated concentrations in other aquifers with similar biogeochemical conditions.     

Feldspar dissolution rates in the Topopah Spring Tuff, Yucca Mountain, Nevada

Two different field-based methods are used here to calculate feldspar dissolution rates in the Topopah Spring Tuff, the host rock for the proposed nuclear waste repository at Yucca Mountain, Nevada. The center of the tuff is a high silica rhyolite, consisting largely of alkali feldspar (60 wt%) and quartz polymorphs (35 wt%) that formed by devitrification of rhyolitic glass as the tuff cooled. First, the abundance of secondary aluminosilicates is used to estimate the cumulative amount of feldspar dissolution over the history of the tuff, and an ambient dissolution rate is calculated by using the estimated thermal history. Second, the feldspar dissolution rate is calculated by using measured Sr isotope compositions for the pore water and rock. Pore waters display systematic changes in Sr isotopic composition with depth that are caused by feldspar dissolution. The range in dissolution rates determined from secondary mineral abundances varies from 10⁻¹⁶ to 10⁻¹⁷ mol s⁻¹ kg tuff⁻¹ with the largest uncertainty being the effect of the early thermal history of the tuff. Dissolution rates based on pore water Sr isotopic data were calculated by treating percolation flux parametrically, and vary from 10⁻¹⁵ to 10⁻¹⁶ mol s⁻¹ kg tuff⁻¹ for percolation fluxes of 15 mm a⁻¹ and 1 mm a⁻¹, respectively. Reconciling the rates from the two methods requires that percolation fluxes at the sampled locations be a few mm a⁻¹ or less. The calculated feldspar dissolution rates are low relative to other measured field-based feldspar dissolution rates, possibly due to the age (12.8 Ma) of the unsaturated system at Yucca Mountain; because oxidizing and organic-poor conditions limit biological activity; and/or because elevated silica concentrations in the pore waters (50 mg L⁻¹) may inhibit feldspar dissolution.     

Geological sources of boron and fluoride anomalies in Silurian–Ordovician aquifer system,Estonia

The objective of this study was to examine the possible natural sources of fluorides and boron in Silurian–Ordovician (S–O) aquifer system, as the anomaly of these elements has been distinguished in groundwater of western Estonia. Water–rock interactions, such as dissolution and leaching of the host rock, are considered to be the main source of high fluoride and boron concentrations in groundwater. Altogether 91 rock samples were analysed to determine if high F⁻ and B levels in groundwater could be attributed to certain aquifer forming rock types. Fluorine and boron contents in limestones and dolomites vary from 100 to 500 mg/kg and 5 to 20 mg/kg, reaching up to 1,000 and 150 mg/kg in marlstones, respectively. K-bentonites, altered volcanic ash beds, are rich in fluorine (400–4,500 mg/kg) and boron (50–1,000 mg/kg). Thus, clay-rich sediments, providing ion-exchange and adsorption sites for F⁻ and B, are the probable sources of both elements in S–O aquifer system in western Estonia.