Applied Geochemistry (v.26, #12)

Biogeochemical modelling of anaerobic vs. aerobic methane oxidation in a meromictic crater lake (Lake Pavin, France) by F. Lopes; E. Viollier; A. Thiam; G. Michard; G. Abril; A. Groleau; F. Prévot; J.-F. Carrias; P. Albéric; D. Jézéquel (1919-1932).
► Aerobic and anaerobic methane oxidations likely occur in the water column of Lake Pavin. ► Seasonal differences in aerobic and anaerobic methane oxidation rates are detected. ► Iron dependent anaerobic methane oxidation may contribute to methane consumption in the water column. ► Aerobic methane oxidation appears as the major sink of methane in the Lake Pavin.Methane is a powerful greenhouse gas and its concentration in the atmosphere has increased over the past decades. Methane produced by methanogenic Archae can be consumed through aerobic and anaerobic oxidation pathways. In anoxic conditions found in freshwater environments such as meromictic lakes, CH4 oxidation pathways involving different terminal electron acceptors such as NO 3 - , SO 4 2 - , and oxides of Fe and Mn are thermodynamically possible. In this study, a reactive transport model was developed to assess the relative significance of the different pathways of CH4 consumption in the water column of Lake Pavin. In most cases, the model reproduced experimental data collected from the field from June 2006 to June 2007. Although the model and the field measurements suggest that anaerobic CH4 oxidation may contribute to CH4 consumption in the water column of Lake Pavin, aerobic oxidation remains the major sink of CH4 in this lake.

► We present the origin of solutes in a saline fluid collected at the sediment/basement interface. ► We show the relations ships of the present fluid with solutes in the rock matrix microporosity. ► These solutes originate from fluid circulation events which occurred 160–200 Myrs ago. ► A two-stage fluid flow regime may explain the chemistry of present and paleo fluids. ► Other saline fluids elsewhere could be explained by these phases of fluid circulation.This paper investigates the isotopic composition (O, D, Sr, O SO 4 , S SO 4 , Cl, He) of a present saline fluid sample collected at the sediment/basement interface in the Permian continental formation at 634 m depth in the SE margin of the Massif Central shield (Ardèche margin of the Southeast basin of France). The fluid sample shows clear water–rock interaction processes, such as feldspar dissolution and kaolinite precipitation, which have led to high Na concentrations and water stable isotopes above the local meteoric water line. The geological formations of the SE margin of the Massif Central shield show that intensive fluid circulation phases occurred across the margin from the late Triassic to the middle Jurassic. The fluids most probably originated from fluid expulsion during burial of the thick Permo-Carboniferous sedimentary succession. These circulation phases were responsible for cementation of the margin and for the solutes in the matrix microporosity which were extracted by leaching core samples.The chemical and isotopic composition of the saline fluid sample at 634 m in the Permian rock is very similar to that of fluids in the microporosity of the rock matrix. Their S SO 4 , O SO 4 and Sr isotopic compositions are close to those of cements investigated in fracture fillings in the same geological formations. Simple diffusion computations and comparison of the chemical composition of the present free fluid sample with matrix porosity fluids indicate that the solutes in the present free fluid sample are related to solutes originating from fluid circulation events which occurred 160–200 Ma ago through their diffusion from the matrix microporosity.A two-stage fluid flow regime is proposed to interpret the chemistry of present and paleo-fluids. (1) During the extensional context (Permian to Cretaceous), basinal brines migrated along the basement/sediment interface after expulsion from the subsiding basin. This fluid migration would be responsible for the solutes in the rock matrix microporosity and the solutes in the present free fluid sample. (2) Following the Alpine and Pyrenean compressive phases, gravity-driven meteoritic fluids slowly migrated from the surface down to the basement along major faults. This fluid regime would be responsible for the meteoric water collected in the present free fluid sample. Several investigations in Europe have shown that the existence of other saline fluids sampled elsewhere could be explained by these phases of fluid circulation related to specific geodynamic events.

Arsenic mobility and impact on recovered water quality during aquifer storage and recovery using reclaimed water in a carbonate aquifer by J.L. Vanderzalm; P.J. Dillon; K.E. Barry; K. Miotlinski; J.K. Kirby; C. Le Gal La Salle (1946-1955).
► Desorption from iron oxide surfaces and pyrite oxidation both contribute to arsenic mobilisation in reclaimed water ASR. ► Arsenic recovered from ASR consists of As(V) and As(III), while As(V) is dominant in the injectant and ambient groundwater. ► Reduction during aquifer storage leads to partial reduction of As(V) to As(III).Arsenic release from aquifers can be a major issue for aquifer storage and recovery (ASR) schemes and understanding the processes that release and attenuate As during ASR is the first step towards managing this issue. This study utilised the first and fourth cycles of a full scale field trial to examine the fate of As within the injectant plume during all stages of the ASR cycle, and the resultant water quality. The average recovered As concentration was greater than the source concentration; by 0.19 μmol/L (14 μg As/L) in cycle 1 and by 0.34 μmol/L (25 μg As/L) in cycle 4, indicating that As was being released from the aquifer sediments during ASR and the extent of As mobilisation did not decline with subsequent cycles. In the injection phase, As mobilisation due to oxidation of reduced minerals was limited to an oxic zone in close proximity to the ASR well, while desorption from Fe oxyhydroxide or oxide surfaces by injected P occurred further in the near well zone (0–4 m from the ASR well). With further aquifer passage during injection and greater availability of sorption sites there was evidence of attenuation via adsorption to Fe oxyhydroxides which reduced concentrations on the outer fringes of the injectant plume. During the period of aquifer storage, microbial activity resulting from the injection of organic matter resulted in increased As mobility due to reductive Fe oxyhydroxide dissolution and the subsequent loss of sorption sites and partial reduction of As(V) to the more mobile As(III). A reduced zone directly around the ASR well produced the greatest As concentration and illustrated the importance of Fe oxyhydroxides for controlling As concentrations. Given the small spatial extent of this zone, this process had little effect on the overall recovered water quality.

► Carnotite saturation in groundwater successfully targets secondary U mineralisation. ► Regional 5 km sample spacing is effective. ► Separation of channel and upland groundwater types improved targeting and background characterisation. ► Surface flow and erosion is important for determining prospective terrane. ► Primary sources of secondary deposits can also be inferred from groundwater data.Shallow calcrete aquifers in the central north of the Yilgarn Craton in Western Australia are the host to numerous secondary carnotite U deposits. Sampling and analysis of approximately 1400 shallow aquifer groundwaters were conducted to test if U mineralisation of this type may be found using a >5 km sample spacing. Results show this can be achieved. All the economic deposits and most of the minor deposits and occurrences are associated with groundwater that has carnotite (KUO2VO4) approaching or exceeding saturated conditions. Soluble U concentrations alone identified the largest deposit (Yeelirrie) and several smaller deposits, but this parameter was not as successful as the mineral saturation indices. Palaeodrainage distribution and thickness of cover combined with surface drainage and catchment boundaries provided background information of U primary sources and for areas with the highest exploration potential for channel and playa U deposits. Granites in the SE of the study area are less prospective with regard to secondary U deposits. Groundwater geochemistry in conjunction with palaeodrainage mapping may greatly improve exploration through cover where radiometric geophysics is not effective. The study of regional, shallow groundwater for U shows multiple benefits for mineral exploration, the economy and potable water quality.

Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine by Binyam L. Alemu; Per Aagaard; Ingrid Anne Munz; Elin Skurtveit (1975-1989).
► Two shale samples were reacted in batch reactor with brine and CO2. ► Carbonate-rich shales display higher reactivity compared to clay mineral-rich shales. ► CO2 was permanently trapped as calcite by reacting with Ca released from silicates. ► Zeolites formed only in samples reacted without CO2 (control reactions).Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO2 (CO2–brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO2 storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO2 (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N2, in order to mimic O2-depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO2-free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks.Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO2 increased in all experiments. Release rates of Fe and SiO2 were more pronounced in solutions reacted with CO2–brine as compared to those reacted with CO2-free brine. For samples reacted with the CO2–brine, lower temperature reactions (80 °C) released much more Fe and SiO2 than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO2–brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO2-free brine; while no trace of analcime was observed in either of the samples reacted with CO2–brine.

Geochemistry of mine waters draining a low-sulfide, gold-quartz vein deposit, Bralorne, British Columbia by Alexandre J. Desbarats; Michael B. Parsons; Jeanne B. Percival; Suzanne Beauchemin; Y.T. John Kwong (1990-2003).
► Mine waters contain As and Sb concentrations up to 3304 and 349 μg/L, respectively. ► Oxidation rate of arsenopyrite is 25 times slower than that of pyrite. ► Oxidation rate of stibnite is 1.5 times faster than that of pyrite. ► 57% of As and 46% of Sb released by sulfide oxidation is attenuated by sorption. ► Drainage alkalinity and hardness may inhibit the adsorption of oxyanions.The Bralorne and Pioneer mines, now inactive, produced over 4 million ounces of Au from an orogenic lode Au deposit located on the eastern edge of the Coastal Mountains of SW British Columbia. Between 2007 and 2009, drainage from a recently developed exploration adit was investigated in order to better understand and anticipate potential environmental management issues associated with the development of this type of deposit in the future. Portal discharge rate and specific conductance were monitored continuously over a 14-month period during which 36 water samples were collected. Additional samples were collected from flooded workings within the adit. Concentrations of As and Sb at the portal range as high as 1738 and 316 μg/L, respectively, while those in the mine pool reach 3304 and 349 μg/L, respectively. Effluent chemistry is mildly alkaline (pH = 8.7) and is dominated by Na, Ca, Mg, HCO3 and SO4. Geochemical inverse modeling of effluent composition indicates weathering of albite (2515 kg/a), ferroan dolomite (718 kg/a), pyrite (456 kg/a), arsenopyrite (23 kg/a) and stibnite (2 kg/a). Modeled sulfide reaction coefficients, normalized by their corresponding host rock concentrations, suggest that oxidation of arsenopyrite is 25 times slower than that of pyrite whereas oxidation of stibnite is 1.5 times faster. Oxidative dissolution of arsenopyrite and stibnite releases 10.6 kg/a of As and 1.1 kg/a of Sb of which 57% and 46%, respectively, are sorbed to ferrihydrite and gibbsite on the bed of the shallow channel through which the mine pool drains to the portal. Although mass balance calculations predict the formation of sufficient ferrihydrite to sorb 100% of the As dissolved in the mine pool, this attenuation process was ineffective possibly because the precipitated sorbents settled to the bottom of the water column or because of competition for sorption sites from Ca and HCO3. The dissolved Sb/As molar ratio in portal effluent (0.082) is much greater than the Sb/As ratio of the mineralization (0.002) because of slower arsenopyrite oxidation and somewhat lesser sorption of Sb.

Influence of hardpan layers on arsenic mobility in historical gold mine tailings by Stephanie L. DeSisto; Heather E. Jamieson; Michael B. Parsons (2004-2018).
► Hardpan cements range from nearly pure scorodite to predominantly HFA with minor HFO. ► Localized, pH-dependent, As-cycling occurs within each hardpan. ► Most advanced sulfide alteration shows As-cycling is dependent on sulfide persistence. ► Long-term As sequestration in hardpan cements is not expected. ► Detailed hardpan characterization can guide remediation of high-As mine waste.Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO4·2H2O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (<1 mg/L at pH 3–4) and high As content (43–52 wt.% As2O5, this study). Co-existing waters and solids were sampled at Montague Gold Mines to identify present-day field conditions influencing scorodite precipitation and dissolution, and to better understand the mineralogical and chemical relationship between hardpan and tailings. In addition to scorodite, hardpan cements were found to include amorphous Fe arsenate and Fe oxyhydroxide. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity, As5+ and Fe3+ for secondary mineral precipitation. Pore waters sampled from the hardpan have pH values ranging from 2.43 to 7.06. Waters with the lowest pH values also have the highest As concentrations (up to 35.8 mg/L) and are associated with the most extensive hardpan and greatest amount of weathered sulfide. Samples from areas with discontinuous hardpan and less sulfide have near-neutral pH and lower As concentrations. Detailed petrographic observations indicate that the identity and stability of As-bearing secondary minerals depends on the continued availability of sulfide concentrate. The results of this study are being used to develop remediation strategies for highly weathered, hardpan-bearing tailings that consider the stability of both primary and secondary minerals under various cover scenarios.

► Propagation of uncertainty in uranium speciation is examined by derivative and Monte Carlo methods. ► Predictions of solution speciation are robust with minimal amplification of input uncertainty. ► Predictions of sorption equilibria at low total U(VI) are not robust, with bimodal distributions of predicted speciation.Remediation of U-contaminated sites relies upon thermodynamic speciation calculations to predict U(VI) movement in the subsurface. However, reliability and applicability of geochemical speciation and reactive transport models may be limited by determinate (model) errors and random (uncertainty) errors in the equilibrium speciation calculations. This study examines propagated uncertainty in two types of subsurface calculations: I. Dissolved U(VI) speciation based on measured analytical constraints and solution phase equilibria and II. Overall U(VI) speciation which combined the dissolved phase equilibria with previously published adsorption reactions. Three levels of uncertainty, instrumental uncertainty, temporal variation and spatial variation across a site, were investigated using first-derivative sensitivity calculations and Monte Carlo simulations. Dissolved speciation calculations were robust, with minimal amplification of uncertainty and normal output distributions. The most critical analytical constraints in the dissolved system are pH, DIC, total U and total Ca, with some effect from dissolved SO 4 2 - . When considering adsorption equilibria, calculations were robust with respect to adsorbed U(VI) concentration prediction, but bimodal distributions of dissolved U(VI) concentrations were observed in simulations with background levels of total U(VI) and higher (spatial and temporal variability) estimates of input uncertainty. Consequently, sorption model predictions of dissolved U(VI) may not be robust with respect these higher levels of uncertainty.

