Applied Geochemistry (v.22, #12)
Biogeochemical Gradients: Microbes, Measurements, and Modeling by Danielle Fortin; Thomas Pichler; Chad Saltikov (2567-2568).
Biogeochemical reactive–diffusive transport of heavy metals in Lake Coeur d’Alene sediments by S. Sevinç Şengör; Nicolas F. Spycher; Timothy R. Ginn; Rajesh K. Sani; Brent Peyton (2569-2594).
Decades of runoff from precious-metal mining operations in the Lake Coeur d’Alene Basin, Idaho, have left the sediments in this lake heavily enriched with toxic metals, most notably Zn, Pb and Cu, together with As. The bioavailability, fate and transport of these metals in the sediments are governed by complex biogeochemical processes. In particular, indigenous microbes are capable of catalyzing reactions that detoxify their environments, and thus constitute an important driving component in the biogeochemical cycling of these metals. Here, the development of a quantitative model to evaluate the transport and fate of Zn, Pb and Cu in Lake Coeur d’Alene sediments is reported. The current focus is on the investigation and understanding of local-scale processes, rather than the larger-scale dynamics of sedimentation and diagenesis, with particular emphasis on metal transport through reductive dissolution of Fe hydroxides. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network including consortium biodegradation kinetics with multiple terminal electron acceptors and syntrophic consortium biotransformation dynamics of redox front. The model captures the mobilization of metals initially sorbed onto hydrous ferric oxides, through bacterial reduction of Fe(III) near the top of the sediment column, coupled with the precipitation of metal sulfides at depth due to biogenic sulfide production. Key chemical reactions involve the dissolution of ferrihydrite and precipitation of siderite and Fe sulfide. The relative rates of these reactions play an important role in the evolution of the sediment pore-water chemistry, notably pH, and directly depend on the relative activity of Fe and SO4 reducers. The model captures fairly well the observed trends of increased alkalinity, sulfide, Fe and heavy metal concentrations below the sediment–water interface, together with decreasing terminal electron acceptor concentrations with depth, including the development of anoxic conditions within about a centimeter below the lake bottom. This effort provides insights on important biogeochemical processes affecting the cycling of metals in Lake Coeur d’Alene and similar metal-impacted lacustrine environments.
Enhanced geochemical gradients in a marine shallow-water hydrothermal system: Unusual arsenic speciation in horizontal and vertical pore water profiles by Roy E. Price; Jan P. Amend; Thomas Pichler (2595-2605).
The shallow marine hydrothermal vents near Ambitle Island in eastern Papua New Guinea discharge hot, slightly acidic, As-rich, chemically reduced fluid into cool, slightly alkaline, oxygenated seawater. Gradients in temperature, pH, and total As (AsT), among others, are established as the two aqueous phases mix. The hydrothermal fluid contained ∼900 μg/L AsT, almost exclusively present as the reduced AsIII, while local seawater measured between 1.2 and 2.4 μg/L As, with approximately equal levels of AsIII and AsV. Of particular interest in this study was As speciation and abundance in pore waters as a function of sediment depth and as a function of distance from the area of focused venting. With increasing distance, AsT concentration in the pore water decreased rapidly, but remained elevated up to 300 m from the area of focused venting when compared to a non-hydrothermal control site. As a function of depth (to ∼100 cm) AsT concentration in the pore water profiles was elevated and generally increased with depth. Surprisingly, aqueous AsV far exceeded aqueous AsIII at almost all distances and depths investigated, while at the control site the AsIII concentration exceeded that of AsV. In the Tutum Bay hydrothermal system, chemical disequilibria among As species provide potential metabolic energy for arsenite oxidizing microorganisms where hydrothermal fluid mixes with seawater near the vent orifice, and for arsenate reducing microorganisms with increasing distance and depth from the hydrothermal point source.
Gradients controlling natural attenuation of ammonium by Uli Maier; Hermann Rügner; Peter Grathwohl (2606-2617).
Oxidation of reduced pollutants such as NH 4 + in groundwater often takes place at steep redox gradients where oxygenated water is being mixed into polluted water such as landfill leachate. In order to identify controlling parameters and quantify the influence of environmental factors for NH 4 + degradation, sensitivity analysis was performed by means of scenario specific numerical modelling. Geometrical factors such as aquifer thickness have been shown to be very influential on the capability of natural attenuation of pollutants in groundwater. The scenarios investigated here include biodegradation at redox gradients in groundwater, so called fringe processes, for (i) a partly contaminated aquifer with two reaction fronts, (ii) and a spatially variable aquifer thickness. In addition, (iii) the influence of groundwater recharge and (iv) restricted supply of O2 to contaminated water by slow dispersion and diffusion across the capillary fringe are investigated. Contaminated aquifer thickness, zones of enhanced mixing due to flow focussing and diffusion/dispersion coefficients in the capillary fringe are identified qualitatively as controlling factors for natural attenuation under complex conditions, whereas predictive functions will require further research.
