Applied Geochemistry (v.61, #C)

The Water–Sediment Regulation Scheme (WSRS) is an important water conservancy project in the Yellow River basin, which is usually operated annually from June to July to control water and sediment release from the Xiaolangdi Reservoir in the middle reaches. As a greatly concentrated period of delivering terrigenous materials from the Yellow River to the sea, the WSRS can serve as a natural laboratory to examine the geochemical behavior of elements during their transport along the river. Uranium isotopes (234U and 238U) were measured in Yellow River waters at stations Xiaolangdi (located in the middle reaches of the Yellow River) and Lijin (the last hydrologic station near the Yellow River estuary) during the WSRS 2012. Compared with station Xiaolangdi, dissolved uranium concentration at station Lijin was markedly higher, showing a significant impact from the WSRS. Budget calculation for dissolved uranium during the WSRS indicated that two major sources of new added dissolved uranium in the section of the Yellow River between Xiaolangdi and Lijin: suspended particles (46%) and porewater of bottom sediment (45%). The flux of dissolved uranium from the Yellow River to the sea was estimated to be 2.40 × 107  g during the WSRS 2012.

Thermodynamic and fully-coupled reactive transport models of a steel–bentonite interface by James C. Wilson; Steven Benbow; Hiroshi Sasamoto; David Savage; Claire Watson (10-28).
Engineered barrier system (EBS) designs for the geological disposal of high-level radioactive waste often include a bentonite buffer, the primary function of which is to protect metal waste containers or overpacks from mechanical shearing. The buffer also acts as a barrier to solute transport. One potentially deleterious process that may occur in the buffer is the alteration of swelling clay to iron-rich minerals, some of which have a limited capacity to swell. There is a dearth of relevant natural analogues of iron–bentonite interactions, and experimental data do not provide an unequivocal indication of the conditions that will promote non-swelling clay minerals (such as berthierine) to form rather than swelling clays (such as iron-rich saponite). In addition, many of the previously-published reactive transport models of iron–bentonite or iron–claystone interfaces have not considered how evolution of mineral–fluid equilibria in the bentonite buffer could affect the nature and rate of steel corrosion. In this study, new thermodynamic models of iron-rich clay minerals are presented which suggest that the activities of major ions, especially Fe2+, Fe3+, Al3+, H+ and SiO2(aq), act as key controls on the relative stabilities of iron-rich clay minerals. In particular, they suggest that Fe-saponite is stabilised under low f  O2(g) conditions when SiO2(aq) activities are buffered by quartz or more soluble silica polymorphs (e.g. chalcedony). Iron-rich 1:1 clay minerals, such as berthierine, tend to be stabilised in fluids that are quartz under-saturated. The thermodynamic models were used to inform the development of three fully-coupled reactive transport models of a steel–bentonite interface: (1) steel corrosion reaction applied on a boundary directly in contact with bentonite at a fixed rate; (2) steel corrosion reaction applied on a boundary directly in contact with bentonite at a diffusion-limited rate; and (3) a ‘corrosion cell’ representation with a fixed steel corrosion rate.The extent and nature of the alteration predicted by the models was found to be sensitive to model conceptualisation. The corrosion cell assumption leads to steel corrosion products including magnetite and siderite and the alteration of primary minerals to berthierine (∼2 cm thick layer forming over 100 000 years, with partial loss of montmorillonite to a depth of ∼9 cm). In contrast, the boundary corrosion assumption with a fixed steel corrosion rate leads to quicker alteration to iron-rich clay minerals. If the diffusion-limited corrosion rate assumption is made, the steel corrosion rate varies over time as the bentonite porewater composition evolves, and the spatial extent of alteration is much more limited (millimetre scale). Although much progress has been made, a number of uncertainties associated with modelling bentonite evolution remain, especially with regard to ion transport through smectite interlayers and the potential for complex couplings between chemical and physical processes.

Optimizing aquifer storage and recovery performance through reactive transport modeling by E.A. Antoniou; B.M. van Breukelen; P.J. Stuyfzand (29-40).
Water quality deterioration is a common phenomenon that may limit the recovery of injected water during aquifer storage and recovery (ASR). Quality deterioration is often caused by the oxidation of reduced aquifer components by oxygenated source water, the subsequent pH decline, and induced dissolution of carbonate minerals. We use a previously calibrated reactive transport model (PHREEQC) to optimize ASR depending on source water quality and kind of pretreatment. We give quantitative projections on the performance increase over successive cycles with respect to specific water quality indicators. We simulate the response of a representative, deeply anoxic aquifer upon injection of three different commonly applied oxygenated water types: pre-treated drinking water, desalinated seawater, and urban storm water. The model is coupled to a Python script that automatically stops recovery and starts the next injection phase when certain specified concentration thresholds are exceeded. This setup enables realistic simulations to gradually create a buffer zone around the ASR well that allows 100% recovery at a specific stage of aquifer development. Each source water type was associated with different issues causing the deterioration of the abstracted water quality with respect to Fe(II), Mn(II), and As. The injection of pre-treated drinking water caused Mn(II) exceedances that disappeared after a number of cycles, provided that the recovery would halt as soon as the Mn(II) exceedances would occur. The injection of desalinated water caused persisting Fe(II) exceedances, which substantially slowed the creation of an effective buffer zone; whereas, the injection of urban storm water caused similar issues with respect to arsenic. For both cases, it was shown that enriching the source water with O2 and/or NaOH had major positive effects by accelerating the creation of an efficient buffer zone. Finally, we simulated a long-term operational rest of the ASR plant to evaluate water quality effects during potential migration of the stored water due to lateral groundwater flow, as dependent on source water composition and pretreatment method. Exceedances of drinking water guidelines occurred long before the arrival of the native water. Fe(II) and Mn(II) exceedances, after having used desalinated and drinking water, respectively, were observed after a bubble migration of 9% and 40%, respectively, and were associated with the slightly acidic pH conditions promoting the dissolution of Mn-carbonate and preventing an efficient sorptive removal. The arsenic exceedances, after using urban storm water, were associated with the arsenic wave deriving from the pyrite oxidation and reaching the ASR well after 34% of bubble migration. Enrichment of the source water with O2 and/or NaOH was also helpful in protecting the water quality around the ASR well for a longer time during a bubble migration scenario. The Fe(II) breakthrough occurred after 59% of desalinated bubble migration (instead of 9%) whereas As broke through after 70% of urban storm bubble migration (instead of 34%). This study illustrates that reactive transport modeling with a calibrated model is a useful tool to a-priori test the potential effectiveness of various operational options in ASR application on improving recovered water quality and the recovery efficiency.

