Applied Geochemistry (v.23, #6)
The influence of extraction rate on the reduced sulphur content of Aix-les-Bains’ thermal spring waters: Consequences for resource-quality monitoring by Stéphanie Gallino; Myriam Bulloz; Emmanuel Naffrechoux; Marc Dzikowski; Dominique Gasquet (1367-1382).
Sulphurous thermal springs account for a few percent of all thermal springs. As each degree of oxidation of elemental S corresponds to a different S species, sulphurous spring waters may contain a variety of S species. Total S content is one of the parameters measured when analysis is carried out for issuing a spa’s operating licence. Under French law these parameters must be stable over time. The two spas in Aix-les-Bains are fed by a number of natural springs and boreholes, whose waters have total reduced S concentrations of between 30 and 800 mg L−1. To ensure compliance with the requirements of the spas’ operating licences, particularly in terms of total reduced S content, official analysis of the waters is carried out every month at random dates and times. Monthly analyses have revealed seasonal variations in the S content of some of Aix-les-Bains’ springs; therefore, more frequent monitoring was carried out in order to investigate the extent and causes of these fluctuations. As well as seasonal variations, this monitoring has revealed daily and hourly fluctuations that appear to be related to extraction rates. For some of the springs and boreholes, S concentrations were correlated with extraction rates; for others, an increase in extraction rate led to a reduction in total S content. Concentrations of sulphide ions (one of the species included in the total sulphur analyses) were monitored at the same time as the total S content. Variations in sulphide ion concentrations and in sulphide concentration/total S ratios did not appear to be linked to variations in flow rate. These tests show that random monthly testing is not suitable for monitoring the stability of S contents, as this parameter can only be considered stable in terms of its yearly mean value.
Experimental and modelling investigations of the biogeochemistry of gas production from low and intermediate level radioactive waste by Joe Small; Mikko Nykyri; Mika Helin; Ulla Hovi; Tuija Sarlin; Merja Itävaara (1383-1418).
The degradation of organic wastes and the corrosion of metallic wastes and steel containers in low and intermediate level radioactive waste (LLW/ILW) repositories are important processes that affect repository geochemistry and the speciation and transport of radionuclides. Gas is generated in association with these degradation processes and this has the potential to overpressure the repository, which can promote transport of groundwater and gas, and consequently radionuclide transport. Microbial activity plays an important role in organic degradation, corrosion and gas generation through the mediation of reduction–oxidation reactions.A large-scale gas generation experiment has been established at the LLW/ILW repository, Olkiluoto, Finland to examine gas generation from LLW in waste drums disposed of in the operational VLJ Repository (VLJ is a Finnish acronym which translates to “reactor operating waste”). The experiment has monitored, for a period of 9 a, the rate and composition of gas generated, and the aqueous geochemistry and microbe populations present at various locations within the experiment. There is considerable heterogeneity within the experiment, such that pH is observed to vary from pH 5.5 to pH 10 between organic-rich waste and water associated with concrete. The heterogeneity results in competing anaerobic processes occurring together in the experiment but within different niches. Microbial activity initially dominant in organic waste has after 7 a reduced the alkalinity of the concrete influenced regions.The experiment has been modelled using a biogeochemical reaction-transport code (GRM) using a blind testing approach. Using independent data, the model was able to reproduce, within a factor of two, the rate of gas production. In addition, the model represented the main anaerobic microbial processes leading to methanogenesis and the observed spatial and temporal variations in aqueous and gaseous species. In order to model the experiment, its heterogeneity was considered such that individual waste containers were represented and assumptions were made concerning transport rates of chemical species. Cellulose waste and H2 produced by corrosion provide microbial substrates for reduction processes and CH4 generation. However, gas generation is a complex interaction of waste degradation processes. Simple repository gas generation models that consider corrosion and cellulose degradation in isolation will tend to overestimate H2 content and gas generation. The GRM model is more realistic and utilises information concerning SO 4 2 - , NO 3 - and other oxidised species present in LLW/ILW to consider the competition between microbial groups for electron donors that reduce the extent of H2 and CH4 generation. Models such as GRM could be applied to other repository systems, such as for high level waste and spent nuclear fuel, to evaluate how H2 gas generation from corrosion and radiolysis may be affected by microbial activity. However, this will require estimation of appropriate microbial kinetic parameters for these more extreme environments.
Combined effects of spatially variable flow and mineralogy on the attenuation of acid mine drainage in groundwater by Maria E. Malmström; Sten Berglund; Jerker Jarsjö (1419-1436).
