A chromate-contaminated site in southern Switzerland – Part 1: Site characterization and the use of Cr isotopes to delineate fate and transport

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 1000 kg 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 120 m long flow path was estimated at an average linear groundwater velocity of 3.3 m/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.     

Reactive transport modeling of uranium 238 and radium 226 in groundwater of the Königstein uranium mine, Germany

Knowledge of the transport behavior of radionuclides in groundwater is needed for both groundwater protection and remediation of abandoned uranium mines and milling sites. Dispersion, diffusion, mixing, recharge to the aquifer, and chemical interactions, as well as radioactive decay, should be taken into account to obtain reliable predictions on transport of primordial nuclides in groundwater. This paper demonstrates the need for carrying out rehabilitation strategies before closure of the Königstein in-situ leaching uranium mine near Dresden, Germany. Column experiments on drilling cores with uranium-enriched tap water provided data about the exchange behavior of uranium. Uranium breakthrough was observed after more than 20 pore volumes. This strong retardation is due to the exchange of positively charged uranium ions. The code TReAC is a 1-D, 2-D, and 3-D reactive transport code that was modified to take into account the radioactive decay of uranium and the most important daughter nuclides, and to include double-porosity flow. TReAC satisfactorily simulated the breakthrough curves of the column experiments and provided a first approximation of exchange parameters. Groundwater flow in the region of the Königstein mine was simulated using the FLOWPATH code. Reactive transport behavior was simulated with TReAC in one dimension along a 6000-m path line. Results show that uranium migration is relatively slow, but that due to decay of uranium, the concentration of radium along the flow path increases. Results are highly sensitive to the influence of double-porosity flow.     

Fractionation,distribution and transport of mercury in rivers and tributaries around Wanshan Hg mining district,Guizhou Province,Southwestern China: Part 2 – Methylmercury

Water samples were collected during normal flow (2007) and during a drought period (2008) from five rivers and tributaries draining the Wanshan Hg mining district, Guizhou, China. Unfiltered methylmercury (MeHg) as well as particulate and dissolved fractions of MeHg (P-MeHg, D-MeHg) were measured to assess the spatial and temporal variation of MeHg contamination in the local river system. Most locations (about 80%) displayed higher MeHg concentrations during drought period than during normal discharge conditions. Concentrations of MeHg during the drought period ranged from <0.035 to 11 ng L⁻¹ (geometric mean: 0.43), while during normal flow the concentrations ranged from <0.035 to only 3.4 ng L⁻¹ (geometric mean: 0.21). Concentrations of MeHg were positively correlated with total Hg (THg) concentrations (R² = 0.20–0.58, P < 0.001) and inversely related to distance from the calcines, during both sampling periods (R² = 0.34 and 0.23, P < 0.001, for low and normal flow, respectively) indicating that calcines may be important sources of MeHg to the downstream environment. Approximately 39% of MeHg was bound to particulates and the rest was transported in the dissolved phase along stretches of the entire river, which was different from THg, as this was mainly transported bound to particulates (commonly more than 80%).     

Reactive transport simulation study of geochemical CO2 trapping on the Tokyo Bay model – With focus on the behavior of dawsonite

A long-term (up to 10 ka) geochemical change in saline aquifer CO₂ storage was studied using the TOUGHREACT simulator, on a 2-dimensional, 2-layered model representing the underground geologic and hydrogeologic conditions of the Tokyo Bay area that is one of the areas of the largest CO₂ emissions in the world. In the storage system characterized by low permeability of reservoir and cap rock, the dominant storage mechanism is found to be solubility trapping that includes the dissolution and dissociation of injected CO₂ in the aqueous phase followed by geochemical reactions to dissolve minerals in the rocks. The CO₂–water–rock interaction in the storage system (mainly in the reservoir) changes the properties of water in a mushroom-like CO₂ plume, which eventually leads to convective mixing driven by gravitational instability. The geochemically evolved aqueous phase precipitates carbonates in the plume front due to a local rise in pH with mixing of unaffected reservoir water. The carbonate precipitation occurs extensively within the plume after the end of its enlargement, fixing injected CO₂ in a long, geologic period.Dawsonite, a Na–Al carbonate, is initially formed throughout the plume from consumption of plagioclase in the reservoir rock, but is found to be a transient phase finally disappearing from most of the CO₂-affected part of the system. The mineral is unstable relative to more common types of carbonates in the geochemical evolution of the CO₂ storage system initially having formation water of relatively low salinity. The exception is the reservoir-cap rock boundary where CO₂ saturation remains very high throughout the simulation period.     