Diffusion of cations in clayrocks is widely investigated, because deep clay-rich formations are currently considered as one of the potential host rocks for radioactive waste repositories. However, several authors have already reported that sorbing cations seem to diffuse at rates larger than those predicted by a simple pore diffusion model from their sorption coefficients and from the diffusive flux of non-sorbing water tracers. This process has been attributed to the migration of cations within the electrical double layer, next to the mineral surfaces, called the surface diffusion phenomenon. The aim of this work was to verify whether this “enhanced” cation diffusion compared to neutral species was observed for strontium and, if so, to what extent this effect might vary with the salinity of the synthetic solutions. These questions were addressed by performing batch sorption, through-diffusion and out-diffusion experiments on rock samples from the Callovo-Oxfordian claystone formation (France).The results showed that there was a good agreement of the distribution ratios (RD ) determined on crushed and intact rocks by batch and through-diffusion methods with a RD decrease related to the increase of the sodium concentration, a sorption competitor. Such a trend was also well reproduced by means of a geochemical modeling based on the multi-site ion exchange (MSIE) theory. Moreover, the “enhanced” diffusion for strontium was clearly observed in this study: the Sr diffusive flux was almost five times higher than that for HTO in the cell with the lowest ionic strength, and diminished to less than 1.5 times in the cell with the highest ionic strength. The evolution of the Sr diffusive flux with the ionic strength was qualitatively reproduced by a surface diffusion model, based on the concept of relative diffusive mobility or mobile fraction.

Photopromoted oxidative dissolution of stibnite by Xingyun Hu; Mengchang He; Linghao Kong (53-61).
Light irradiation plays an important role in the oxidative dissolution of Sb2S3. A qualitative mechanism was proposed through simulated experiments to describe the dissolution reaction under various ambient conditions, specifically darkness/light and aerobic/anaerobic conditions. The pH-dependent solubility of Sb2S3(s) and photo-induced oxidative dissolution are two types of reaction mechanisms. The oxidation of Sb(III) and its sulfides by light irradiation induce the prompt dissolution of Sb2S3. Photo-oxidants in the form of hydroxyl radicals ( • OH ) , superoxide anion free radicals ( • O 2 - ) and photo-generated holes h vb + were produced in the photo-induced oxidative dissolution reaction of Sb2S3. Among these, O2 and • O 2 - are the most significant oxidants. The main oxidation products of Sb2S3 are Sb(V), S0 and S 2 O 3 2 - . This study may help elucidate the geochemistry and fate of antimony in the environment.

Action of a clay suspension on an Fe(0) surface under anoxic conditions: Characterization of neoformed minerals at the Fe(0)/solution and Fe(0)/atmosphere interfaces by Pierre Le Pape; Camille Rivard; Manuel Pelletier; Isabelle Bihannic; Renaud Gley; Sandrine Mathieu; Lise Salsi; Sylvie Migot; Odile Barres; Frédéric Villiéras; Nicolas Michau (62-71).
Display OmittedTo better understand the reaction mechanisms involved at the Fe(0)/clay minerals interface, we investigate in the present study the reaction between an Fe(0) surface and a clay suspension extracted from the Callovo-Oxfordian claystone (COx). Batch experiments were carried out under anoxic conditions in sealed autoclave, at 90 °C to mimic predicted radioactive waste disposal conditions. An Fe(0) foil was introduced into the autoclave so that the lower part of the foil was immersed in the clay suspension while the upper part was contacted with the atmosphere of the experimental setup. After two months, the mineralogical deposits that precipitated at the surface of the Fe(0) foil were analyzed using multiple techniques, namely X-ray diffraction (XRD), scanning/transmission electron microscopy associated to microanalysis (SEM/TEM–EDXS), and micro-spectroscopic measurements (μ-FTIR and μ-Raman). Both parts of the Fe(0) foil were then shown to react: magnetite was the main resulting mineral formed at the Fe(0) surface in the atmospheric conditions whereas serpentine 1:1 phyllosilicates were the main end-products in the clay suspension. The analyses performed on the immersed part of the foil revealed a spatial heterogeneity in both serpentine cristallochemistry and morphology, with a gradient from the Fe(0) contact point toward the clay suspension. A pure Fe–Si phyllosilicate ring was observed at the direct contact point with the Fe(0) foil and a progressive incorporation of Al instead of Fe into the clay phases was identified as deposit thickness increased from the Fe(0) surface to the clay suspension. Our findings suggest that reaction mechanisms include several steps, corresponding to successive regimes depending on the availability of the main reactive elements at the Fe(0)/solution interface, namely Fe, Si and Al. Thus, our results provide new information to support the understanding of both mineral organization and composition at the clay/Fe(0) interface.