The gold-in-calcrete anomaly at the ET gold prospect, Gawler Craton, South Australia by Mel Lintern; Malcolm Sheard; Nicky Buller (2027-2043).
► The distribution of more than 60 elements in the regolith at the ET Au prospect is investigated. ► Gold is concentrated in the surficial environment, including calcrete, at the ET Au prospect. ► The Au-in-calcrete distribution compares favourably with other prospective sample media. ► Other media tested for Au included shallow drilling, soil, vegetation and Bacillus cereus. ► Gold appears the best element to analyse for to locate prospective mineralisation.Much of Australia has an extensive regolith cover that conceals basement rocks and hinders mineral exploration and this situation is particularly acute in the Western Gawler Craton (South Australia), where, in addition to fluvial, marine and colluvial sediments, the land surface is extensively cloaked by sand dunes. This study documents the Au distribution at the ET Au prospect (Great Victoria Desert) in the Western Gawler Craton (South Australia). Although no economic Au mineralisation has yet been found at ET, the prospect hosts one of the larger Au-in-calcrete anomalies in Australia and is typical of many such prospects identified in the region. In addition to calcrete, the distribution of Au in regolith and biotic sample media was also examined. The study at ET shows that:Calcrete sampling has been a successful exploration technique to reveal cohesive Au anomalies within in situ regolith. Where transported regolith dominates and landforms are favourable e.g. sloping, calcrete (containing Au) will disperse (in solution and/or mechanically eroding) and thus provide a spatially larger target area for mineral exploration purposes. However, in these settings, the actual source of mineralisation may be difficult to locate due to various factors related to the dispersion processes and weathering history. At ET, other sampling media such as bacteria (Bacillus cereus) and vegetation have a greater uncertainty associated with them and their anomalies are not as cohesive.

Distribution, characterization, and geochemical controls of elements of concern in uranium mine tailings, Key Lake, Saskatchewan, Canada by Sean A. Shaw; M. Jim Hendry; Joseph Essilfie-Dughan; Tom Kotzer; Dirk Wallschläger (2044-2056).
► Characterize the aqueous and solid geochemistry of a uranium mine tailings deposit. ► As, Mo, and Se aqueous phase concentrations stable over the initial 15 years. ► As, Mo, and Se concentrations controlled by adsorption/co-precipitation with ferrihydrite. ► Determine present day source concentrations for As, Ca, Fe, Mo, Ni, Se, and SO4 aqueous values.The distribution of As, Mo, Ni and Se in the aqueous and solid phases of U mine tailings in the Deilmann Tailings Management Facility (DTMF), located at the Key Lake mine in northern Saskatchewan, Canada, was defined using as-discharged tailings samples collected monthly starting in 1996 and core samples collected from the DTMF from 3 locations on 3 occasions between 2004 and 2009. These data indicated that the DTMF can be divided into two geochemical zones; tailings generated from the Deilmann ore body between 1996 and 2000 and tailings from the McArthur River ore body, mined after July 2000. The Deilmann tailings solids are generally characterized by greater elemental concentrations than those in the McArthur River tailings, particularly for As, Co and Ni. These elevated concentrations are attributed to the abundance of Ni–Co–As–S minerals in this ore. The mean aqueous concentrations of As, Mo, Ni, and Se are 3.7, 23.2, 0.14, and 0.02 mg L−1, respectively, in the Deilmann porewaters and 0.20, 4.16, 0.06, and 0.03 mg L−1, respectively, in the McArthur River porewaters. Similarly, the mean As, Mo, Ni, and Se solid phase concentrations are 5.89 × 103, 69.9, 3.20 × 103, and 17.4 μg g−1, respectively, for the Deilmann tailings and 440, 13.6, 551, and 3.03 μg g−1 in the McArthur River tailings. Statistically, pH remained unchanged from the time of discharge while Eh values have remained oxic with no significant change between the two tailings types over 15 a of residence time. Results from sequential extractions, thermodynamic modeling, and results of complimentary research indicated that ferrihydrite solubility is the dominant control on aqueous As concentrations and also plays a role in controlling dissolved Mo, Ni and Se.

► Treated WWTP effluents were a major source of fresh hydrophilic organic Hg-ligands. ► Their stability constants (pH ∼ 6.8, I  = 0.5 M, T  = 25 °C) were greater than 1024. ► They commonly dominated Hg-DOM speciation in the downstream Seine River.Dissolved organic matter (DOM) samples were obtained from a low-density urbanized area located upstream of Paris (along the Marne River, France) and from the treated effluents at the Paris main wastewater treatment plant. These samples were then fractionated according to their hydrophobicity. DOM fractions consisted of nanomolar to sub-micromolar fresh organic substances with extremely strong Hg-complexing ligands. The conditional stability constants (i.e. K HgL ′ ; pH ∼ 6.8, I NaCl  = 0.5 M, T  = 25 °C) of the Hg-DOM complexes formed were greater than 1024  M−1, for the reaction Hg2+  + L = HgL (with L as ligand). For upstream of Paris, thermodynamic calculations indicated that the vast majority of Hg-DOM was associated with hydrophobic DOM. In contrast, in the Paris main wastewater treatment plant effluents, Hg-DOM was mainly bound to hydrophilic DOM. Simple dilution calculations highlighted that due to the large DOM loading of urban discharges, the hydrophilic urban DOM ligands may commonly dominate Hg-DOM speciation in the downstream Seine River, except under extreme dilution (i.e. high water episodes or floods).

Chloride and Boron behavior in fluids of Los Humeros geothermal field (Mexico): A model based on the existence of deep acid brine by Ruben Bernard; Yuri Taran; Maddalena Pennisi; Enrique Tello; Alejandro Ramirez (2064-2073).
► Chloride–boron decoupling in fluids of high-temperature Los Humeros geothermal field, Mexico. ► A theoretical model for the Cl–B decoupling and high B content based on the boiling of acid brine. ► Model results are in a semi-quantitative agreement with the observed data. ► Magmatic origin of B in Los Humeros fluids and prediction of the hypothetical brine properties.Geothermal field Los Humeros, Mexico, is characterized by a high steam fraction in the well fluids, by extremely high B concentrations in separated water (grams per liter, with a magmatic B signature, δ 11B ± 2σ  = −0.8 ± 1.6‰), by the absence of correlation between B and Cl concentrations and by positive correlation between B content in separated water and fluid enthalpy. Such behavior is consistent with the existence of moderately acid brine (pH 3–5) at depth with a high B concentration (500–600 mg/kg). In this case a 3-level model can be suggested for Los Humeros: (1) immature, moderately acid brine at depth which is boiling at a temperature ∼350 °C producing the HCl-bearing vapor with a high B content; (2) partial condensation of this vapor at the upper level accompanying water–rock interaction and neutralization; (3) formation of a shallow water-dominated aquifer above a lithologic low-permeability boundary as has been proposed by other authors. A thermochemical computer code has been used to model boiling of an aqueous fluid at 350 °C with 0.1 M of NaCl, 0.05 M of H3BO3 and variable amount of HCl (0.001–0.1 M), then a partial condensation of the produced vapor at 250 °C and then separation of the steam–water mixture at 150 °C. Results of simulation are in a qualitative agreement with the observed data.