Dissolution kinetics of selected natural minerals relevant to potential CO2-injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2-bearing brines

Chemical interaction processes among injected CO₂, saline fluids and potential reservoir materials are experimentally simulated to derive dissolution rates of natural materials (minerals) that can be used as input parameters for modeling of CO₂ storage in deep saline formations and risk analyses. In order to study dissolution processes, mineral aliquots were exposed to CO₂-bearing brines at elevated temperature (60, 100, 150 °C) and pressure (85 bar) and at various run durations. Several potential reservoir rocks include carbonates as cement. Calcite and dolomite grains were therefore mainly used as solid starting material. Experiments with the two feldspar varieties alkali feldspar and almost pure anorthite were performed in addition. Grain sizes of the mineral starting materials varied between <63 μm and 500 μm with most experiments performed at grain size fractions of 160 – 250 μm and 250 – 500 μm. All experiments run with a complex synthetic brine (total dissolved solids: ∼156 g/l) according to a natural upper cretaceous formation water. Dry ice was used as CO₂-source. All experiments were done in closed batch reactors. These reactors allow mimicking reservoir conditions far from the injection site as well as reservoir conditions after finishing the CO₂ injection. The concentration changes during the experiment were monitored by ICP-OES measurements of the initial and the post-run fluids. Dissolution rates were derived based on the concentration changes of the brine.Most of the studied experimental variables and parameters (temperature, run duration, grain size, brine composition – expressed as pH-value and ionic strength) impact alteration of the reacting agents, i.e. they change the chemical composition of the brine, change the surfaces of the mineral aliquots exposed to the CO₂-bearing brine, and induce formation of secondary minerals. Hence, all influencing parameters on dissolution processes have to be considered and time-resolved changes of the dissolution behavior have to be implemented in numerical simulations of processes at CO₂ injection sites and CO₂ storage reservoirs.     

Large and small numbers: options for quantifying the costs of extremes on transport now and in 40 years

The past 15 years has been characterized by a high density of record-breaking weather extremes in Europe. These include the extratropical cyclones Lothar, Kyrill and Xynthia across western and mid Europe; the major floods in the UK, Germany and eastern Europe; the heat waves in 2003 and 2007 and, after a long period of mild winters, the heavy winter seasons in 2009/2010 and 2010/2011. Inspired by the rising trends in weather-related damages worldwide and the studies on the topic carried out in overseas, the EC funded the sister projects such as Weather Extremes-Assessing the Impacts on Transport and Hazards for European Regions (WEATHER), EWENT and ECCONET to determine the magnitude of current and future risks to the European transport sector and to assess suitable adaptation strategies. This paper presents the assessment framework of the WEATHER project and the results of weather-inflicted damage costs now and in 2040–2050. Total annual damages are found to be around €2.5 billion, which largely attribute to road traffic. However, broken down to passenger and ton kilometers the highest risk is borne by rail traffic due to its expensive infrastructures and its comparably complex operating structure. This indication even amplifies when looking four decades ahead: while average road transport costs will only raise by 7 % due to milder winters, rail traffic costs may increase by up to 80 % due to more floods and less predictable winter periods. A comparison with the results of the EWENT study uncovers a high range of uncertainty concerning methodological approaches and data treatment. Thus, the final figures might even be much higher.     

