Applied Geochemistry (v.26, #7)

Natural tracer profiles across argillaceous formations by Martin Mazurek; Peter Alt-Epping; Adrian Bath; Thomas Gimmi; H. Niklaus Waber; Stéphane Buschaert; Pierre De Cannière; Mieke De Craen; Andreas Gautschi; Sébastien Savoye; Agnès Vinsot; Isabelle Wemaere; Laurent Wouters (1035-1064).
► Solute transport processes in clay and shale formations at nine sites are examined. ► Conservative pore-water tracers (e.g. Cl, δ18O, δ2H, He) show regular profiles. ► These indicate the dominance of diffusive transport over times of 105–106 years. ► The contribution of vertical advection to transport is limited or negligible. ► Modelled evolution times are in line with independent palaeo-hydrogeological data.Argillaceous formations generally act as aquitards because of their low hydraulic conductivities. This property, together with the large retention capacity of clays for cationic contaminants, has brought argillaceous formations into focus as potential host rocks for the geological disposal of radioactive and other waste. In several countries, programmes are under way to characterise the detailed transport properties of such formations at depth. In this context, the interpretation of profiles of natural tracers in pore waters across the formations can give valuable information about the large-scale and long-term transport behaviour of these formations. Here, tracer-profile data, obtained by various methods of pore-water extraction for nine sites in central Europe, are compiled. Data at each site comprise some or all of the conservative tracers: anions (Cl, Br), water isotopes (δ18O, δ2H) and noble gases (mainly He). Based on a careful evaluation of the palaeo-hydrogeological evolution at each site, model scenarios are derived for initial and boundary pore-water compositions and an attempt is made to numerically reproduce the observed tracer distributions in a consistent way for all tracers and sites, using transport parameters derived from laboratory or in situ tests. The comprehensive results from this project have been reported in . Here the results for three sites are presented in detail, but the conclusions are based on model interpretations of the entire data set. In essentially all cases, the shapes of the profiles can be explained by diffusion acting as the dominant transport process over periods of several thousands to several millions of years and at the length scales of the profiles. Transport by advection has a negligible influence on the observed profiles at most sites, as can be shown by estimating the maximum advection velocities that still give acceptable fits of the model with the data. The advantages and disadvantages of different conservative tracers are also assessed. The anion Cl is well suited as a natural tracer in aquitards, because its concentration varies considerably in environmental waters. It can easily be measured, although the uncertainty regarding the fraction of the pore space that is accessible to anions in clays remains an issue. The stable water isotopes are also well suited, but they are more difficult to measure and their values generally exhibit a smaller relative range of variation. Chlorine isotopes (δ37Cl) and He are more difficult to interpret because initial and boundary conditions cannot easily be constrained by independent evidence. It is also shown that the existence of perturbing events such as the activation of aquifers due to uplift and erosion, leading to relatively sharp changes of boundary conditions, can be considered as a pre-requisite to obtain well-interpretable tracer signatures. On the other hand, gradual changes of boundary conditions are more difficult to parameterise and so may preclude a clear interpretation.

A geochemical study on mud volcanoes in the Junggar Basin, China by Ryoichi Nakada; Yoshio Takahashi; Urumu Tsunogai; Guodong Zheng; Hiroshi Shimizu; Keiko H. Hattori (1065-1076).
► Gases released from Xinjiang mud volcanoes are dominated by thermogenic origin. ► Secondary microbial activities occurring closer to the surface dramatically changed the δ13CCO2. ► The water–rock interaction occurred at deeper level than gas and petroleum reservoir.A comprehensive study was performed to characterize, for the first time, the mud, water, and gases released from onshore mud volcanoes located in the southern margin of the Junggar Basin, northwestern China. Chemical compositions of mud, along with the geology of the basin, suggest that a source of the mud is Mesozoic or Cenozoic shale. Oxygen and H isotope compositions of the released water suggest a local meteoric origin. Combined with the positive Eu anomalies of the water, a large 18O shift of the water suggests extensive interaction with rocks. Gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons, and the high δ13C values (>+20‰ VPDB) for CO2 gases and dissolved carbonate in muddy water suggest secondary methanogenesis with CO2 reduction after oil biodegradation.The enrichments of Eu and 18O in water and the low thermal gradient of the area suggest that the water–rock interactions possibly occur deeper than 3670 ± 200 m. On the other hand, considering the relationship to the petroleum reservoir around the mud volcanoes, the depth of the gases can be derived from about 3600 m, a depth that is greater than that generally estimated for reservoirs whose gas is characterized by 13C-enriched CO2. Oil biodegradation with CO2 reduction likely occurs at a shallower depth along the seepage system of the mud volcano. The results contribute to the worldwide data set of gas genesis in mud volcanoes. Moreover, they further support the concept that most terrestrial mud volcanoes release thermogenic gas produced in very deep sediments and may be early indicators of oil biodegradation, an important problem in the petroleum industry.