Evaluation of sulfate reduction at experimentally induced mixing interfaces using small-scale push–pull tests in an aquifer–wetland system by Tara A. Kneeshaw; Jennifer T. McGuire; Erik W. Smith; Isabelle M. Cozzarelli (2618-2629).
This paper presents small-scale push–pull tests designed to evaluate the kinetic controls on SO 4 2 - reduction in situ at mixing interfaces between a wetland and aquifer impacted by landfill leachate at the Norman Landfill research site, Norman, OK. Quantifying the rates of redox reactions initiated at interfaces is of great interest because interfaces have been shown to be zones of increased biogeochemical transformations and thus may play an important role in natural attenuation. To mimic the aquifer–wetland interface and evaluate reaction rates, SO 4 2 - -rich anaerobic aquifer water ( ∼ 100 mg / L SO 4 2 - ) was introduced into SO 4 2 - -depleted wetland porewater via push–pull tests. Results showed SO 4 2 - reduction was stimulated by the mixing of these waters and first-order rate coefficients were comparable to those measured in other push–pull studies. However, rate data were complex involving either multiple first-order rate coefficients or a more complex rate order. In addition, a lag phase was observed prior to SO 4 2 - reduction that persisted until the mixing interface between test solution and native water was recovered, irrespective of temporal and spatial constraints. The lag phase was not eliminated by the addition of electron donor (acetate) to the injected test solution. Subsequent push–pull tests designed to elucidate the nature of the lag phase support the importance of the mixing interface in controlling terminal electron accepting processes. These data suggest redox reactions may occur rapidly at the mixing interface between injected and native waters but not in the injected bulk water mass. Under these circumstances, push–pull test data should be evaluated to ensure the apparent rate is actually a function of time and that complexities in rate data be considered.
A novel approach to estimate iron distribution within different pore domains of structured media by Wiwat Kamolpornwijit; Scott C. Brooks; Young-Jin Kim; Timothy D. Scheibe (2630-2636).
The Fe content of soils and aquifer solids is usually quantified using different extraction solutions performed with homogenized samples in a well-mixed batch experiment. For structured media where preferential flow prevails over the matrix flow, however, the Fe content determined from homogenized samples may not well represent the Fe available for biogeochemical reactions. In this study ammonium oxalate extraction was performed on a core of intact saprolite where physical structure was preserved. An unsaturated flow setup was modified with the intent of allowing the extraction under two pore tensions, 15 and 0 cm of water, although a malfunctioning vacuum regulator made this more difficult than anticipated. Approximately 85% of the oxalate-extractable Fe was contained within the finer pore domain (matrix potential larger than 15 cm). Less than 15.5% of the extracted Fe mass (an upper bound) was present in domains of pore tension less than15 cm. To the extent that Fe(III) oxides play an important role in contaminant biogeochemistry and solute transport, their distribution in structured subsurface media is critical to the understanding of these processes.
Relative contributions of sulfate- and iron(III) reduction to organic matter mineralization and process controls in contrasting habitats of the Georgia saltmarsh by Jung-Ho Hyun; April C. Smith; Joel E. Kostka (2637-2651).
The objectives of this study were to partition out the predominant anaerobic respiration pathways coupled to C oxidation and to further elucidate the controls of anaerobic C respiration in three major saltmarsh habitats at Skidaway Island, GA; the short form of Spartina alterniflora (SS), the tall form of S. alterniflora (TS), and unvegetated, bioturbated creekbank (CB). Geochemical analysis of pore water and solid phase constituents revealed that the SS site experienced highly reducing conditions with two orders of magnitude higher pore water sulfide inventories (1.884 mmol m−2) than TS (0.003 mmol m−2) and CB (0.005 mmol m−2), respectively. Conversely, reactive Fe(III) inventories at TS (2208 mmol m−2) and CB (2881 mmol m−2) were up to 7–9 times higher than at SS (338 mmol m−2). Incubations and intact core experiments indicated that SO 4 2 - reduction accounted for 95% (SS), 37% (TS) and 66% (CB) of total anaerobic respiration. There was no detectable Fe(III) reduction at SS, while Fe(III) reduction accounted for up to 70% of C oxidation in the 3–6 cm depth interval at TS and 0–3 cm depth of CB, and on average, approximately 55% of C oxidation over two-thirds of marsh surface area. Laboratory manipulations provided further evidence for the importance of Fe(III) reduction as the accumulation rates of fermentation products were high when Fe(III) reduction was inhibited by removing the Fe(III) minerals from highly bioturbated CB sediments with higher Fe(III) mineral contents. Anaerobic C oxidation, SO 4 2 - - and Fe(III)-reduction rates appeared to be highest at the TS site during active plant growth in summer. Overall results suggest that bioturbation by macrofauna is the overriding factor in modulating the pathway of C mineralization in the saltmarsh, whereas availability of organic substrates from plants is a key factor in controlling the C oxidation rate.
