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Abstract: Marine euxinic sediments, particularly organic-rich black shales, are important sinks for oceanic molybdenum (Mo), and the determination of Mo concentration and isotopic composition are used to constrain oxygenation state and specifically expansion of marine anoxic and sulfidic (euxinic) waters in ancient oceans. The use of Mo as a paleo-redox tracer is based on its distinct geochemical behavior in oxic and sulfidic environments. Mo removal from sulfidic waters starts with MoO4 2− reacting with aqueous H2S to form particle reactive thiomolybdates, MoO4− x S x 2−, but the post-thiomolybdate steps and the ultimate Mo host in euxinic sediments are not well understood. We used X-ray absorption fine structure (XAFS) spectroscopy to determine the oxidation state and the molecular coordination environment of pristine, solid phase Mo in sediments from permanently euxinic Lake Cadagno, Switzerland. Samples were taken from a 9-meter piston core representing the last ten thousand years of deposition. The euxinic lake sediments provide unequivocal evidence that the post-thiomolybdate steps along the burial pathway involve Mo(VI) reduction to Mo(IV) before it is ultimately hosted in the sediments. Anoxic samples contain Mo(IV)–S compounds that, when oxidized upon air-exposure, are transformed into Mo(VI)–O, thus confirming our results. This observation is important to better understand the Mo burial pathway from oxic waters to euxinic sediments. Our results support a model where post-thiomolybdate MoVI reduction proceeds via reactions with zero-valent sulfur, S(0) (mainly S8, Vorlicek et al., 2004). In this model, Mo is scavenged from sulfidic waters as reactive Mo-polysulfide species, Mo(IV)O(S4)S2− or Mo(IV)S(S4)S2−, and not as thiomolybdate (MoOS3 2− or MoS4 2−) as has been previously assumed. This result can have important implications for how paleoenvironmental redox conditions are inferred from sedimentary Mo enrichments, Mo/TOC ratios, and δ 98Mo, if the Mo accumulation rate is accelerated via the S8-assisted pathway in settings where partial oxidation of S is possible. For example, euxinic Mo/TOC ratios are predicted not only as a function of Mo concentration in overlying seawater. Instead, the ratio will be higher in sediments deposited under highly sulfidic waters where S8 is also abundant, such as in settings where the chemocline depth fluctuates considerably and/or oxygen intrusions occur to the overlying bottom waters. Partial oxidation of dissolved sulfide is an important step in polysulfide production. Finally, we conclude that XAFS measurements are a powerful way to study the Mo burial pathway in sediments and enhance our ability to infer past environmental conditions from the sedimentary record. [Copyright &y& Elsevier]

Abstract: In this study, the chemical composition of PM10 at various sites in Switzerland during a one year period in 2008/2009 and changes since the time of a similar characterisation campaign in 1998/1999 are investigated. The concentrations of main components of PM10 were found to be similar at different site types north of the Alps (i.e. urban background, suburban and rural sites). Secondary inorganic PM10 components (nitrate, sulphate and ammonium) constituted the largest PM10 mass fraction, followed by carbonaceous matter (OM and EC), while the abundance of mineral dust and trace elements was small (both <10%). At a rural site south of the Alps the concentrations of secondary inorganic components were much lower and the concentrations of OM and EC clearly higher than north of the Alps. Enhanced concentrations of EC, trace elements and to a lesser extent of OM and mineral dust were found at an urban roadside site. At this site, typical traffic related trace elements like Cu, Fe, Mo, Sb and Ba were highly enriched. This paper indicates clear positive effects of emission reduction strategies for PM10 and precursors implemented during the past ten and more years. The concentrations of sulphate and EC declined during the considered time period, EC reductions were especially strong at the urban roadside site (−2.4μgm−3). In contrast, the concentration of nitrate slightly increased at all of the considered sites (0.2–0.5μgm−3). Large reductions in the concentrations of many trace elements in PM10 during the past ten years are evident. As an example, the observed relative decrease of Pb is about −80%, which is a success of the ban of leaded gasoline in Switzerland and in the EU. [Copyright &y& Elsevier]