Avoiding high concentrations of arsenic, manganese and salinity in deep tubewells in Munshiganj District, Bangladesh by Stephan J. Hug; Dominique Gaertner; Linda C. Roberts; Mario Schirmer; Thomas Ruettimann; Thomas M. Rosenberg; A.B.M. Badruzzaman; M. Ashraf Ali (1077-1085).
► Deep tubewells are the second most successful arsenic mitigation option in Bangladesh. ► Manganese concentrations and salinities are often high and not systematically tested. ► Distinctly different groundwater types are typically found at different depths. ► Monitoring wells are advisable to find the optimal depth for new tubewells. ► Groundwater abstraction by hand pumps should not lead to draw-down of As-rich water.Over 85% of shallow tubewells (STWs) in the district of Mushiganj, Bangladesh, are affected by As concentrations above 50 μg/L. Deep tubewells (DTWs) are among the preferred mitigation options for reducing exposure to As. Around the town of Sreenagar (30 km South of Dhaka) water samples were collected from existing STWs and DTWs, monitoring wells (5–210 m depth) and newly installed DTWs. Analysis of water samples from 2005 to 2010 indentified three types of groundwater currently used for drinking: (1) Shallow water from 20 to 100 m deep dark-grey sediments with high As-concentrations (100–1000 μg/L), intermediate-high Fe (2–11 mg/L), intermediate Mn (0.2–1 mg/L) relatively low EC (400–900 μS/cm) dominated by Ca–Mg– HCO 3 - . (2) Water from 140 to 180 m deep light-grey sediments with low As (<10 μg/L), intermediate Mn (0.2–1.0 mg/L), intermediate Fe (1–5 mg/L), low total organic C, N and NH 4 + , and intermediate EC (1200–1800 μS/cm) dominated by Ca–Mg– HCO 3 - –Na–Cl. (3) Deep water from 190 to 240 m deep brown sediments with low As (<10 μg/L), high Mn (2–5 mg/L), low Fe (<3 mg/L), and high EC (2000–3000 μS/cm) dominated by Ca–Mg–Na–Cl with high Ca and Cl concentrations. Drillers have traditionally used the transition from grey to brown sediments as an indicator for the depth from which safe drinking water can be obtained. However, in most of the tubewells in the studied area of over 190 m depth, Mn exceeds the WHO-limit of 0.4 mg Mn/L by a factor 2–5 and the water tastes noticeably saline. Based on the depth-resolved water compositions and a small survey of DTWs in a 2.5 × 2.5 km2 area, a depth range of 150–180 m with light-grey sediments is recommended for the construction of new DTWs. Pumping tests have shown that the deeper aquifer is largely separated from the upper aquifer, such that small volume water abstraction for drinking with hand pumps can be deemed safe as long as wells are periodically tested. DTWs are an excellent option for avoiding high As concentrations. However, before installing larger numbers of new DTWs, monitoring wells should be installed to determine the optimal depth for water withdrawal in each area.

► There is felsic volcanic input into the Late Carboniferous coals from the Weibei coalfield. ► Al-hydroxide/oxyhydroxide minerals are derived from the incongruent dissolution of kaolinite. ► Al-hydroxide/oxyhydroxide minerals are responsible for higher Ga and lower Si2O/Al2O3.This paper mainly describes mineralogy and geochemistry of coals from the Weibei coalfield in the southeastern Ordos Basin, North China. A number of Al-hydroxide/oxyhydroxide minerals were detected in the Late Carboniferous coals (Nos. 5, 10 and 11 coals), especially in the No. 10 coal. Aluminum-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite and diaspore) in the No. 10 coal are associated with kaolinite, suggesting that these minerals are derived from the breakdown of kaolinite. A model in which Al-hydroxide/oxyhydroxide minerals form from the incongruent dissolution of kaolinite is presented. Nordstrandite and boehmite mainly occur as massive lenses (<500 μm in length). Diaspore appears as massive aggregates and as single euhedral crystals (<50 μm in length) in a kaolinite matrix. The presence of high temperature quartz, and zircon indicates that there was input of felsic volcanic debris during accumulation of the Late Carboniferous coals. These volcanic materials have also had a significant influence on the enrichment of certain trace elements including Li, Be, Ga, Zr, Nb, Mo, Sn, W and U in the Late Carboniferous coals (Nos. 5, 10, and 11 coals). SEM-EDX results show that Ga in the No. 10 coal (whole coal average 33.4 μg/g; n  = 2) mainly occurs within Al-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite, and diaspore), kaolinite and organic matter.

