A.E. Milodowski

Distal deposition of tephra from the Eyjafjallajökull 2010 summit eruption

The 2010 Eyjafjallajokull lasted 39 days and had 4 different phases, of which the first and third (14-18 April and 5-6 May) were most intense. Most of this period was dominated by winds with a northerly component that carried tephra toward Europe, where it was deposited in a number of locations and was sampled by rain gauges or buckets, surface swabs, sticky-tape samples and air filtering. In the UK, tephra was collected from each of the Phases 1-3 with a combined range of latitudes spanning the length of the country. The modal grain size of tephra in the rain gauge samples was 25 mu m, but the largest grains were 100 mu m in diameter and highly vesicular. The mass loading was equivalent to 8-218 shards cm(-2), which is comparable to tephra layers from much larger past eruptions. Falling tephra was collected on sticky tape in the English Midlands on 19, 20 and 21st April (Phase 2), and was dominated by aggregate clasts (mean diameter 85 mu m, component grains <10 mu m). SEM-EDS spectra for aggregate grains contained an extra peak for sulphur, when compared to control samples from the volcano, indicating that they were cemented by sulphur-rich minerals e. g. gypsum (CaSO4 center dot H2O). Air quality monitoring stations did not record fluctuations in hourly PM10 concentrations outside the normal range of variability during the eruption, but there was a small increase in 24-hour running mean concentration from 21-24 April (Phase 2). Deposition of tephra from Phase 2 in the UK indicates that transport of tephra from Iceland is possible even for small eruption plumes given suitable wind conditions. The presence of relatively coarse grains adds uncertainty to concentration estimates from air quality sensors, which are most sensitive to grain sizes <10 mu m. Elsewhere, tephra was collected from roofs and vehicles in the Faroe Islands (mean grain size 40 mu m, but 100 mu m common), from rainwater in Bergen in Norway (23-91 mu m) and in air filters in Budapest, Hungary (2-6 mu m). A map is presented summarizing these and other recently published examples of distal tephra deposition from the Eyjafjallajokull eruption. It demonstrates that most tephra deposited on mainland Europe was produced in the highly explosive Phase 1 and was carried there in 2-3 days.

Rate and mechanism of the reaction of silicates with cement pore fluids

Abstract The reaction mechanisms and rates of reaction of a number of the common rock-forming silicates with synthetic cement pore fluids have been evaluated in a series of laboratory experiments at 70°C. Mass transfer is dominated by the dissolution of the primary silicate and the precipitation of a range of Na-K-Al substituted calcium silicate hydrates, and a possible zeolite. Calcium was lost from, and silicon gained by, the fluid phase as a result of the reactions. Secondary solids formed thick layers on primary silicates, but dissolution of the silicates was not diffusion-limited. The rate of dissolution of the silicates was determined to be 2–3 orders of magnitude greater at pH 12–13 than at neutral pH, and confirm measurements by other authors. The rate of growth of calcium silicate hydrates was limited by the rate of supply of silicon from the primary silicates. Although the results of the laboratory experiments were dominated by the loss of calcium from the fluid and the precipitation of calcium silicate hydrates, thermodynamic modelling suggests that these may be replaced by zeolites and/or feldspars when groundwater residence times are considered.

A natural analogue of high pH cement pore waters from the Maqarin area of northern Jordan. I: introduction to the site

Abstract The highly alkaline springs of the Maqarin area of northern Jordan are currently under examination as part of an international project testing the models used to analyse the safety of repositories for low and intermediate level radioactive waste (L/ILW). The Maqarin area contains a rock-groundwater system which is an ideal natural analogue of a concrete-filled L/ILW repository emplaced in a sedimentary host rock. The high pH (12.5) groundwaters at this site are the product of interaction with naturally occurring cement minerals and not of the alteration of ultramafic minerals. Stable isotope data for the hyperalkaline groundwaters lie below both the local meteoric water line and the eastern Mediterranean water line. At least part of the shift appears to be the result of groundwater interaction with the cement minerals. This paper introduces the site of the natural analogue study and, in presenting novel data on the chemistry and stable isotopic signature of the groundwater, provides background information which is used in a companion paper (Alexander et al., 1992) on testing the predictive capabilities of geochemical thermodynamic codes (and their associated databases) which will be utilised as part of the safety assessment of a L/ILW repository.

