Investigating the relationship between naturally occurring radionuclides and grain size of sediment from the Helskloof Basin, South Africa

Abstract: This study investigates the relationship between sediment grain size and naturally occurring radionuclide concentrations (40K, 238U and 232Th) within the Helskloof Basin, South Africa, an arid region underlain by complex volcanic and plutonic lithologies. Sediment samples were collected from a part of the Helskloof Basin and analyzed using laboratory-based gamma spectroscopy following granulometric separation. The results revealed distinct spatial variations in radionuclide concentrations, reflecting lithological differences and sediment transport dynamics. All three radionuclides exhibited a characteristic trend with grain size, marked by an exponential decline from fine to intermediate fractions, followed by a linear increase toward coarser fractions. Curve fitting of these trends consistently identified the minimum activity at grain sizes between 612 and 678 µm, corresponding to coarse sand. This minimum likely reflects the dominance of quartz and feldspar in this size range, minerals that typically host low radionuclide concentrations, whereas finer fractions are more enriched by adsorption onto clay-rich minerals. The combined analysis of grain size and radionuclide activity demonstrates the utility of radionuclides as indicators of sediment provenance and depositional processes in mechanically weathered arid environments. These findings contribute to improved interpretations of sediment dynamics and highlight the need for further investigation into endmember mineralogy and its role in radionuclide separation.