Quantifications of the spreading of acid mine drainage (AMD) in groundwater are needed for risk assessments of mining sites. However, due to subsurface heterogeneity, available field data may prove insufficient for deterministic process descriptions, even at well-characterized sites. Here, the probabilistic LaSAR-PHREEQC model is used to consider multicomponent reactions and transport in heterogeneous (flow and geochemistry) groundwater surrounding a mine waste site, with specific focus on the spreading of Zn. Model results, using field data from a mill tailings impoundment in northern Sweden (including major component geochemistry), indicate that precipitation of smithsonite (ZnCO3) may drastically delay the downstream arrival of Zn, but may also cause a peak concentration once the retained Zn is released. The amount of smithsonite formed is, however, minute and its spatial variation large, such that detection of smithsonite in soil samples may be difficult. Results further show that even a low degree of flow heterogeneity can effectively smooth otherwise distinctive temporal concentration changes attributed to the considered chemical reactions, and thereby mask the attenuation processes. By contrast, the existence of preferential flow paths can cause temporally separated concentration peaks in response to a single chemical reaction chain, even in a geochemically homogeneous domain, making the interpretation of the concentration curves non-trivial. The stochastic modelling results for Zn considering flow and/or mineralogical heterogeneity indicate a less efficient Zn attenuation than predicted by standard, deterministic reactive-transport models. In addition, in all considered probabilistic Zn and SO 4 2 - scenarios, the spatial variability in downstream pollutant concentration was high, implying that a relatively large number of point samples are needed to determine field-scale mean concentrations.
The arsenic source term for an in-pit uranium mine tailings facility and its long-term impact on the regional groundwater by Brett J. Moldovan; M. Jim Hendry; Glenn A. Harrington (1437-1450).
Detailed field sampling and analyses and laboratory-based diffusion-cell experiments were used in conjunction with 3-D reactive transport modeling (MODFLOW and MT3D99) to quantify the fate and long-term (10 ka) transport of As in the Rabbit Lake In-pit Tailings Management Facility (RLITMF), northern Saskatchewan, Canada. The RLITMF (300 m × 425 m × 90 m thick) was engineered to ensure solute transport within the RLITMF is dominated by diffusion. Concentrations of As in the tailings pore fluids ranged from 0.24 to 140 mg/L (n = 43). Arsenic speciation analyses indicate 90% of this arsenic exists as As5+. This observation is supported by pH–Eh measurements of pore fluids (n = 135). Geochemical analyses yielded a strong inverse correlation between the Fe/As molar ratio in the tailings solids and the corresponding concentration of dissolved As, which is attributed to the adsorption of As to secondary 2-line ferrihydrite present in the tailings. Diffusion-cell testing yielded values for the effective diffusion coefficient, sorption coefficient, and effective porosity of As in the tailings of 4.5 × 10−10 m2/s, 2–4 cm3/g and 0.36, respectively. Reactive transport simulations using the field and laboratory data show adsorption of As to the tailings and diffusive transport of dissolved As in the tailings should reduce the source term concentration of As to between 40% and 70% of the initial concentrations over the 10 ka simulation period. Based on these simulations, the As concentrations in the regional groundwater, 50 m down gradient of the tailings facility, should be maintained at background concentrations of 0.001 mg/L over the 10 ka period. These findings suggest the engineered in-pit disposal of U mine tailings can provide long-term protection for the local groundwater regime from As contamination.
Effect of major anions on arsenate desorption from ferrihydrite-bearing natural samples by Franco Frau; Riccardo Biddau; Luca Fanfani (1451-1466).
The influence of background electrolytes (Na2HPO4 · 2H2O, NaHCO3, Na2SO4, NaNO3 and NaCl) on arsenate (As(V)) desorption from 3 environmental samples (a tailings sample, a stream-bed sediment and a top soil) containing ferrihydrite as the main As-bearing phase has been studied by means of kinetic batch experiments and geochemical simulations. The experimental results indicate that As(V) release increases greatly in the presence of dissolved phosphate and carbonate species. Similarly to PO 4 3 + , a strong surface interaction of inner-sphere type between ferrihydrite and aqueous carbonate species is suggested. Nitrate and Cl− have negligible effects on the As(V) desorption reaction, whereas SO 4 2 - exhibits intermediate behavior depending on its dissolved concentration that probably influences the type of surface complex (i.e. outer-sphere or inner-sphere). The process of As(V) release follows the first-order rate equation of Lagergren modified for desorption; most values of the desorption rate constant k des are in the range of 0.0012–0.0030 min−1. Modeling of the desorption experiments with PHREEQC, with ferrihydrite as the main As-bearing phase, indicates that the influence of pH is notably less important than the displacement action of carbonate species in determining the amount of As(V) released to solution. Simulation of As(V) desorption totally fails when the carbonate surface complexes are excluded from the model. In the NaHCO3 experiments with the tailings sample the best match between observed and calculated data is obtained also including dissolution of scorodite and arsenopyrite in the model. Moreover, modeling has stressed the poor quality of the adsorption constants for sulfate species that leads to strong overestimation of As(V) desorption at pH 4 and underestimation at pH 7.5. Although the findings of this study are consistent with the results of recent studies from other authors, they cannot be generalized or directly applied to natural systems. However, environmental implications concerning As mobility, as well as possible application in various fields (e.g. irrigation agriculture, soil decontamination, water treatment and mine site remediation), might be derived from these findings.
