The work programme included: (1) sampling of sediment cores from 5 estuarine saltmarshes in SW UK and associated end member samples; (2) gamma-ray spectrometry analysis for geochronology; (3) geochemical analysis for heavy metal content; (4) particle size determination by laser granulometry; (5) organic C and N content; (6) collection of available historical flood events, river flow data, and regional land uses data, (7) Pb isotope ratios analysis. Collected data were integrated and analysed within a multiproxy approach.
Data derived within Objective 1 were expected to deliver high-resolution geochronological information covering the period from the major mining activities to contemporary diffuse pollution. Complications arisen in applying the well-established Constant Rate of Supply (CRS) model for Pb-210 geochronology due to the complex deposition processes which occur during tidal periods or high flooding events. A new model from the literature was successfully applied (TERESA model), which is based on statistical correlation between excess Pb-210 fluxes and sediment accumulation rates. A key finding was that Pb-210 geochronology is not a trivial task and more attention should be paid in understanding the nature of the processes involved in marsh sediment deposition. The Researcher contributed scientific knowledge to the host institution improving the laboratory practices for determination of Pb-210. This methodological work in gamma-ray spectrometry was presented at the ICRM2017 conference and a paper containing the main findings was sent for publication in the peer-reviewed journal Applied Radiation and Isotopes. Partial results on Pb-210 geochronology for saltmarshes were presented at an international capacity building workshop (funded by UK Newton Fund) on “Soil erosion and impacts on river ecosystems: a global challenge”, co-organized by the Researcher and her Supervisor at University of Plymouth, UK, with specific focus on the soil erosion problems in African countries.
Second objective delivered data for the evaluation of anthropogenic impacts on sediment quality and provided the necessary proxies for understanding the processes involved in heavy metal accumulation in saltmarsh sediments. In terms of sediment quality with respect to mining pollution, core sections at 1 cm resolution were analysed using Q-ICP-MS techniques. The study focused on Pb, but also other heavy metals that are normally associated with mining and smelting activities (e.g. Cu, Sn, Zn, As, Cr, Cd, etc.) and lithogenic elements (e.g. Fe, Al, Ti, Rb) were determined.
The multi-proxy data (e.g. Pb-210 derived sedimentation rates, metal content, Pb stable isotope ratios, granulometric data, C/N ratio) corroborate well in achieving the goal of the third objective. The granulometric data indicated an increase in the catchment disturbance for the last decades, due to the increased frequency of extreme rainfall events and the intensification of land use following the mechanization of agricultural practices in the middle of 1950s. The organic carbon content showed a decreasing trend in all cores, with higher values towards surface layers, while the C/N ratio revealed mixed inputs from marine and terrestrial plants (with marine component being preponderant). The extent of contamination by individual metal species was dependent upon the nature of the historic site-specific inputs but present diffuse pollution from the mining legacy sites is not significant.
Partial results on the extent of the metal contamination were presented at the EGU2016 meeting, while the final corroborated data were presented at the EGU2017 and IASWS2017 conferences. Contact has also been established with the local stakeholders (e.g. Devonport Naval Base), which were interested to further apply the SEDiLINK approach for the identification of the Pb sources in river sediments from the mouth of the Plymouth Sound. This is linked to an EC Intereg initiative on recycling of dredge material (Using Sediment as a Resource- USAR) project represented in UK by the Westcountry Rivers Trust, a research partner of the research coordinator.
