D. J. Assinder

Plutonium in intertidal coastal and estuarine sediments in the Northern Irish Sea

Abstract Surface intertidal sediments from 35 sites in the Irish Sea have been analysed for their 238 Pu and 239,240 Pu activities, together with an intensive study of plutonium in sediments of the Esk Estuary (NW England). The range of plutonium activities for the whole survey were 0·14–4118 and 1·3–16 026 Bq kg −1 for 238 Pu and 239,240 Pu, respectively. The levels of Pu activity, derived from the Sellafield nuclear fuel reprocessing effluents, in sediments are controlled by lithological factors and the influence of transport and post-depositional processes. Grain size distribution is particularly important, the major part of plutonium activity being in the mud fraction of all sediments. The data suggest that over the Irish Sea coastline, dynamic mixing of sediment grains by reworking and resuspension and/or by dispersion in tidal currents are important in determining plutonium distributions. The exponential decrease in sediment plutonium activities away from the Sellafield source is attributed to the progressive mixing with older contaminated and uncontaminated sediments.

Seasonal fluxes across the sediment—water interface, and processes within sediments

Measurements of oxygen uptake across the sediment-water interface suggested that between 17-45% of the net primary production in the southern North Sea was degraded in the bottom sediments. Similar measurements of nutrient exchange fluxes showed that the sediments were significant sources of nutrients transferred to the water column. The sediments are, therefore, important sites of organic matter degradation and nutrient recycling, and must be included in any models for the North Sea. The sediments are also accumulators of radionuclides, particularly associated with the silt/clay fraction. At one site in the more central area of the North Sea where the water column stratifies during summer, vertical profiles of radionuclides (137Cs, 239,240Pu, 210Pb) suggested a deposition rate of sediment of 0.3-0.6 cm a-1, but at other sites vertical sediment profiles were unsuitable to measure deposition.

Holocene evolution of seasonal stratification in the Celtic Sea: refined age model, mixing depths and foraminiferal stratigraphy

Published stable isotopic (oxygen, carbon) and preliminary foraminiferal data from a Holocene vibrocore from the Celtic Sea (Northwest European continental shelf) have been interpreted in terms of the progressive replacement of tidally mixed by seasonally stratified water, the first study of the long-term dynamics of seasonal stratification. This study was hampered by poor age control and the foraminiferal data were based on processing with a 125-μm sieve which has been shown not to recover critical small taxa. We present here a new age model for this vibrocore (BGS 51/-07/199) based on 12 accelerator mass spectrometry 14C dates on foraminifera and molluscs, radionuclide (137Cs, 40K) profiles from the vibrocore and juxtaposed multicores, and a complete foraminiferal stratigraphy based on sieving at 63 μm. Together, the three datasets enable a mixing model to be proposed which clarifies the resolution of the record which is confirmed as extending from the Late-Glacial to the late Holocene. Correction for autocompaction reveals an increase in sedimentation rate and mixing depth from the early to the late Holocene. A temporary increase in sedimentation rate at 6650 yr cal BP is attributed to migration of the productive frontal zone across the core site. Some time after 3000 yr cal BP sedimentation either slowed abruptly or ceased completely, giving the modern mixed layer an apparent age of ∼3000 yr cal BP. The mixed layer depth indicates strongly that the apparent transition to stratification during the early Holocene is primarily a threshold change attenuated by bioturbation; secondary attentuation is related to reworking. The mixed layer model adopted suggests that this isotopic transition occured between 8990 and 8440 yr cal BP (8720±2σ). The foraminiferal analyses, in the light of modern foraminiferal distributional data, support the inference that the succession can be interpreted as a response to the progressive seasonal stratification of the Celtic Sea during the Holocene. The data highlight the value of key taxa as indicators of shelf palaeostratification in the geological record, notably of Textularia bocki and Stainforthia fusiformis as mixed-frontal and frontal-stratified indicators, respectively. The critical change to this frontal assemblage occurs at the same depth as the isotopic threshold, based on the mixing model. These data affirm the earlier interpretation of the isotopic record as registering bottom water temperature and water column productivity changes driven by the evolution of seasonal stratification. The clear association between grain size and sedimentation, as well as these isotopic and faunal data, indicates that the shelf record of highstand sedimentation is preferentially biased towards the preservation of sequences deposited under seasonally stratified rather than mixed water masses.

Tidal variations in dissolved and particulate phase radionuclide activities in the Esk estuary, England, and their distribution coefficients and particulate activity fractions

Abstract Measurements have been made of the radionuclide activities, particularly for plutonium isotopes, of the particulate and dissolved fractions of the tidal waters of the Esk Estuary. A wide range of K d values indicates the lack of equilibrium conditions in this dynamically changing environment. The particulate activity contributes a major part of the activity of the suspension, with reworked estuarine sediment providing an important component of this. The estuarine circulation system results in the dispersion of particulate radionuclides into the low salinity upper reaches of the estuary, where plutonium isotopes are apparently mobilised, enriching the dissolved fraction activity.