Mineralogical controls on environmental mobility of arsenic from historic mine processing residues, New Zealand by Laura Haffert; Dave Craw (1467-1483).
Processing of arsenopyrite ore took place at Blackwater Au mine, New Zealand, between 1908 and 1951 and no rehabilitation was undertaken after mine closure. High As concentrations in solid processing residues (up to 40 wt% As) are due to secondary As minerals. Site pH regimes vary from 4.1 to circum-neutral. Originally, all processed As was present as arsenolite (arsenic trioxide polymorph, AsIII), a by-product of arsenopyrite roasting. Near the roaster, scorodite precipitated as a result of the high dissolved As concentration during arsenolite dissolution. The formation of scorodite has two major consequences. Firstly, the scorodite precipitate cements the ground in the vicinity of the roaster area, thereby creating an impermeable surface crust (up to 30 wt% As) and encapsulating weathered arsenolite grains within the cement. Secondly, formation of scorodite temporarily immobilizes some of the dissolved As that is generated during nearby arsenolite dissolution. Where all the available arsenolite has dissolved, scorodite becomes soluble, and the dissolved As concentrations are controlled by scorodite solubility, which is at least two orders of magnitudes lower than arsenolite solubility. Downstream Eh conditions fall below the AsV/AsIII boundary, so that scorodite does not precipitate and dissolved As concentrations are controlled by arsenolite solubility. Dissolved As reaches up to 52 mg/L in places, and exceeds the current WHO drinking water guideline of 0.01 mg/L by 5200 times. This study shows that dissolved As concentrations in discharge waters at historic mine sites are dependent on the processing technology and associated mineralogy.
Role of bentonite colloids on europium and plutonium migration in a granite fracture by Tiziana Missana; Úrsula Alonso; Miguel García-Gutiérrez; Manuel Mingarro (1484-1497).
To establish when the presence of colloids can favour contaminant migration is a critical task for the performance assessment of a high level radioactive waste repository. In this study, the effects of the presence of bentonite colloids on the migration of highly sorbing elements, such as Eu and Pu, in a granite environment were investigated. Special efforts were made to quantify the filtration of bentonite colloids in this medium and to determine the experimental uncertainties that could bias the interpretation of the results.The migration of bentonite colloids in a granite fracture was studied in the laboratory by dynamic column experiments under low water flow rates (4–11 mL/h). The breakthrough curves of bentonite colloids always showed a peak in a similar position to conservative tracers, but the colloids recovery critically depended on their initial concentration and on the water flow rate. In the presence of colloids, the breakthrough curves of Eu and Pu always showed a peak in the same position as conservative tracers, thus indicating the migration of these radionuclides to be mostly colloid-driven. The recovery of Pu adsorbed onto the colloids was approximately that expected when the experimentally determined bentonite colloid filtration in the column was accounting for but the Eu recovery was always significantly lower.
Modelling retention of heavy metals in the Oostriku peat bog, Estonia: Comparison of predicted and observed results by K. Syrovetnik; M.E. Malmström; I. Neretnieks (1498-1512).
In this study, the retention of Ca and other metals (Pb, Cu, Fe, Zn and Mn) in the Oostriku peat bog (central Estonia) was modelled. Equilibrium sorption of metals on amorphous ferric oxyhydroxide and solid organic matter was simulated at steady-state. Ferric oxyhydroxide formation and possible precipitation of other metals (Mn, Pb and Cu) in the peat was also assessed. Evolution of metal sorption fronts along a peat profile over time was simulated with a dynamic model to test if metal–metal competition effects could cause Pb and Cu to sorb at higher amounts in the uppermost peat than in the lower peat, as observed in the field. The predicted sorbed amounts of metals were compared with those previously observed in the peat. In general, good agreement between both batch and dynamic model results and the independent observations at the Oostriku peat site was obtained. This suggests that the relatively simple model approach employed here might be generally useful for assessing other peat sites and similar applications.