Sequestration of CO2 after reaction with alkaline earth metal oxides CaO and MgO by Martin Back; Markus Bauer; Helge Stanjek; Stefan Peiffer (1097-1107).
► Gas phase CO2 transfer into the aqueous phase was the rate limiting step for CaO carbonation. ► MgO carbonation is a slower and much more complex process. ► Temperatures >50 °C are favourable for precipitation of CaCO3 and Mg carbonates. ► Ca rich materials should be utilized preferentially for aqueous mineral carbonation. ► CaO and MgO can be used as proxies to estimate most common alkaline waste reaction with CO2.The uptake of CO2 by CaO and MgO in aqueous solution has been studied in batch experiments at different temperatures (15–75 °C), stirring rates (300–600 rpm) and solid liquid ratios (0.0125–0.125 mol L−1) in order to identify the kinetic controls and limitations. Both minerals are constituents in many combustion residues and their carbonation is considered as an option for CO2 sequestration. Generally the uptake of CO2 by metal (hydr)oxide suspension increased with higher turbulence due to better gas/water exchange and higher temperature due to enhanced rates of mineral dissolution and carbonate precipitation. The reaction of CaO and MgO with CO2 in solution occurred at distinctly different pH values. While in the CaO systems a pH between 12.8 (0.1 mol L−1) and 11.7 (0.0125 mol L−1) was established, suspensions with MgO only reached maximum pH values of ∼10.3 even though the mineral surface area was larger in these assays (140 m2  L−1 for MgO compared to 7.5 m2  L−1 for CaO). This difference in reaction pH affected the transfer of CO2 from the gas into the liquid phase as well as dissolution rates of the minerals themselves.Maximum CO2 uptake rates were 0.027 mmol L−1  s−1 for the CaO containing experiments at pH 12.8. Dissolution of CaO was fast and calcite precipitation occurred instantaneously at all tested temperatures and concentrations. Thus, the CO2 dissolution step was limiting the overall carbonation reaction. Under otherwise similar conditions maximum CO2 uptake rates were lower for the MgO system. Magnesium carbonate precipitation was slower than calcite precipitation and depended strongly on the different boundary conditions, in particular temperature and the degree of supersaturation. Temperatures greater than 50 °C were required for assays with low dissolved Mg2+ and CO 3 2 - concentrations in order to precipitate Mg carbonate in the form of hydromagnesite (Mg5(CO3)4(OH)2·4H2O). Below 50 °C the use of high concentrations of Mg and C species were necessary to initialise precipitation of Mg carbonate as nesquehonite (MgCO3·3H2O). This has implications for the application of mineral carbonation in an aqueous system as a means of C capture and storage. Calcium-rich materials can be utilized efficiently and almost independently of reaction regime if an optimum supply of CO2 is provided. In contrast specific conditions are required to use the CO2 binding potential of alkaline Mg minerals, as T  > 50 °C or high concentrations are required for Mg carbonate precipitation.

► Slow recovery rates present challenges for water sampling of aquitard wells. ► Geochemical changes occur within the standing water columns of aquitard wells. ► Changes in dissolved oxygen occur as groundwater slowly enters the well. ► Geochemical zonation in the water column may be a guide for proper sampling depth.Monitoring wells are often installed in aquitards to verify effectiveness for preventing migration of surface contaminants to underlying aquifers. However, water sampling of aquitard wells presents a challenge due to the slow recovery times for water recharging the wells, which can take as long as weeks, months or years to recharge depending on the sample volume needed. In this study, downhole profiling and sampling of aquitard wells was used to assess geochemical changes that occur in aquitard wells during water level recovery. Wells were sampled on three occasions spanning 11 years, 1 year and 1 week after they were purged and casing water showed substantial water chemistry variations. Temperature decreased with depth, whereas pH and specific conductance increased with depth in the water column after 11 years of water level recovery. Less stable parameters such as dissolved O2 (DO) and Eh showed strong zonation in the well column, with DO stratification occurring as the groundwater slowly entered the well. Oxidation of reduced till groundwater along with degassing of CO2 from till pore water affects mineral solubility and dissolved solid concentrations. Recommendations for sampling slowly recovering aquitard wells include identifying the zone of DO and Eh stratification in the well column and collecting water samples from below the boundary to better measure unstable geochemical parameters.

► It is planned to seal conductive fractures near a repository with cementitious grout. ► Modeling includes simultaneous hydration and leaching of the grout. ► Modeling results show a very limited formation of the high-pH plume. ► Results are in qualitative agreement with borehole monitoring data.Grouting of water-conducting fractures with low-alkali cement is foreseen for the potential future repository for spent nuclear fuel in Finland (ONKALO). A possible consequence of the interaction between groundwater and grout is the formation of high-pH solutions which will be able to react with the host rock (gneisses) and alter its mineralogy and porosity. A reactive transport modeling study of this possible alteration has been conducted. First, the hydration of the low-alkali cementitious grout has been modeled, using results from the literature as a guide. The hydrated cement is characterized by the absence of portlandite and the presence of a C–S–H gel with a Ca/Si ratio about 0.8 after tens of years (Ca/Si is about 1.7 in Ordinary Portland Cement). Second, calculations have simulated the interaction between flowing water and grout and the formation of an alkalinity plume, which flows beyond the grouted section of the fracture. The calculations include the hydration and simultaneous leaching of the grout through diffusive exchange between the porewater in the grout and the flowing water in the fracture. The formation of an alkaline plume is extremely limited when the low-pH grout is used. Even when using a grout with a lower silica fume content, the extent and magnitude of the alkaline plume is quite minor. These results are in qualitative agreement with monitoring at ONKALO.