Indications for pedogenic formation of perylene in a terrestrial soil profile: Depth distribution and first results from stable carbon isotope ratios by Tilman Gocht; Johannes A.C. Barth; Michaela Epp; Maik Jochmann; Michaela Blessing; Torsten C. Schmidt; Peter Grathwohl (2652-2663).
In order to distinguish between pyrogenic and natural generation compound-specific 13C/12C ratios (δ 13C) were compared between perylene and other PAHs in samples from the top-soil and sub-soil. Despite successful clean-up of the extracts, low perylene concentrations and peak overlaps with benzo(e)pyrene and benzo(a)pyrene prevented determination of a unique δ 13C value for perylene in the upper horizon. However, the δ 13C value of perylene in the sub-soil was 5.7 permille more negative than other equal-mass PAHs (with m/z of 252) in the top-soil, which rather supports in situ generation of perylene in the sub-soil.
Centimeter-scale characterization of biogeochemical gradients at a wetland–aquifer interface using capillary electrophoresis by Susan Báez-Cazull; Jennifer T. McGuire; Isabelle M. Cozzarelli; Anne Raymond; Lisa Welsh (2664-2683).
Steep biogeochemical gradients were measured at mixing interfaces in a wetland–aquifer system impacted by landfill leachate in Norman, Oklahoma. The system lies within a reworked alluvial plain and is characterized by layered low hydraulic conductivity wetland sediments interbedded with sandy aquifer material. Using cm-scale passive diffusion samplers, “peepers”, water samples were collected in a depth profile to span interfaces between surface water and a sequence of deeper sedimentary layers. Geochemical indicators including electron acceptors, low-molecular-weight organic acids, base cations, and NH 4 + were analyzed by capillary electrophoresis (CE) and field techniques to maximize the small sample volumes available from the centimeter-scale peepers. Steep concentration gradients of biogeochemical indicators were observed at various interfaces including those created at sedimentary boundaries and boundaries created by heterogeneities in organic C and available electron acceptors. At the sediment–water interface, chemical profiles with depth suggest that SO 4 2 - and Fe reduction dominate driven by inputs of organic C from the wetland and availability of electron acceptors. Deeper in the sediments (not associated with a lithologic boundary), a steep gradient of organic acids (acetate maximum 8.8 mM) and NH 4 + (maximum 36 mM) is observed due to a localized source of organic matter coupled with the lack of electron acceptor inputs. These findings highlight the importance of quantifying the redox reactions occurring in small interface zones and assessing their role on biogeochemical cycling at the system scale.
Water fluxes and their control on the terrestrial carbon balance: Results from a stable isotope study on the Clyde Watershed (Scotland) by J.A.C. Barth; H. Freitag; H.J. Fowler; A.P. Smith; C. Ingle; A. Karim (2684-2694).
The gradients between precipitation and runoff quantities as well as their water isotopes were used to establish a water balance in the Clyde River Basin (Scotland). This study serves as an example for a European extreme with poorly vegetated land cover and high annual rainfall and presents novel water stable isotope techniques to separate evaporation, interception and transpiration with annual averages of 0.029 km3 a−1, 0.220 km3 a−1 and 0.489 km3 a−1, respectively. Transpiration was further used to determine CO2 uptake of the entire basin and yielded an annual net primary production (NPP) of 352 × 109 g C (Giga gram) or 185.2 g C m−2. Compared to other temperate areas in the world, the Clyde Basin has only half the expected NPP. This lower value likely results from the type of vegetation cover, which consists mostly of grasslands. Subtracting the annual heterotrophic soil respiration flux (R h) of 392 Gg (206.1 g C m−2 a−1) from the NPP yielded an annual Net Ecosystem Productivity (NEP) of −40 Gg C, thus showing the Clyde Watershed as a source of CO2 to the atmosphere. Despite the unusual character of the Clyde Watershed, the study shows that areas with predominant grass and scrub vegetation still have transpirational water losses that by far exceed those of pure evaporation and interception. This infers that vegetation can influence the continental water balances on time scales of years to decades.
Discharge of weathering products from acid sulfate soils after a rainfall event, Tweed River, eastern Australia by B.C.T. Macdonald; I. White; M.E. Åström; A.F. Keene; M.D. Melville; J.K. Reynolds (2695-2705).
The oxidation of the iron sulfide, pyrite, in acid sulfate soil floodplains generate substantial acidity and this acid has caused further weathering of the soil profile. The movement of groundwater from these soils is an important geochemical control on surface water quality. The flux of acidified and metal-rich water during a wet season rainfall event has been examined at two study catchments on the Tweed River in eastern Australia. At the sites, 81 kg/ha and 60 kg/ha of oxidisable acidity are exported, along with Al, Fe and Zn during the flood event. The main contributors to the acid flux are H+, Fe and Al at the first site and whilst Fe and Al are present in the drainage waters at the second site, the main contributor is likely to be H+. The different flux characteristics at the sites may be caused by different surface soil hydraulic conductivities and oxidation history.