The Serchio River catchment, northern Tuscany: Geochemistry of stream waters and sediments, and isotopic composition of dissolved sulfate by Gianni Cortecci; Enrico Dinelli; Tiziano Boschetti; Paola Arbizzani; Loredana Pompilio; Mario Mussi (1513-1543).
The Serchio River and its tributaries in northern Tuscany were investigated for the chemical and isotopic compositions of waters and bed sediments. Bedrocks are mostly limestone/dolomite and siliciclastics, thermal spring systems are present in the catchment, and the main industrial activity is represented by paper-mills. Main results obtained are: (1) major ions in solution appear to be basically controlled by precipitation and lithology, as well as subordinately by direct inputs of thermal springs, (2) human influence on metals in the waters along the main Serchio and Lima rivers is indicated at a number of sites by increases in concentration compared to the chemical composition of upstream tributaries, (3) S and O isotope compositions delineate two main sources for aqueous SO 4 2 - , that is dissolution of Triassic evaporite (directly or via thermal springs) and oxidation of sulfide dispersed in siliciclastic rocks. Anthropogenic contributions are probable, but they cannot be quantitatively assessed. Only SO 4 2 - in the notoriously polluted Ozzeri tributary is suspected to be largely anthropogenic, and (4) the chemical composition of bed sediments is mainly influenced by lithology, apart from a number of technogenic elements in the upper part of the Serchio River and in some tributaries. Contamination possibly occurs at other sites, but geochemical indications are weak.
A consistent geochemical modelling approach for the leaching and reactive transport of major and trace elements in MSWI bottom ash by Joris J. Dijkstra; Johannes C.L. Meeussen; Hans A. Van der Sloot; Rob N.J. Comans (1544-1562).
To improve the long-term environmental risk assessment of waste applications, a predictive “multi-surface” modelling approach has been developed to simultaneously predict the leaching and reactive transport of a broad range of major and trace elements (i.e., pH, Na, Al, Fe, Ca, SO4, Mg, Si, PO4, CO3, Cl, Ni, Cu, Zn, Cd, Pb, Mo) and fulvic acids from MSWI bottom ash. The geochemical part of the model approach incorporates surface complexation/precipitation on Fe/Al (hydr)oxides, complexation with humic and fulvic acids (HA and FA, respectively) and mineral dissolution/precipitation. In addition, a novel approach is used to describe the dynamic leaching of FA, based on the surface complexation of FA on Fe/Al (hydr)oxides. To enable reactive transport calculations, the geochemical part of the model is combined with advective/dispersive transport of water and first-order mass transfer between mobile and stagnant zones. Using a single, independently determined set of input parameters, adequate model predictions are obtained for the leaching of a broad range of elements under widely different conditions, as verified with data from the European standardised pH-static and percolation leaching tests (TS 14997 and TS 14405, respectively). The percolation tests were operated at different flow velocities and with flow interruptions to enable verification of the local equilibrium assumption. Although the combination of experimental and modelling results indicates that the leaching of major solubility-controlled elements occurs largely under local equilibrium conditions, this study has led to the identification of physical non-equilibrium processes for non-reactive soluble salts, as well as possible sorption-related non-equilibrium processes for the leaching of Mo, FA and associated trace metals. Further improvement of the reactive transport model can be achieved by a more mechanistic description of the (dynamic) leaching behaviour of humic substances. As the modelling approach outlined in this study is based on the fundamental processes that underlie leaching, the approach is expected to be also applicable to other granular contaminated materials application scenarios and conditions. Therefore, the combination of standardized leaching test methods, selective chemical extractions and mechanistic modelling, constitutes a promising generic approach to assess the long-term environmental impact of the application of granular contaminated materials in the environment.
The hydrochemistry of a semi-arid pan basin case study: Sua Pan, Makgadikgadi, Botswana by Frank D. Eckardt; Robert G. Bryant; Graham McCulloch; Baruch Spiro; Warren W. Wood (1563-1580).
