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Uranium-thorium disequilibrium in north-east Atlantic waters


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K.J. Smith a, L. Leôn Vintro a, P.I. Mitchell a, P. Bailly de Bois b, D. Boust b

Journal of Environmental Radioactivity 74 (2004) 199-210

Type de document > *Article de revue

Mots clés > eaux douces/de mer, thorium, uranium

Unité de recherche > IRSN/DEI/STEME/LMRE, IRSN/DEI/SECRE/LRC

Auteurs > BAILLY DU BOIS Pascal, BOUST Dominique

Date de publication > 14/10/2004

Résumé

In this paper we report and compare the concentrations of 234Th and 231U measured in surface and subsurface waters collected in the course of a sampling campaign in the north east Atlantic in June-July 1998. Dissolved 234Th concentrations in surface waters ranged from 5 to 20 Bq m-3, showing a large deficiency relative to 238U concentrations (typically 42 Bq m3). This disequilibrium is indicative of active 234Th scavenging from surface waters. Observed 234Th/238U activity ratios, together with corresponding 234Th particulate con­centrations, were used to calculate mean residence times for 234Th with respect to scavenging onto particles (tpart) and subsequent removal from surface waters (part). Residence times in the range 5-30 days were determined for (tdiss) an 4-18 days for an 4-18 days for tpart.

In addition, ultrafiltration experiments at six stations in the course of the saine expedition revealed that in north-east Atlantic surface waters a significant fraction (46 ± 17%; n = 6) of the thorium in the (operationally-defined) dissolved phase (<0.45 gym) is in colloidal form. These observations are consistent with the 'colloidal pumping' model in which it is assumed that 234Th is rapidly absorbed by colloidal particles, which then aggregate, albeit at a slower rate, into larger filterable particles. In essence, colloids act as intermediaries in the transition from the fully dissolved to the filter-retained (>0.45 µm) phase. Thus, the time (tc) for fully dissolved 234Th to appear in the filter-retained fraction is dependent on the rate of colloidal aggregation. Here, we determined i,, values in the range 3-17 days.

a Department of Experimental Physics, University College Dublin, Stillorgan Road, Belfield, Dublin 4, Ireland
b Laboratoire d'Études Radioécologiques de la Façade Atlantique, Institut de Radioprotection et de Sûreté Nucléaire, Cherbourg- Octeville, France