Dissolution kinetics of selected natural minerals relevant to potential CO2-injection sites − Part 1: A review

This publication provides a literature review on experimental studies of dissolution kinetics of mainly carbonates and feldspar group minerals, i.e. most common minerals at potential CO₂-injection and/or storage sites. Geochemical interaction processes between injected CO₂ and coexisting phases, namely reservoir and cap rock minerals and formation fluids close to the CO₂-injection site can be simulated by flow-through or mixed flow reactors, while processes far from the injection site and long-term processes after termination actual CO₂-injection can be mimicked by batch reactors. At sufficient small stirring rates or fluid flow rates as well as low solute concentrations flow-through reactors are also able to simulate processes far from the injection site. The experimental parameter temperature not only intensifies the dissolution process, the dominant dissolution mechanisms are also influenced by temperature. The dissolution mechanisms change from incongruent and surface controlled mechanisms at lower temperatures to congruent and transport controlled mechanisms at higher temperatures. The CO₂ partial pressure has only a second order influence on dissolution behavior compared to the influence of pH-value and ionic strength of the CO₂-bearing brine. Minerals exposed to CO₂-bearing brines at elevated temperatures and pressures are subject of alteration, leading to severe changes of reactive surfaces and potential precipitation of secondary minerals.Computational simulations of mineral reactions at potential CO₂ storage sites have therefore to include not only the time-resolved changes of dissolution behavior and hence kinetics of mineral dissolution, but also the influence of secondary minerals on the interaction of the minerals with CO₂-enriched brines.     

Stochastic continuum modeling of flow and transport in a crystalline rock mass: Fanay-Augères,France, revisited

A stochastic discrete-fracture model was used by Cacas et al.^a,b to interpret flow measurements and transport experiments in a fractured crystalline rock mass at Fanay-Augères. They considered continuum models to be incapable of properly interpreting small-scale measurements or tracer tests in fractured systems, which, in their view, require three-dimensional modeling of numerous discrete channels; in their opinion, continuum modeling applies only to average flow on a relatively large scale. Cacas et al. considered their discrete fracture model to have been validated by its demonstrated ability to reproduce selected experimental results. In this paper, flow and transport at Fanay-Augères are modeled by viewing the fractured rock as a stochastic continuum in a manner originally proposed by Neuman^c,d. The stochastic continuum approach obviates the need for detailed information about fracture geometry or assumptions about how individual fractures control flow and transport. All it requires is the delineation of a few dominant features, which can be embedded into the stochastic continuum model as heterogeneous porous slabs. Though a fault zone has been identified at the Fanay-Augères experimental site, it has been modeled neither by Cacas et al. nor in this paper. In fact, in this paper, a larger selection of experimental results than those considered by Cacas et al. are reproduced merely by modeling the rock as a statistically homogeneous continuum in two dimensions. These results demonstrate that a continuum approach may be well suited for the analysis of flow and transport in fractured rock. This does not constitute a validation of the continuum approach, just as the results of Cacas et al. fall short of validating the discrete fracture approach. Instead, the two sets of results illustrate jointly the well-established principle that an open system, especially one as complex as fractured hydrogeologic environments tend to be, cannot be described uniquely on the basis of sparse data and need not be described in great detail to capture its salient behavior by a model.^a Cacas MC, Ledoux E, de Marsily G, Barbreau A, Calmels P, Gaillard B, Margritta R (1990a) Modelling fracture flow with a stochastic discrete fracture network: calibration and validation. 1. The flow model. Water Resour Res 26(3):479–489^b Cacas MC, Ledoux E, de Marsily G, Barbreau A, Calmels P, Gaillard B, Margritta R (1990b) Modelling fracture flow with a stochastic discrete fracture network: calibration and validation. 2. The transport model. Water Resour Res 26(3):491–500^c Neuman SP (1987) Stochastic continuum representation of fractured rock permeability as an alternative to the REV and fracture network concepts, in Rock Mechanics. In: Farmer IW, Daemen JJK, Desai CS, Glass CE, Neuman SP (eds) Proceedings of the 28th U.S. Symposium, Tucson, Arizona. Balkema, Rotterdam, pp 533–561^d Neuman SP (1988) A proposed conceptual framework and methodology for investigating flow and transport in Swedish crystalline rocks. SKB Swedish Nuclear Fuel and Waste Management Co., Stockholm, September, Arbetsrapport 88–37, 39 pp     