Weathering and transport of chromium and nickel from serpentinite in the Coast Range ophiolite to the Sacramento Valley, California, USA by Jean M. Morrison; Martin B. Goldhaber; Christopher T. Mills; George N. Breit; Robert L. Hooper; JoAnn M. Holloway; Sharon F. Diehl; James F. Ranville (72-86).
A soil geochemical study in northern California was done to investigate the role that weathering and transport play in the regional distribution and mobility of geogenic Cr and Ni, which are both potentially toxic and carcinogenic. These elements are enriched in ultramafic rocks (primarily serpentinite) and the soils derived from them (1700–10,000 mg Cr per kg soil and 1300–3900 mg Ni per kg soil) in the Coast Range ophiolite. Chromium and Ni have been transported eastward from the Coast Range into the western Sacramento Valley and as a result, valley soil is enriched in Cr (80–1420 mg kg−1) and Ni (65–224 mg kg−1) compared to median values of U.S. soils of 50 and 15 mg kg−1, respectively. Nickel in ultramafic source rocks and soils is present in serpentine minerals (lizardite, antigorite, and chrysotile) and is more easily weathered compared to Cr, which primarily resides in highly refractory chromite ([Mg,Fe2+][Cr3+,Al,Fe3+]2O4). Although the majority of Cr and Ni in soils are in refractory chromite and serpentine minerals, the etching and dissolution of these minerals, presence of Cr- and Ni-enriched clay minerals and development of nanocrystalline Fe (hydr)oxides is evidence that a significant fractions of these elements have been transferred to potentially more labile phases.

An elevated level of arsenic (As) in the Indo-Gangetic delta plain aquifers has been reported since the 1990s. Organic matter (OM) present in groundwater and aquifer sediments supports the microbial communities in these aquifers. During installation of a drinking water well, 26 sediment intervals of 6 m each were retrieved up till 156 m from a Pleistocene brown sand aquifer (BSA). Grain size distribution, sequential extraction of metals and total extractable lipids were analyzed in each sample. These parameters were statistically correlated in order to establish relationship between the physical vs. inorganic and organic characteristics, and how these properties affected the distribution of As in BSAs. The aquifer sediments consisted of medium to coarse sand except the surface sediments and those at the bottom of the well, which had high clay and slit content. Arsenic (As) concentration in sediments ranged from 2 to 21 mg/kg and indicated a strong correlation with grain size. Arsenic was mostly associated with crystalline oxides and silicate-rich minerals. Arsenic showed significant correlation with Fe in all fractions, and suggests presence of pyrite bound As-bearing minerals in these sediments. The diagnostic sedimentary lipid biomarkers indicated presence of compounds derived from vascular plants and microbial cell wall. This inference was supported by various diagnostic lipid ratios. The biomarkers were abundant in surface and deeper layers, which had high clay and silt content. The BSA sediments indicated preferential preservation of n-alkanes over other functional compounds, which were more reactive and subject to degradation. The thick clay layer at 132–156 m contained visible plant fragments, and OM in this layer indicated preferential preservation of organic carbon most likely due to the absence of specific microbial communities that degraded these compounds and mobilized As. Statistical analyses indicated the influence of selective inorganic and organic components (As, Fe and fatty acids) controlling the co-distribution of various inorganic and organic components in the aquifer.

An established engineering model is used to identify conditions where diffusion controls the dissolution of quartz and forsterite in packed beds. The model shows that diffusion control is favored at low advection flux, large grain size, high temperature, and high pH (if the reaction consumes H+). Quartz dissolution is chemical reaction controlled for most geochemically reasonable combinations of temperature, grain size, and flow rate. On the other hand, forsterite dissolution rates can be diffusion controlled for typical advection fluxes, grain sizes, temperatures, and pH’s. The apparent activation energy for diffusion-controlled reactions in a packed bed is much higher than the <∼20 kJ/mol value that is often used to identify diffusion controlled reactions. The models are quite general and can be adapted to deal with other mineral dissolution and precipitation reactions.

The dissolution of olivine added to soil: Implications for enhanced weathering by P. Renforth; P.A.E. Pogge von Strandmann; G.M. Henderson (109-118).
Chemical weathering of silicate minerals consumes atmospheric CO2 and is a fundamental component of geochemical cycles and of the climate system on long timescales. Artificial acceleration of such weathering (“enhanced weathering”) has recently been proposed as a method of mitigating anthropogenic climate change, by adding fine-grained silicate materials to continental surfaces. The efficacy of such intervention in the carbon cycle strongly depends on the mineral dissolution rates that occur, but these rates remain uncertain. Dissolution rates determined from catchment scale investigations are generally several orders of magnitude slower than those predicted from kinetic information derived from laboratory studies. Here we present results from laboratory flow-through dissolution experiments which seek to bridge this observational discrepancy by using columns of soil returned to the laboratory from a field site. We constrain the dissolution rate of olivine added to the top of one of these columns, while maintaining much of the complexity inherent in the soil environment. Continual addition of water to the top of the soil columns, and analysis of elemental composition of waters exiting at the base was conducted for a period of five months, and the solid and leachable composition of the soils was also assessed before and after the experiments. Chemical results indicate clear release of Mg2+ from the dissolution of olivine and, by comparison with a control case, allow the rate of olivine dissolution to be estimated between 10−16.4 and 10−15.5 moles(Mg) cm−2  s−1. Measurements also allow secondary mineral formation in the soil to be assessed, and suggest that no significant secondary uptake of Mg2+ has occurred. The olivine dissolution rates are intermediate between those of pure laboratory and field studies and provide a useful constraint on weathering processes in natural environments, such as during soil profile deepening or the addition of mineral dust or volcanic ash to soils surfaces. The dissolution rates also provide critical information for the assessment of enhanced weathering including the expected surface-area and energy requirements.