Abstract: In Thun, Switzerland, a permeable reactive barrier (PRB) for Cr(VI) reduction by gray cast iron was installed in May 2008. The PRB is composed of a double array of vertical piles containing iron shavings and gravel. The aquifer in Thun is almost saturated with dissolved oxygen and the groundwater flow velocities are ca. 10–15m/day. Two years after PRB installation Cr(VI) concentrations still permanently exceed the Swiss threshold value for contaminated sites downstream of the barrier at selected localities. Groundwater δ 53/52CrSRM979 measurements were used to track Cr(VI) reduction induced by the PRB. δ 53/52CrSRM979 values of two samples downstream of the PRB showed a clear fractionation towards more positive values compared to four samples from the hotspot, which is clear evidence of Cr(VI) reduction induced by the PRB. Another downstream sample did not show a shift to more positive δ 53/52CrSRM979 values. Because this latter location correlates with the highest downstream Cr(VI) concentration it is proposed that a part of the Cr(VI) plume is bypassing the barrier. Using a Rayleigh fractionation model a minimum present-day overall Cr(VI) reduction efficiency of ca. 15% was estimated. A series of 2D model simulations, including the fractionation of Cr isotopes, confirm that only a PRB bypass of parts of the Cr(VI) plume can lead to the observed values. Additionally, the simulations revealed that the proposed bypass occurs due to an insufficient permeability of the individual PRB piles. It is concluded that with this type of PRB a complete and long-lasting Cr(VI) reduction is extremely difficult to achieve for Cr(VI) contaminations located in nearly oxygen and calcium carbonate saturated aquifer in a regime of high groundwater velocities. Additional remediation action would limit the environmental impact and allow to reach target concentrations. [Copyright &y& Elsevier]

Abstract: The risk of groundwater contamination by chromate at a former chromite ore processing industrial site in Rivera (Switzerland) was assessed by determining subsoil Cr(VI) concentrations and tracking naturally occurring Cr(VI) reduction with Cr isotopes. Using a hot alkaline extraction procedure, a total Cr(VI) contamination of several 1000kg was estimated. Jarosite, KFe3((SO4) x (CrO4)1− x )2(OH)6, and chromatite (CaCrO4) were identified as Cr(VI) bearing mineral phases using XRD, both limiting groundwater Cr(VI) concentrations. To track assumed Cr(VI) reduction at field scale δ53Cr values of contaminated subsoil samples in addition to groundwater δ53Cr data are used for the first time. The measurements showed a fractionation of groundwater δ53Cr values towards positive values and subsoil δ53Cr towards negative values confirming reduction of soluble Cr(VI) to insoluble Cr(III). Using a Rayleigh fractionation model, a current Cr(VI) reduction efficiency of approximately 31% along a 120m long flow path was estimated at an average linear groundwater velocity of 3.3m/d. Groundwater and subsoil δ53Cr values were compared with a site specific Rayleigh fractionation model proposing that subsoil δ53Cr values can possibly be used to track previous higher Cr(VI) reduction efficiency during the period of industrial activity. The findings strongly favor monitored natural attenuation to be part of the required site remediation measures. [Copyright &y& Elsevier]

Abstract: The Cr(VI) reduction behaviour of five different types of grey cast iron shavings meant for construction of a permeable reactive barrier (PRB) in Thun, Switzerland, was investigated by performing batch and column experiments. Closed system batch experiments clearly demonstrated that the largest shavings also containing the largest spherical C inclusions, were best qualified for the PRB due to their fast Cr(VI) reduction rate. However, the column experiment performed with this type of material revealed that a complete and long term Cr(VI) treatment by the planned PRB was questionable due to the site-specific high groundwater flow velocities and the almost O2 and CaCO3 saturated aquifer conditions. The experimentally observed Cr(VI) breakthrough is explained as a result of a decline of reactivity provoked by a strong passivation by observed Fe hydroxides (FeOOH-polymorphs) and carbonates (calcite and aragonite). The column experiment was simulated using the reactive transport modelling code CrunchFlow. Iron cycling and intra-aqueous reactions were incorporated into a previously described reaction network in order to model the strong decline in reactivity of grey cast iron. All key parameters like aqueous species concentrations and mineralogical evolution of the column were successfully modelled. The modelling results confirmed that the observed Cr(VI) breakthrough was caused by surface passivation of the Fe shavings. Complete oxidation of the initially present mass of shavings is not predicted to occur during the expected PRB lifetime of several decades. The model seems to be robust, and it is expected that an application of the calibrated model in 2D to field sites will allow a quantitative evaluation for the performance of planned PRB’s in such environments. [Copyright &y& Elsevier]