Temporal alteration of fracture permeability in granite under hydrothermal conditions and its interpretation by coupled chemo-mechanical model by Hide Yasuhara; Naoki Kinoshita; Hiroaki Ohfuji; Dae Sung Lee; Shinichiro Nakashima; Kiyoshi Kishida (2074-2088).
► Flow-through experiments in a granite fracture are conducted under different pressure and temperature conditions. ► The evolutions of the fracture permeability and the effluent concentrations are examined. ► A dominant mechanism for the evolution of the fracture permeability may be attributed to the dissolution-induced process. ► A chemo-mechanical conceptual is developed and relatively well-replicates the actual measurements.Examining the evolution of fracture permeability under stressed and temperature-elevated conditions, a series of flow-through experiments on a single rock fracture in granite has been conducted under confining pressures of 5 and 10 MPa, under differential water pressures ranging from 0.04 to 0.5 MPa, and at temperatures of 20–90 °C, for several hundred hours in each experiment. Measurements of fluid and dissolved mass fluxes, and post-experimental microscopy, were conducted to constrain the progress of mineral dissolution and/or precipitation and to examine its effect on transport properties. Generally, the fracture aperture monotonically decreased with time at room temperature, and reached a steady state in relatively short periods (i.e., <400 h). However, once the temperature was elevated to 90 °C, the aperture resumed decreasing and kept decreasing throughout the rest of the experimental periods. This reduction may result from the removal of the mineral mass from the bridging asperities within the fracture. Post-experimental observations by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM-EDX), revealed the formation of several kinds of secondary minerals such as silica and calcite. However, the precipitated minerals seemed to have had little influence on the flow characteristics within the fracture, because the precipitation was limited to quite local and small areas. The evolving rates and ultimate magnitudes of the fracture aperture are likely to be controlled by the stress exerted over the contacting asperities and temperatures, and by the prescribed flow conditions. Thus, this complex behavior should be attributed to the coupled chemically- and mechanically-induced effect. A coupled chemo–mechano conceptual model, accounting for pressure and free-face dissolutions, is presented in this paper to follow the evolution of the fracture permeability observed in the flow-through experiments. This model addresses the two dissolution processes at the contacting asperities and the free walls within the fractures, and is also capable of describing multi-mineral dissolution behavior. The model shows that the evolution of a fracture aperture (or related permeability) and of element concentrations may be followed with time under arbitrary temperature and pressure conditions. The model predictions for the evolving fracture aperture and elements concentrations show a relatively good agreement with the experimental measurements, although it is not possible to replicate the abrupt reduction observed in the early periods of the experiments, which is likely to be due to an unaccounted mechanism of more stress-mediated fracture compaction driven by the fracturing of the propping asperities.

► Br/Cl versus I/Cl mixing diagrams enable organic Br to be quantified. ► Organic Br contributions are significant in saline formation waters related to Zn–Pb deposits. ► Halogens and noble gases provide complementary constraints on groundwater-hydrocarbon interactions. ► Fluid mixing and hydrocarbons are critical in Zn–Pb mineralisation.Halogen ratios (Br/Cl and I/Cl) and concentrations provide important information about how sedimentary formation waters acquire their salinity, but the possible influence of organic Br derived from sedimentary wall-rocks is rarely quantified. Here, it is demonstrated that Br/Cl versus I/Cl mixing diagrams can be used to deconvolve organic Br contributions; that organic matter has a limited range of Br/I ratios; and that organic Br is a more significant component in Zn–Pb deposit ore fluids than previously recognised. The significance of these findings is illustrated for the Lennard Shelf Zn–Pb deposits of Western Australia.Fluid inclusions related to Lennard Shelf Zn–Pb mineralisation have variable salinity and hydrocarbon contents. The halogen data from these fluid inclusions require mixing of three fluid end-members: (1) an evaporated seawater bittern brine (30 wt.% NaCl equiv.) with greater than seawater Br/Cl ratio; (2) a lower salinity pore fluid (⩽5 wt.% NaCl equiv.) with moderately elevated Br/Cl and I/Cl; and (3) fluids with Br/Cl ratios of ∼5 times seawater and extremely elevated I/Cl ratios of ∼11,500 times seawater. The first two fluids have 40Ar/36Ar of 300–400 and greater than air saturated water 36Ar concentrations that are typical of fluid inclusions related to Zn–Pb mineralisation. The third ‘organic-rich’ fluid has the highest 40Ar/36Ar ratio of up to 1500 and a depleted 36Ar concentration.Mineralisation is interpreted to have resulted from mixing of Zn-rich evaporitic brines and H2S present in hydrocarbons. It is suggested that aqueous fluids acquired organic Br and I from hydrocarbons, and that hydrocarbons exsolving from the aqueous fluid removed noble gases from solution. Interaction of variably saline brines and hydrocarbons could account for the variable Br/Cl and I/Cl composition, and 36Ar concentrations, recorded by Lennard Shelf fluid inclusions. The distinct 40Ar/36Ar signature of the fluid with the highest I/Cl ratio suggests the hydrocarbons and brines were sourced independently from different parts of the sedimentary basin. These data indicate the complementary nature of halogen and noble gas analysis and provide new constraints on important mixing processes during sediment-hosted Zn–Pb mineralisation.

Impact of mining activities on sediments in a semi-arid environment: San Pedro River, Sonora, Mexico by Agustín Gómez-Álvarez; Jesús L. Valenzuela-García; Diana Meza-Figueroa; Margarita de la O-Villanueva; Jorge Ramírez-Hernández; Javier Almendariz-Tapia; Efrén Pérez-Segura (2101-2112).
► High total concentrations of metals were detected in the San Pedro River (SPR) (Fe > Cu > Mn > Zn > Pb > Cd). ► Residual and Fe/Mn oxide fractions were the most important with regard to retaining of metals. ► Geoaccumulation index shows slight to moderate contamination in most studied samples. ► Enrichment factors demonstrate anthropogenic origins for Pb, Cd and Cu. ► Sediments are impacted by the mining industry and untreated wastewater discharges.A study of the San Pedro River (SPR), which is located in a semi-arid region in Sonora, Mexico, was conducted to evaluate the chemical, spatial and temporal (mobilization) trends of potentially harmful metals in its sediment in the rainy and dry seasons. High total concentrations of metals were detected in the following order: Fe > Cu > Mn > Zn > Pb > Cd. All studied metals except for Pb were increased during the dry season showing the effect of climate on the metal distribution in sediments. The results of sequential extraction indicated that the residual and Fe/Mn oxide fractions were the most important with regard to retaining potentially harmful metals in the sediments. In the exchangeable carbonate and Fe oxide fractions, high concentrations of metals were detected, representing high environmental risk. The geoaccumulation index shows slight to moderate contamination in most samples, and sampling point E4 (related to cattle activity) shows strong contamination for Cd, Cu, Pb and Zn. Enrichment factors (EFs) demonstrate anthropogenic origins for Pb (EF: 3–57), Cd (EF: 6–73) and Cu (EF: 1.5–224). This study shows that sediments are impacted by anthropogenic activities related to the mining industry, untreated wastewater discharges from the city of Cananea and cattle activities. Metal mobility in the SPR can disrupt the development of aquatic species in the river.