Reaction and diffusion at the reservoir/shale interface during CO2 storage: Impact of geochemical kinetics by Victor N. Balashov; George D. Guthrie; Christina L. Lopano; J. Alexandra Hakala; Susan L. Brantley (119-131).
We use a reactive diffusion model to investigate what happens to CO2 injected into a subsurface sandstone reservoir capped by a chlorite- and illite-containing shale seal. The calculations simulate reaction and transport of supercritical (SC) CO2 at 348.15 K and 30 MPa up to 20,000 a. Given the low shale porosity (5%), chemical reactions mostly occurred in the sandstone for the first 2000 a with some precipitation at the ss/sh interface. From 2000 to 4000 a, ankerite, dolomite and illite began replacing Mg–Fe chlorite at the sandstone/shale interface. Transformation of chlorite to ankerite is the dominant reaction occluding the shale porosity in most simulations: from 4000 to 7500 a, this carbonation seals the reservoir and terminates reaction. Overall, the carbonates (calcite, ankerite, dolomite), chlorite and goethite all remain close to local chemical equilibrium with brine. Quartz is almost inert from the point of its dissolution/precipitation. However, the rate of quartz reaction controls the long-term decline in aqueous silica activity and its evolution toward equilibrium. The reactions of feldspars and clays depend strongly on their reaction rate constants (microcline is closer to local equilibrium than albite). The timing of porosity occlusion mostly therefore depends on the kinetic constants of kaolinite and illite. For example, an increase in the kaolinite kinetic constant by 0.25 logarithmic units hastened porosity closure by 4300 a. The earliest simulated closure of porosity occurred at approximately 108 a for simulations designed as sensitivity tests for the rate constants.These simulations also emphasize that the rate of CO2 immobilization as aqueous bicarbonate (solubility trapping) or as carbonate minerals (mineral trapping) in sandstone reservoirs depends upon reaction kinetics – but the relative fraction of each trapped CO2 species only depends upon the initial chemical composition of the host sandstone. For example, at the point of porosity occlusion the fraction of bicarbonate remaining in solution depends upon the initial Na and K content in the host rock but the fraction of carbonate mineralization depends only on the Ca, Mg, Fe content. Since ankerite is the dominant mineral that occludes porosity, the dissolved concentration of ferrous iron is also an important parameter. Future efforts should focus on cross-comparisons and ground-truthing of simulations made for standard case studies as well as laboratory measurements of the reactivities of clay minerals.

The geochemistry of saline spring waters in the Athabasca oil sands region (AOSR) in Alberta (Canada) discharging from Devonian carbonate rocks into the Athabasca and Clearwater rivers was characterized for major ions, trace elements, dissolved gases, and polycyclic aromatic hydrocarbons (PAHs). In addition, stable isotope analyses of H2O, SO4, dissolved inorganic carbon (DIC), Sr, and CH4 were used to trace the sources of spring waters and their dissolved solutes, and to identify subsurface processes affecting water chemistry. The spring waters had δ18O values as low as −23.5‰, suggesting they are composed of up to 75% Laurentide glacial meltwater. Tritium and radiocarbon age-dating results, analyzed for three spring waters, supported a glacial origin. The high salinity of the spring waters (TDS 7210–51,800 mg/L) was due to dissolution of Devonian evaporite and carbonate deposits in the subsurface. Spring waters were affected by bacterial (dissimilatory) sulfate reduction, methanogenesis, and methane oxidation. Trace elements were present in spring waters at varying concentrations, with only one spring containing several predominant oil sands metals (As, Fe, Mo, Ni, Se, Zn) suggesting bitumen as a source. Five springs contained elements (Al, As, B, Fe, Se) at concentrations exceeding water quality guidelines for the protection of aquatic life. Seven PAHs were detected in spring waters (total PAH concentrations ranged from 7.3 to 273.6 ng/L), but most springs contained a maximum of two PAHs (phenanthrene and naphthalene), with more PAHs being detected in springs along the Athabasca River. This geochemical characterization of the saline groundwater discharging from the Devonian carbonates underlying oil sands deposits contributes to the knowledge of baseline groundwater chemistry in the AOSR, which is of importance in the delineation of natural versus anthropogenic effects on regional surface water and groundwater quality.

Hydrochemical processes in a shallow coal seam gas aquifer and its overlying stream–alluvial system: implications for recharge and inter-aquifer connectivity by Clément Duvert; Matthias Raiber; Daniel D.R. Owen; Dioni I. Cendón; Christelle Batiot-Guilhe; Malcolm E. Cox (146-159).
In areas of potential coal seam gas (CSG) development, understanding interactions between coal-bearing strata and adjacent aquifers and streams is of highest importance, particularly where CSG formations occur at shallow depth. This study tests a combination of hydrochemical and isotopic tracers to investigate the transient nature of hydrochemical processes, inter-aquifer mixing and recharge in a catchment where the coal-bearing aquifer is in direct contact with the alluvial aquifer and surface drainage network. A strong connection was observed between the main stream and underlying alluvium, marked by a similar evolution from fresh Ca–Mg–HCO3 waters in the headwaters towards brackish Ca–Na–Cl composition near the outlet of the catchment, driven by evaporation and transpiration. In the coal-bearing aquifer, by contrast, considerable site-to-site variations were observed, although waters generally had a Na–HCO3–Cl facies and high residual alkalinity values. Increased salinity was controlled by several coexisting processes, including transpiration by plants, mineral weathering and possibly degradation of coal organic matter. Longer residence times and relatively enriched carbon isotopic signatures of the downstream alluvial waters were suggestive of potential interactions with the shallow coal-bearing aquifer. The examination of temporal variations in deuterium excess enabled detection of rapid recharge of the coal-bearing aquifer through highly fractured igneous rocks, particularly at the catchment margins. Most waters collected from the coal-bearing aquifer also showed an enhanced influence of weathering during the wet season, which was likely triggered by the water–rock interaction with fresh recharge waters. An increase in both residual alkalinity and carbon isotopic ratios at two locations indicated inter-aquifer mixing between alluvium and bedrock during the wet season. The results of this study emphasise the need for conducting baseline hydrochemical surveys prior to CSG development in order to describe the transient nature of recharge and inter-aquifer mixing processes.