Mercury distribution and transport in a contaminated river system in Kazakhstan and associated impacts on aquatic biota by Susanne M. Ullrich; Mikhail A. Ilyushchenko; Grigory A. Uskov; Trevor W. Tanton (2706-2734).
The River Nura in Central Kazakhstan has been heavily polluted by Hg originating from an acetaldehyde plant. A number of studies were undertaken to investigate the transport, fate and bioavailability of Hg in this river system. The sediments within a 20 km section of the river downstream of the effluent outfall canal are highly polluted and are acting as a strong source of surface water contamination. Mercury transport in the river is dominated by the remobilization of contaminated bed sediments and river bank erosion during the annual spring flood. Peak Hg concentrations in unfiltered surface water samples during a larger than usual flood event in 2004 were in the order of 1600–4300 ng L−1. The majority of the particulate-bound Hg appears to be sedimented in the shallow Intumak reservoir ∼75 km downstream of the source of the pollution, leading to a drop in aqueous Hg concentrations by an order of magnitude. Nevertheless, background concentrations of Hg in surface water are not reached until at least 200 km downstream, and during the flood period Hg is also detected in the terminal wetlands of the river.Mercury concentrations in sediment cores taken from the river bed in the most contaminated section of the Nura ranged from 9.95 to 306 mg kg−1. Methylmercury (MeHg) levels in shallow sediment cores were highest in surface sediments and ranged between 4.9 and 39 μg kg−1, but were generally less than 0.1% of total Hg (THg). A significant inverse relationship was found between THg concentrations and the percentage of MeHg formed in the sediments, irrespective of the sampling depth. The observed relationship was confirmed by comparison with results from a different river system, indicating that it may be true also for other highly contaminated aquatic systems. It is hypothesized that at high THg levels in severely contaminated sediments, the accumulation of MeHg may be limited by increasingly efficient demethylation processes, and that this underlying trend in sediments is the reason why MeHg levels in surface water are often found to be higher at less contaminated sites compared to upstream sites.Mercury concentrations in biota in the most contaminated section of the river were 15–20 times higher than background levels. Fish were found to be impacted for more than 125 km downstream from the source, indicating significant transport of dissolved MeHg to downstream areas and/or in-situ MeHg production in less contaminated downstream reaches. There were also indications that impoundments may increase the bioavailability of Hg.
Characterization of the reactivity of riverine heterogeneous sediments using a facies-based approach; the Rhine–Meuse delta (The Netherlands) by Pieter-Jan van Helvoort; Jasper Griffioen; Niels Hartog (2735-2757).
Large groundwater resources are found in densely populated lowland areas, which consist often of young unconsolidated and reduced sediments. When anthropogenic activities lead to oxygenation of the aquifer, breakdown of the main reduced fractions, i.e. sedimentary organic matter (SOM) and pyrite, could lead to severe groundwater deterioration such as acidification, heavy metal mobilization, and increased hardness. The characterization of the reactive properties of these sediments is important in predicting groundwater deterioration, but is often complicated by the high degree of heterogeneity of these sediments. In this study, the potential reduction capacity (PRC, based on SOM and pyrite content), the potential buffer capacity (PBC, based on carbonate content), potential acidification capacity (PAC, based on the potential acid production by sulfide oxidation), and the measured reduction capacity (MRC) of five facies, which are typical of the riverine sediments in the Rhine–Meuse delta (The Netherlands) were determined. A universal facies-classification model was used to classify the deposits into more homogeneous sub-units based on lithologic and geogenic properties, with a further sub-division into oxic or anoxic redox environment based upon groundwater data and field observations. The bulk chemical data show strong variation across facies for the median values of PRC (186–9093 mmol O2 kg−1), PBC (17–132 mmol O2 kg−1), and PAC (36–1530 mmol H+ kg−1). The MRC was measured as reactivity to molecular O2 exposure and was 0.5–567.3 mmol O2 kg−1. Steady-state oxidation rates were in the wide range of 0.001–10.355 mmol O2 kg−1 day−1 but were typically about 3–8 times faster in fine facies than in coarse facies. Both the PRC and MRC depend strongly on grain size, but also on the syn/post-depositional environment and redox conditions. The main part of the PRC consists of SOM, but pyrite reactivity is higher than SOM reactivity as shown by the relative depletion of pyrite in oxic subfacies and the preferential oxidation during the oxidation experiments. Some facies are very prone to acidification because the PAC is higher than the PBC, but the oxidation experiments also show that acidification could already start before the PRC is fully exhausted. This study, is one of the few that combines bulk chemical data, groundwater data, and reactivity measurements and shows that a facies-based approach is a practical tool in characterizing the reactivity of heterogeneous deposits.