This study presents results on the fluid and salt chemistry for the Makgadikgadi, a substantial continental basin in the semi-arid Kalahari. The aims of the study are to improve understanding of the hydrology of such a system and to identify the sources of the solutes and the controls on their cycling within pans. Sampling took place against the backdrop of unusually severe flooding as well as significant anthropogenic extraction of subsurface brines. This paper examines in particular the relationship between the chemistry of soil leachates, fresh stream water, salty lake water, surface salts and subsurface brines at Sua Pan, Botswana with the aim of improving the understanding of the system’s hydrology. Occasionally during the short wet season (December–March) surface water enters the saline environment and precipitates mostly calcite and halite, as well as dolomite and traces of other salts associated with the desiccation of the lake. The hypersaline subsurface brine (up to TDS 190,000 mg/L) is homogenous with minor variations due to pumping by BotAsh mine (Botswana Ash (Pty) Ltd.), which extracts 2400 m3 of brine/h from a depth of 38 m. Notable is the decrease in TDS as the pumping rate increases which may be indicative of subsurface recharge by less saline water. Isotope chemistry for Sr (87Sr/86Sr average 0.722087) and S (δ 34S average 34.35) suggests subsurface brines have been subject to a lithological contribution of undetermined origin. Recharge of the subsurface brine from surface water including the Nata River appears to be negligible.
Investigating the origin of Pb pollution in a terrestrial soil–plant–snail food chain by means of Pb isotope ratios by M.J.M. Notten; N. Walraven; C.J. Beets; P. Vroon; J. Rozema; R. Aerts (1581-1593).
Lead isotope ratios were used to trace the origin of Pb in a soil–plant (Urtica dioica)–snail (Cepaea nemoralis) food chain in two polluted locations in the floodplains of the rivers Meuse and Rhine (Biesbosch National Park) and one reference location in the Netherlands. Lead isotope ratios and concentrations were determined in soil, litter, plant leaves, snails, rainwater and airborne particulate matter. Anthropogenic Pb in the soils of all locations was found to be derived from deposition of Pb polluted river sediments. Discharging rivers influenced the reference location before being reclaimed from the sea. The river sediment contains anthropogenic Pb from various sources related to industrial activities in the hinterland of the rivers Meuse and Rhine. Lead in the atmosphere contributed substantially to Pb pollution and Pb transfer in plant leaves and snails in all locations. Lead pollution in plant leaves and snails can be explained from a mixture of river sediment-Pb and atmospheric Pb from various transfer routes that involve low concentrations.
Meteorological impacts on the water quality in the Pajuluoma acid sulphate area, W. Finland by Peter Österholm; Mats Åström (1594-1606).
Water samples (n = 354) from a small catchment (7.4 km2) covered by acid sulphate soils (pH < 4) were collected during all seasons and all types of hydrological conditions in 1990–2001. The electric conductivity (EC) and pH, i.e. the key indicators of acid sulphate soil impact in the current setting, were determined. Representative daily runoff and precipitation data was available for the whole study period. The 10th and 90th percentiles for EC and pH were 29–140 mS/m and 3.8–4.6, respectively. While the water quality varied remarkably from year to year, and even within seasons, some regularity was found. The water quality was generally worst in late autumn (water temperature < 5 °C) and in spring. Of all seasons the variations were clearly smallest in spring, indicating that most representative samples can be obtained in this season. There were significant correlations between autumn, early winter and spring water quality within hydrological years. Thus acid and metal surges in spring are somewhat predictable. At base flow conditions (runoff about 1 L/s km2 or less), the water quality was relatively good in all seasons. Above base flow conditions, the impact of acid sulphate soils tended to slightly increase with increasing runoff and precipitation, especially in early summer, but not in late summer. No significant signs of dilution during flood conditions (up to 100 L/s km2) were found and neither were there any correlations to rising or falling limbs. The severity of individual summer droughts, which in theory should increase the oxidation of S and acidity in the soils, had little or no impact on the water quality in subsequent autumn and spring. On the other hand, there was a remarkable long-term increase in EC and a corresponding decrease in pH (starting in 1995) after a suite of several very dry summers. After that the water quality did not improve even if the dry summers were followed by some wet summers. This indicates that the temporary pool of readily leachable acidity in the soils is fairly large. Moreover, it indicates that the potential shift towards more extreme global weather conditions (with more severe dry spells) may have significant impacts on the water quality in midwestern Finland, a region that is heavily affected by acidity and metals from acid sulphate soils.
Influence of sulfate input on freshwater sediments: Insights from incubation experiments by Anna Szynkiewicz; Mariusz Orion Jędrysek; Marta Kurasiewicz; Maria Mastalerz (1607-1622).
Incubation experiments were carried out under high and low SO 4 2 - conditions to investigate the buffering capacity of lake sediments. Increased SO 4 2 - content in the water column enhanced microbial SO 4 2 - reduction, causing a continuous decrease of SO 4 2 - content from 1086 to 83 mg/L paralleled by an increase of pH in the water column from 3.76 to 7.20. These changes were accompanied by decreased methanogenesis in the incubated sediments. The results demonstrate that the buffering capacity resulted from a variety of biodegradation pathways controlled to a large extent by SO 4 2 - reduction, rather than by direct anaerobic oxidation of CH4. This is documented by distinctly lower δ 13C values (from −73.99 to −65.24‰) of the CH4 generated under higher SO 4 2 - conditions compared to higher δ 13C values (from −68.98 to −61.37‰) of the CH4 generated under lower SO 4 2 - conditions.
