Erosion, vegetation and the evolution of hillslopes in upland landscapes

The geomorphic and geochemical characteristics of landscapes impose a physical template on the establishment and development of ecosystems. Conversely, vegetation is a key geomorphic agent, actively involved both soil production and sediment transport. The evolution of hillslopes and the ecosystems that populate them, are thus intimately coupled; their co-dependence potentially has a profound impact on the way in which landscapes respond to environmental change. This thesis explores how rates of erosion, integrated over millennia, impact on the structural characteristics of the mixed conifer forest that presently mantles this landscape, the development of the underlying soils and emergence of bedrock. The focus for this investigation is the Feather River Region in the northern Sierra Nevada in California, a landscape characterised by a striking geomorphic gradient accompanied by spatial variations in erosion rate spanning over an order of magnitude, from 20 mm ka-1 to over 250 mm ka-1. Using LiDAR data to quantify forest structure, I demonstrate that increasing rates of erosion drive a reduction in canopy height and aboveground biomass. Subsequently, I exploit a novel method to map rock exposure, based on a metric of topographic roughness, to show that as erosion rates increase and soil thickness consequently decreases, the degree of bedrock exposed on hillsides increases. Importantly, this soil-bedrock transition is gradual, with rapidly eroding hillslopes frequently possessing a mosaic of bedrock outcrop and intermittent soil mantle. Both the ecological and geomorphic trends are shown to be impacted by the underlying bedrock, which provides an additional source of heterogeneity in the evolution of the Feather River landscape. The negative correlation between AGB and erosion rate has potential implications for soil production. Using a simple hillslope model I show that if this decrease in AGB is associated with a drop in biotic soil production, then feedbacks between soil thickness and biotic soil production are capable of generating a complex response to geomorphic forcing, such that hillslopes possess multiple stable states: for intermediate rates of erosion, equilibrium hillslopes may be either soil mantled or bedrock. Hillslope evolution in these simulations is path dependent; once exposed at the surface, significant patches of bedrock exposure may persist over a wide range of incision rates

Understanding radionuclide migration from the D1225 Shaft, Dounreay, Caithness, UK

A 65 m vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the offshore discharge of radioactive effluent from the various nuclear facilities then under construction. In 1959, the Shaft was licensed as a disposal facility for radioactive wastes and was routinely used for the disposal of ILW until 1970. Despite the operation of a hydraulic containment scheme, some radioactivity is known to have leaked into the surrounding rocks. Detailed logging, together with mineralogical and radiochemical analysis of drillcore has revealed four distinct bedding-parallel zones of contamination. The data show that Sr-90 dominates the bulk beta/gamma contamination signal, whereas Cs-137 and Pu-248/249 are found only to be weakly mobile, leading to very low activities and distinct clustering around the Shaft. The data also suggest that all uranium seen in the geosphere is natural in origin. At the smaller scale, contamination adjacent to fracture surfaces is present within a zone of enhanced porosity created by the dissolution of carbonate cements from the Caithness flagstones during long-term rockwater interactions. Quantitative modelling of radionuclide migration, using the multiphysics computer code QPAC shows the importance of different sorption mechanisms and different mineralogical substrates in the Caithnesss flagstones in controlling radionuclide migration

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