Assessment of the relative importance of biological, physical and chemical processes on the transport and speciation of pollutants, particularly radionuclides, in the Irish Sea AE1129 Final Project report for DEFRA

Jones, D.G.; Bouch, J.; Burt, G.R.; Campbell, N.; Cave, M.R.; Chesman, B.S.; Coombs, P.; Davies, A.M.; Davis, J.R.; Hall, P.; Hards, V.L.; Harrison, I.; Higgo, J.J.W.; Humphreys, B.; Hyslop, E.K.; James, J.W.C.; Jones, J.E.; Langston, W.J.; McKervey, J.; Metcalfe, R.; Milodowski, A.E.; Moore, Y.; Pearce, J,M.; Pope, N.D.; Ridgway, J.; Rochelle, C.; Strutt, M.H.; Vane, C.H. ORCID: https://orcid.org/0000-0002-8150-3640; Wallis, D.; West, J.M.; Wragg, J.; Xing, J.. 2002 Assessment of the relative importance of biological, physical and chemical processes on the transport and speciation of pollutants, particularly radionuclides, in the Irish Sea AE1129 Final Project report for DEFRA. Nottingham, UK, British Geological Survey, 23pp. (CR/02/295N) (Unpublished)

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Abstract/Summary

This study examined the relative influence of physical, chemical and biological processes on the transport and speciation of pollutants in the Irish Sea. The focus was on radionuclides discharged from Sellafield, but other contaminants were considered from a wide range of sources. The results will help to underpin DEFRA policy by providing robust science to help understand the processes and influences that impact on the marine environment. DEFRA recognises that policy making needs to be developed on the basis of a proper understanding of pollutant distribution and behaviour in the marine environment. This assists the UK in meeting its obligations under international conventions such as OSPAR. The report is timely because a framework for nature conservation in the Irish Sea is being piloted as the next step in DEFRA’s Review of Marine Nature Conservation. The first stage of the work was a review of existing knowledge, presented as a report with new maps and backed by a database of recent references. This was followed by detailed studies of sediment cores from the NE Irish Sea from the area close to Sellafield. This included the mud patch and the inshore region with highest radionuclide levels. Intertidal cores were also collected, to greater depths than previously. These provided a contrast to the offshore cores, in a zone more accessible to man, and improved our knowledge of the inventory of radionuclides in this environmental compartment. The sediment studies were supplemented by modelling work that included 3-D hydrodynamic modelling and chemical modelling. The study has shown that physical, chemical and biological factors are all important. Their relative importance varies from place to place and differs markedly when comparing intertidal and offshore environments. Further work is needed to move from a qualitative assessment of relative impact to a more quantitative approach. Some of the methodologies developed for this study offer this possibility, particularly with respect to biological processes. The behaviour of the offshore sediments is considered next and then compared to the intertidal sediments. Typically, the offshore sediments are bioturbated near to the surface (commonly to depths of 10-20cm) but at greater depth show preserved physical sedimentary structures. Profiles of alpha, beta and gamma emitting radionuclides with depth are consistent with a time-integrated Sellafield discharge signature modified by mixing by burrowing organisms. This is supported by ratios of Pu isotopes that reflect progressively earlier discharges with depth. However, it is at odds with earlier work indicating very low sedimentation rates. The apparent discrepancy may be explained by the earlier focus on the mud patch (15-20km offshore) relative to a bias in this study in favour of the inshore zone (3-4 km offshore) adjacent to and north of Sellafield, where radionuclide contents are highest. Mesocosm work has shown that mud shrimps (Callianassa, Upogebia) are effective redistributors of radionuclides in the sediments, whereas other species studied appear to have relatively little impact. Experiments with another major Irish Sea bioturbator (Maxmulleria) were, unfortunately, not successful due to poor survival rates. Modelling techniques were used to estimate the degree of mixing produced by different species. Taken in conjunction with information on species density and geographical distribution, the impact of bioturbation could, in future, be estimated quantitatively. The principal influence on the distribution of the biotic communities appears to be the grain size of the sediments, along with the related parameter of water depth. This has been established using multivariate statistical methods applied to both the identified biota and sediment parameters. Physical processes are more important offshore below the surface oxygenated zone. Sedimentary structures are apparent below the top 10-30cm and include the interpreted products of storms. The impact of storms was also examined by 3-D hydrodynamic modelling. In particular, three storms were studied where there were some direct observations of water movements that could be used to test the models. Modelling examined the relative impact of far-field (shelf-wide) effects and local wind forcing. Water movements in detail were controlled by the direction and duration of the storm, but showed the potential to move contaminated sediment into the estuaries of the eastern Irish Sea and more soluble phases over wider areas. Removal of oxygenated surface sediment could expose reduced sediments to aerated water and this could have implications for the release of redox sensitive contaminants. Bioturbation also has the potential to introduce local oxic conditions or recycle material from depth to the oxic near-surface environment. Geochemical