Early diagenetic vivianite [Fe3(PO4)2 · 8H2O] in a contaminated freshwater sediment and insights into zinc uptake: A μ-EXAFS, μ-XANES and Raman study by Kevin G. Taylor; Karen A. Hudson-Edwards; Andrew J. Bennett; Vladimir Vishnyakov (1623-1633).
The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption.
Experimental investigations of the reaction path in the MgO–CO2–H2O system in solutions with various ionic strengths, and their applications to nuclear waste isolation by Yongliang Xiong; Anna Snider Lord (1634-1659).
The reaction path in the MgO–CO2–H2O system at ambient temperatures and atmospheric CO2 partial pressure(s), especially in high-ionic-strength brines, is of both geological interest and practical significance. Its practical importance lies mainly in the field of nuclear waste isolation. In the USA, industrial-grade MgO, consisting mainly of the mineral periclase, is the only engineered barrier certified by the Environmental Protection Agency (EPA) for emplacement in the Waste Isolation Pilot Plant (WIPP) for defense-related transuranic waste. The German Asse repository will employ a Mg(OH)2-based engineered barrier consisting mainly of the mineral brucite. Therefore, the reaction of periclase or brucite with carbonated brines with high-ionic-strength is an important process likely to occur in nuclear waste repositories in salt formations where bulk MgO or Mg(OH)2 will be employed as an engineered barrier. The reaction path in the system MgO–CO2–H2O in solutions with a wide range of ionic strengths was investigated experimentally in this study. The experimental results at ambient laboratory temperature and ambient laboratory atmospheric CO2 partial pressure demonstrate that hydromagnesite (5424) (Mg5(CO3)4(OH)2 · 4H2O) forms during the carbonation of brucite in a series of solutions with different ionic strengths. In Na–Mg–Cl-dominated brines such as Generic Weep Brine (GWB), a synthetic WIPP Salado Formation brine, Mg chloride hydroxide hydrate (Mg3(OH)5Cl · 4H2O) also forms in addition to hydromagnesite (5424).The observation of nesquehonite (MgCO3 · H2O) and subsequent appearance of hydromagnesite (5424) in the experiments in a Na–Cl-dominated brine (ERDA-6) at room temperature and P CO 2 = 5 × 10 - 2 atm allows estimation of the equilibrium constant (log K) for the following reaction: Mg 5 ( CO 3 ) 4 ( OH ) 2 · 4H 2 O + CO 2 ( g ) + 10H 2 O = 5MgCO 3 · 3H 2 O as ∼2.5 at 25 °C. The log K for the above reaction at 5 °C is calculated to be ∼4.0 by using the Van’t Hoff equation. By using these equilibrium constants, the co-existence of hydromagnesite (5424) with nesquehonite in various, natural occurrences such as in weathering products of the meteorites from the Antarctic and serpentine-rich mine tailings, can be well explained. Since the stoichiometric ratio of Mg to C is higher in hydromagnesite (5424) than in nesquehonite, this finding could have important implications for the sequestration of anthropogenic CO2 in mafic and ultramafic rocks, suggesting that the sequestration of anthropogenic CO2 is optimal in the stability field of nesquehonite.
Field application of calcite Dispersed Alkaline Substrate (calcite-DAS) for passive treatment of acid mine drainage with high Al and metal concentrations by Tobias S. Rötting; Manuel A. Caraballo; José A. Serrano; Carlos Ayora; Jesús Carrera (1660-1674).
Passive treatment systems are widely used for remediation of acid mine drainage (AMD), but existing designs are prone to clogging or loss of reactivity due to Al- and Fe-precipitates when treating water with high Al and heavy metal concentrations. Dispersed alkaline substrate (DAS) mixed from a fine-grained alkaline reagent (e.g. calcite sand) and a coarse inert matrix (e.g. wood chips) had shown high reactivity and good hydraulic properties in previous laboratory column tests. In the present study, DAS was tested at pilot field scale in the Iberian Pyrite Belt (SW Spain) on metal mine drainage with pH near 3.3, net acidity 1400–1650 mg/L as CaCO3, and mean concentrations of 317 mg/L Fe (95% Fe(II)), 311 mg/L Zn, 74 mg/L Al, 20 mg/L Mn, and 1.5–0.1 mg/L Cu, Co, Ni, Cd, As and Pb. The DAS-tank removed an average of 870 mg/L net acidity as CaCO3 (56% of inflow), 25% Fe, 93% Al, 5% Zn, 95% Cu, 99% As, 98% Pb, and 14% Cd, but no Mn, Ni or Co. Average gross drain pipe alkalinity was 181 mg/L as CaCO3, which increased total Fe removal to 153 mg/L (48%) in subsequent sedimentation ponds. Unfortunately, the tank suffered clogging problems due to the formation of a hardpan of Al-rich precipitates. DAS lifetime could probably be increased by lowering Al-loads.
