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

This study investigates the relationship between sediment grain size distributions and the concentrations of naturally occurring radionuclides (40K, 238U and 232Th) within the Helskloof Basin, an extremely arid and geologically complex region of the Richtersveld, South Africa. Sediment samples were collected from dry river valleys and the Helskloof floodplain and mechanically separated into grain-size fractions ranging from <63 μm to 2 mm. Gamma-ray spectroscopy using a NaI(Tl) scintillation detector was employed to determine radionuclide activity concentrations within each fraction. The results demonstrate clear grain-size-dependent behaviour for all three radionuclides, characterised by an exponential decline in activity concentrations from fine to intermediate grain sizes, followed by a linear increase or stabilisation toward coarser fractions. Spatial variations in radionuclide concentrations further revealed distinct lithological and sediment sorting influences along the Klein Helskloof Valley. Elevated 40K concentrations were associated with potassium-rich feldspar and mica minerals derived from granodioritic lithologies, while increasing upstream concentrations of 238U and 232Th reflected enhanced felsic volcanic contributions from the Windvlakte Subgroup. A strong positive correlation (r = 0.80) was observed between average grain size and 40K concentrations, suggesting that potassium may serve as a useful proxy for granulometric trends in this environment. An empirical model describing radionuclide concentration as a function of grain size demonstrated consistent minimum activity grain sizes between 612 and 678 μm for all radionuclides, corresponding to coarse sand which is dominated by quartz and feldspar. The study demonstrates that integrated gamma spectrometry and granulometric analysis provide an effective approach for characterising sediment provenance, transport dynamics, and depositional processes in arid terrains. The consistent minimum activity grain size identified across the naturally occurring radionuclides warrants further investigation as a potential indicator of sediment composition and geological provenance.