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Geological disposal of intermediate level radioactive waste in the UK is planned to involve the use of cementitious materials, facilitating the formation of an alkali-disturbed zone within the host rock. The biogeochemical processes that will occur in this environment, and the extent to which they will impact on radionuclide migration, are currently poorly understood. This study investigates the impact of biogeochemical processes on the mobility of the radionuclide technetium, in column experiments designed to be representative of aspects of the alkali-disturbed zone. Results indicate that microbial processes were capable of inhibiting 99m Tc migration through columns, and X-ray radiography demonstrated that extensive physical changes had occurred to the material within columns where microbiological activity had been stimulated. The utilisation of organic acids under highly alkaline conditions, generating H 2 and CO 2 , may represent a mechanism by which microbial processes may alter the hydraulic conductivity of a geological environment. Column sediments were dominated by obligately alkaliphilic H 2 -oxidising bacteria, suggesting that the enrichment of these bacteria may have occurred as a result of H 2 generation during organic acid metabolism. The results from these experiments show that microorganisms are able to carry out a number of processes under highly alkaline conditions that could potentially impact on the properties of the host rock surrounding a geological disposal facility for intermediate level radioactive waste. [ABSTRACT FROM AUTHOR]

Abstract: Protection of the environment post-mining is an important issue, especially where runoff and erosion can lead to undesirable material leaving post-mining landscapes and contaminating surrounding land and watercourses. Methods for assessment of the environmental impact and long-term behaviour of post-mining landforms based on scientific methodology are needed especially where field data are absent or poor. An appraisal of the former Nabarlek uranium mine was conducted to assess the site from a soil erosion perspective as part of an independent evaluation of overall rehabilitation success. Determination of the gross erosion occurring, sediment discharge to Cooper Creek and the resultant sediment associated radionuclide load in Cooper Creek were the primary objectives of the study. These objectives were achieved through the application of several models using parameter values collected from the site. The study found that the area containing the mill tailings repository is extremely stable and meets the guidelines established for long-term storage of uranium mill tailings. Most other areas on the site are stable; however there are some areas with a high sediment loss. Sediment concentration in Cooper Creek, which drains the site, was found to be within the Australian water quality guidelines for fresh water, however sediment concentrations in tributaries were found to exceed recommended levels. Radionuclide determinations on soil samples showed that the highest specific activities (Bq kg−1) were present on a small (0.44 ha) area with a relatively high erosion rate. This small area contributed the majority of the estimated flux to Cooper Creek of uranium-series radionuclides sorbed or structurally incorporated to eroded soil particles sourced from the mine site. This study provides a methodology for assessment of the erosional stability of such a landscape and consequent impact on water quality, using extensive field data and readily available and well known models and methodologies. [Copyright &y& Elsevier]

Approaches to the modelling of long-term, large-scale (1–100 year and drainage basin to coastal zone) behaviour are outlined for the catchments of the Humber and Tamar estuaries, UK, and San Francisco Bay, USA. The estuarine environment is particularly difficult to model. Once fluvial materials enter an estuary, sharp chemical gradients and complex physical and biochemical dynamics can result in large changes to the amount, chemical nature and timing of the flux of nutrients and other materials that are transported to the coastal zone. Examples are given both of catchment models for the three systems and of an estuarine model that has been used to estimate contemporary fluxes of some nitrogen species from the Humber catchment to the North Sea, in an attempt to forecast the possible effects associated with climate change. Other applications of long-term models are discussed, including predictions of oxygen isotope ratio distributions in San Francisco Bay, which are of value to palaeo studies, and the dependence of salt intrusion on rising sea level and drought conditions there. Some of the more difficult modelling problems involve the determination of estuarine sediment transport and its consequences. A problem which has received little attention up to now, but which is becoming a very active area of research, is the influence of biota on sediment transport. Long-term modelling of fine sediment transport and its application to contaminant (including radionuclide) transport and dispersion to the coastal zone are discussed for the Tamar Estuary. Other long-term estuarine problems that are discussed include the modelling of seasonal heat fluxes from the Tamar to its coastal zone, and the dispersion of tracers within the coastal zone using three-dimensional simulations of heat, salinity and flow. The article is mainly concerned with simulations that require only reasonably short amounts of time to run on a PC and with models that are not intended to give solutions of great temporal or spatial resolution. Generally, they are intended to provide insights into phenomena and systems. To achieve this, a common feature of the long-term models has been their parameterisation of some intratidal processes, such as sediment resuspension and deposition, or tidal mixing. Provided the parameterisations are physically based and lead to good comparisons with available data, when these exist, then the approach would appear to be a valid ‘use’ rather than an invalid ‘abuse’ of models. [Copyright &y& Elsevier]