modelling has shown that Pu is partly soluble under oxygenated conditions, with up to 10% being available for removal in the aqueous phase. This fits well with the observed loss of Pu from the sediments over time. The much greater proportionate loss of Cs is well documented from both subtidal and intertidal sediments. Sequential extraction techniques suggest that much of the remaining Cs is relatively tightly bound in clays and therefore loss from the sediment may be expected to now be much reduced. Cs and U are more prevalent in the deeper sediments, perhaps also indicating their removal from near surface material. Pore water measurements, microbiological work and modelling all confirm the presence of a shallow (typically 10-15cm) oxygenated layer with increasingly reducing conditions below this. Sulphate-reducing bacteria are probably controlling sulphide production at depth, seen as dark colouration in the cores and microscopically as pyrite framboids within the sediments. Bulk mineralogy does not appear to be a controlling factor in the uptake of radionuclides by the sediments – changes in the clay content or the available surface area are not well correlated with radionuclide levels. Grain size does, however, play a part. Radionuclides are distributed in the fine matrix of the sediments but are also associated with hot particles. Autoradiography has been used in combination with optical and backscatter scanning electron microscopy to investigate these particles. Contrary to previous studies, hot particles of possible spent nuclear fuel origin, or associated with iron minerals, have not been observed. Rather, the hot spots seen are organic gel-like particles with a high magnesium content or consist of mud pellets (possible faecal) or pellicles around shell fragments. It appears the latter may be associated with local reducing conditions, perhaps related to the breakdown of an organic coating. This is supported by the presence of pyrite. Shells lacking an organic coating (possibly older fragments) appear not to have the more radioactive rim. The difference in hot particle characteristics may reflect diagenetic breakdown of fuel particles, with the high Mg perhaps reflecting the Mg-alloy casing on Magnox fuel. Multivariate statistics and sequential extraction techniques indicated that the most extractable U was in carbonates. Th was mostly non-extractable, but the more available Th was held in Fe oxides. A new method of sequential extraction suggests that a potential redistribution of elements occurs between phases in traditional Tessier type schemes. This can lead to erroneous conclusions regarding solid phase speciation. As and V show possible redox effects, being associated with relatively labile forms in near-surface sediments and bound in Fe-oxyhydroxides or carbonates under deeper more reducing conditions. The intertidal sediments appear to be dominated by physical sedimentary processes. Although burrowing is evident on the surface, the cores are characterised by well-developed laminations that are little affected by bioturbation. It seems that the migration of tidal channels across the estuary is responsible for the extensive reworking of the sediments and may obliterate the smaller scale mixing by bioturbation. The estuaries, with their tidal dominance, are filling with sediment from seawards with little landward input from rivers. They therefore remain a sink for particle-associated contaminants, although they have clearly released significant amounts of more mobile Cs since Sellafield discharges were reduced. Deeper coring of the intertidal sediments has, in places, reached uncontaminated sediment at depths of less than 1m. In many places, further from the shore, radionuclides are still present down to at least 2.5m. Further deeper coring is required to finally define the volume of contaminated sediment, but inventory estimates for 137Cs, 241Am and plutonium isotopes in the Solway Firth and Morecambe Bay can now be revised upwards by factors of at least 4.5 and 3.5 respectively. Using the known depth of contaminated sediment (where the base is reached) and radionuclide profiles showing a pronounced subsurface peak, sedimentation rates can be estimated. They range from <1 to 6 cmy-1, consistent with the range observed on Solway salt marshes and in the Esk Estuary. In general, heavy metal contamination is not a problem in the sediments studied. The highest values of a range of metals (Ni, Cu, Cr, Pb, Zn) are seen in a sample off the coast between Whitehaven and Workington. Relatively high levels of Cd are seen within all the Solway Firth cores, consistent with the seaward source of the sediments and the previously observed higher Cd levels off Whitehaven that have been linked to phosphate processing. Analysis of PAHs in the offshore and intertidal sediments showed typical ranges for marine sediments. The patterns of relative proportions of different PAHs were characteristic of assemblages derived from high temperature fossil fuel combustion (pyrolitic). Petrogenic (petroleum-derived) patterns were only seen in one offshore sample, from near Whitehaven, and may relate to fuel spillages associated with the port. PAH contents were lower in the intertidal cores, consistent with the lower TOC levels and coarser grain size. However, there was also an appreciable petrogenic contribution. This may result from spillages from small craft operating in the shallow waters. A number of recommendations for future work are made as a result of the study and are outlined at the end of this report.

Item Type:	Publication - Report
Programmes:	BGS Programmes > Other
Funders/Sponsors:	British Geological Survey, DEFRA
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Date made live:	19 Dec 2024 13:32 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/538573

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