40K, 226Ra, 232Th, 238U and 137Cs relationships and behaviour in sedimentary rocks and sediments of a karstic coastal area (Kaštela Bay, Croatia) and related rocks and sediments’ differentiation

Natural (⁴⁰K, ²²⁶Ra, ²³²Th, ²³⁸U) and anthropogenic (¹³⁷Cs) radionuclides in consolidated sedimentary rocks (limestones s.l. and marls) and unconsolidated sediments (stream sediments and soils) of the Kaštela Bay (Adriatic Sea, Croatia) coastal area were studied. Kaštela Bay is a typical karstic environment and it is under strong anthropogenic influence due to industrialisation and urbanisation. Relationships between radionuclides, their behaviour in different types of samples and possible influencing factors were studied. Radionuclides were also studied in relation to selected metals (Al, K, Ca, Fe, Ti, Mn, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Pb and As) and distribution in limestones s.l., marls, stream sediments and soils. Radionuclides’ massic activities were determined by gamma-spectrometry and metal mass fractions were measured by EDXRF technique. Relationships were studied using statistical tools (principal component analysis, factor analysis). It was found that stream sediments were more similar to source rocks (limestones s.l. and marl) than to soils in terms of natural radionuclides’ activities. ⁴⁰K and ²³²Th in consolidated rocks were preferentially bound to alumosilicates and associated with the majority of studied metals. However, this was not the case with ²²⁶Ra and ²³⁸U, which did not show any preferences regarding binding to mineral components or associating with metals. On the other hand, all four natural radionuclides presented the same behaviour in unconsolidated sediments, i.e. they were all preferentially bound to alumosilicates and associated with the majority of metals. ¹³⁷Cs did not associate with any of the metals or other radionuclides in sediments. The statistical approach applied in this study revealed differences in radionuclides’ relationships and behaviour in a karstic environment and implied possible influencing processes or factors. This approach can be applied in other types of environments as well to study preferential radionuclides’ binding to certain mineral components that influences radionuclides’ mobility and transport to other environmental compartments.