Radioisotopes of Thirteen Elements in Intertidal Coastal and Estuarine Sediments in the Irish Sea

A survey has been carried out of40K,60Co,99Tc,106Ru,125Sb,129I,134Cs,137Cs,154Eu,226Ra,228Ra,237Np,238Pu,239,240Pu,241Pu,241Am,242Cm, and243,244Cm activities in sediments from the Irish Sea. Several of these radionuclides were measured independently at two separate laboratories to enable the intercomparison of the results produced. The comprehensive data set generated allows the present spatial distributions of these radionuclides in coastal sediments to be examined in relation to sediment textural characteristics and by comparison with earlier survey, enables the temporal trends in their activities to be determined. In general all artificial radionuclides showed lower activities in sediments in response to falling discharges from Sellafield and relationships with either grain size, distance from Sellafield or both. Isotope ratios were similar to cumulative weighted activity ratios rather than present day values due to sediment mixing. Pu levels supplied evidence for the lag in response of sediment deposits with increasing distance from Sellafield due to sediment transport.

Stepwise chemical degradations of a UK bituminous coal

Abstract Organic materials extracted from a UK high volatile bituminous coal, its solvent extracted O-alkylation product and its solvent extracted O-alkylated, LiAlH4 reduction product have been analysed using g.c.-m.s. to determine changes in the distributions of extractable components with increasing extent of solubilization. Changes in the distribution of functional groups in the insoluble residues have been followed after each step using FT-i.r. Although the range of compounds released at each step is similar, the distribution of extractable lipid classes and their individual components varies. Hopanes (αβ and βα), acyclic isoprenoids, n-alkanes and alkyl phenanthrenes are present in all extracts, with n-alkanes predominating. Maturity parameters based on hopane, phenanthrene and n-alkane distributions give conflicting results for the relative maturities of the three extracts, possibly due to the differential association of individual components within the coal matrix. The organic components extracted after O-alkylation and after LiAlH4 reduction are likely to arise from the release of physically trapped material as a consequence of cleavage of specific bonds within the coal macromolecular structure.

Conservative and non‐conservative behaviour of radionuclides in an estuarine environment, with particular respect to the behaviour of plutonium isotopes.

Abstract Of the radionuclides entering the Esk Estuary from the Sellafield effluent discharges into the Irish Sea, the behaviour in the dissolved phase of 106Ru, 134Cs and 137Cs is conservative whilst that of 238Pu and 239,240Pu is non‐conservative. The latter leads to the enrichment of Pu in low salinity waters, probably by transfer from the particulate phase. This has repercussions on the distribution of Pu activity in the estuary, and also on the long term fate of sediment stored Pu.

Radiological assessment of the ribble estuary—I. Distribution of radionuclides in surface sediments

Abstract Surface sediment samples were collected from the estuary of the River Ribble, UK, which is contaminated by radionuclides derived from BNFL Sellafield and Springfields. Activity concentrations were measured by gamma and alpha-spectrometry for 137 Cs, 238 Pu, 239,240 Pu, 241 Am (Sellafield-derived) and 228 Th, 230 Th, 232 Th, 234 Th, 234m Pa, 234 U and 238 U (Springfields-derived). Activity concentrations for Sellafield-derived and Springfields-derived radionuclides, with the exception of 234 Th and 234m Pa, showed significant relationships with sediment grain size with higher levels in finer-grained sediments at upstream sites in the estuary. For 234 Th and 234m Pa, the relationship with grain size was obscured by the effect of the short half-life for 234 Th. Their highest activity concentrations were also found at upstream sites with fine-grained sediments but were more likely to show variability at any given grain size. Repeated surveys showed that activity concentrations fluctuated over relatively short periods of time due to sediment redistribution throughout the estuary.

Determination of low-level99Tc in environmental samples by high resolution ICP-MS

An analytical method has been developed for the determination of low-level99Tc in environmental samples by High Resolution ICP-MS. The method consists of leaching of99Tc by HNO3 and separation by three different solvent extractions with 30% TOA-xylene, MEK, and cyclohexanone. Finally, purification of99Tc was made by using an anion exchange resin column to reduce dissolved solids content. The final solution was adjusted to 1M HNO3 for introducing into the HR-ICP-MS. The accuracy and precision of the method was confirmed to be satisfactory by applying this technique to the determination of99Tc in IAEA marine algae sample (AG-B-1). Measurements of99Tc using 0.5–2.5 g of sediment samples from the Irish Sea, UK, were successfully performed by the present method.

Distribution and behaviour of99Tc,237Np,239,240Pu, and241Am in the coastal and estuarine sediments of the Irish Sea

Intertidal coastal and estuarine sediments from 24 sites in the Irish Sea have been analyzed for99Tc,237Np,238Pu,239,240Pu and241Am. The237Np activity and239Pu/240Pu ratio were measured simultaneously by ICP-MS, and99Tc was determined by HR-ICP-MS which is ten times more sensitive than Q-ICP-MS.The activities of99Tc,237Np,239,240Pu and241Am were distributed over a wide range of 1.5–70.5, 0.01–13.3, 2.3–1589, 2.6–1894 Bq/kg, respectively. Activities of these radionuclides decreased exponentially with distance from the Sellafield source. The241Am/239,240Pu and237Np/239,240Pu ratios were almost constant with distance from the Sellafield. This result suggests that the distribution and behavior of Np and Pu are controlled by complicated factors such as the influence of transport, the variation with time of Np/Pu ratio in the Sellafield discharges and sedimentary mixing processes in the Irish Sea.