Control of As and Ni releases from a uranium mill tailings neutralization circuit: Solution chemistry, mineralogy and geochemical modeling of laboratory study results by John Mahoney; Maynard Slaughter; Donald Langmuir; John Rowson (2758-2776).
Processing U ores in the JEB Mill of the McClean Lake Operation in northern Saskatchewan produces spent leaching solutions (raffinates) with pH ⩽ 1.5, and As and Ni concentrations up to 6800 and 5200 mg L−1, respectively. Bench-scale neutralization experiments (pH 2–8) were performed to help optimize the design of mill processes for reducing As and Ni concentrations in tailings and raffinates to ⩽1 mg L−1 prior to their disposal. Precipitate mineralogy determined by chemical analysis, XRD, SEM, EM, XM and EXAFS methods, included gypsum (the dominant precipitate), poorly crystalline scorodite (precipitated esp. from pH 2–4), annabergite, hydrobasaluminite, ferrihydrite, green rust II and theophrastite. The As was mostly in scorodite with smaller amounts in annabergite and trace As adsorbed and/or co-precipitated, probably by ferrihydrite. Geochemical modeling indicated that above pH 2, the ion activity product (IAP) of scorodite lies between the solubility products of amorphous and crystalline phases (log K sp = −23.0 and −25.83, respectively). The IAP decreases with increasing pH, suggesting that the crystallinity of the scorodite increases with pH. Forward geochemical models support the assumption that during neutralization, particles of added base produce sharp local pH gradients and disequilibrium with bulk solutions, facilitating annabergite and theophrastite precipitation.
Transport of trace elements under different seasonal conditions: Effects on the quality of river water in a Mediterranean area by Rosa Cidu; Riccardo Biddau (2777-2794).
Concentrations of total and dissolved elements were determined in 35 water samples collected from rivers in Sardinia, a Mediterranean island in Italy. The overall composition did not change for waters sampled in both winter and summer (i.e., January at high-flow condition and June at low-flow condition), but the salinity and concentrations of the major ions increased in summer. Concentrations of elements such as Li, B, Mn, Rb, Sr, Mo, Ba and U were higher in summer with only small differences between total and dissolved (i.e., in the fraction <0.4 μm) concentrations. The fact that these elements are mostly dissolved during low flow periods appears to be related to the intensity of water–rock interaction processes that are enhanced when the contribution of rainwater to the rivers is low, that is during low-flow conditions. In contrast, the concentrations of Al and Fe were higher in winter during high flow with total concentrations significantly higher than dissolved concentrations, indicating that the total amount depends on the amount of suspended matter. In waters filtered through 0.015 μm pore-size filters, the concentrations of Al and Fe were much lower than in waters filtered through 0.4 μm pore-size filters, indicating that the dissolved fraction comprises very fine particles or colloids. Also, Co, Ni, Cu, Zn, Cd and Pb were generally higher in waters collected during the high-flow condition, with much lower concentrations in 0.015 μm pore-size filtered waters; this suggests aqueous transport via adsorption onto very fine particles. The rare earth elements (REE) and Th dissolved in the river waters display a wide range in concentrations (∑REE: 0.1–23 μg/L; Th: <0.005–0.58 μg/L). Higher REE and Th concentrations occurred at high flow. The positive correlation between ∑REE and Fe suggests that the REE are associated with very fine particles (>0.015 and <0.4 μm); the abundance of these particles in the river controls the partitioning of REE between solution and solid phases.Twenty percent of the water samples had dissolved Pb and total Hg concentrations that exceeded the Italian guidelines for drinking water (>10 μg/L Pb and >1 μg/L Hg). The highest concentrations of these heavy metals were observed at high-flow conditions and they were likely due to the weathering of mine wastes and to uncontrolled urban wastes discharged into the rivers.
Feldspar dissolution rates measured using phase-shift interferometry: Implications to CO2 underground sequestration by M. Sorai; T. Ohsumi; M. Ishikawa; K. Tsukamoto (2795-2809).
To assess CO2 underground sequestration from a geochemical viewpoint, the anorthite dissolution rate, which is an important parameter of risk analysis, was measured in a CO2–water system. The authors sought to obtain precise dissolution rate data in a short time observing a crystal surface on a nanoscale. For this purpose, phase-shift interferometry was applied. Using this method, uncertainty of the reactive surface area that is imparted on calculation of the dissolution rate constant can also be avoided. The time-course profile of vertical retreat of the surface revealed that the anorthite dissolution process changes from the initial transient state to a later steady state, which is consistent with results of numerous precedent studies. The transient dissolution rate depends strongly on local features (e.g., density of defects, variation of chemical compositions) of the crystal surface, rather than on temperature. Therefore, it is very important to determine the original properties of the anorthite surface for the examination of subsequent dissolution process. Contrary to general expectations, the anorthite dissolution can alter the physical properties of reservoir rock immediately after CO2 injection. The simple estimation using the anorthite dissolution rate obtained in this study, which was done as a test case for the CO2 underground sequestration project conducted by RITE, revealed that porosity of reservoir rock increased about 2% (23–23.4%) of initial values during 60 a. That change in physical property in such a short time might enhance the diffusion of injected CO2 and formation water, and therefore accelerate further geochemical reactions. Results of this study demonstrate that the geochemical water–rock interaction, which is generally regarded as a longer-term phenomenon than various physical processes, can also affect the reservoir system from the initial stage.