Origin and spatial distribution of gas at seismogenic depths of the San Andreas Fault from drill-mud gas analysis by Thomas Wiersberg; Jörg Erzinger (1675-1690).
Data are presented on the molecular composition of drill-mud gas from the lower sedimentary section (1800–3987 m) of the SAFOD (San Andreas Fault Observatory at Depth) Main Hole measured on-line during drilling, as well as C and H isotope data from off-line mud gas samples. Hydrocarbons, H2 and CO2 are the most abundant non-atmospheric gases in drill-mud when drilling seismogenic zones. Gas influx into the well at depth is related to the lithology and permeability of the drilled strata: larger formation gas influx was detected when drilling through organic-rich shales and permeable sandstones. The SAF (San Andreas Fault), encountered between approximately 3100 m and 3450 m borehole depth, is generally low in gas, but is encompassed by two gas-rich zones (2700–2900 m and below 3550 m) at the fault margins with enhanced 222Rn activities and distinct gas compositions. Within the fault, two interstratified gas-rich lenses (3150–3200 m and 3310–3340 m) consist of CO2 and hydrocarbons (upper zone), but almost exclusively of hydrocarbons (lower zone).The isotopic composition indicates an organic source of hydrocarbons and CO2 in the entire sedimentary section of the well. Hydrocarbons in sedimentary strata are partly of microbial origin down to ∼2500 m borehole depth. The contribution of thermogenic gas increases between ∼2500 m and 3200 m. Below ∼3200 m, hydrocarbons fully derive from thermal degradation of organic matter. The lack of H2 in the center of the fault and the high concentration of H2 in the fractured zones at the fault margins are consistent with H2 formation by interaction of water with fresh silica mineral surfaces generated by tectonic activities, however, this needs to be verified by laboratory experiments. Based on these studies, it is concluded that the fault zone margins consist of strata with enhanced permeability, separated by a low-permeability fault center.
Hyperfiltration of sodium chloride through kaolinite membranes under relatively low-heads – Implications for groundwater assessment by Megan Hart; T.M. Whitworth; Eliot Atekwana (1691-1702).
Three relatively low-head hyperfiltration experiments were conducted to investigate clay membrane behavior at lower hydraulic heads. Dilute Cl− solutions (187 and 336 mg/L) were forced through thin layers of kaolinite (0.67–1.62 mm) under heads ranging between 1.45 and 1.89 m. At the end of each experiment, there was a Cl− concentration increase (15–29%) within the cell. The concentration increase is attributable to solute-sieving by the kaolinite. Calculated final concentration increases at the membrane face ranged between 1.51 and 1.65 times the initial concentration and the calculated values of the reflection coefficient ranged between 0.35 and 0.40. These experiments show that clays are capable of significant hyperfiltration effects at lower pressure heads more typical of natural aquifers. Consequently, hyperfiltration effects may need to be considered in a broad spectrum of shallow subsurface processes in which these effects have been regarded as inconsequential; particularly in perched aquifers.
Baseline determination of the atmospheric Pb, Sr and Nd isotopic compositions in the Rhine valley, Vosges mountains (France) and the Central Swiss Alps by M. Lahd Geagea; P. Stille; F. Gauthier-Lafaye; Th. Perrone; D. Aubert (1703-1714).
The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. 206Pb/207Pb and 87Sr/86Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with 206Pb/207Pb and 87Sr/86Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (206Pb/207Pb: 1.158–1.167; 87Sr/86Sr: 0.719–0.725; ε Nd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (87Sr/86Sr: 0.714 − 0.716; ε Nd: −3.6 to −8.1). The 143Nd/144Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the 143Nd/144Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.
High-resolution monitoring of biogeochemical gradients in a tar oil-contaminated aquifer by Bettina Anneser; Florian Einsiedl; Rainer U. Meckenstock; Lars Richters; Frank Wisotzky; Christian Griebler (1715-1730).
