65 research outputs found
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
A simple reactive-transport model of calcite precipitation in soils and other porous media
Calcite formation in soils and other porous media generally occurs around a localised source of reactants, such as a plant root or soil macro-pore, and the rate depends on the transport of reactants to and from the precipitation zone as well as the kinetics of the precipitation reaction itself. However most studies are made in well mixed systems, in which such transport limitations are largely removed. We developed a mathematical model of calcite precipitation near a source of base in soil, allowing for transport limitations and precipitation kinetics. We tested the model against experimentally-determined rates of calcite precipitation and reactant concentrationâdistance profiles in columns of soil in contact with a layer of HCO3â-saturated exchange resin. The model parameter values were determined independently. The agreement between observed and predicted results was satisfactory given experimental limitations, indicating that the model correctly describes the important processes. A sensitivity analysis showed that all model parameters are important, indicating a simpler treatment would be inadequate. The sensitivity analysis showed that the amount of calcite precipitated and the spread of the precipitation zone were sensitive to parameters controlling rates of reactant transport (soil moisture content, salt content, pH, pH buffer power and CO2 pressure), as well as to the precipitation rate constant. We illustrate practical applications of the model with two examples: pH changes and CaCO3 precipitation in the soil around a plant root, and around a soil macro-pore containing a source of base such as urea
PADAMOT : project overview report
Background and relevance to radioactive waste management
International consensus confirms that placing radioactive wastes and spent nuclear fuel deep
underground in a geological repository is the generally preferred option for their long-term
management and disposal. This strategy provides a number of advantages compared to leaving it
on or near the Earthâs surface. These advantages come about because, for a well chosen site, the
geosphere can provide:
⢠a physical barrier that can negate or buffer against the effects of surface dominated natural
disruptive processes such as deep weathering, glaciation, river and marine erosion or
flooding, asteroid/comet impact and earthquake shaking etc.
⢠long and slow groundwater return pathways from the facility to the biosphere along which
retardation, dilution and dispersion processes may operate to reduce radionuclide
concentration in the groundwater.
⢠a stable, and benign geochemical environment to maximise the longevity of the engineered
barriers such as the waste containers and backfill in the facility.
⢠a natural radiation shield around the wastes.
⢠a mechanically stable environment in which the facility can be constructed and will
afterwards be protected.
⢠an environment which reduces the likelihood of the repository being disturbed by inadvertent
human intrusion such as land use changes, construction projects, drilling, quarrying and
mining etc.
⢠protection against the effects of deliberate human activities such as vandalism, terrorism and
war etc.
However, safety considerations for storing and disposing of long-lived radioactive wastes must
take into account various scenarios that might affect the ability of the geosphere to provide the
functionality listed above. Therefore, in order to provide confidence in the ability of a repository
to perform within the deep geological setting at a particular site, a demonstration of geosphere
âstabilityâ needs to be made. Stability is defined here to be the capacity of a geological and
hydrogeological system to minimise the impact of external influences on the repository
environment, or at least to account for them in a manner that would allow their impacts to be
evaluated and accounted for in any safety assessments.
A repository should be sited where the deep geosphere is a stable host in which the engineered
containment can continue to perform according to design and in which the surrounding
hydrogeological, geomechanical and geochemical environment will continue to operate as a
natural barrier to radionuclide movement towards the biosphere. However, over the long periods
of time during which long-lived radioactive wastes will pose a hazard, environmental change at
the surface has the potential to disrupt the stability of the geosphere and therefore the causes of
environmental change and their potential consequences need to be evaluated.
As noted above, environmental change can include processes such as deep weathering,
glaciation, river and marine erosion. It can also lead to changes in groundwater boundary
conditions through alternating recharge/discharge relationships. One of the key drivers for
environmental change is climate variability. The question then arises, how can geosphere stability be assessed with respect to changes in climate? Key issues raised in connection with
this are:
⢠What evidence is there that 'going underground' eliminates the extreme conditions that
storage on the surface would be subjected to in the long term?
⢠How can the additional stability and safety of the deep geosphere be demonstrated with
evidence from the natural system?
As a corollary to this, the capacity of repository sites deep underground in stable rock masses to
mitigate potential impacts of future climate change on groundwater conditions therefore needs to
be tested and demonstrated. To date, generic scenarios for groundwater evolution relating to
climate change are currently weakly constrained by data and process understanding. Hence, the
possibility of site-specific changes of groundwater conditions in the future can only be assessed
and demonstrated by studying groundwater evolution in the past. Stability of groundwater
conditions in the past is an indication of future stability, though both the climatic and geological
contexts must be taken into account in making such an assertion
The Mersey Estuary : sediment geochemistry
This report describes a study of the geochemistry of
the Mersey estuary carried out between April 2000 and
December 2002. The study was the first in a new programme
of surveys of the geochemistry of major British estuaries
aimed at enhancing our knowledge and understanding of the
distribution of contaminants in estuarine sediments.