Deposition rates of polysilicic acid with up to 10−3 M calcium ions by Taiji Chida; Yuichi Niibori; Osamu Tochiyama; Hitoshi Mimura; Koichi Tanaka (2810-2816).
Cementitious materials used for radioactive waste repository construction complicate the performance assessment of radioactive waste systems because the use of cement may greatly alter the pH (8–13) of groundwater and release constituents such as calcium ions. Under such conditions, it is important to clarify also the dynamic behavior of silica (silicic acid), in order to evaluate the alteration in the chemical and physical properties of the fractured layer or the host rock surrounding the repository. Since silica undergoes polymerization, precipitation or dissolution depending on the pH and/or temperature, the behavior of silica would be greatly complicated in the presence of other ions. This study is focused on the deposition rates of polysilicic acid and soluble silicic acid with up to 10−3 M Ca ions. In the experiment, Na2SiO3 solution (250 mL, pH > 10, 298 K) was poured into a polyethylene vessel containing amorphous silica powder (0.5 g), and a buffer solution, HNO3, and CaNO3 as Ca ions were sequentially added into the vessel. The pH of the solution was set to 8. The silica, initially in a soluble form at pH > 10 (1.4 × 10−2 M), became supersaturated and either deposited on the solid surface or changed into the polymeric form. Then the concentrations of both poly- and soluble silicic acid were monitored over a 40-day period. The decrease of polysilicic acid became slow with an increase in the concentration of Ca ions in the range of up to 10−3 M. In general, the addition of electrolytes to a supersaturated solution accelerates the aggregation and precipitation of polymeric species. However, the experimental result showed that polysilicic acid in the presence of Ca ions is apparently stable in solution, compared with that under a Ca-free condition. On the other hand, the concentration of soluble silicic acid in the presence of Ca ions immediately became metastable, that is, slightly higher than the solubility of soluble silicic acid. Its dynamic behavior was similar to that in the Ca-free condition.
Chemistry of fluids from a natural analogue for a geological CO2 storage site (Montmiral, France): Lessons for CO2–water–rock interaction assessment and monitoring by Hélène Pauwels; Irina Gaus; Yves Michel le Nindre; Jonathan Pearce; Isabelle Czernichowski-Lauriol (2817-2833).
Chemical and isotope studies of natural CO2 accumulations aid in assessing the chemical effects of CO2 on rock and thus provide a potential for understanding the long-term geochemical processes involved in CO2 geological storage. Several natural CO2 accumulations were discovered during gas and oil exploration in France’s carbogaseous peri-Alpine province (south-eastern France) in the 1960s. One of these, the Montmiral accumulation at a depth of more than 2400 m, is currently being exploited. The chemical composition of the water collected at the wellhead has changed in time and the final salinity exceeds 75 g/L. These changes in time can be explained by assuming that the fraction of the reservoir brine in the recovered brine–CO2–H2O mixture varies, resulting in variable proportions of H2O and brine in the sampled water. The proportions can be estimated in selected samples due to the availability of gas and water flowrate data. These data enabled the reconstruction of the chemical and isotope composition of the brine. The proportions of H2O and brine can also be estimated from isotope (δ 2H, δ 18O) composition of collected water and δ 18O of the sulfates or CO2. The reconstituted brine has a salinity of more than 85 g/L and, according to its Br− content and isotope (δ 2H, δ 18O, δ 34S) composition, originates from an evaporated Triassic seawater that underwent dilution by meteoric water. The reconstitution of the brine’s chemical composition enabled an evaluation of the CO2–water–rock interactions based on: (1) mineral saturation indices; and (2) comparison with initial evaporated Triassic seawater. Dissolution of K- and SO4-containing minerals such as K-feldspar and anhydrite, and precipitation of Ca and Mg containing minerals that are able to trap CO2 (carbonates) are highlighted. The changes in concentration of these elements in the brine, which are attributed to CO2 interactions, illustrate the relevance of monitoring the water quality at future industrial CO2 storage sites.
Hydrochemical and isotopic tracing of mixing dynamics and water quality evolution under pumping conditions in the mine shaft of the abandoned Frances Colliery, Scotland by Trevor Elliot; Paul L. Younger (2834-2860).