► log β n of REE complexes with naturally occurring organic ligands are critically reviewed. ► Formation constants of REE–organic complexes at infinite dilution, 25 °C and 1 bar are recommended. ► The dataset covers complexes of La to Lu with acetate, citrate, malonate, oxalate, and succinate. ► Solubility constants of La- to Gd-oxalate are evaluated from studies in HCl and H2SO4 media. ► Solubility product constants of La-, Ce-, Pr-, Nd-, Sm-, and Gd-oxalates are recommended.Naturally occurring organic ligands, such as acetate, citrate, malonate, oxalate, and succinate, play important roles in mobility and accumulation of La and other rare earth elements in low temperature systems under Earth surface conditions. However, a comprehensive and consistent thermodynamic database covering the complexes of rare earth elements with those naturally occurring organic ligands is lacking. In this study, thermodynamic data of organic species of rare earth elements (REE) represented by La, with an emphasis on their aqueous complexes with organic ligands, are critically reviewed. The organic ligands covered by this study include acetate, citrate, malonate, oxalate and succinate. In this critical review, the Specific Interaction Theory (SIT) model is adopted for extrapolation to infinite dilution. This model is a reliable activity coefficient model valid for a wide range of ionic strengths. These critically reviewed data, including complex formation constants, SIT interaction coefficients and solubility product constants, would enable accurate modeling of the speciation and solubility of REE in various environments including high ionic strength environments, providing insight into mobility and enrichment of REE in various environments.

A comparative study of the modelling of cement hydration and cement–rock laboratory experiments by David Savage; Josep M. Soler; Kohei Yamaguchi; Colin Walker; Akira Honda; Manabu Inagaki; Claire Watson; James Wilson; Steven Benbow; Irina Gaus; Joerg Rueedi (1138-1152).
► A modelling intercomparison has shown that the dominant reaction pathways are well understood. ► However, significant differences in model parameterisation produced similar results. ► The modelling showed that there is benefit in keeping the numerical models as simple as possible.The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The ‘Long-term Cement Studies’ (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement–rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan).This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement–rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability.The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral–fluid reactions at these longer timescales.

► A hydrothermal process transformed Fe–Mg smectites into corrensite. ► This transformation was favoured by the intrusion of the Morrón de Mateo dome. ► The intrusion caused a temperature increased and a supply of Fe–Mg rich solutions. ► The system can be a good natural analogue of bentonite barrier in a radwaste disposal. ► Experimental studies of stability of bentonite are in agreement with the results.The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical effects on a bentonite barrier in a deep geological repository of high level radioactive wastes. This bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work, the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic (white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo deposit are described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites, while mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates are dioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical composition of smectites reflects the different nature of the parent rocks, in such a way that smectites from white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to montmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mg contents and a mean structural formulae corresponding to Fe–Mg-rich beidellite and/or to an intermediate smectite member between beidellite and saponite. In addition, chemical composition and textural features of corrensite-like clay minerals in relation to Fe–Mg-rich smectites in the samples have also been studied, suggesting that the former seems to be formed from Fe–Mg-rich smectites. The presence of corrensite in the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration process occurred after bentonite formation, which transformed Fe–Mg-rich smectites into corrensite. This transformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, which produced a temperature increase in the geological media and a supply of Fe–Mg-rich solutions. These physicochemical conditions were also responsible for the metasomatic transformations observed in the biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrón de Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite barrier related to the radioactive decay of fission products and the interaction between the corrosion products of steel over-packs and the bentonite. These circumstances would favour the transformation of the candidate Al-rich smectites into Fe–Mg-rich smectites and corrensite, as steps prior to formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectites would disappear and the clayey barrier would fail.