► Samples of uranium mill raffinate (pH 1.2) were neutralized to pH 10 with Ca(OH)2 and NaOH ± BaCl2. ► Neutralization experiments showed 226Ra concentrations decreased from 150 to <4 Bq/L with all neutralizing agents. ► Ca(OH)2  + BaCl2 was the most effective treatment (final concentration ∼1.0 Bq/L; ∼99.3% removal). ► Aqueous concentrations of 226Ra did not attain steady-state during neutralization and should decrease with time. ► Experiments showed that adsorption onto ferrihydrite can remove most of 226Ra in the raffinate.This study was conducted to define the geochemical controls on 226Ra during raffinate (pH = 1.2) neutralization to pH 10 at the Key Lake U mill in northern Saskatchewan, Canada. High activities (120–150 Bq/L) of aqueous phase 226Ra are present in raffinate produced during milling of U ore. The solubility control of 226Ra in the SO4-rich, hydrometallurgical raffinate solutions often involves the addition of BaCl2 to form a radium-barite co-precipitate (Ba(Ra)SO4). As such, neutralization experiments were conducted with samples of mill raffinate using Ca(OH)2 or NaOH with and without the addition of BaCl2. Radium-226 activity decreased from 150 to <4 Bq/L for all combinations of neutralizing agents with Ca(OH)2  + BaCl2 being the most effective combination (final activity ∼1.0 Bq/L; ∼99.3% removal). In the absence of BaCl2, Ca(OH)2 more efficiently removed 226Ra than NaOH between pH 4 and 8, due to the co-precipitation of 226Ra with gypsum. Overall, neutralization with the addition of BaCl2 reduced 226Ra activities at lower pH values (by pH 4.5), due to co-precipitation of 226Ra with BaSO4. At varying concentrations of BaCl2, aqueous phase activities of 226Ra converged, but did not attain steady-state values during neutralization and would continue to decrease with time. Sequential extractions indicated that 226Ra in precipitates formed during neutralization of the mill raffinate is dominated by amorphous and crystalline Fe hydroxide phases, consistent with raffinate neutralization experiments that showed that adsorption onto ferrihydrite can remove most 226Ra in the raffinate. Data generated in this study are being used to define the long-term geochemical controls on 226Ra in U mill processes and tailings.

Geochemical changes during neutralisation of acid mine drainage in a dynamic mountain stream, New Zealand by Hugh Davies; Paul Weber; Phil Lindsay; Dave Craw; Barrie Peake; James Pope (2121-2133).
► Voluminous AMD was neutralised with ultrafine limestone slurry. ► Stream pH rises from 3 to >5 in the first 250 m, and to 7 at 4 km. ► Partial neutralisation removes most dissolved ferric iron in the first 10 m. ► Dissolved Al decreases over ∼100 m in the limestone mixing zone. ► Dissolved zinc is extracted by adsorption but dissolved Ni remains.The Mangatini Stream drains a coal mining area in the mountains of northwestern South Island of New Zealand. Abundant rainfall on pyritic rocks yields acid mine drainage (AMD) to the stream, which flows through a steep gorge at discharges that rapidly increase from <1 to >100 m3/s during frequent rain events. The AMD is treated with finely ground limestone, which is discharged as a slurry at a point in the middle of the gorge. The limestone slurry mixes and reacts with the AMD during flow ∼4 km downstream over ∼12 h. Neutralisation reactions increase stream pH from near 3 (untreated Mangatini Stream water impacted by AMD) to 5–6 in the first 250 m downstream, although mixing is commonly incomplete in this zone. Large stream discharge volumes in rain events dilute the neutralising material input, thus driving the pH back towards 4 downstream of treatment. More complete neutralisation is achieved 4 km downstream, even in major rain events, and pH can rise to >7. Partial neutralisation is sufficient to remove most of the dissolved Fe(III) (typically ∼30 mg/L) from the Mangatini Stream in the first 10 m, and remaining dissolved Fe is essentially all Fe(II), which decreases over time as it oxidises and precipitates. Dissolved Al in the Mangatini Stream (typically ∼50 mg/L) decreases steadily downstream over ∼100 m in the limestone mixing zone. Precipitated Fe and Al form amorphous oxyhydroxides that are transported as suspended solids and deposited on the stream bed with excess limestone in zones of low flow velocity. Dissolved Zn is removed from solution by adsorption to Fe oxyhydroxide when pH reaches ∼5, but dissolved Ni remains in solution despite the neutralisation process. Gypsum precipitation occurs throughout the limestone mixing zone, resulting in at least 30% decrease in dissolved SO 4 2 - . Minor ettringite forms in the first 100 m, but then probably redissolves. The limestone dosing system is an effective method of neutralising the effects of AMD and removing most dissolved metals in a steep mountain stream with frequent rain events where this dynamic environment places many constraints on treatment options.

Geogas transport in fractured hard rock – Correlations with mining seismicity at 3.54 km depth, TauTona gold mine, South Africa by Johanna Lippmann-Pipke; Jörg Erzinger; Martin Zimmer; Christian Kujawa; Margaret Boettcher; Esta Van Heerden; Armand Bester; Hannes Moller; Nicole A. Stroncik; Zeev Reches (2134-2146).
► New methodologies enable real-time geogas monitoring in Tau Tona gold mine. ► Geogas transport through an inactive fault system in 3.54 km depth was observed. ► CO2, CH4, He, and H2 variations correlate with mining seismicity. ► Time series and data analysis identify two processes influencing gas composition. ► Monitoring system sensitive enough to quantify earthquake induced fluid transport.An on-site gas monitoring study has been conducted in the framework of an earthquake laboratory (The International NELSAM–DAFGAS projects) at the TauTona gold mine, South Africa. Five boreholes up to 60 m long were drilled at 3.54 km depth into the highly fractured Pretorius Fault Zone and instruments for chemical and seismic monitoring installed therein. Over the span of 4 years sensitive gas monitoring devices were continuously improved to enable the direct observation of geogas concentration variations in the DAFGAS borehole. The major gas concentrations are constant and air-like with about 78% N2, 21% O2, 1% Ar. The geogas components CO2, CH4, He and H2 show the most interesting trends and variations on the minute-by-minute basis and significantly correlate with seismic data, while the 222Rn activity remains constant. Time series and cross correlation analysis allow the identification of different gas components (geogas and tunnel air) and the identification of two processes influencing the borehole gas composition: (1) pumping-induced tunnel air breakthrough through networks of initially water-saturated fault fractures; and (2) seismicity induced permeability enhancement of fault fractures to above ∼5 × 10-10  m2. The current set-up of the gas monitoring system is sensitive enough to quantify the resulting geogas transport during periods of intense blasting activities (including recorded blasts with seismic moment ⩽1 × 109  Nm, located within 1000 m of the cubby) and, it is suggested, also during induced earthquakes, a final goal of the project.