Influence of freeze–thaw dynamics on internal geochemical evolution of low sulfide waste rock by Sean A. Sinclair; Nam Pham; Richard T. Amos; David C. Sego; Leslie Smith; David W. Blowes (160-174).
Continuous monitoring of a 15 m high heavily instrumented experimental waste rock pile (0.053 wt.% S) since 2006 at the Diavik diamond mine in northern Canada provided a unique opportunity to study the evolution of fresh run-of-mine waste rock as it evolved over annual freeze–thaw cycles. Samples were collected from soil water solution samplers to measure pore water properties, from twelve 4 to 16 m2 basal collection lysimeters to measure basal leachate properties in the region underlying the crest of the pile (the core), and from basal drains to measure aggregate total pile leachate properties. By 2012, monitoring of pore water geochemistry within the core structure of the test pile revealed an apparent steady state with respect to weathering geochemistry, represented by (i) a flush of pre-existing blasting residuals and applied tracers, (ii) declining pH, (iii) a stepwise progression and subsequent equilibrium with acid-neutralizing phases (depletion of available carbonates; equilibrium with respect to aluminum hydroxide phases and subsequent iron (III) hydroxide phases), and (iv) concordant release of SO4, major cations (Ca, Mg, K, Na, Si), and trace metals (Al, Fe, Ni, Co, Cu, Zn). Distinct, high concentration ‘spring flushes’, characteristic of drainage in northern environments and primarily explained by a combination of fluid residence time and the build-up of oxidation products over the winter, were released from core drainage each season. Following the initial flush, the concentration of all dissolved constituents steadily declined, with distinct minimums prior to freeze-up. The opposite trend was observed in the cumulative pile drainage, in which early season leachate dominated by snowmelt and batter flow had low concentrations and late season leachate dominated by contributions from the core of the pile (indicated by season end merging of core and cumulative drainage geochemistry) had higher concentrations. Northern waste rock pile drainage geochemistry is strongly influenced by freeze–thaw cycling and varying core and batter subsystem contributions to total drainage. A comprehensive understanding of thermal cycling in waste rock piles is an important component of temporal predictions of drainage water composition based on up-scaling or reactive transport modeling.

Measuring the effective diffusion coefficient of dissolved hydrogen in saturated Boom Clay by E. Jacops; K. Wouters; G. Volckaert; H. Moors; N. Maes; C. Bruggeman; R. Swennen; R. Littke (175-184).
Boom Clay is studied as a potential host formation for the disposal of high-and intermediate level long-lived radioactive waste in Belgium. In such a geological repository, generation of gases (mainly H2 from anaerobic corrosion) will be unavoidable. In order to make a good evaluation of the balance between gas generation vs. gas dissipation for a particular waste form and/or disposal concept, good estimates for gas diffusion coefficients of dissolved gases are essential. In order to obtain an accurate diffusion coefficient for dissolved hydrogen in saturated Boom Clay, diffusion experiments were performed with a recently developed through-diffusion set-up for dissolved gases. Due to microbial activity in the test set-up, conversion of hydrogen into methane was observed within several experiments. A complex sterilisation procedure was therefore developed in order to eliminate microbiological disturbances. Only by a combination of heat sterilisation, gamma irradiation and the use of a microbial inhibitor, reliable, reproducible and accurate H2(g) diffusion coefficients (measured at 21 °C) for samples oriented parallel (D eff  = 7.25 × 10−10  m2/s and D eff  = 5.51 × 10−10  m2/s) and perpendicular (D eff  = 2.64 × 10−10  m2/s) to the bedding plane were obtained.

Reactivity of Fe from a natural stream water towards As(V) by Anneli Sundman; Torbjörn Karlsson; Per Persson (185-191).
Display OmittedInteractions between iron (Fe) and arsenic (As) play a vital role in aquatic and terrestrial ecosystems influencing the reactivity and transport of arsenic. A key aspect is the effect of natural organic matter (NOM) on these interactions, and previous investigations have reported the existence of ternary As–Fe–NOM species. In this study, the reactivity of Fe, from a boreal stream water, towards As(V) was investigated using Fe and As K-edge X-ray absorption spectroscopy (XAS). The native stream water was shown to contain mononuclear Fe–NOM complexes together with Fe(III) (hydr)oxides associated with the NOM. Addition of As(V) to this water at Fe to As ratios of 2.0–15.6 resulted in substantial changes in the Fe speciation; the Fe(III) (hydr)oxides were partly converted into FeAsO4(s) or a solid solution where As(V) was incorporated into Fe(III) (hydr)oxide structures. Under the same conditions no or only small effects of As(V) on the Fe–NOM complexes initially present were observed, and the concurrent existence of these complexes and free As(V) showed that a large fraction of the Fe–NOM complexes were non-reactive towards As(V). This study suggests that complexation of Fe by NOM in organic rich environments may lead to elevated free, aqueous arsenic levels as these complexes do not interact with As(V). Moreover, the formation of Fe–NOM complexes also reduce the tendency of Fe to form reactive Fe(III) (hydr)oxides particles and Fe(III)–arsenate precipitates.

A highly efficient, low-density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in Central Norway. Soil O and C horizon samples (N  = 752) were collected in Nord-Trøndelag and parts of Sør-Trøndelag, and analysed for 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr) and Pb isotopes in an aqua regia extraction. At the sample density of one site/36 km2 the four metal deposits, which have been mined in the area within the last 50 years were all detected as geochemical anomalies. In addition, a number of new anomalies that may warrant follow-up surveys were found. In terms of geology the Grong–Olden Culmination is marked by a distinct 206Pb/207Pb isotope anomaly. Geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area. Though the Fosdalen iron ore deposit is only marked in the soil O horizon, the C horizon outlines the more prominent anomalies of possibly economic interest. Climatic factors like the input of marine aerosols along the coast are clearly visible in the soil O horizon. Low-density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in Nord-Trøndelag county.