Abstract: The aim of this study was to compare the composition of bacterial and archaeal communities in contaminated sediments (Vidy Bay) with uncontaminated sediments (Ouchy area) of Lake Geneva using 16S rRNA clone libraries. Sediments of both sites were analysed for physicochemical characteristics including porewater composition, organic carbon, and heavy metals. Results show high concentrations of contaminants in sediments from Vidy. Particularly, high contents of fresh organic matter and nutrients led to intense mineralisation, which was dominated by sulphate-reduction and methanogenesis. The bacterial diversity in Vidy sediments was significantly different from the communities in the uncontaminated sediments. Phylogenetic analysis revealed a large proportion of Betaproteobacteria clones in Vidy sediments related to Dechloromonas sp., a group of dechlorinating and contaminant degrading bacteria. Deltaproteobacteria, including clones related to sulphate-reducing bacteria and Fe(III)-reducing bacteria (Geobacter sp.) were also more abundant in the contaminated sediments. The archaeal communities consisted essentially of methanogenic Euryarchaeota, mainly found in the contaminated sediments rich in organic matter. Multiple factor analysis revealed that the microbial community composition and the environmental variables were correlated at the two sites, which suggests that in addition to environmental parameters, pollution may be one of the factors affecting microbial community structure. [ABSTRACT FROM AUTHOR]

Abstract: Surface subsidence associated with tunnelling in fractured crystalline rock masses is rarely considered to be large enough to be a cause for concern. Recent high precision levelling measurements along the Gotthard Pass road in Central Switzerland, however, have revealed up to 12cm of subsidence along sections that pass several hundred metres above the Gotthard highway tunnel. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage is believed to be the contributing mechanism. Although these settlements may appear to be small compared to those associated with groundwater or oil and gas withdrawal from more compliant porous media, they are large enough to adversely affect the structural integrity of sensitive concrete structures on the surface (e.g. thin-arch concrete dams). This is a concern for the 57km long Gotthard Base Tunnel, which is currently under construction, as it will pass through similar geological conditions as the Gotthard highway tunnel and underneath several important dams. The prediction of the expected settlements requires a more complete understanding of the processes underlying the settlements than exist at present. This paper (Part 1) and a companion paper (Part 2) are a contribution towards this end. In Part 1 we examine the question of consolidation in fractured crystalline rock, introduce the geodetic data and the geological setting of the subsidence associated with the Gotthard highway tunnel. Possible mechanisms for the consolidation are described and estimates obtained for the key hydro-mechanical parameters that govern their action, such as the normal stiffness of fault zones. These data are used to condition analytical scoping calculations of expected subsidence based upon poro-elastic theory, and to provide input for a series of numerical models used to obtain a more detailed understanding of the data, which are presented in Part 2. [Copyright &y& Elsevier]

Wetlands have important filter functions in landscapes but are considered to be the biggest unknowns regarding their element dynamics under global climate change. Information on sink and source function of sulphur, nitrogen, organic matter and acidity in wetlands is crucial for freshwater regeneration. Recent results indicate that redox processes are not completely controlled by the sequential reduction chain (that is electron acceptor availability) but that electron donor availability may be an important regulator. Our hypothesis was that only sites which are limited in their electron donor availability (low concentrations of dissolved organic carbon (DOC)) follow the concept of the sequential reduction chain. We compared the results of two freshwater wetland systems: 1) three forested fens within a boreal spruce catchment in a low mountain range in southern Germany (high DOC regime) and 2) three floodplain soils within a groundwater enrichment area in the Rhein valley in northwest Switzerland (low DOC regime). Micro scale investigations (a few cm3) with dialyse chambers as well as soil solution and groundwater concentrations at the forested fens (high DOC regime) indicated simultaneous consumption of nitrate and sulphate with release of iron, manganese and methane (CH4) as well as an enrichment in stable sulphur isotopes indicating a co-existence of processes attributed to different redox gradients. Soil and aquifer gas measurements down to 4.6 m at the groundwater enrichment site (low DOC regime and carbon limitation) showed extreme high rates of metabolism with carbon dioxide (CO2), dinitrous oxide (N2O) and CH4 concentrations reaching fifty, thirty and three times atmospheric concentrations, respectively. Simultaneously, groundwater oxygen (O2) saturation was between 50 and 95%. We concluded that independent of DOC regime the sequential reduction chain was not a suitable concept in our systems. Instead of electron acceptor or donor availability micro site variability might explain the co-existence of redox processes within our sites. [Copyright &y& Elsevier]