[en] The solubility control of uranium in selected groundwater samples from the cliff and sediments at the Needle's Eye natural analogue site is investigated using the speciation code PHREEQE and the CHEMVAL thermodynamic database (release 3). Alkali-earth bearing uranyl carbonate secondary minerals are likely to exert influence on the solubility . Other candidates are UO₂ and arsenates, depending on the prevailing redox conditions. In the absence of literature data, solubility products for important arsenates have been estimated from analogy with other arsenates and phosphates. Phosphates themselves are unlikely to exert control owing to their comparatively high solubilities. The influence of seawater flooding into the sediments is also discussed. The importance of uranyl arsenates in the retardation of uranium in shallow sediments has been demonstrated in theory, but there are some significant gaps in the thermodynamic databases used. (author)

[en] The British Geological Survey (BGS) has been co-ordinating a research programme centred on the Needle's Eye natural analogue site in Dumfries and Galloway District, southwest Scotland. This study of a natural radioactive geochemical system has been carried out with the aim of improving confidence in using geochemical and predictive models of radionuclide (uranium) migration in the geosphere. The Needle's Eye site is located on the Solway Coat within the Southwick Coast Reserve of the Scottish Wildlife Trust who gave permission to sample for soils, sediments and waters for analysis. This report is one of a series and covers the period from June 1989 to June 1990. It presents the results of applying the PHREEQE code to the hydrochemistry of the site using the CHEMVAL thermodynamic database. (author)

[en] One of the four natural analogue sites being investigated by the British Geological Survey is Loch Lomond, Scotland. Naturally occurring halogen elements (Cl, Br and I) have been migrating from a thin marine horizon into overlying freshwater deposits by a diffusion process which has been occurring for at least 5 400 years. This report summarizes the main findings accumulated since 1983 when the work was first begun, and provides a modelling interpretation of the measured concentration-depth profiles using a new numerical code called Diagen. The release rates of I and Br from the organic matter association in the shallow buried marine layer are very slow; subsequent anion movement by diffusion is affected by tortuosity differences in the sediments rather than by chemical reaction with the sediments. The bulk of the evidence supports conservative transport of iodide, bromide and chloride anions towards the sediment/loch interface. The report discusses some implications of the findings

[en] The study of natural geomchemical systems can help us to gain a better understanding of the different processes of radionuclide mobilisation, transport and retardation in the far-field, and to provide a means of testing thermodynamic databases and transport codes which underpin performance assessments of radioactive waste repositories. Over the past three years, the British Geological Survey has been co-ordinating a programme of natural analogue studies of radionuclide migration in Britain directed at four sites which are characterized by surfical sedimentary deposits. In various way these sediments have played host to the transport and retardation of U/Th decay series isotopes, rare earth elements and iodine over time-scale ranging to about 10.000 years ago and over distances up to 100 m or more. The different attributes of each site and the approaches adopted for defining and measuring the elemental enrichments, migration and retardation are briefly described. (orig.)

[en] Published in summary form only

[en] The British Geological Survey has been carrying out a research investigation of the Needle's Eye site at Southwick on the Solway coast in south-western Scotland. This study of a naturally radioactive geochemical system has the aim of improving our confidence in using predictive models of radionuclide migration in the geosphere. This summary report describes results from the integrated use of hydrogeological, mineralogical and geochemical techniques applied to the study of the transport and distribution of uranium. Pitchblende veins exposed in the cliffs are a major source of soluble uranium in ground-waters flowing into organic-rich post-glacial flood plain and intertidal mudflat deposits. Organic matter both living and dead has played a key role in the retardation of uranium in these sediments. Chemical transport modelling of the uranium dispersion/retardation is described and the implications for performance assessment work are discussed. Computer codes used: CHEMVAL (thermodynamic data base constants). CHIMERE (chemical equilibrium code). METIS (flow code). PHREEQE (chemical equilibrium code). STELE (coupled chemical transport code)

[en] It is of pressing concern to understand the behaviour of radionuclides in the environment and in particular long-lived ones (e.g. Np-237 and Te-99) in argillaceous rocks. Clay formations have been chosen as likely candidates for holding low level radioactive waste repositories and in the event of leakage of radionuclides into the geosphere some knowledge of their fate is required in a far-field safety assessment study. The objectives of this present work were to examine the properties of neptunium and technetium in ground-waters associated with clay-rich materials and to ascertain the variations in sorption of these radionuclides under different environmental conditions and to use the information in a forecast of transport through a clay layer

[en] Natural analogues based on uranium deposits are commonly used to represent the long-term behaviour of radioactive waste repositories or the processes that influence their radioactive contents. The geochemical dispersion of naturally occurring uranium can also be used to model natural radioactivity fluxes in the vicinity of a planned repository. These fluxes can be estimated for erosional and groundwater discharge processes and compared with calculated future fluxes of radioactivity that would be released from a repository. The methodology is outlined and the benefits of the approach for supporting the derivation of a safety case for a repository are indicated. (author)

[en] It is now well-recognized that the study of natural geochemical systems can help us to gain a better understanding of the different processes of radionuclide mobilization, transport and retardation in the far field, and provide a means of testing the thermodynamic databases and transport codes which underpin performance assessments of radioactive waste repositories. The British Geological Survey has been carrying out a programme of natural analogue studies of radionuclide migration at four sites: the sediments of Loch Lomond, Scotland, and the sediments at Broubster in Caithness, N Scotland, at Needle's Eye near Dalbeattie, SW Scotland, and at the disused South Terras mine, near St. Stephan, Cornwall. In various ways these post-glacial sediments have played host to the transport and retardation of U/Th decay series isotopes, rare earth elements and iodine over long time-scales and over distances up to 100 m or more

[en] This report describes the results of sorption experiments designed to provide essential data for migration modelling. Sorption of ²³³U from natural ground-water onto peat from Broubster and silt from Needle's Eye was studied under atmospheric conditions and different pH regimes. The temperature was maintained at 10⁰C and, in the case of Needle's Eye silt, the kinetics of sorption were followed. The results were analyzed in conjunction with speciation modelling in an attempt to understand the sorption mechanisms and to extrapolate the findings to cover the range of conditions likely to be met in the field. This work is part of the CEC project Mirage - Second phase, research area 'Natural analogues'