Screening of natural (238U and 232Th-series, 40K) and technogenic (137Cs) radionuclides along with heavy metals in bottom sediments of rivers in Transcarpathia

Bottom sediments are important indicators of a river’s condition, accumulating natural and industrial substances that reflect both background geochemical characteristics and anthropogenic impact. This study aimed to analyse the content of natural and technogenic radionuclides and heavy metals in Transcarpatian rivers to identify their distribution patterns and assess bioaccumulation within the «bottom sediments - ichthyofauna» system. Samples were collected in 2023-2024 from eight locations along the Uzh, Latorica, Borzhava and Tisza rivers. The specific activity of gamma-active nuclides in bottom sediments was determined using an ORTEC semiconductor γ-spectrometer. Heavy metal concentrations were determined using an ElvaX pro X-ray fluorescence analyser. The data obtained indicate the presence of local differences in radionuclide content levels. These differences are due to natural geochemical factors. The total specific activity of natural radionuclides (U- and Th-series) ranged from 340 to 534 Bq/kg. Maximum values of the sum of natural radionuclides were found within the settlements of Uzhhorod, Vylok and Chop areas. The highest activity of the natural radionuclide K-40 (522-533 Bq/kg) was recorded in areas with potassium-rich minerals and possible agricultural load. These areas include Pidpolozzia, Kolchyno and Chop. Technogenic Cs-137 was predominantly at low levels (2.9-7.0 Bq/kg), but a local increase was observed in fine-grained silt. Analysis of the degree of correlation between radionuclide content was expressed through the geochemical index Th/U. Fluctuations within the range of 0.63-1 revealed the mineralogical diversity of bottom sediments. Heavy metal content also varied - Zn 1.4-89.9 mg/kg, Pb 4.4-8.9 mg/kg, Cu 3.9-9.7 mg/kg, Ni 5.1-8.3 mg/kg and Cr 6.3-13.0 mg/kg. Maximum values were observed in urbanised areas (Uzhhorod, Chop), while minimum levels occurred in mountainous sites with minimal anthropogenic impact. Although concentrations did not exceed environmental standards, local maxima indicated potential pollution. The practical significance of this study value in using the results for ecological monitoring, predicting the migration of hazardous substances and assessing their bioaccumulation in aquatic food chains