Clay mineralogy insights and assessment of the natural (228Ra, 226Ra, 210Pb, 40K) and anthropogenic (137Cs) radionuclides dispersion in the estuarine and lagoon systems along the Atlantic coast of Portugal

The activity concentrations of natural (Ra-228, Ra-226, Pb-210, K-40) and anthropogenic (Cs-137) radionuclides were measured by gamma spectrometry on sediments collected from aquatic systems. The mineralogy of clay fractions identified by X-ray diffraction (XRD) is composed by an illitic material (detrital mica, illite/smectite, and illite/ smectite/vermiculite mixed-layers) +/- kaolinite and illite. The Mg2+-saturated clay fractions show three distinct illitic populations within the illitic material: detrital mica, illite/vermiculite mixed-layers, and illite/smectite. Transmission electron microscopy (TEM) images show illitic material aggregates with a flaky- and lath-shapes, pseudohexagonal platy kaolinite and lath-shape illite. The Ra-228 activity concentration values range from 30.1 to 403 Bq Kg(-,) the Ra-226 from 24.6 to 290 Bq Kg(-1) and from 44 to 368 Bq Kg 1 for Pb-210. The Ra fluxes suggest a unique source where the mixing process and radioactive decay equilibrated the losses of short- (Ra-228) and long-lived (Ra-226) isotopes. The Pb-210/Ra-226 ratio shows an average value of 2.165, indicating that the radioactive secular equilibrium (similar to 1) was reached. A decrease of the natural radionuclide (Ra-228, Ra-226, Pb-210) activity concentrations was observed in estuaries and near shore coastal sediments from north to south of Atlantic Portuguese coast. Higher Pb-210 and Ra-226 activity concentrations were obtained in samples composed by illitic material + kaolinite and organic matter. The illite/smectite mixed-layers and kaolinite are suitable sorbents for Ra and Pb-210 isotopes. Higher 40K values (473-1001 Bq K-1) were measured on feldspar, mica and clay sediments provided from granite weathering (north region). The Cs-137 activity values range from 1.25 to 10.1 +/- 1.9 Bq Kg(- 1) for samples collected from the northern and central part of estuarine areas. A higher Cs-137 activity (31.2 +/- 3.5 Bq Kg(- 1)) was measured in the southern part of the Atlantic coast, corresponding to a sediment with a higher amount of illite.