Radiocaesium dispersion and fixation in the lagoon systems of bRia de AveiroQ, Portugal Iuliu Bobos a, * , Fernando Rocha b a Department of Geology, Faculty of Sciences, University of Oporto, Prac ¸a Gomes Teixeira, 4099-002 Oporto, Portugal b Department of Geosciences, University of Aveiro, Campus Santiago, 3810-001 Aveiro, Portugal Received 1 April 2005; accepted 19 August 2005 Available online 8 November 2005 Abstract Radiocaesium ( 137 Cs) dispersion and Cs + fixation were studied in the sediments collected from the lagoon systems of bRia de AveiroQ. The Cs + sorption was tested for the fine mica grains and for the b 2 Am clay fractions extracted from silty clays. The Cs + exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs + engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the 137 Cs were found associated with mica-rich silty clays. The 137 Cs ranges from 3.2 to 3.9 Bq kg À 1 in the b 38 Am fractions and from 2.9 to 3.3 Bq kg À 1 in the b 64 Am fractions. D 2005 Elsevier B.V. All rights reserved. Keywords: Radiocaesium; Distribution coefficient; Silty-clay sediments; Lagoon systems of bRia de AveiroQ; Portugal 1. Introduction Large concentrations of 137 Cs were released to en- vironment around the world in the last 50 years. The 137 Cs with half-lives of 30.2 years found out in sedi- ments is connected to the worldwide fallout, resulting from atmosphere nuclear tests and the Chernobyl acci- dent (Ukraine). Also, it is an important constituent of radioactive wastes held in storage by several nuclear potentially countries. The migration of the 137 Cs to the biosphere and its subsequent bioavailability depends on the mechanisms of clay minerals adsorption (Higgo, 1987) and organic matter (Staunton et al., 2002). The interface of river– ocean, via estuaries or lagoon systems, acts as sinks for natural and anthropogenic radionuclides. Weathering effects on the structure of mica-type clay mineral is a sensitive consequence of the relative 137 Cs concentrations in highly selective fashion to bfrayed edgeQ sites (Jackson, 1963). The 137 Cs may also be viewed as a potential radioisotope to data very recent sedimentation rates and ages of contaminated core (Siegel et al., 1994). The aim of this study is focused on the 137 Cs dispersion, Cs + ion-exchange reaction and estimation of the sedimentation rates in the living eco- system of bRia de AveiroQ based on the dispersion of 137 Cs. 2. Materials and methods The so called bRia de AveiroQ is a barrier type lagoon located in the central north-western littoral of 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.08.077 * Corresponding author. Tel.: +351 223401471; fax: +351 222056456. E-mail address: ibobos@fc.up.pt (I. Bobos). Journal of Geochemical Exploration 88 (2006) 367 – 372 www.elsevier.com/locate/jgeoexp