Evaluation of a method for identification of host physico-chemical phases for trace metals and measurement of their solid-phase partitioning in soil samples by nitric acid extraction and chemometric mixture resolution

A sequential extraction methodology, designed to measure the solid phase partitioning of metals in soils and sediments, is described. The method uses centrifugation to pass increasing concentrations of HNO3 through the sample, followed by ICP-AES analysis of major and trace elements of the extracts. A data-processing algorithm is used to identify the number of physico-chemical components extracted, their composition and the proportion of each in each extract. The algorithm has been successfully tested on a synthetic data set and the combination of the extraction methodology and data-processing algorithm have been tested on a contaminated soil sample (NIST SRM 2710). The 14 extracts from each duplicate experiment were analysed for 19 elements and data analysis identified nine chemically distinct soil components: pore-water residual solutes; organic, easily exchangeable; a Cu–Zn dominated phase; a Pb-dominated phase; amorphous Fe oxide/oxyhydroxide; crystalline Fe oxide; Fe–Ti oxide; and Mn oxide.

The migration of uranium into peat-rich soils at Broubster, Caithness, Scotland, U.K.

Abstract Uranium is being actively transported from uraniferous sedimentary rocks into a peat bog at the Broubster natural analogue site in Caithness, Scotland. Massive calcareous sandstone within the Caithness Flags sequence is the main source of uranium which resides primarily within diagenetic apatite and dispersed USiTi phases. Supergene weathering processes have decalcified the sandstone and are effective in mobilising uranium by groundwater leaching, primarily along a fault zone. Uranium transport in solution by means of groundwater and surface flows is effectively terminated by retardation within 4-kyr-old peat deposits laid down on boulder clay. This process of secondary fixation has resulted in a young uranium anomaly in the peat which comprises in excess of 0.1 wt% U. The site has been investigated comprehensively to define the geometry of the anomaly together with the hydrogeology, hydrochemistry, petrology, mineralogy and the nature of the peat sink-term. The main physical and geochemical properties of the system, including the uranium decay series radionuclide distributions in water and solid samples, are documented in this paper. From these data, the processes governing the distribution of uranium have been quantified using a three-dimensional groundwater flow package and an equilibrium speciation model incorporating a recently developed electrostatic surface complexation model to account for cation-organic interactions. The results described form part of a coordinated project on natural radionuclide migration undertaken to improve confidence in predictive methods used for radiological assessment.

Hydrothermal alteration of granite by meteoric fluid: an example from the Carnmenellis Granite, United Kingdom

The interaction of granitic rock with meteoric fluid is instrumental in determining the chemistry of pore fluids and alteration mineralogy in downflow portions of convective groundwater circulation cells associated with many hydrothermal systems in the continental crust. Hydrothermal experiments and a detailed mineralogical study have been carried out to investigate the hydrothermal alteration of the Carnmenellis Granite, Cornwall, UK. Samples of drill chippings from a borehole 2 km deep in the Carnmenellis Granite have been reacted with a dilute Na-HCO3-Cl fluid in hydrothermal solution equipment at temperatures of 80°, 150° and 250° C and a pressure of 50 MPa, with a water/rock mass ratio of 10, for experiment durations up to 200 days. Fluid samples were analysed for seventeen different chemical components, and solids were examined prior to, and after reaction using SEM, electron microprobe and conventional light optic techniques. Experimental fluids were mildly alkaline (pH 7–8.5) and of low salinity (TDS <800 mgl−1). Mineral-fluid reaction was dominated by the dissolution of plagioclase and the growth of smectite, calcite (at all temperatures), laumontite (at 150° C), wairakite and anhydrite (at 250° C). Final fluids were saturated with respect to quartz and fluorite. Certain trace elements (Li, B, Sr) were either incorporated into solids precipitated during the experiments or sorbed onto mineral surfaces and cannot be considered as ‘conservative’ (partitioned into the fluid phase) elements. Concentrations of all analysed chemical components showed net increases during the experiments except for Ca (at 250° C) and Mg (at all temperatures). A comparison of the alteration mineralogy observed in the experiments with that present as natural fracture infills in drillcore from the Carnmenellis Granite reveals that the solid products from the experiments correspond closely to mineral assemblages identified as occurring during the later stages of hydrothermal circulation associated with the emplacement of the granite.