Origin and in situ concentrations of hydrocarbons in the Kumano forearc basin from drilling mud gas monitoring during IODP NanTroSEIZE Exp. 319 by Thomas Wiersberg; Anja M. Schleicher; Keika Horiguchi; Mai-Linh Doan; Nobuhisa Eguchi; Jörg Erzinger (206-216).
NanTroSEIZE Exp. 319 of the Integrated Ocean Drilling Program (IODP) was the first cruise in the history of scientific ocean drilling with drilling mud circulation through a riser. Drilling mud was pumped through the drill string and returned to the drill ship through the riser pipe during drilling of hole C0009A from 703 to 1604 mbsf (meter below sea floor) and hole enlargement from 703 to 1569 mbsf. During riser drilling, gas from returning drilling mud was continuously extracted, sampled and analyzed in real time to reveal information on the gas composition and gas concentrations at depth.Hydrocarbons were the only formation-derived gases identified in drilling mud and reached up to 14 vol.% of methane and 48 ppmv of ethane. The chemical and isotopic compositions of hydrocarbons exhibit a microbial origin. Hydrocarbons released from drilling mud and cuttings correlate with visible allochthonous material (wood, lignite) in drilling cuttings. At greater depth, addition of small but increasing amounts of hydrocarbons probably from low-temperature thermal degradation of organic matter is indicated. The methane content is also tightly correlated with several intervals of low Poisson’s ratio from Vp/Vs observed in sonic velocity logs, suggesting that the gas is situated in the pore space of the rock as free gas. The gas concentrations in the formation, determined from drilling mud gas monitoring, reaching up to 24 Lgas/Lsediment for methane in hole C0009A, in line with gas concentrations from interpreted downhole sonic logs.

In this study a new geochemical reaction modelling methodology is used to shed light on the geochemical processes within Witwatersrand tailings impoundments and the evaluation of the geochemical impacts for future mining projects is evaluated. Information from international and local studies on tailings sulphidic tailings impoundments is used to develop a conceptual understanding on a typical Witwatersrand gold tailings impoundment. The tailings impoundments consist of 3 distinct geochemical zones: Oxidation Zone (OZ), a Transition Zone (TZ) and a Reduction Zone (RZ). Individual reaction models are developed for each of the 3 zones. The output of one model is used as an input to the next in spatial order. The results of the final model represent the tailings basal seepage characteristics, which indicate the most likely impacts on groundwater resources. The model results agree with existing information on AMD in the Witwatersrand. The results indicate that the tailings basal seepage is likely to be acidic (pH of ∼3.5), containing elevated concentrations of SO4 and trace metals (Al, Co, Cr, Cu, Mn, Ni, Zn and U). Predicted Fe concentrations are low, due to the low f O 2 in the TZ and RZ and the buffering effect of the precipitation of K-jarosite in the OZ and TZ. This study therefore indicates that the methodology employed produces results that can be correlated to existing information and can thus be used as a methodology in the assessments of impacts from sulphidic tailings material for future mining projects.

Display OmittedArsenate and antimonate are water-soluble toxic mining waste species which often occur together and can be sequestered with varying success by a hydrous ferric oxide known as ferrihydrite. The competitive adsorption of arsenate and antimonate to thin films of 6-line ferrihydrite has been investigated using primarily adsorption/desorption kinetics monitored by in situ attenuated total reflectance infrared (ATR-IR) spectroscopy on flowed solutions containing 10−3 and 10−5  mol L−1 of both species at pH 3, 5, and 7. ICP-MS analysis of arsenate and antimonate adsorbed to 6-line ferrihydrite from 10−3  mol L−1 mixtures in batch adsorption experiments at pH 3 and 7 was carried out to calibrate the relative surface concentrations giving rise to the IR spectral absorptions. The kinetic data from 10−3 and 10−5  mol L−1 mixtures showed that at pH 3 antimonate achieved a greater surface concentration than arsenate after 60 min adsorption on 6-line ferrihydrite. However, at pH 7, the adsorbed arsenate surface concentration remained relatively high while that of adsorbed antimonate was much reduced compared with pH 3 conditions. Both species desorbed slowly into pH 3 solution while at pH 7 most adsorbed arsenate showed little desorption and adsorbed antimonate concentration was too low to register its desorption behaviour. The nature of arsenate which is almost irreversibly adsorbed to 6-line ferrihydrite remains to be clarified.

Thermodynamic modelling of alkali-activated slag cements by Rupert J. Myers; Barbara Lothenbach; Susan A. Bernal; John L. Provis (233-247).
Display OmittedThis paper presents a thermodynamic modelling analysis of alkali-activated slag-based cements, which are high performance and potentially low-CO2 binders relative to Portland cement. The thermodynamic database used here contains a calcium (alkali) aluminosilicate hydrate ideal solid solution model (CNASH_ss), alkali carbonate and zeolite phases, and an ideal solid solution model for a hydrotalcite-like Mg–Al layered double hydroxide phase. Simulated phase diagrams for NaOH- and Na2SiO3-activated slag-based cements demonstrate the high stability of zeolites and other solid phases in these materials. Thermodynamic modelling provides a good description of the chemical compositions and types of phases formed in Na2SiO3-activated slag cements over the most relevant bulk chemical composition range for these cements, and the simulated volumetric properties of the cement paste are consistent with previously measured and estimated values. Experimentally determined and simulated solid phase assemblages for Na2CO3-activated slag cements were also found to be in good agreement. These results can be used to design the chemistry of alkali-activated slag-based cements, to further promote the uptake of this technology and valorisation of metallurgical slags.