► A flooded area was studied using δ2H, δ18O, 87Sr/86Sr, 3H and CFC/SF6. ► Complex hydraulic connections between the water bodies were highlighted by isotopes. ► The influence of the Seine river to groundwater was shown using 3H and 87Sr/86Sr. ► The impact on the aquifer of water of the gravel-pit lakes was shown by δ2H, δ18O. ► Multiple geochemical tools were needed to understand this complex water system.Understanding water exchange between groundwater and streams, or groundwater/surface-water relationships, is of primary importance for solving conflicts related to water use and for restoring water ecosystems. To this end, a combination of classic geochemical tools and isotopic tools were tested on the Bassée study site, located in the alluvial plain of the River Seine, to see whether they are relevant for tracing multiple and complex groundwater/surface-water relationships. The Ca/Sr ratio associated with Sr isotopes shows contrasted values and suggests that infiltration of surface water to groundwater increases when approaching the Seine. Furthermore, stable isotopes of the water molecule indicate that water from gravel-pit lakes may infiltrate into groundwater. Tritium and CFC tools confirmed surface-water influence on the Alluvial and Chalk aquifers. This geochemical approach, tested on the Bassée site, clearly demonstrates the need of using various geochemical tools for describing groundwater/surface-water relationships, and can be conclusively addressed to other case studies for helping decision makers in their management of natural water resources.

► We characterise the porosity of an in situ cement–argillite interaction. ► Multi-scale characterisation of the pore space is proposed with classic and innovative techniques. ► Porosity measurements was done in both cement and clay materials. ► These sets of data are unique.The solution selected by some countries to isolate radioactive wastes from the biosphere for up to one million years in deep geological repositories includes a multi-barrier disposal design, with steel canister, bentonite and cement materials. The geochemical contrast between such materials and the host rock formation creates perturbations potentially altering the confinement properties of the formation. In this context, the French Institute for the Radiological protection and Nuclear Safety (IRSN) have developed an in situ experimental programme based on the study of cement/argillaceous formation interfaces in their Underground Research Laboratory at Tournemire (Aveyron, France). An in situ engineered analogue of a cement/clay-rock interface which has undergone 15 years of interaction has been characterised. Such important interaction time for an in situ engineered analogue provides a bridge between laboratory-derived data and the long time scale of safety assessment modelling. As the mineralogical and petrological investigations have already been published, this work presents for the first time a quantitative characterisation of the spatial distribution of the porosity in the cement and the clay-rock in terms of time scale and design. Interfaces have been characterised using an autoradiography technique in addition to petrophysical measurements. This technique enables visualisation and quantification of the spatial distribution of the porosity using 2D mapping of decimetric-scale specimens. Thus autoradiographs allow highlighting the relationship between the field heterogeneities and the pore space evolution in each material in contact. Moreover, the porosity measurements show a clogging of the porosity in the clay-rock while the porosity increases in the cement. The extension of the porosity evolution extends to a centimetre on both sides of the interface but is heterogeneously distributed in space as a function of the fissure network and interface geometries. The connected fissure network visualised using autoradiography in the clogged area could permit solute (e.g. radionuclide) transport and may also be interpreted as an evolution of the mechanical properties of the clay-rock formation upon alkaline perturbation. This set of data, with the spatial quantification of the porosity in both cement and clay materials will be useful to constrain reactive transport modelling and thus to predict long term evolution of an engineered barrier.

A case study on the correlation of micro-contaminants and potassium in the Leine River (Germany) by Karsten Nödler; Tobias Licha; Steffi Fischer; Bianca Wagner; Martin Sauter (2172-2180).
Display Omitted► We examine the correlation of micro-contaminants and potassium in the Leine River (Germany). ► We examine its temporal and spatial variation in the river. ► Excellent correlations found for selected micro-contaminants. ► K+-equivalents of the micro-contaminants depend on land use, season and population structure.River monitoring with focus on the correlation of 41 micro-contaminants with K+ and its temporal and spatial variation was carried out in the Leine River (Germany). A positive correlation of concentrations of wastewater-related micro-contaminants and K+ is to be expected in receiving waters of wastewater treatment plant (WWTP) effluent since urine is a significant source of K+ in WWTP effluent. This correlation was found for compounds, which meet the following criteria: (1) WWTP effluent is the dominating source of the compound, (2) variability of its mass flux in the WWTP is negligible, and (3) the compound is persistent in WWTPs and in the environment. The excellent positive correlation of carbamazepine with K+ resulted in the fitting of a universal linear equation to the summer and winter data. For almost all other correlating compounds (1H-benzotriazole, citalopram, diclofenac, metoprolol, sotalol, sulfamethoxazole and tolyltriazole), slopes of the line fittings were steeper in winter (x-axis: K+, y-axis: respective micro-contaminant). This has been attributed to a presumed lower degree of degradation and attenuation within WWTPs and in the environment due to low temperatures as well as an increase in consumption of these compounds in the winter months. As part of this research, a sampling event along the entire flow path of the Leine River (∼280 km) was conducted to identify compounds demonstrating stable ratios of various respective micro-contaminants with K+. Among other compounds, carbamazepine, sulfamethoxazole, and tolyltriazole demonstrated the best correlations with R 2  > 0.89. Potassium-equivalents of the individual micro-contaminants depended on land use and population structure of the investigated river section.

► Isotopic compositions of S, N and C in the soil and the vegetation of three boreal forests were studied. ► They increased with soil depth and decreased with latitude and atmospheric inputs. ► Isotopic fractionation in ecosystem processes was higher for S than for N. ► Almost all S in the stream of the site with the lowest atmospheric inputs originated from the mineral soil.The concentrations and the isotopic compositions of S, N and C were studied in soils and in the dominant plant species of three forested watersheds (Québec, Canada) located along a latitudinal and atmospheric deposition gradient. Large increases in S, N and C isotope ratios (up to 3.9‰, 10‰, 2.6‰, respectively) were observed with increasing soil depth at the three watersheds. These increases were accompanied by a strong decrease in elemental concentrations resulting in a strong negative relationship between these two variables. Both S and N concentrations throughout the soil profile and δ 34S and δ 15N in the mineral soil appeared to increase with increasing S and N deposition rates and decreasing latitude. A strong positive linear relationship was found between δ 34S and δ 15N (R 2  = 0.72) values and between organic S and N concentrations (R 2  = 0.96) in soils. The slope of the linear relationship between δ 34S and δ 15N (δ 34S = f(δ 15N)) indicated that isotopic fractionation was almost 4 times higher for S than for N during transformations that occurred in soil. However, this difference might reflect a higher degree of openness of the S cycle compared to the N cycle rather than an isotope effect per se. Overall, the results suggest that N and S inputs significantly impact the isotope ratios and the concentrations of N and S in the soils, and that S and N were closely associated and subject to similar processes with the same isotopic effects throughout the soil profile. Contrary to most studies, δ 34S-SO4 in stream water of the most northerly site with the lowest S deposition rate was significantly higher than δ 34S-SO4 in atmospheric depositions but similar to the δ 34S of the bulk mineral soil. It suggests that the mineral soil actually contributes a large portion of the stream S-SO4 for this site.

