Dominique Boust

Distribution and inventories of some artificial and naturally occurring radionuclides in medium to coarse-grained sediments of the channel

Abstract Concentrations of artificial ( 60 Co , 137 Cs , 238 Pu and 239,240 Pu ) and naturally occurring radionuclides ( 40 K , 212 Pb and 214 Pb , daughter nuclides of the 232 Th and 238 U series) in bottom sediments of the Channel are reported. They are grain-size modulated but usual grain-size normalisation methods fail due to the strong heterogeneity of the sediment admixture and/or the occurrence of rock debris in the area of concern. When plotted versus distance from Cap La Hague, 60 Co and Pu isotope concentrations display a maximum in the Central Channel, but 137 Cs do not. This is further explained by the contribution of the releases from the La Hague plant relative to other radionuclide inputs, especially Atlantic inflow and direct atmospheric fallout. Apparent transit times from Cap La Hague are derived from Pu isotopic ratios and yield average sediment velocities ranging from some kilometres to some tens of kilometres per year. Sediment inventories of artificial radionuclides show that a significant part of the input of 60 Co and Pu isotopes is immobilised in the Channel seabed while most of the 137 Cs input has been evacuated by water mass circulation.

Temporal record of Pu isotopes in inter-tidal sediments from the northeastern Irish Sea

A depth profile of (239)Pu and (240)Pu specific activities and isotope ratios was determined in an inter-tidal sediment core from the Esk Estuary in the northeastern Irish Sea. The study site has been impacted with plutonium through routine radionuclide discharges from the Sellafield nuclear reprocessing plant in Cumbria, NW England. A pronounced sub-surface maximum of ~10 k Bq kg(-1) was observed for (239+240)Pu, corresponding to the peak in Pu discharge from Sellafield in 1973, with a decreasing trend with depth down to ~0.04 k Bq kg(-1) in the deeper layers. The depth profile of (239+240)Pu specific activities together with results from gamma-ray spectrometry for (137)Cs and (241)Am was compared with reported releases from the Sellafield plant in order to estimate a reliable sediment chronology. The upper layers (1992 onwards) showed higher (239+240)Pu specific activities than would be expected from the direct input of annual Sellafield discharges, indicating that the main input of Pu is from the time-integrated contaminated mud patch of the northeastern Irish Sea. The (240)Pu/(239)Pu atom ratios ranged from ~0.03 in the deepest layers to >0.20 in the sub-surface layers with an activity-weighted average of 0.181. The decreasing (240)Pu/(239)Pu atom ratio with depth reflects the changing nature of operations at the Sellafield plant from weapons-grade Pu production to reprocessing spent nuclear fuel with higher burn-up times in the late 1950s. In addition, recent annual (240)Pu/(239)Pu atom ratios in winkles collected during 2003-2008 from three stations along the Cumbrian coastline showed no significant spatial or temporal differences with an overall average of 0.204, which supports the hypothesis of diluted Pu input from the contaminated mud patch.

Solid partitioning and solid-liquid distribution of 210Po and 210Pb in marine anoxic sediments: roads of Cherbourg at the northwestern France

A sequential extraction protocol has been used to determine the solid-phase partition of (210)Po and (210)Pb in anoxic marine sediment from the roads of Cherbourg (France) in the central English Channel. Measurements were also obtained in pore waters, in which (210)Po activities range between 1 and 20 mBq L(-1) and (210)Pb activities between 2.4 and 3.8 mBq L(-1), with highest activities in the topmost layer. These activities are higher than in seawater, suggesting that sediment act as a source of both (210)Po and (210)Pb for overlying water. The (210)Po profile in the pore waters is apparently correlated with those obtained for Fe, Mn and SO(4)(2)(-), suggesting an influence of early diagenetic processes on the (210)Po solid-liquid distribution. In the sediment, (210)Po is predominantly bound to organic matter or chromium reducible sulphides, and residuals (clay minerals and refractory oxides). Our results indicate that (210)Po is not significantly bound to AVS, i.e. acid volatile sulphides: bioturbation could play a role by the early redistribution of (210)Po bound to acid volatile sulphides in the sediment. (210)Po, (210)Pb and Pb exhibit differences in terms of distribution, probably due to a different mode of penetration in the sediment. This work provides information on solid and liquid distribution of (210)Po and (210)Pb in marine sediment. These data are very scarce in the literature.

Dating of sediment record at two contrasting sites of the Seine River using radioactivity data and hydrological time series

Sediment cores were collected at the outlet of the highly anthropogenized catchment of the Seine River at two contrasting sites: a flood plain of the lower Seine River and a quasi-permanently submerged harbour basin (or wet dock) in the upper tidal estuary. Analyses of artificial radionuclides ((137)Cs and plutonium isotopes), coupled with hydrological and bathymetric data, lead to a precise dating of the sediment cores collected at the two sites. (137)Cs signals originating from global fallout (early 1960s) and from the Chernobyl accident (1986) are identified, but at different levels due to the incomplete nature or variable continuity of the records. Anomalous (238)Pu concentrations found at both sites (1-2 Bq kg(-1)) are attributed to unknown industrial releases originating from upstream. Interpolating (137)Cs sediment activities under the assumption of a constant sediment rate, those releases were dated back to 1975 ± 1, thus providing a local but reliable time-marker. Age models have highlighted a very contrasting sediment filling dynamics in these two sites. This study presents the first sediment record of alpha- and gamma-emitting artificial radionuclides obtained at the outlet of the huge catchment area of the River Seine, over a period covering the last 50 years.

Distinct diversity of the czcA gene in two sedimentary horizons from a contaminated estuarine core

In estuarine ecosystems, trace metals are mainly associated with fine grain sediments which settle on mudflats. Over time, the layers of sediments accumulate and are then transformed by diagenetic processes, recording the history of the estuary’s chemical contamination. In such a specific environment, we investigated to what extent a chronic exposure to contaminants could affect metal-resistant sedimentary bacteria in subsurface sediments. The occurrence and diversity of cadmium resistance genes (cadA, czcA) was investigated in 5- and 33-year-old sediments from a highly contaminated estuary (Seine France). Primers were designed to detect a 252-bp fragment of the czcA gene, specifically targeting a transmembrane helice domain (TMH IV) involved in the proton substrate antiport of this efflux pump. Although the cadA gene was not detected, the highest diversity of the sequence of the czcA gene was observed in the 5-year-old sediment. According to the percentage of identity at the amino acid level, the closest CzcA relatives were identified among Proteobacteria (α, β, γ, and δ), Verrucomicrobia, Nitrospirae, and Bacteroidetes. The most abundant sequences were affiliated with Stenotrophomonas. In contrast, in the 33-year-old sediment, CzcA sequences were mainly related to Rhodanobacter thiooxydans and Stenotrophomonas, suggesting a shaping of the metal-resistant microbial communities over time by both diagenetic processes and trace metal contamination.