Tracing sources of ammonium in reducing groundwater in a well field in Hanoi (Vietnam) by means of stable nitrogen isotope (δ15N) values by Jenny Norrman; Charlotte J. Sparrenbom; Michael Berg; Duc Nhan Dang; Gunnar Jacks; Peter Harms-Ringdahl; Quy Nhan Pham; Håkan Rosqvist (248-258).
Display OmittedIn the Southern part of Hanoi, high ammonium (NH4 +) concentrations in reducing groundwater have been an issue over the last 25 years. Elevated NH4 + concentrations in groundwater, in general, are an indicator of influences from anthropogenic sources, but the buried peat layers in the Red River delta formation are also hypothesized to contribute to the high NH4 + levels (up to 100 mg/l). We traced the sources of NH4 + at the Nam Du well field of the Hanoi water works by means of isotope ratios (15N/14N). The δ15N values were determined for total sedimentary N and exchangeable NH4 + of the peat material, and for NH4 + dissolved in deep and shallow groundwater, sewage, and surface water. Groundwater NH4 + of the upper (Holocene) and the lower (Pleistocene) aquifers had higher δ15N values than did total N and NH4 + of the sediments, and were somewhat higher than the δ15N values of NH4 + in sewage and surface water. We conclude that the present conditions of temperature and pH tend to promote deprotonation of NH4 + to ammonia (NH3), which eventually degasses from the groundwater table to the unsaturated pore space. This can cause an enrichment of 15N in the remaining NH4 +, as the lighter 14N in NH3 is volatilized at a slightly faster rate. The intermediate δ15N values within the Pleistocene aquifer can be explained by the recharge thereto, which is a mixture of the high δ15N values of the Holocene aquifer and the low δ15N values of water infiltrating from the Red River into the Pleistocene aquifer. Some part of the increased groundwater NH4 + is likely to arise from anthropogenic activities, as supported by several indications: a large drawdown in the Pleistocene aquifer caused by Hanoi’s extensive water abstraction and subsequent downward gradient from the upper Holocene aquifer; the presence of coliforms in groundwater; and a positive correlation between ammonium and DOC, Cl, Br and Ni, but a lack of correlation with As. However, the much higher concentrations of NH4 + in the groundwater compared to the potential surface sources, the positive correlation between NH4 + and DOC, the abundance of natural organic matter (OM), the amount of exchangeable NH4 + in the sediments, and the highly reducing conditions in the aquifers indicate that N-mineralization of organic N from the peat contribute substantially to the high NH4 + levels in groundwater of the Nam Du well field.
Keywords: Ammonium; δ15N; Reducing conditions; Arsenic; Groundwater; Red River delta;

Serpentinite rock dissolution experiments at 25 °C and 2 < pH < 9 were used to retrieve antigorite dissolution rates, as its high abundance and comparatively fast dissolution dominate element release from the rock. Retrieved antigorite dissolution rates at pH 2.02 and 3.06 are similar to previous literature data for serpentine minerals at corresponding pH. In contrast, retrieved rates at higher pH are considerably lower than corresponding literature data. Nonetheless, the retrieved rates at pH 5.76 and 7.28 are consistent with the linear log rate – pH relationship of Orlando et al. (2011), suggesting that rates follow a single ‘acidic’ mechanism to a pH of at least 7.3. The relatively low dissolution rates of serpentine minerals found in this study suggests that significantly more time is required for serpentine weathering than that estimated using the dissolution rates of these solids reported by Marini (2006) based on a review of literature data. As such, using enhanced serpentine weathering as a carbon capture/storage strategy may be less efficient than previously assumed.

Kinetics of CO2–fluid–rock reactions in a basalt aquifer, Soda Springs, Idaho by Alexandra Maskell; Niko Kampman; Hazel Chapman; Daniel J. Condon; Mike Bickle (272-283).
The dissolution of silicate minerals by CO2-rich fluids and the subsequent precipitation of CO2 as carbonate minerals represent a means of permanently storing anthropogenic CO2 waste products in a solid and secure form. Modelling the progression of these reactions is hindered by our poor understanding of the rates of mineral dissolution–precipitation reactions and mineral surface properties in natural systems. This study evaluates the chemical evolution of groundwater flowing through a basalt aquifer, which forms part of the leaking CO2-charged system of the Blackfoot Volcanic Field in south-eastern Idaho, USA. Reaction progress is modelled using changes in groundwater chemistry by inverse mass balance techniques. The CO2-promoted fluid–mineral reactions include the dissolution of primary plagioclase, orthoclase, pyroxene and gypsum which is balanced by the precipitation of secondary albite, calcite, zeolite, kaolinite and silica. Mineral mole transfers and groundwater flow rates estimated from hydraulic head data are used to determine the kinetics of plagioclase and orthoclase feldspar dissolution. Plagioclase surface area measurements were determined using the evolution of the U-series isotope ratios in the groundwater and are compared to published surface area measurements. Calculated rates of dissolution for plagioclase range from 2.4 × 10−12 to 4.6 × 10−16  mol/m2/s and orthoclase from 2.0 × 10−13 to 6.8 × 10−16  mol/m2/s respectively. These feldspar reaction rates, correlate with the degree of mineral–fluid disequilibrium and are similar to the dissolution rates for these mineral measured in other natural CO2-charged groundwater systems.
Keywords: Carbon sequestration; CO2–water–rock interaction; Feldspar dissolution; Gibbs free energy; Blackfoot Volcanic Field; Soda Springs;