Organic carbon amendments for passive in situ treatment of mine drainage: Field experiments by Matthew B.J. Lindsay; David W. Blowes; Peter D. Condon; Carol J. Ptacek (1169-1183).
► Organic carbon amendments can support passive treatment of mine drainage. ► Decreased transport of sulfide-oxidation products under sulfate-reducing conditions. ► Treatment effectiveness dependent on organic carbon source and amendment rate.A field-scale experiment was conducted to evaluate various organic C sources as amendments for passive treatment of tailings pore water. Varied mixtures of peat, spent-brewing grain (SBG) and municipal biosolids (MB) were assessed for the potential to promote dissimilatory sulfate reduction (DSR) and metal-sulfide precipitation. Five amended cells and one control were constructed in the vadose zone of a sulfide- and carbonate-rich tailings deposit, and the geochemistry, microbiology and mineralogy were monitored for 4 a. Increases in pore-water concentrations of dissolved organic C (DOC) and decreases in aqueous SO4 concentrations of >2500 mg L−1 were observed in cells amended with peat + SBG and peat + SBG + MB. Removal of SO4 was accompanied by shifts in δ34S-SO4 values of >+30‰, undersaturation of pore water with respect to gypsum [CaSO4·2H2O], and increased populations of SO4-reducing bacteria (SRB). Decreases in aqueous concentrations of Zn, Mn, Ni, Sb and Tl were observed for these cells relative to the control. Organic C introduction also supported growth of Fe-reducing bacteria (IRB) and increases in Fe and As concentrations. Enhanced Fe and As mobility occurred in all cells; however, maximum concentrations were observed in cells amended with MB. Subsequent decreases in Fe and As concentrations were attributed to DSR and metal-sulfide precipitation. The common presence of secondary Zn-S and Fe-S phases was observed by field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDS) spectroscopy. Selective extractions indicated that large decreases in water-soluble SO4 occurred in cells that supported DSR. Furthermore, amendments that supported DSR generally were characterized by slight decreases in solid-phase concentrations of extractable metal(loid)s. Amendment of tailings with organic C amendments that supported ongoing DOC production and DSR was essential for sustained treatment.

► Isotopic equilibrium attained between oxygen in H2O and supercritical CO2. ► δ 18O values of H2O change predictably due to oxygen isotope exchange with CO2. ► δ 18O values can be used to assess role of trapping mechanisms in CCS settings.Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on δ 13C values of CO2 to trace the migration of injected CO2 in the subsurface. More recently the use of δ 18O values of both CO2 and reservoir fluids has been proposed as a method for quantifying in situ CO2 reservoir saturations due to O isotope exchange between CO2 and H2O and subsequent changes in δ 18OH2O values in the presence of high concentrations of CO2. To verify that O isotope exchange between CO2 and H2O reaches equilibrium within days, and that δ 18OH2O values indeed change predictably due to the presence of CO2, a laboratory study was conducted during which the isotope composition of H2O, CO2, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 °C and up to 19 MPa) and varying CO2 pressures. Conditions typical for the Pembina Cardium CO2 Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that δ 18O values of CO2 were on average 36.4 ± 2.2‰ (1σ, n  = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5‰ for the experimental temperatures of 50 °C. By using 18O enriched water for the experiments it was demonstrated that changes in the δ 18O values of water were predictably related to the fraction of O in the system sourced from CO2 in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO2 is related to the total volumetric saturation of CO2 and water as a fraction of the total volume of the system, it is concluded that changes in δ 18O values of reservoir fluids can be used to calculate reservoir saturations of CO2 in CCS settings given that the δ 18O values of CO2 and water are sufficiently distinct.

Hydrothermal Fe–Si–Mn oxide deposits from the Central and South Valu Fa Ridge, Lau Basin by Zhilei Sun; Huaiyang Zhou; Qunhui Yang; Zhixue Sun; Shenxu Bao; Huiqiang Yao (1192-1204).
► The Fe–Mn crust in the HHF has seawater contribution, whereas the Fe–Si oxide in the MHF is dominated by hydrothermal fluid ► The Nd isotope of diffuse flow Fe–Si–Mn deposits indicates the obvious hydrothermal origin. ► The Mn/Fe ratio in hydrothermal deposit may be a good indicator of propagating activities of the Valu Fa Ridge.A series of samples from the Hine Hina hydrothermal field (HHF) and the Mariner hydrothermal field (MHF) in the Central and Southern Valu Fa Ridge (VFR), Lau Basin were examined to explain the source origin and formation of the hydrothermal Fe–Si–Mn oxide deposits. The mineralogy was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, and energy-dispersive spectroscopy (EDS). For the Fe–Mn oxide crusts in the HHF, varying amounts of volcanic fragments and some seawater contributions were recognized, along with higher concentrations of Mn, Al, Co, Ni, Zn, Sr, Mo, elevated ∑REE and negative Ce anomalies. In contrast, the Si-rich oxide samples of the MHF were enriched in Cu, Pb and Ba, indicative of proximity to a hydrothermal jet. Moreover, conductive cooling of hydrothermal fluid evoked the Si-rich deposit formation in the MHF. The Sr, Nd and Pb isotope data provided further constraints regarding the source and formation of the Fe–Si–Mn deposits in the VFR by showing that the samples of the HHF are a mixture of three components, namely, hydrothermal fluid, seawater and volcanic materials, whereas the samples of the MHF were dominated by hydrothermal fluids. The seawater had a minor influence on the Nd isotope data, and the Pb isotope data exhibited a close association with the substrate rock and preformed volcaniclastic layers in this area. The occurrence of relatively high Mn/Fe ratios in the hydrothermal deposits of this area may be a good indicator of the propagating activities of the VFR over geological time.

