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

Abstract The activity concentrations of natural (228Ra, 226Ra, 210Pb, 40K) and anthropogenic (137Cs) 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 228Ra activity concentration values range from 30.1 to 403 Bq Kg−1, the 226Ra from 24.6 to 290 Bq Kg−1 and from 44 to 368 Bq Kg−1 for 210Pb. The Ra fluxes suggest a unique source where the mixing process and radioactive decay equilibrated the losses of short- (228Ra) and long-lived (226Ra) isotopes. The 210Pb/226Ra ratio shows an average value of 2.165, indicating that the radioactive secular equilibrium (~1) was reached. A decrease of the natural radionuclide (228Ra, 226Ra, 210Pb) activity concentrations was observed in estuaries and near shore coastal sediments from north to south of Atlantic Portuguese coast. Higher 210Pb and 226Ra 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 210Pb isotopes. Higher 40K values (473–1001 Bq Kg−1) were measured on feldspar, mica and clay sediments provided from granite weathering (north region). The 137Cs 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 137Cs 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.