Can CO2 hydrate assist in the underground storage of carbon dioxide

Abstract The sequestration of CO2 in the deep geosphere is one potential method for reducing anthropogenic emissions to the atmosphere without necessarily incurring a significant change in our energy-producing technologies. Containment of CO2 as a liquid and an associated hydrate phase, under cool conditions, offers an alternative underground storage approach compared with conventional supercritical CO2 storage at higher temperatures. We briefly describe conventional approaches to underground storage, review possible approaches for using CO2 hydrate in CO2 storage generally, and comment on the important role CO2 hydrate could play in underground storage. Cool underground storage appears to offer certain advantages in terms of physical, chemical and mineralogical processes, which may usefully enhance trapping of the stored CO2. This approach also appears to be potentially applicable to large areas of sub-seabed sediments offshore Western Europe.

Influence of bacteria on rock-water interaction and clay mineral formation in subsurface granitic environments

Abstract Studies of the subsurface microbiology of the Äspö Hard Rock Laboratory, Sweden have revealed the presence of many different bacteria in the deep groundwaters which appear to maintain reducing conditions. Experiments were conducted to study the rock-water and microbial interactions. These used crushed Äspö diorite, Äspö groundwater and iron- and sulphate-reducing bacteria in flowing systems under anaerobic conditions. In column experiments, there was evidence of loss and mobilization of fine-grained crushed material (<5 μm) which had originally adhered to grain surfaces in the starting material. The mobilized fines were trapped between grains. The degree of mineralogical alteration was greater in the experiments when bacteria were present. In both column and continuously stirred reactor experiments, there is evidence for the formation of a secondary clay. These experiments have shown that microbial activity can influence rock-water interactions even in nutrient-poor conditions.

Evidence for gas-induced pathways in clay using a nanoparticle injection technique

Abstract Corrosion, water radiolysis and microbial degradation will result in the generation of gas within repositories designed for the geological disposal of high-level radioactive waste. It is therefore crucial in the design of such facilities that the relevant mechanisms allowing gas migration through repository materials, both engineered barriers and clay-based candidate host rocks, are correctly identified. In Belgium, the Boom Clay represents a candidate host material for which the advective gas breakthrough characteristics and transport properties have been extensively tested and are well defined by numerous studies. The Boom Clay displays a significant capacity for self-sealing and both laboratory and field tests indicate that advective gas transport occurs not by visco-capillary flow, but instead through the formation of pressure-induced dilatant pathways. In this study, we present results from a gas injection test designed to demonstrate the presence of these features by injecting nanoparticulate tracers with helium gas into a sample of Boom Clay. The results provide conclusive evidence for the formation of transient, dilatant gas pathways within a candidate clay-based host rock. This technique provides a novel diagnostic tool for the identification of processes governing multi-phase flow, supporting robust long-term assessments of repository performance.

Analcime reactions at 25–90°C in hyperalkaline fluids

Abstract Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi2O6‧H2O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite. Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 90°C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12-13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 70°C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi2O6. Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25-70°C and with pH at each temperature. Absolute rates of dissolution ranged from 10-10 mol m-2 s-1 at pH 9.5 at 25°C to 10-7 mol m-2 s-1 at pH 12 at 70 and 90°C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (aH⁺)n, where k is the rate constant and n is -0.3 at 25°C, -0.4 at 50°C, -0.6 at 70°C and -0.7 at 90°C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 90°C were unsuccessful, although a limiting rate at 70°C, pH 10 was calculated to be 4 × 10-11 mol m-2 s-1, roughly 100× less than the rate of dissolution under the same conditions. These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K+ in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids.