Final Report Summary - SEDSRES (Quantifying sources and residence time of contaminated sediment in human-impacted river basins: an integrated approach)
Marie Curie International Incoming Fellowship PIIF-GA-2010-273069:
Quantifying sources and residence time of contaminated sediment in human-impacted river basins: an integrated approach
Contacts: Dr Hugh Smith, University of Liverpool: Hugh.Smith@liverpool.ac.uk; Dr Will Blake, Plymouth University: wblake@plymouth.ac.uk
1. Summary description of the project objectives.
The overall aim of this research was to develop an integrated approach for quantifying diffuse sources and residence times of sediment and associated contaminants in European river basins. It was proposed that this integrated approach, henceforth referred to as SedSRes (Sediment Source and Residence time), could be used to (i) determine diffuse sources of sediment and associated contaminants stored in channel deposits and transported in suspension over event and seasonal timescales and (ii) estimate residence times for sediment supply and transport through catchments over a range of timescales, thereby providing a timescale for river and catchment recovery from sediment contamination. Working with the Tamar catchment in southwest UK, the following objectives were undertaken to develop and demonstrate the SedSRes approach:
Objective 1: Determine diffuse sources of sediment and associated contaminants stored in channel deposits and transported in suspension over event and seasonal timescales
Objective 2: Determine residence times for sediment supply and transport through the study catchment over a range of timescales.
2. Description of the work performed since the beginning of the project
The work programme was based on (i) intensive field sampling of catchment source materials and channel suspended sediment, (ii) laboratory analysis for gamma emitting fallout radionuclides, major and minor element geochemistry, particle size and organic carbon (iii) data analysis to include modelling of sediment sources and modelling of sediment residence time and (iv) modelling of sediment flux using the spatially distributed erosion/sediment yield model, SedNet.
3. Description of the main results achieved so far
Data derived within objective 1 were expected to deliver a straightforward pattern of sediment source dynamics to provide part for the SedSRes framework. However, complications in the data arose since source discrimination in the Tamar catchment was shown to be limited by (i) rotation of cultivated and pasture fields resulting in reduced differences between these two sources and (ii) the cultivated source signature resembling a mix of the pasture and channel bank sources for many tracer properties. This was compounded by metal pollution from historic abandoned mines and organic enrichment of sediment from upland areas of peaty soils which resulted in the non-conservative behaviour of some tracer properties in several catchments. Consequently, the agenda of this objective was altered slightly to address these short-comings in the application of the sediment tracer approach to disturbed catchment systems. In particular, rather than applying standard data corrections to address non-conservative behaviour of sediment tracer properties, attention was focussed on the particle size and organic carbon content of source soils which were shown to explain much of the variation in tracer properties in downstream sediment, rather than selective transport effects. Sensitivity analysis showed that correcting source tracer data for organic matter can introduce large errors in source apportionment results and should not be used. A key finding was that confounding factors related to poor source discrimination and non-conservative behaviour are highly likely to affect source fingerprinting studies in many agricultural catchments. This important development in use of fingerprinting technology was published in the peer-reviewed journal Geomorphology in 2014. Discussion continues to develop with the international scientific community with Dr Smith as lead convenor of a now established European Geosciences Union session on sediment tracing for source and residence time (successfully run in 2013 and scheduled again for 2104).
Objective 2 was pursued as the first application of the Australian soil erosion and sediment delivery model SedNet in a European context. This is an important aspect of the knowledge transfer undertaken within the programme and this input to EU science continues with Dr Smith’s appointment to a permanent Faculty position at the University of Liverpool in July 2013 and appointment to this competitive position was strongly supported by the MC IIF programme. The sediment budget model SedNet was coupled with a mass balance model of particle residence times based on atmospheric and fluvial fluxes of three fallout radionuclide tracers (Be-7, excess Pb-210 and Cs-137). Mean annual fluxes of suspended sediment were simulated in seven river basins in south-west England for three land cover surveys (1990, 2000 and 2007). Suspended sediment flux increased across the basins in response to increasing arable land area between consecutive surveys. The residence time model divided basins into slow (upper surface soil) and rapid (river and connected sediment source areas) transport compartments. Estimated theoretical residence times in the slow compartment decreased with the increase in basin sediment exports. In contrast, the short residence times for the rapid compartment increased as the modelled connected source area expanded with increasing sediment supply from more arable land. This increase in sediment residence time was considered to correspond to longer sediment travel distances linked to the larger source area. The coupled modelling approach provides unique insight into river basin responses to land use and other recent environmental change not otherwise available from conventional monitoring alone. A manuscript reporting these findings is currently in revision for the peer-reviewed journal Earth Surface Processes and Landforms.
4. Final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
The immediate impacts of the research anticipated are related to assessment and management of diffuse water pollution from agricultural land (DWPA) and the role of resident channel sediment as a secondary source of contaminants. The Water Framework Directive (2000/60/EC) requires UK rivers to reach good ecological status by 2015. A large proportion of UK catchments currently at risk of failure are adversely affected by DWPA. While considerable effort is being made to reduce DWPA (sediment nutrients and contaminants ) in rural lowland catchments and also quantify the benefits of on farm measures, little attention has been paid to the potential role of channel stored sediment as a secondary source of nutrients that could continue to enhance nutrient flux after on farm measures have been completed. This presents a serious gap in our knowledge with tangible political and socio-economic consequences. While sediment stores may protract the impact of DWPA beyond expected targets for improvement, in the longer-term improvement is likely but we need to know how much longer it might take. This is critical to manage both public and political expectations of the shorter-term benefits of government investment in catchment management. Building on the SedSRes foundation, Blake and Smith are currently undertaking work for key UK catchment management stakeholder groups to explore the risk that stored sediment poses to overlying waters. These data will contribute directly to catchment management decision to improve chances of achieving WFD targets.