A locally conservative Eulerian–Lagrangian numerical method and its application to nonlinear transport in porous media

Eulerian-Lagrangian and Modified Method of Characteristics (MMOC) procedures provide computationally efficient techniques for approximating the solutions of transport-dominated diffusive systems. The original MMOC fails to preserve certain integral identities satisfied by the solution of the differential system; the recently introduced variant, called the MMOCAA, preserves the global form of the identity associated with conservation of mass in petroleum reservoir simulations, but it does not preserve a localized form of this identity. Here, we introduce an Eulerian-Lagrangian method related to these MMOC procedures that guarantees conservation of mass locally for the problem of two-phase, immiscible, incompressible flow in porous media. The computational efficiencies of the older procedures are maintained. Both the original MMOC and the MMOCAA procedures for this problem are derived from a nondivergence form of the saturation equation; the new method is based on the divergence form of the equation. A reasonably extensive set of computational experiments are presented to validate the new method and to show that it produces a more detailed picture of the local behavior in waterflooding a fractally heterogeneous medium. A brief discussion of the application of the new method to miscible flow in porous media is included. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fractionation,distribution and transport of mercury in rivers and tributaries around Wanshan Hg mining district,Guizhou province,southwestern China: Part 1 – Total mercury

The Wanshan Hg mining area in Guizhou, China, was one of the world’s largest Hg producing regions. Numerous mine-waste and calcines still remain, leaching Hg to local rivers and streams and potentially impacting the local population. Several studies have been published on local environmental impacts of these mining and retorting residues, but a comprehensive, regional survey on the distribution of Hg in the rivers in the region, as presented in this paper, has not previously been conducted. This study focuses on the regional distribution and temporal variation of aqueous Hg fractions in the five main watercourses draining the Wanshan Hg mining and retorting area, covering more than 700 km². Three sampling campaigns were carried out in 2007 and 2008, covering high flow, normal flow and low flow periods. Total (THg), particulate (PHg), dissolved (DHg) and reactive (RHg) Hg fractions were determined. All rivers had the highest Hg concentrations at sample sites about 100–500 m downstream of the mine wastes. Total Hg concentrations ranged from extremely high (up to 12,000 ng L⁻¹) at the sample site just 100 m below mine wastes, to quite low in tributary streams (1.9 ng L⁻¹, about 14 km downstream of the mine wastes). Total Hg and PHg concentrations were usually highest during high flow periods in the Hg-contaminated areas (i.e. THg ? 50 ng L⁻¹), while in the less-impacted downstream areas (with THg < 50 ng L⁻¹) the Hg concentrations were usually lowest during high flow periods. Although highly elevated concentrations of Hg in water samples were found just downstream of the mine wastes, the concentrations decreased sharply to well below 50 ng L⁻¹ (US EPA Hg concentration standard for protection of fresh water), within only 6–8 km downstream. Concentrations of THg were highly dominated by and correlated with PHg (R² = 0.996–0.999, P < 0.001); PHg constituted more than 80% of THg in Hg-contaminated areas, and could account for 99.6% of the THg close to the mine wastes.     