Lead and stable Pb-isotope characteristics of tropical soils in north-eastern Brazil by Anne Schucknecht; Jörg Matschullat; Clemens Reimann (2191-2200).
► Rather low average Pb-concentrations with minor differences between the biomes. ► Distinct Pb-isotopic signature differences between the biomes. ► Both data sets are quite unique (southern hemisphere and tropical environment). ► Data do not contradict Clemens Reimann’s hypothesis. ► More data are needed to fully confirm or to reject his hypothesis.Stable Pb-isotope ratios are widely used as tracers for Pb-sources in the environment. Recently, a few publications have challenged the predominating view of environmental applications of Pb-isotopes. Present applications of Pb-isotopic tracers in soils largely represent the northern hemisphere. This study focuses on tropical soils from Paraíba, north-eastern Brazil. Lead concentrations and Pb-isotopic signatures (both 7N HNO3) were determined at 30 sites along a 327 km E–W-transect, from the Atlantic coast at João Pessoa to some kilometers west of Patos, to identify possible processes for the observed (and anticipated) distribution pattern. Thirty samples each of litter (ORG) and top mineral soil (TOP) were taken on pasture land at suitable distance from roads or other potential contamination sources. Lead-content was determined by inductively-coupled plasma atomic emission spectrometry (ICP-AES) and the ratios of 206Pb/207Pb, 206Pb/208Pb, and 208Pb/207Pb by ICP-sector field mass spectrometry (ICP-SFMS). Both sample materials show similarly low Pb-concentrations with a lower median in the ORG samples (ORG 3.4 mg kg−1 versus TOP 6.9 mg kg−1). The 206Pb/207Pb ratios revealed a large spread along the transect with median 206Pb/207Pb ratios of 1.160 (ORG) and 1.175 (TOP). The 206Pb/207Pb ratios differ noticeably between sample sites located in the Atlantic Forest biome along the coast and sample sites in the inland Caatinga biome. The “forest” sites were characterised by a significant lower median and a lower spread in the 206Pb/207Pb and 206Pb/208Pb ratios compared to the Caatinga sites. Results indicate a very restricted influence of anthropogenic activities (individual sites only). The main process influencing the spatial variability of Pb-isotope ratios is supposed to be precipitation-dependent bioproductivity and weathering.

Using 87Sr/86Sr, δ18O and δ2H isotopes along with major chemical composition to assess groundwater salinization in lower Shire valley, Malawi by Maurice Monjerezi; Rolf D. Vogt; Per Aagaard; Asfaw Gebretsadik Gebru; John D.K. Saka (2201-2214).
► A combined assessment of 87Sr/86Sr, δ18O and δ2H and major ions in water from lower Shire valley, Malawi, was conducted. ► End-members (fresh- and saline groundwater, and evaporated recharge) were identified. ► Saline groundwater is inferred to result from dissolution of halite. ► Mixing models show that brackish water results from a three component mixture comprising the identified end-membersGroundwater resources in some parts of the lower section of Shire River valley, Malawi, are not useable for rural domestic water supply due to high salinity. In this study, a combined assessment of isotopic (87Sr/86Sr, δ18O and δ2H) and major ion composition was conducted in order to identify the hydro-geochemical evolution of the groundwater and thereby the causes of salinity. Three major end-members (representing fresh- and saline groundwater, and evaporated recharge) were identified based on major ion and isotopic composition. The saline groundwater is inferred to result from dissolution of evaporitic salts (halite) and the fresh groundwater shows influence of silicate weathering. Conservative mixing models show that brackish groundwater samples result from a three component mixture comprising the identified end-members. Hence their salinity is interpreted to result from mixing of fresh groundwater with evaporated recharge and saline groundwater. On the other hand, the groundwater with low TDS, found at some distance from areas of high salinity, is influenced by mixing of evaporated recharge and fresh groundwater only. Close to the Shire marshes, where there is shallow groundwater, composition of stable isotopes of water indicates that evaporation may also be an important factor.

Quantification of glacial till chemical composition by reflectance spectroscopy by Maarit Middleton; Paavo Närhi; Viljo Kuosmanen; Raimo Sutinen (2215-2225).
► Visible and near-infrared spectroscopy can predict till soil element concentrations. ► Element concentrations of mineral powder fraction were modelled with chemometry. ► Prediction of Al, Ba, Co, Cr, Cu, Fe, Mg, Mn, Ni, V, and Zn was successful. ► Cross-correlation to spectrally active mineralogy enables trace element detection.Chemometric modelling of soil element concentrations from diffuse visible and near-infrared (VSWIR, 350–2500 nm) reflectance spectroscopic measurements holds potential for soil element analyses. Research has demonstrated it particularly for organic agricultural soils, yet little is known about the VSWIR response of glacial tills. Soils with low organic matter content developed on unstratified glacial materials were studied at two geologically similar sites on the mafic metavolcanic rocks of the Lapland Greenstone belt in northern Finland. The till samples (n  = 217) were composed primarily of quartz, plagioclase and amphibole having less than 3% of clinochlore, talc and illite. VSWIR spectra of mineral powder (<0.06 mm) samples were measured in the laboratory, and the spectral reflectance was regressed against partial elemental concentrations of till obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) following hot aqua regia digestion. Partial least-squares regression (PLSR) analyses resulted in simultaneous prediction (R 2  = 0.80–0.89) of several soil chemical elements such as Al (validation RMSE 1802 mg kg−1), Ba (5.85 mg kg−1), Co (0.86 mg kg−1), Cr (6.94 mg kg−1), Cu (2.54 mg kg−1), Fe (2088 mg kg−1), Mg (449.6 mg kg−1), Mn (0.82 mg kg−1), Ni (3.24 mg kg−1), V (4.88 mg kg−1), and Zn (0.80 mg kg−1). The electronic and vibrational molecular processes causing absorption might be responsible for accurate predictions of major elements such as Al, Fe and Mg. However, the concentrations of other major and trace elements could be predicted by the PLSR because they were cross-correlated to spectrally active soil elements or extraneous soil properties. Therefore, the applicability of the results is highly sample set specific. Further, the results show that in local scale studies at geologically fairly homogenous areas the limited spread of the data may restrict the use of the spectroscopic–chemometric approach. This paper demonstrates the capability of laboratory VSWIR spectroscopy for determining element concentrations of glacial tills. Further work should focus on overcoming the issues of sampling scale and understanding the causality for cross-correlation in quantification of the elements.