Since 1995, when pumps were withdrawn from deep mines in East Fife (Scotland), mine waters have been rebounding throughout the coalfield. Recently, it has become necessary to pump and treat these waters to prevent their uncontrolled emergence at the surface. However, even relatively shallow pumping to surface treatment lagoons of the initially chemically-stratified mine water from a shaft in the coastal Frances Colliery during two dynamic step-drawdown tests to establish the hydraulic characteristics of the system resulted in rapid breakdown of the stratification within 24 h and a poor pumped water quality with high dissolved Fe loading. Further, data are presented here of hydrochemical and isotopic sampling of the extended pump testing lasting up to several weeks. The use in particular of the environmental isotopes δ18O, δ2H, δ34S, 3H, 13C and 14C alongside hydrochemical and hydraulic pump test data allowed characterisation of the Frances system dynamics, mixing patterns and water quality sources feeding into this mineshaft under continuously pumped conditions. The pumped water quality reflects three significant components of mixing: shallow freshwater, seawater, and leakage from the surface treatment lagoons. In spite of the early impact of recirculating lagoon waters on the hydrochemistries, the highest Fe loadings in the longer-term pumped waters are identified with a mixed freshwater–seawater component affected by pyrite oxidation/melanterite dissolution in the subsurface system.
Citrate sorption and biodegradation in acid soils with implications for aluminum rhizotoxicity by Yohey Hashimoto (2861-2871).
Citrate and other organic acids play an important role in the rhizosphere and pedogenic processes. Although secreting citrate from roots in response to Al and heavy metal stress has been recognized as a central mechanism for plants to avoid toxicity, the efficiency of root citrate on metal detoxification is still contradictory in acid soil with abundant oxide minerals that serve as a potential sorption site for citrate. The objective of this study was to investigate sorption and biodegradation of citrate in subtropical acid soils with different mineralogical properties. A batch experiment was conducted to assess the possible fates (adsorption and biodegradation) of citrate in the three acid soils (Cecil, Creedmoor and Norfolk) under microbial-active and inactive conditions. Citrate adsorption isotherms for all soils were adequately described by the Freundlich equation with the R 2 value being over 0.90. The Cecil soil had the highest affinity for citrate adsorption among the soils with 99% adsorption observed throughout the citrate concentration range, which was due primarily to the abundant Al and Fe oxides. Citrate sorption to the mineral phase significantly reduced its biodegradation by 56%, 65% and 99% for the Creedmoor, Norfolk and Cecil soils, respectively. The results suggest the efficiency of rhizosphere processes for Al detoxification by root-secreted citrate would be significantly reduced in acid soil with abundant Al and Fe oxides.
Elemental distribution of coastal sea and stream sediments in the island-arc region of Japan and mass transfer processes from terrestrial to marine environments by Atsuyuki Ohta; Noboru Imai; Shigeru Terashima; Yoshiko Tachibana; Ken Ikehara; Takashi Okai; Masumi Ujiie-Mikoshiba; Ran Kubota (2872-2891).
A total of 49 elements have been identified in 338 coastal sea sediment samples collected from an area situated off the Ise-Tokai region of Japan for a nationwide marine geochemical mapping project. The spatial distribution patterns of the elemental concentrations in coastal seas along with the existing geochemical maps in terrestrial areas were used to define the natural geochemical background variation, mass transport, and contamination processes. The elemental concentrations of coastal sea sediments are determined primarily by particle size and regional differences. Most elemental concentrations increase with a decrease in particle size. Some elements such as Ca, Mn, and Yb are found to exist in large quantities in coarse particles containing calcareous shells, Fe–Mn oxides, and felsic volcanic sediments. Regional differences reflect the mass transfer process from terrestrial areas to coastal seas and the influence of the local marine geology. An analysis of variance (ANOVA) reveals that for many elements, the particle size effect is predominant over regional difference. The mean chemical compositions of coastal sea sediments are similar to those of stream sediments in adjacent terrestrial areas and in the upper crust of Japan. This observation supports the fact that coastal sea sediments have certainly originated from terrestrial materials. However, the spatial distributions of elemental concentrations are not always continuous between the land and coastal seas. The scale of mass movement observed in marine geochemical maps occurs at a distance of 20 km from the river mouth. A detailed examination of the spatial distribution patterns of K (K2O) and Cr concentrations suggests that terrestrial materials supplied through rivers are deposited near the shore initially, and then gravity-driven processes shift the sediments deeper into the basin. Contamination with heavy metals such as Zn, Cd and Pb was observed in coastal bays surrounded by urban and industrial areas. It is noteworthy that the areas with the highest concentration of these elements usually do not occur near the shore (not near the contamination source) but at the center of the bay. Unexpected low concentrations of Zn, Cd and Pb near shore may either be due to a decreased anthropogenic load in the most recent sediments or to dilution by unpolluted flood sediments.
Sorption of Th(IV) on Na-rectorite: Effect of HA, ionic strength, foreign ions and temperature by Di Xu; Changlun Chen; Xiaoli Tan; Jun Hu; Xiangke Wang (2892-2906).