Mercury (Hg) is a pollutant of global concern due to its ability to accumulate as methylmercury (MeHg) in biota. Mercury is methylated by anaerobic microorganisms such as sulfate reducing bacteria (SRB) in water and sediment. Throughout North America, reservoirs tend to have elevated methylmercury (MeHg) concentrations compared to natural lakes and rivers. This impact is most pronounced in newly created reservoirs where methylation is fueled by the decomposition of flooded organic material, which can release Hg and enhance microbial activity. Much less is known about the longer-term water-level management impacts on Hg cycling in older reservoirs. The objective of our study was to understand the role of on-going water-level fluctuations on sediment MeHg concentrations and sulfur speciation within a reservoir 75 years after initial impoundment. The study was performed at the Cottage Grove Reservoir located 15 km downstream of the historical Black Butte Hg mine. For 8 months each year, the water level is lowered resulting in roughly half of the reservoir’s sediment being exposed to the atmosphere. Water samples from the inflow, water-column, outflow, and sediment were collected seasonally over a year for total-Hg, MeHg, and several ancillary parameters. The results showed that conditions in the reservoir were favorable to methylation with a much higher %MeHg observed in the outflowing water (34%) compared to the inflow (7%) during the late-summer. An anoxic hypolimnion did not develop in the reservoir indicating that methylation was predominantly occurring in the sediments. In the sediments subjected to seasonal inundation, MeHg production was highest in the top 2 cm of the sediments and declined with depth. The seasonally inundated sediments also had significantly higher methylation activity than the permanently inundated area of the reservoir. Oxidizing conditions in the sediments during periods of exposure to air resulted in an increase in sulfate concentrations which likely stimulated SRB methylation following the raising of the water levels. In contrast, the sulfur in the permanently inundated sediments was all in a reduced form (sulfide) and sulfate remained below detection throughout the year. Overall, our results indicate that reservoir water level fluctuations can affect sediment redox conditions and enhance MeHg production. This process can result in a continued elevation of MeHg concentrations in older reservoirs after the initial impact of landscape flooding has subsided.
Keywords: Mercury mine; Mercury methylation; Reservoir; Water-level; Sediment;

Thermodynamic properties of zýkaite, a ferric sulfoarsenate by Juraj Majzlan; Felix Y. Amoako; Helena Kindlová; Petr Drahota (294-301).
Zýkaite, nominally Fe4(AsO4)3(SO4)(OH)⋅15H2O, is a common ferric arsenate in damp, moist environments, especially in underground spaces where the humidity does not decline during the year and the temperature fluctuations are minimal. The natural sample used in this study had chemical composition Fe1.23(AsO4)0.93(PO4)0.07(SO4)0.31(OH)0.07⋅5.89H2O and typical poor crystallinity. The solubility product, derived from solubility experiments, is log Ksp= −25.77 for the reaction Fe1.23(AsO4)0.93(PO4)0.07(SO4)0.31(OH)0.07⋅5.89H2O + 0.07 H+  = 1.23Fe3+  + 0.93AsO4 3−  + 0.07PO4 3−  + 0.31SO4 2−  + 5.96H2O. Enthalpy of formation, determined by acid-solution calorimetry, is −2952.2 ± 2.9 kJ mol−1. Gibbs free energy of formation, calculated from the log Ksp value, is −2485.1 kJ mol−1. The entropies of zýkaite, estimated for a solid and calculated from its enthalpy and Gibbs free energy of formation, are in a fair agreement and document the accuracy of the thermodynamic data. Variable-temperature powder X-ray diffraction showed that the structure of zýkaite gradually collapses and turns amorphous at ∼90 °C. No crystalline phase formed upon heating to 200 °C. Zýkaite is one of the least stable phases in the system Fe2O3–As2O5–SO3–H2O, restricted to environments with low pH and very high activities of As(V) and S(VI). Zýkaite and kaňkite (FeAsO4⋅3.5H2O) are often seen on the gently sloping rock faces or on the rock walls, whereas stalactites of hydrous ferric arsenate forms below them on the overhangs. Thermodynamic modeling with our data show that the transformation of zýkaite to other ferric arsenates and sulfoarsenates (scorodite, parascorodite, kaňkite, bukovskýite, hydrous ferric arsenates) is thermodynamically driven. Comparing and combining our thermodynamic data, modeling, variable-temperature X-ray diffraction, and field observations, it seems that zýkaite transforms readily to other phases in the system Fe2O3–As2O5–SO3–H2O via dissolution and precipitation; a solid-state transformation is unlikely.

We report the modelling and experimental studies of SON68 glass dissolution in hyperalkaline solution (pH = 13.7) at 30 and 70 °C for more than 300 days. In the experiments at 30 °C and the first 100 days at 70 °C, the concentrations of B, Mo, alkalis and even Si showed square root of time dependence indicative of a diffusion-controlled process. At 70 °C fast dissolution resumed after about 100 days due to crystallization of aluminosilicates, and glass dissolution became surface reaction-controlled. For the experiment at 30 °C, a simple model coupling dissolution of the glass and precipitation of secondary phases can also produce a good result for non-conservative elements, such as Ca and Al.
Keywords: Hyperalkaline water; SON68 glass; Transport-controlled dissolution; Dissolution and precipitation; Coupled modelling;

Complex formation of Cm(III) with formate studied by time-resolved laser fluorescence spectroscopy by Daniel R. Fröhlich; Andrej Skerencak-Frech; Petra J. Panak (312-317).
Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log  K′(T)) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log  K 0 n (T) values for the formation of [Cm(Form) n ](3− n )+ (n  = 1, 2) and the ion interaction coefficients (ε(i,k)) for [Cm(Form) n ](3− n )+ (n  = 1, 2) with Cl are obtained. The log  K 0 1(T) (2.11 (20 °C)–2.49 (90 °C)) and log  K 0 2(T) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log  K 0 n (T) values are used to derive the standard reaction enthalpies and entropies (ΔrH 0 m , ΔrS 0 m ) of the respective complexation reactions according to the Van’t Hoff equation. In all cases, positive ΔrH 0 m and ΔrS 0 m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.
Keywords: Curium; Formate; Complexation; Thermodynamics; Saline solution; Elevated temperature;