Carbonation of borehole seals: comparing evidence from short-term laboratory experiments and long-term natural analogues

It is crucial that the engineered seals of boreholes in the vicinity of a deep storage facility remain effective for considerable timescales if the long-term geological containment of stored CO2 is to be effective. These timescales extend beyond those achievable by laboratory experiments or industrial experience. Study of the carbonation of natural Ca silicate hydrate (CSH) phases provides a useful insight into the alteration processes and evolution of cement phases over long-timescales more comparable with those considered in performance assessments. Samples from two such natural analogues in Northern Ireland have been compared with samples from laboratory experiments on the carbonation of Portland cement. Samples showed similar carbonation reaction processes even though the natural and experimental samples underwent carbonation under very different conditions and timescales. These included conversion of the CSH phases to CaCO3 and SiO2, and the formation of a well-defined reaction front. In laboratory experiments the reaction front is associated with localised Ca migration, localised matrix porosity increase, and localised shrinkage of the cement matrix with concomitant cracking. Behind the reaction front is a zone of CaCO3 precipitation that partly seals porosity. A broader and more porous/permeable reaction zone was created in the laboratory experiments compared to the natural samples, and it is possible that short-term experiments might not fully replicate slower, longer-term processes. That the natural samples had only undergone limited carbonation, even though they had been exposed to atmospheric CO2 or dissolved in groundwater for several thousands of years, may indicate that the limited amounts of carbonate mineral formation may have protected the CSH phases from further reaction

Palaeohydrogeology using geochemical, isotopic and mineralogical analyses: salinity and redox evolution in a deep groundwater system through Quaternary glacial cycles

Mineralogical, geochemical and isotopic analyses of secondary calcites are interpreted as part of an investigation of deep groundwaters in fractured metavolcanic rock overlain by sedimentary rock. Drillcore rock samples and groundwater samples from deep boreholes (maximum depth 1950 m) were analysed. This produces information about the evolution of salinity and redox in relation to past groundwater movements including the impacts of climatic change through the Quaternary period. Salinities of present-day groundwaters vary from dilute to brine concentrations and are related to three distinct groundwater flow regimes. Crystal morphology, stable isotopic analyses and isotopic dating, cathodoluminescence and microanalyses of Fe, Mn and REEs in the latest generation of secondary calcite, plus other analyses, have provided insights into variations of salinity over time and of redox in past groundwaters. Interpretation suggests that groundwater in the depth range of the transition from dilute to brackish/saline concentrations has been gradually diluted over time by meteoric water ingress. 230Th/234U whole-crystal ages indicate that at least part of the late-stage calcite mineralisation in the present groundwater flow system is of Quaternary age, although the mineralisation may have been initiated much earlier by meteoric invasion in the Miocene, following regional uplift. The calcites exhibit a wide range in oxygen isotope composition (δ18OPDB −2 to −22‰), although no extremely light or heavy δ13C values indicative of microbial methane oxidation or deep methanogenesis were observed. The very light δ18O values suggest that glacial or other cold-climate waters flowed to more than 700 m depth in the centre of the study area and formed a greater proportion of groundwater at that depth than at present. Fe and Ce are interpreted as semi-quantitative proxies for past redox conditions over the period when secondary calcite was deposited. Variability of Fe and Mn contents of secondary calcites in deeper rock, presently containing saline groundwater, is evidence of reducing conditions being maintained in the long term, though the strength of negative redox has probably fluctuated due to other redox-active chemistry. Depth-wise changes of groundwater redox in the past are also indicated by Ce concentrations versus other REEs in secondary calcites. Shallow calcites show a negative Ce anomaly in some growth zones due to oxidation to CeIV whilst deeper calcites do not exhibit this Ce behaviour, indicating that reducing conditions prevailed. Distribution of Fe-Mn oxyhydroxides and pyrite confirm, at a broader scale over depth and time, the findings about redox variations that secondary calcites indicate. Mineralogical and geochemical studies add further information to the understanding of past geochemical conditions in deep groundwaters in this area. Interpretations provide semi-quantitative constraints on the evolution and likely variations and directions of movement of groundwater salinity and redox over the Quaternary timescale

Biotic and abiotic controls on calcium carbonate formation in soils

Over half of the carbon (C) taking part in the global C cycle is held in terrestrial systems. Because of the sensitivity of the C cycle to changes in such soil-based pools of carbon, it is important to understand the basic mechanisms by which soil C is stored and cycled between the range of di erent pools which occur belowground. In the context of climate change mitigation, it is considered that increasing soil-based stocks of C, either by reducing losses from soils, or by actively sequestering new carbon, is a potentially important strategy . Organic carbon is the main form of carbon in soil and as such has received most focus. Cont/d.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Retention of technetium-99 by grout and backfill cements: Implications for the safe disposal of radioactive waste