The detailed understanding of in situ biodegradation of petroleum hydrocarbons in porous aquifers requires knowledge on biogeochemical gradients, the distribution of individual redox species and microorganisms. The generally limited spatial resolution of conventional monitoring wells, however, hampers appropriate characterization of small-scale gradients and thus localization of the relevant processes. Groundwater sampling across a BTEX plume in a sandy aquifer by means of a novel high-resolution multi-level well (HR-MLW) is presented here. The presence of distinct and steep biogeochemical gradients is demonstrated in the centimeter and decimeter scale, which could not be resolved with a conventional multi-level well. The thin BTEX plume with a vertical extension of only 80 cm exhibited a decline of contaminant concentrations by two orders of magnitude within a few centimeters in the upper and lower fringe zone. The small-scale distribution of sulfate, sulfide and Fe(II) in relation to the contaminants and elevated δ 34S and δ 18O values of groundwater sulfate strongly indicated sulfate and iron reduction to be the dominant redox processes involved in biodegradation. High microbial activities and biomass especially at the plume fringes and the slope of chemical gradients supported the concept that the latter are regulated by microbial processes and transverse dispersion, i.e. vertical mixing of electron donors and acceptors. Transverse dispersion therefore was suggested to be a driving factor controlling biodegradation in porous aquifers, but not exclusively limiting natural attenuation processes at this site. Broad overlapping zones of electron donors and electron acceptors point towards additional factors limiting anaerobic biodegradation in situ. The identification of small-scale gradients substantially contributed to a better understanding of biodegradation processes and hence is a prerequisite for the development of reliable predictive mathematical models and future remediation strategies.
Anthropogenic influences on the input and biogeochemical cycling of nutrients and mercury in Great Salt Lake, Utah, USA by David Naftz; Cory Angeroth; Terry Kenney; Bruce Waddell; Nathan Darnall; Steven Silva; Clay Perschon; John Whitehead (1731-1744).
Despite the ecological and economic importance of Great Salt Lake (GSL), little is known about the input and biogeochemical cycling of nutrients and trace elements in the lake. In response to increasing public concern regarding anthropogenic inputs to the GSL ecosystem, the US Geological Survey (USGS) and US Fish and Wildlife Service (USFWS) initiated coordinated studies to quantify and evaluate the significance of nutrient and Hg inputs into GSL. A 6‰ decrease in δ 15N observed in brine shrimp (Artemia franciscana) samples collected from GSL during summer time periods is likely due to the consumption of cyanobacteria produced in freshwater bays entering the lake. Supporting data collected from the outflow of Farmington Bay indicates decreasing trends in δ 15N in particulate organic matter (POM) during the mid-summer time period, reflective of increasing proportions of cyanobacteria in algae exported to GSL on a seasonal basis. The C:N molar ratio of POM in outflow from Farmington Bay decreases during the summer period, supportive of the increased activity of N fixation indicated by decreasing δ 15N in brine shrimp and POM. Although N fixation is only taking place in the relatively freshwater inflows to GSL, data indicate that influx of fresh water influences large areas of the lake. Separation of GSL into two distinct hydrologic and geochemical systems from the construction of a railroad causeway in the late 1950s has created a persistent and widespread anoxic layer in the southern part of GSL. This anoxic layer, referred to as the deep brine layer (DBL), has high rates of SO 4 2 - reduction, likely increasing the Hg methylation capacity. High concentrations of methyl mercury (CH3Hg) (median concentration = 24 ng/L) were observed in the DBL with a significant proportion (31–60%) of total Hg in the CH3Hg form. Hydroacoustic and sediment-trap evidence indicate that turbulence introduced by internal waves generated during sustained wind events can temporarily mix the elevated CH3Hg concentrations in the DBL with the more biologically active upper brine layer (UBL). Brine shrimp collected during the summer/fall time periods contained elevated Hg concentrations (median concentration = 0.34 mg/kg, dry weight (dw)) relative to samples collected during the spring (median concentration < 0.2 mg/kg, dw). Higher Hg in brine shrimp during the summer and fall may reflect the higher proportion of adult brine shrimp during this time period, resulting in an increased time for bioaccumulation of Hg. Eared grebes (Podiceps nigricollis) consume brine shrimp from GSL during the fall molting period. Median Hg concentrations in eared grebe livers increased by almost three times during the 3–5 month fall molting period. Selected duck species utilizing GSL have consistently exceeded the US Environmental Protection Agency (USEPA) screening level for Hg (0.3 mg/kg Hg wet weight), resulting in the issuance of warnings against unlimited human consumption of breast muscle tissue.