Rockfalls into forests: Analysis and simulation of rockfall trajectories — considerations with respect to mountainous forests in Switzerland

Rockfalls are a major threat to settlements and transportation routes in many places. Although the general protective effect of forests against rockfalls is currently not questioned, little is known about the ideal properties of a forest stand that provides good protection. Therefore, in this study the question was assessed of how mountainous forests may influence rockfalls of single boulders. An actual rockfall trajectory was measured, recorded, analysed and simulated with a rockfall model. Rockfalls into different forest scenarios were also modelled for the site. Results showed that the actual rockfall event can be well simulated. Furthermore, a completely forested slope reduces velocity and energy of the falling blocks much better than a sparsely forested slope. For the profile discussed in this paper, the largest effect upon falling 3 m³ blocks was obtained with a high forest containing 350 trees per ha. The results confirmed common assumptions on ideal properties of a protective forest stand against rockfalls.     

A surge response function approach to coastal hazard assessment – part 1: basic concepts

This paper reviews historical methods for estimating surge hazards and concludes that the class of solutions produced with Joint Probability Method (JPM) solutions provides a much more stable estimate of hazard levels than alternative methods. We proceed to describe changes in our understanding of the winds in hurricanes approaching a coast and the physics of surge generation that have required recent modifications to procedures utilized in earlier JPM studies. Of critical importance to the accuracy of hazard estimates is the ability to maintain a high level of fidelity in the numerical simulations while allowing for a sufficient number of simulations to populate the joint probability matrices for the surges. To accomplish this, it is important to maximize the information content in the sample storm set to be simulated. This paper introduces the fundamentals of a method based on the functional specification of the surge response for this purpose, along with an example of its application in the New Orleans area. A companion paper in this special issue (Irish et al. 2009) provides details of the portion of this new method related to interpolating/extrapolating along spatial dimensions.     

Petrogenesis of post – orogenic Lower Permian andesites in southern Europe: insights into the collapse of the Variscan range

In the post-Variscan Early Permian deposits of southwestern Europe, andesites belong to successions characterized by the constant occurrence of a basal rhyolite ignimbrite, followed by andesites, and by abundant dacite-rhyodacite products, interbedded with lacustrine sediments. The subalkaline andesite to K-andesite volcanism developed within intramontane basins following the collapse of the Variscan orogen. The compositional features, including trace element ratios and initial Sr (and Nd) of post-Variscan intermediate products allow excluding continental or island arc settings. The andesite composition of Lower Permian lavas was reproduced starting from two potential primary magmas: picrobasaltic and enriched MORB compositions. In particular, the picrobasalt was modified, by adding incremental amounts of a felsic granulite, from the lower continental crust. Mixing and fractional crystallization (MFC) were modelled by the MELTS software in a closed system under isobaric, isenthalpic conditions (P = 0.3 GPa). The computed residual liquid evolves to match closely the composition of Lower Permian andesite lavas after ~34 % crystallization by adding ~27 % of contaminant to the parental magma.     

The Bastar craton,central India: A window to Archean – Paleoproterozoic crustal evolution

The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3 Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.     

Samovar: a thermomechanical code for modeling of geodynamic processes in the lithosphere—application to basin evolution

We present a new 2D finite difference code, Samovar, for high-resolution numerical modeling of complex geodynamic processes. Examples are collision of lithospheric plates (including mountain building and subduction) and lithosphere extension (including formation of sedimentary basins, regions of extended crust, and rift zones). The code models deformation of the lithosphere with viscoelastoplastic rheology, including erosion/sedimentation processes and formation of shear zones in areas of high stresses. It also models steady-state and transient conductive and advective thermal processes including partial melting and magma transport in the lithosphere. The thermal and mechanical parts of the code are tested for a series of physical problems with analytical solutions. We apply the code to geodynamic modeling by examining numerically the processes of lithosphere extension and basin formation. The results are directly applicable to the Basin and Range province, western USA, and demonstrate the roles of crust–mantle coupling, preexisting weakness zones, and erosion rate on the evolutionary trends of extending continental regions. Modeling of basin evolution indicates a critical role of syn-rift sedimentation on the basin depth and a governing role of Peierls deformation in cold lithospheric mantle. While the former may increase basin depth by 50%, the latter limits the depth of rift basins by preventing faulting in the subcrustal lithosphere.