Spatial distribution of lead and lead isotopes in soil B-horizon, forest-floor humus, grass (Avenella flexuosa) and spruce (Picea abies) needles across the Czech Republic by Julie Sucharová; Ivan Suchara; Clemens Reimann; Rognvald Boyd; Peter Filzmoser; Peter Englmaier (1205-1214).
► Pb-concentrations and 206Pb/207Pb isotope ratios are provided for four different sample materials for the Czech Republic. ► The paper demonstrates the local impact of a number of different contamination sources. ► The data provide clear evidence that traffic emissions are no major source of Pb to the Czech environment. ► The data demonstrate that the B-horizon provides no valid “background” for Pb-concentration or the 206Pb/207Pb isotope ratio. ► Pb isotope ratios change during soil weathering and at the interface biosphere/pedosphere.Lead concentrations were determined in samples of soil B-horizon (N  = 258), forest-floor humus (O-horizon, N  = 259), grass (Avenella flexuosa, N  = 251) and spruce (Picea abies, N  = 253) needles (2nd year) collected at the same locations evenly spread over the territory of the Czech Republic at an average density of 1 site/300 km2. Median Pb concentrations differ widely in the four materials: soil B-horizon: 27 mg/kg (3.3–220 mg/kg), humus: 78 mg/kg (19–1863 mg/kg), grass: 0.37 mg/kg (0.08–8 mg/kg) and spruce needles: 0.23 mg/kg (0.07–3 mg/kg). In the Pb distribution maps for humus, grass and spruce a number of well-known Pb-contamination sources are indicated by unusually high concentrations (e.g., the Pb smelter at Pribram, the metallurgical industry in the NE of the Czech Republic and along the Polish border, as well as the metallurgical industry in Upper Silesia and Europe’s largest coal-fired power plant at Bogatynia, Poland). The ratio 206Pb/207Pb was determined in all four materials. The median value of the 206Pb/207Pb isotope ratio in the soil B-horizon is 1.184 (variation: 1.145–1.337). In both humus and grass the median value for the 206Pb/207Pb isotope ratio is 1.162 (variation: 1.130–1.182), in spruce needles the median ratio is 1.159 (variation: 1.116–1.186). In humus, grass and spruce needles the known contamination sources are all marked by higher 206Pb/207Pb isotope ratios in the maps. Furthermore, the soil B-horizon, humus, grass and spruce needles show distinctly different spatial distribution patterns of the 206Pb/207Pb isotope ratios. The B-horizon does not provide a viable background value for metal concentrations in the O-horizon or plant materials. None of the maps provides evidence for the importance of traffic-related emissions for the observed isotope ratios at the scale of the Czech Republic.

Mechanism of tremolite carbonation by Kyoung Won Ryu; Myung Gyu Lee; Young Nam Jang (1215-1221).
► Significant amount of tremolite was transformed to calcite after the reaction for 5 min. ► The tremolite carbonation resulted in the complete change of their morphology. ► The morphology change have an implication on the elimination of their toxicity.The direct aqueous carbonation of fibrous tremolite was investigated by TEM, powder XRD, and FT-IR to elucidate the transformation mechanism of tremolite into calcite. The TEM study revealed that tremolite dissolves along the weak cleavage on (1 0 0) and (1 1 0) and the cleavage fragment fibers. The preferential release of Ca, relative to Si or Mg, into the solution resulted in the crystallization of compositionally homogeneous calcite and in the concurrent formation of proto-saponite. X-ray diffraction analysis revealed that the amount of product calcite increased rapidly within 15 min. Subsequently, after a 5 h carbonation period, it increased slowly to about 60%. The product calcite is euhedral. In contrast, the unreacted tremolite grains retain the original crystal structure even after prolonged carbonation. The morphological modification of tremolite from fibrous to round may have important implications for the elimination of its toxicity.