The report first summarises the physical setting, historical
development, geology, hydrography and bathymetry of the
Mersey estuary and its catchment. Details of the sampling
and analytical programmes are then given followed by a
discussion of the sedimentology and geochemistry. The
chemistry of the water column and suspended particulate
matter have not been studied, the chief concern being with
the geochemistry of the surface and near-surface sediments
of the Mersey estuary and an examination of their likely
sources and present state of contamination
Topographic roughness as a signature of the emergence of bedrock in eroding landscapes
Rock is exposed at the Earth surface when rates of erosion locally exceed
rates of soil production. The thinning of soils and emergence of bedrock has
implications spanning geomorphology, ecology and hydrology. Soil-mantled
hillslopes are typically shaped by diffusion-like sediment transport
processes that act to smooth topography through time, generating the
familiar smooth, convex hillslope profiles that are common in low relief
landscapes. Other processes, however, can roughen the landscape. Bedrock
emergence can produce rough terrain; in this contribution we exploit the
contrast between rough patches of bedrock outcrop and smooth, diffusion-dominated soil to detect bedrock outcrops. Specifically, we demonstrate that
the local variability of surface normal vectors, measured from 1 m resolution
airborne LiDAR data, can be used as a topographic signature to
identify areas within landscapes where rock exposure is present. We then use
this roughness metric to investigate the transition from soil-mantled to
bedrock hillslopes as erosion rates increase in two transient landscapes,
Bald Rock Basin, which drains into the Middle Fork Feather River,
California, and Harrington Creek, a tributary of the Salmon River, Idaho.
Rather than being abrupt, as predicted by traditional soil production
models, in both cases the transition from fully soil-mantled to bedrock
hillslopes is gradual and spatially heterogeneous, with rapidly eroding
hillslopes supporting a patchwork of bedrock and soil that is well
documented by changes in topographic roughness, highlighting the utility of
this metric for testing hypotheses concerning the emergence of bedrock and
adding to a growing body of evidence that indicates the persistence of
partial soil mantles in steep, rapidly eroding landscapes
Inhibition of the formation and stability of inorganic colloids in the alkaline disturbed zone of a cementitious repository
Using microbes to recover rare earths with low environmental impact?
Using Microbes to recover Rare Earths with low environmental impact
Barbara Palumbo Roe, Simon Gregory, Antoni Milodowski, Julia West, Joanna Wragg
British Geological Survey, Nicker Hill, Nottingham NG12 5GG, UK
Steve Banwart, Maria Romero GonzĂĄlez, Wei Huang, Emma Wharfe
Kroto Research Institute, University of Sheffield, Sheffield S3 7HQ, UK
John Harding, Colin Freeman, Shaun Hall
Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, UK
Microbes play an important role in the fate and transport of rare earth elements (REE) in relation to the REE exploitation life cycle. A step change in understanding is needed for key mobilisation, concentration and fractionation processes such as bioleaching, biosorption and biomineralisation and how they can 1) be harnessed to recover REE in situ from low grade ores or secondary deposits, and 2) be quantified for reactive transport in environmental risk assessment and management of mining operations.
Heap/in-situ leaching methods are relatively low impact mining technologies, requiring less energy (for comminution) and in the case of in-situ leaching have a minimal footprint. Furthermore, biologically-assisted leaching and separation processes represent a more sustainable alternative to chemical processes.
We discuss the microbial potential to accelerate dissolution of REEs from source minerals, and how the natural selectivity of mineral and microbial surfaces as ligands for adsorption and biomineralisation of REE dissolved species could be exploited in the recovery of REEs from fluids
Influence of biofilms on transport of fluids in subsurface granitic environments : some mineralogical and petrographical observations of materials from column experiments
Landfill and radioactive waste disposal risk assessments focus on contaminant transport and are principally concerned with understanding the movement of gas, water and solutes through engineered barriers and natural groundwater systems. However, microbiological activity can impact on transport processes changing the chemical and physical characteristics of the subsurface environment. Such effects are generally caused by biofilms attached to rock surfaces. This paper will present some mineralogical and petrographical observations of materials extracted at the completion of an experimental column study which examined the influences of biofilm growth on groundwater flow through crushed diorite from the ĂspĂś Hard Rock Underground Research Laboratory, Sweden
Groundwaterârock 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. +20â°(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
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