► Kinetically controlled bentonite colloid desorption of tri- and tetravalent radionuclides observed. ► Results significantly extent available kinetic data for year-long contact times. ► No Tc(VII) or U(VI) reduction detected under experimental conditions. ► Np(V) reduction on fracture filling material observed. ► Thermodynamic calculations are in good agreement with the experimental findings.The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, 232Th(IV), 242Pu(IV) and 243Am(III) are clearly colloid associated in contrast to 233U(VI), 237Np(V) and 99Tc(VII). Concentrations of colloid-borne 232Th(IV), 242Pu(IV) and 243Am(III) decrease after ∼100 h showing desorption from bentonite colloids while 233U(VI) and 99Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. 232Th(IV) and 242Pu(IV) data yield a slower dissociation from colloids compared to 243Am(III) indicating stronger RN–colloid interaction. In the case of 237Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).

► Uranium is converted to uranyl ions under oxidation conditions in desert terrains. ► The ion complexes are sorbed on clay minerals. ► Fine-grained soils from the clay-rich horizon is an effective sampling medium for deep-penetrating geochemical surveys. ► U and Mo are highly correlated.The Turpan–Hami basin, covering an area of approximately 50,000 km2 in NW China, contains concealed sandstone-type U deposits in a Jurassic sequence of sandstone, mudstone and coal beds. Sampling of soil profiles over the Shihongtan concealed U deposit in this basin shows that fine-grained soil collected from the clay-rich horizon contains U concentrations three times higher than similar soils at background areas. Selective leaching studies of these soils show that U is mainly associated with clay minerals, which comprise from 17.9% to 40% (average 30.4%) of the total mineral content. This may indicate that U is converted to uranyl ions [UO2]2+ under oxidizing conditions and is sorbed on clay minerals to accumulate in anomalous concentrations. Fine-grained soil (<120 mesh, <0.125 mm) from the clay-rich horizon, generally occurring at a depth of 0–40 cm, is shown to be an effective sampling medium for deep-penetrating geochemical surveys. A wide-spaced geochemical survey at a density of approximately 1 site per 100 km2 was carried out throughout the whole basin using this sampling medium. Samples were analyzed for 30 elements by ICP-MS following a 4-acid extraction. Three large-scale geochemical anomalies of U and Mo were delineated over the whole basin. One of the anomalies is consistent with the known U deposit at Shihongtan in the western part of the basin. A new potential target in the eastern part of the basin was selected for a follow-up survey at a density of 1 sample per 4 km2. A drilling exploration programme at the center of the geochemical anomaly delineated by this follow-up survey discovered a new U deposit.

Sphalerite oxidation pathways detected by oxygen and sulfur isotope studies by Claudia Heidel; Marion Tichomirowa; Cornelia Breitkopf (2247-2259).
► Oxygen and sulfur isotopes of sulfate indicate changing oxidation pathways during aerobic sphalerite oxidation. ► Initially, sphalerite was oxidized by molecular oxygen. ► Later on, sphalerite was oxidized by ferric iron generated from released ferrous iron. ► The oxygen isotope enrichment factors ε SO4-O2 and ε SO4–H2O are −22‰ and ⩽4.1‰, respectively. ► There is no general oxidation mechanism for acid-soluble sulfides.Sphalerite oxidation is a common process under acid-mine drainage (AMD) conditions and results in the release of SO 4 2 - , Zn and potentially toxic trace metals, which can pollute rivers and oceans. However, there are only a few studies on the mechanisms of aerobic sphalerite oxidation. Oxygen and S isotope investigations of the produced SO 4 2 - may contribute to the understanding of sphalerite oxidation mechanisms so helping to interpret field data from AMD sites. Therefore, batch oxidation experiments with an Fe-rich sphalerite were performed under aerobic abiotic conditions at different initial pH values (2 and 6) for different lengths of time (2–100 days). The O and S isotope composition of the produced SO 4 2 - indicated changing oxidation pathways during the experiments. During the first 20 days of the experiments at both initial pH values, molecular O2 was the exclusive O source of SO 4 2 - . Furthermore, the lack of S isotope enrichment processes between SO 4 2 - and sphalerite indicated that O2 was the electron acceptor from sphalerite S. As the oxidation proceeded, a sufficient amount of released Fe(II) was oxidized to Fe(III) by O2. Therefore, electrons could be transferred from sphalerite S sites to adsorbed hydrous Fe(III) and O from the hydration sphere of Fe was incorporated into the produced SO 4 2 - as indicated by decreasing δ 18OSO4 values which became more similar to the δ 18OH2O values. The enrichment of 32S in SO 4 2 - relative to the sphalerite may also result from sphalerite oxidation by Fe(III).The incorporation of O2 into SO 4 2 - during the oxidation of sphalerite was associated with an O isotope enrichment factor ε SO4–O2 of ca. −22‰. The O isotope enrichment factor ε SO4–H2O was determined to be ⩽4.1‰. A comparison with O and S studies of other sulfides suggests that there is no general oxidation mechanism for acid-soluble sulfides.

Emission of phosphine in intertidal marshes of the Yangtze Estuary by L.J. Hou; M. Liu; J.L. Zhou; Y. Yang; D. Zhao; G.Y. Yin; Y.L. Zheng (2260-2265).
► Relatively high emission fluxes of phosphine appeared in warm seasons. ► The phosphine fluxes were higher in the freshwater marsh than in the brackish marsh. ► Phosphine emissions were closely associated with sediment characteristics. ► Phosphine production is potentially derived from the microbial transformations of P. ► Phosphine emissions may be a significant internal source of P for the pelagic water.Emission of phosphine, a gaseous form of P, is presently considered a potential pathway of the P biogeochemical cycle in aquatic sediments. This study investigated the emission fluxes of phosphine and its potential production mechanisms in the intertidal marshes of the Yangtze Estuary. It is shown that the relatively high emission fluxes of phosphine were measured in warm seasons, with the values of 3.85–24.9 ng m−2  h−1 and 4.21–36.5 ng m−2  h−1 in August and September, respectively. In contrast, lower fluxes of phosphine appeared in May (1.23–6.32 ng m−2  h−1) and January (0.21–0.91 ng m−2  h−1). Also, the fluxes of phosphine were generally higher in the freshwater marsh, compared with the brackish marsh. The spatio-temporal pattern of phosphine emissions was observed to be mainly associated with sediment structure, temperature and salinity. Meanwhile, the significant correlations of phosphine emissions with sedimentary P and alkaline phosphatase activities reflect that phosphine probably derives from the microbial transformations of PO4 and organic P. In addition, it is estimated that approximately 1.08 × 106  g of phosphine is released annually from sediments into the pelagic water of the Yangtze Estuary. Therefore, it is concluded that phosphine emissions may be an important internal source of P, making a significant contribution to the occurrence of algal blooms especially during warm seasons.