The sorption of Th(IV) on Na-rectorite as a function of pH, ionic strength, temperature, soil humic acid (HA) and foreign ions was studied by using a batch technique under ambient conditions. The results indicated that the sorption of Th(IV) on Na-rectorite is strongly depended on pH, ionic strength and temperature. The presence of HA enhanced Th(IV) sorption at low pH and had no obvious effect on Th(IV) sorption at high pH. The sorption of Th(IV) decreased with increasing temperature, indicating that the sorption process of Th(IV) on rectorite was exothermic. Sodium-rectorite and HA were characterized by acid–base titration to obtain the pK a, and the constant capacitance model (CCM) modeled the sorption data very well with the aid of FITEQL 3.2. HA/Th(IV) addition sequences affected Th(IV) sorption in the ternary systems. The sorption of Th(IV) on Na-rectorite may be dominated by surface complexation, while cation exchange also contributes partly to the sorption.
A method for quantitative pyrite abundance in mine rock piles by powder X-ray diffraction and Rietveld refinement by Erik J. Oerter; George H Brimhall; Jennifer Redmond; Bruce Walker (2907-2925).
The abundance of pyrite and other sulfide minerals in mine rock piles is a potentially significant if not a determinative factor in terms of the geochemical and geomechanical evolution of the dumps as oxidation produces acid solutions that drive hydrolysis reactions. A technique is presented here that supports the quantitative determination of pyrite abundance in mine rock dumps by heavy liquid mineral separation to concentrate pyrite for powder X-ray diffraction and then Rietveld method refinement of the diffraction data on a large number of samples using commonly available laboratory equipment. In order to improve and constrain the accuracy of XRD results, binary (pyrite-quartz) and 6-part mineral mixtures (pyrite and rock-forming andesite minerals) spanning a wide range of pyrite concentrations were prepared gravimetrically and run as standards. These standards were then used to minimize errors in pyrite abundance data by constraining key input parameters in the Rietveld refinement. A new polynomial relationship was derived between diffracting crystallite size and the Brindley microabsorption correction input size. This method provides a means to determine uncertainties in pyrite abundance, whereas conventional Rietveld refinement techniques done without the use of standards yield only statistical measures of the least-squares fit, rather than absolute uncertainties in mineral constituent weight percentages. The technique was applied to a number of mine rock pile samples and the uncertainty in the results determined by applying the relationship derived from the 6-part gravimetric standards to the results of the Brindley corrected Rietveld refinements. Uncertainties determined by this method are found to be on the order of ±10% for samples with pyrite content greater than ∼10 wt% and ±30% for samples with pyrite content less than 10 wt%. In order to evaluate the technique’s improvement upon traditional visual mineral abundance estimation the quantitative results are compared to manual volumetric estimates.
Characterization of dissolved organic matter in anoxic rock extracts and in situ pore water of the Opalinus Clay by Amandine Courdouan; Iso Christl; Sébastien Meylan; Paul Wersin; Ruben Kretzschmar (2926-2939).
Dissolved organic matter (DOM) from the Opalinus Clay, a potential host rock for the disposal of radioactive waste, was isolated under strictly anoxic conditions from ground rock material and compared with DOM of in situ pore water samples. For the extractions, deionized water, synthetic pore water (SPW, water containing all major ions at pore water concentrations but no organic matter) and 0.1 M NaOH were used. The influence of the solid-to-liquid ratio, extraction time, acid-pretreatment and O2 exposure of the rock material on the isolated DOM were investigated. Liquid chromatography coupled with a total organic C detector (LC-OCD) and reverse-phase ion chromatography were used to characterize the DOM size distributions and to determine the low molecular weight organic acid (LMWOA) contents in the pore water samples and the rock extracts.The results revealed that only a small portion of the total organic C of the rock material (<0.38%) was extractable, even after removal of carbonates by acid-pretreatment. The concentrations of dissolved organic C (DOC) were found to range from 3.9 ± 0.4 to 8.0 ± 0.8 mg/L in the anoxic extracts. The pore waters exhibited similar DOC concentrations ranging from 1.2 to 15.8 ± 0.5 mg/L. The analysis by LC-OCD showed that the DOM extracted under anoxic conditions and the pore water DOM mainly consisted of hydrophilic compounds of less than 500 Da. The DOM extracted with SPW was most similar in size to the pore water DOM. Grinding the rock under oxic conditions increased the DOC yields and shifted the size distribution toward higher molecular weight compounds compared to the strictly anoxic treatment. Acetate, lactate and formate were identified in all extracts and in the pore water. In total, LMWOA accounted for 36% of the total DOC in both pore water and SPW extracts. The results imply that controlled anoxic conditions and the use of SPW as an extractant are required to isolate DOM from Opalinus Clay rocks which most resembles the in situ pore water DOM with respect to its size distribution and the LMWOA contents.