Technetium-99 (99Tc) is an important radionuclide when considering the disposal of nuclear wastes owing to its long half-life and environmental mobility in the pertechnetate (Tc(VII)) redox state. Its behaviour in a range of potential cement encapsulants and backfill materials has been studied by analysing uptake onto pure cement phases and hardened cement pastes. Preferential, but limited, uptake of pertechnetate was observed on iron-free, calcium silicate hydrates (C–S–H) and aluminate ferrite monosulphate (AFm) phases with no significant adsorption onto ettringite or calcium aluminates. Diffusion of 99Tc through cured monolithic samples, representative of cements being considered for use in geological disposal facilities across Europe, revealed markedly diverse migration behaviour, primarily due to chemical interactions with the cement matrix rather than differential permeability or other physical factors. A backfill cement, developed specifically for the purpose of radionuclide retention, gave the poorest performance of all formulations studied in terms of both transport rates and overall technetium retention. Two of the matrices, pulverised fuel ash: ordinary Portland cement (PFA:OPC) and a low-pH blend incorporating fly ash, effectively retarded 99Tc migration via precipitation in narrow, reactive zones. These findings have important implications when choosing cementitious grouts and/or backfill for Tc-containing radioactive wastes

Isotopic signals in fracture-filling calcite showing anaerobic oxidation of methane in a granitic basement

Understanding the long-term redox conditions and the related carbon cycle in groundwater is essential for long-term safety assessment because they affect the performance of barrier systems and radionuclide transport in geological disposal. However, it is difficult to identify those long-term changes directly. To help understand this, we conducted a paleohydrogeological study on calcite mineralization associated with fracture-controlled groundwater flow-paths in the Toki Granite in central Japan, focusing on its carbon and oxygen stable isotope characteristics. Previous studies revealed four generations of fracture-filling calcite in the Toki Granite. Therefore, we conducted isotopic analysis on both bulk samples of calcite and spatially-resolved microsamples of discrete generations of calcite within zoned crystals. The δ18OVPDB of calcite ranging between −32.7‰ to −0.59‰ revealed that the groundwater that precipitated the calcite was derived from various origins over the geological history of the area, including early hydrothermal fluids associated with the late-stage cooling of the granite (less than −17.2‰); freshwater invasion from the surface following regional uplift (−18.5‰∼ −8.3‰), and; seawater that penetrated during periods of marine transgression (−8.7‰ ∼ −0.3‰). The range in δ13CVPDB values (−56.5‰ ∼ +6.0‰) was wider than the isotopic range of dissolved inorganic carbon (DIC) that originated from hydrothermal, meteoric, and seawater sources (−25‰ ∼ +2‰). Calcite with low δ13CVPDB values less than −25‰ is believed to have precipitated from groundwater with DIC that was provided by anaerobic oxidation of methane (AOM), whereas calcite with δ13CVPDB higher than +2‰ is believed to have precipitated from groundwater containing 13C-enriched DIC as a carbon source derived during methanogenesis. These processes influencing the formation of calcite mineralization in the Toki Granite are comparable to those at other crystalline rock sites in European countries. The AOM calcite and calcite associated with methanogenesis in the Toki Granite precipitated during the transition of the groundwater origin from meteoric to seawater. Understanding these redox processes and the related carbon cycle in granitic groundwater can provide important insights into processes relevant to assessing the long-term evolution of geoenvironmental systems

Assessing the long-term behaviour of the industrial bentonites employed in a repository for radioactive wastes by studying natural bentonites in the field

Bentonite makes an important contribution to the performance of the engineered barriers in most radioactive waste repository designs. The choice of bentonite results from its favourable properties for waste isolation and its stability in relevant geological environments. However, the longevity of bentonite (especially the resistance to waste container sinking) has been little studied. Modelling results suggest significant bentonite deformation and associated canister sinking is unlikely and, here, long-term natural system data are used as a reality check on model predictions. Results indicate that bentonite from the investigated site shows no significant deviation in bulk physical parameters from repository bentonite. However, micro-scale shear planes can be seen throughout the sampled cores. The presence of multi-directional S- and C-type shears suggests they originate from loading from the overlying limestone, not gravitational tectonics. The plastic limits and angles of shearing resistance for natural and repository bentonites suggest both are susceptible to shearing. The impact of bentonite shear under load could be minimised by appropriate design, but existing lower activity waste container designs do not consider the potentially high external stresses from the bentonite backfill and this should be addressed in future