Distribution and reactivity of oxyanions (Sb, As, V, Mo) in the surface freshwater reaches of the Gironde Estuary (France) by Matthieu Masson; Laurent Lanceleur; Mary-Lou Tercier-Waeber; Jörg Schäfer; Teddy Hezard; Aurélie Larrose; Cécile Bossy; Gérard Blanc (1222-1230).
► Sb showed conservative behavior in the Garonne fluvial estuary. ► High temporal variations in dissolved As, V and Mo were observed at the tidal scale. ► These variations are due to equilibration processes in the MTZ and tidal oscillation. ► Dissolved Mo peaks were attributed to direct inputs from the Bordeaux agglomeration.The present work reports on the temporal variations of Sb, As, V and Mo concentrations in the surface freshwater reaches of the Gironde Estuary (SW France). Dissolved (<0.2 and <0.02 μm) and particulate Sb, As, V and Mo concentrations were measured in samples collected with high temporal resolution (every 30 min) during two consecutive tides at a fixed station located upstream of the city of Bordeaux and the maximum turbidity zone (MTZ). In addition, measurements of suspended particulate matter concentration, physico-chemical parameters, Cl concentration, water depth, current velocity and particulate Th concentration were performed either at the same time resolution or continuously. The data obtained suggested that variations in particulate As, V and Mo concentrations were most probably related to tidal cycling near the sampling site of fine grained particles from the MTZ located downstream. Significant differences in the behavior of the dissolved target oxyanions, mostly present in the <0.02 μm fraction, occurred. The behavior of Sb was conservative. Variations of the dissolved As and V concentrations showed similar cycling trends, strongly related to tidal cycles. These As and V cycles were interpreted as a mixing between upstream freshwater and downstream water enriched in dissolved As and V by desorption from the MTZ particles. The observed trend in the variation of the dissolved Mo was more complex than that of As and Sb and attributed to the mixing of the water bodies mentioned above coupled to a point source input of dissolved Mo from an intra-estuarine source. The contribution of this suspected Mo source to the dissolved Mo concentrations measured at the sampling site was estimated from the ratio of dissolved Mo to dissolved As concentrations. The additional dissolved Mo signal, coupled to the monitored ebb and flood length and associated current velocities, suggested an anthropogenic input which may derive from industrial activity near the city of Bordeaux. Extrapolating these results to the annual scale suggested that this dissolved Mo may be equivalent to 45–90% of the annual dissolved Mo flux into the Garonne Branch, highlighting the importance to further investigate the origin and behavior of Mo in the fluvial estuary near Bordeaux.

Reactions of radium and barium with the surfaces of carbonate minerals by Mark J. Jones; Laura J. Butchins; John M. Charnock; Richard A.D. Pattrick; Joe S. Small; David J. Vaughan; Paul L. Wincott; Francis R. Livens (1231-1238).
► Barium and radium react with carbonate minerals. ► Radium reacts with all the phases studied. ► Barium reacts only with dolomite, magnesite and siderite. ► We study the development of secondary phases formed in these reactions. ► Trace components of the minerals can dictate the outcome of reaction.Radium-226 is a naturally-occurring radioisotope with potentially significant radiological impact and whose environmental behaviour is of concern. The reactions of tracer (0.1–1 nM) dissolved Ra and its chemical analogue Ba with the surfaces of a range of carbonate minerals have been studied. All of the minerals react with Ra but, whereas calcite, dolomite, strontianite, rhodocrosite, ankerite and witherite all show increased uptake with increasing Ra concentration, suggesting a coprecipitation reaction (hence with phase formation limiting uptake), siderite, magnesite and ankerite show behaviour suggesting simple sorption (with decreasing uptake as Ra concentration increases, or with no dependence on [Ra]). Magnesite, in particular, has a low sorption capacity.Barium has been used at higher (0.1–1 mM) concentrations to enable the use of surface analytical and imaging techniques in addition to bulk uptake measurements. Although the same eight carbonates were studied, measurable uptake occurs only on dolomite, magnesite and siderite. For siderite and magnesite, there is an approximately linear relationship between the increasing solid and solution phase Ba concentrations, suggesting a simple sorption process. Dolomite shows more complex behaviour suggesting simple sorption at the lowest concentrations and phase formation at higher concentrations (>0.4 mmol L−1). The latter observation is consistent with spectroscopic evidence for the formation of witherite. Surface analysis and imaging of the three carbonate substrates that react with Ba show a diversity of behaviour, partly as a result of using natural minerals in these experiments. Witherite is commonly formed as a surface precipitate although the presence of even trace SO 4 2 - leads to barite formation. The surface phases display a range of characteristic morphologies, and the surface structure has the effect of templating growth. The presence of even minor amounts of Fe (hydr)oxide phases as alteration products or precipitates on the carbonates is also important, since Ba has a strong affinity for these phases.

► Spatial geochemical modelling can be achieved using geographically weighted regression (GWR). ► The resulting soil organic carbon (SOC) map showed clear spatial patterns influenced by environmental factors. ► The smoothing effect of spatial interpolation by GWR was reduced. ► The GWR provides a promising method for spatial geochemical modelling of SOC and potentially other geochemical parameters.It is challenging to perform spatial geochemical modelling due to the spatial heterogeneity features of geochemical variables. Meanwhile, high quality geochemical maps are needed for better environmental management. Soil organic C (SOC) distribution maps are required for improvements in soil management and for the estimation of C stocks at regional scales. This study investigates the use of a geographically weighted regression (GWR) method for the spatial modelling of SOC in Ireland. A total of 1310 samples of SOC data were extracted from the National Soil Database of Ireland. Environmental factors of rainfall, land cover and soil type were investigated and included as the independent variables to establish the GWR model. The GWR provided comparable and reasonable results with the other chosen methods of ordinary kriging (OK), inverse distance weighted (IDW) and multiple linear regression (MLR). The SOC map produced using the GWR model showed clear spatial patterns influenced by environmental factors and the smoothing effect of spatial interpolation was reduced. This study has demonstrated that GWR provides a promising method for spatial geochemical modelling of SOC and potentially other geochemical parameters.

► The hydrochemical and N–O isotope study to evaluate the source(s) and biogeochemical behavior of nitrate and sulfate in shallow alluvial aquifer. ► Nitrate originated from manure and fertilizers is attenuated by denitrification in the lower oxic and sub-oxic groundwater. ► The δ34Ssulfate values (up to 64.1‰) indicate that sulfate from fertilizers is attenuated by BSR in the sub-oxic groundwater. ► Combined, isotope and hydrochemical data are effective to discriminate different sources for the high sulfate and low nitrate waters.Based on hydrochemical and environmental isotope data (δ15N and δ18O of NO 3 - , and δ34S of SO 4 2 - ) of depth-specific groundwater samples from multi-level samplers, the source(s) and biogeochemical behavior of NO 3 - and SO 4 2 - in a shallow (<25 m below ground level) sandy alluvial aquifer underneath a riverside agricultural area in South Korea were evaluated. The groundwater in the study area was characterized by a large variability in the concentrations of NO 3 - (0.02 to ∼35 mg/L NO 3 ―N) and SO 4 2 - (0.14 to ∼130 mg/L). A distinct vertical redox zoning was observed sub-dividing an oxic groundwater at shallow depths (<8–10 m below ground surface) from sub-oxic groundwater at greater depths. The δ15N and δ18O values indicated that elevated NO 3 - concentrations in the oxic groundwater are due to manure-derived NO 3 - and nitrification of urea- and ammonia-containing fertilizers used on agricultural fields. Chemical and isotopic data also revealed that groundwater NO 3 - concentrations significantly decrease due to denitrification in the lower oxic and sub-oxic groundwater. The δ34Ssulfate values of the oxic groundwater ranged from −14.4‰ to +2.4‰. The relationship between δ34Ssulfate values and SO 4 2 - concentrations with depth showed that increasing SO 4 2 - concentrations were caused by S-bearing fertilizers, not pyrite oxidation. Bacterial (dissimilatory) SO 4 2 - reduction occurred locally in the sub-oxic groundwater, as indicated by increasing δ34Ssulfate values (up to 64.1‰) with concomitant decreases of SO 4 2 - concentrations. This study shows that isotope data are very effective for discriminating different sources for the waters with high SO 4 2 - and low NO 3 - concentrations in the lower oxic zone. It is also suggested that the use of N- and S-containing fertilizers should be better controlled to limit nitrate and SO 4 2 - contamination of shallow groundwater.

Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan Basin, China by Guangyou Zhu; Shuichang Zhang; Haiping Huang; Yingbo Liang; Shucui Meng; Yuegang Li (1261-1273).
► Natural gases discovered from the Zhongba gas field have two genetic types. ► TSR is the main origin of H2S. ► Supportive evidences of TSR are derived from gas carbon isotopic values, sulfur isotopic values and formation water compositions. ► Incomplete TSR reaction ceased in the Late Cretaceous.Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H2S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ34S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T2l3) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H2S and CO2 contents, slightly heavy C isotopic values of CH4 and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T3x2) are reservoired in continental sandstones and characterized by low gas dryness, free of H2S, slightly light C isotopic values of CH4, and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation.The H2S from the Leikoupo Formation is most likely formed by thermochemical SO4 reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H2S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO4 species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH4 δ13C values, wide range variations of ethane, propane and butane δ13C values, and moderately high gas dryness. The δ34S values in SO4, elemental S and H2S fall within the fractionation scope of TSR-derived H2S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T2l reservoir provide supplementary evidence for TSR related alteration.

► General quantitative model of carbonate water–rock interaction. ► First ever presented reaction-path modelling in the water–carbonate environment. ► DSRM first applied to the aqueous release from carbonates.This study reports on the results of a hydrogeochemical survey carried out in the Madonie area, a carbonate massif located in Palermo Province, Northern Sicily. The large dataset (226 collected sites) is used to highlight the processes controlling the distribution of dissolved chemicals in groundwaters; and, more importantly, to develop a general model (based on reaction-path modelling, and using the EQ3/6 code) of rock–water reactions in a carbonate environment. The investigated groundwater samples have conductivity between 31.7 and 8220 μS/cm; their total dissolved solids (TDS) content is higher near the coast area, where the seawater contribution becomes important. Calcium and HCO 3 - excess concentrations in groundwaters, with respect to the meteoric water–seawater mixing line, suggest that water–rock interactions in carbonate aquifers play a major role on water chemistry. Using the dataset, reaction-path modelling is used to simulate the evolution of groundwaters upon interaction with carbonate rocks. Model simulations are performed in time mode, taking into account the mineralogical composition of Madonie carbonate rocks, and the different dissolution rates of each dissolving mineral. The model results of reaction path calculations are in fair agreement with analytical data for natural waters, demonstrating the likelihood of model assumptions, and supporting further the relevance of carbonate dissolution in determining the chemistry of fluids in the investigated area. The developed model is useful for low-temperature weathering of carbonate mineral in general, and is thus likely to apply in a variety of geological contexts.