Subsurface microbial hydrogen cycling: natural occurrence and implications for industry

Hydrogen is a key energy source for subsurface microbial processes, particularly in subsurface environments with limited alternative electron donors, and environments that are not well connected to the surface. In addition to consumption of hydrogen, microbial processes such as fermentation and nitrogen fixation produce hydrogen. Hydrogen is also produced by a number of abiotic processes including radiolysis, serpentinization, graphitization, and cataclasis of silicate minerals. Both biotic and abiotically generated hydrogen may become available for consumption by microorganisms, but biotic production and consumption are usually tightly coupled. Understanding the microbiology of hydrogen cycling is relevant to subsurface engineered environments where hydrogen-cycling microorganisms are implicated in gas consumption and production and corrosion in a number of industries including carbon capture and storage, energy gas storage, and radioactive waste disposal. The same hydrogen-cycling microorganisms and processes are important in natural sites with elevated hydrogen and can provide insights into early life on Earth and life on other planets. This review draws together what is known about microbiology in natural environments with elevated hydrogen, and highlights where similar microbial populations could be of relevance to subsurface industr

Northern Mediterranean climate since the Middle Pleistocene: a 637 ka stable isotope record from Lake Ohrid (Albania/Macedonia)

Lake Ohrid (Macedonia/Albania) is an ancient lake with unique biodiversity and a site of global significance for investigating the influence of climate, geological, and tectonic events on the generation of endemic populations. Here, we present oxygen (δ18O) and carbon (δ13C) isotope data from carbonate over the upper 243 m of a composite core profile recovered as part of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project. The investigated sediment succession covers the past ca. 637 ka. Previous studies on short cores from the lake (up to 15 m, < 140 ka) have indicated the total inorganic carbon (TIC) content of sediments to be highly sensitive to climate change over the last glacial–interglacial cycle. Sediments corresponding to warmer periods contain abundant endogenic calcite; however, an overall low TIC content in glacial sediments is punctuated by discrete bands of early diagenetic authigenic siderite. Isotope measurements on endogenic calcite (δ18Oc and δ13Cc) reveal variations both between and within interglacials that suggest the lake has been subject to palaeoenvironmental change on orbital and millennial timescales. We also measured isotope ratios from authigenic siderite (δ18Os and δ13Cs) and, with the oxygen isotope composition of calcite and siderite, reconstruct δ18O of lake water (δ18Olw) over the last 637 ka. Interglacials have higher δ18Olw values when compared to glacial periods most likely due to changes in evaporation, summer temperature, the proportion of winter precipitation (snowfall), and inflow from adjacent Lake Prespa. The isotope stratigraphy suggests Lake Ohrid experienced a period of general stability from marine isotope stage (MIS) 15 to MIS 13, highlighting MIS 14 as a particularly warm glacial. Climate conditions became progressively wetter during MIS 11 and MIS 9. Interglacial periods after MIS 9 are characterised by increasingly evaporated and drier conditions through MIS 7, MIS 5, and the Holocene. Our results provide new evidence for long-term climate change in the northern Mediterranean region, which will form the basis to better understand the influence of major environmental events on biological evolution within Lake Ohrid

Groundwater–rock interactions in crystalline rocks: evidence from SIMS oxygen isotope data

The diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks

Gel formation at the front of expanding calcium bentonites

The removal of potentially harmful radioactive waste from the anthroposphere will require disposal in geological repositories, the designs of which often favour the inclusion of a clay backfill or engineered barrier around the waste. Bentonite is often proposed as this engineered barrier and understanding its long-term performance and behaviour is vital in establishing the safety case for its usage. There are many different compositions of bentonite that exist and much research has focussed on the properties and behaviour of both sodium (Na) and calcium (Ca) bentonites. This study focusses on the results of a swelling test on Bulgarian Ca bentonite that showed an unusual gel formation at the expanding front, unobserved in previous tests of this type using the sodium bentonite MX80. The Bulgarian Ca bentonite was able to swell to completely fill an internal void space over the duration of the test, with a thin gel layer present on one end of the sample. The properties of the gel, along with the rest of the bulk sample, have been investigated using ESEM, EXDA and XRD analyses and the formation mechanism has been attributed to the migration of nanoparticulate smectite through a more silica-rich matrix of the bentonite substrate. The migration of smectite clay out of the bulk of the sample has important implications for bentonite erosion where this engineered barrier interacts with flowing groundwater in repository host rocks