Analysis of Natural Radioactivity in Sediment Samples from the Cross River Estuary

River and estuarine sediments serve as sinks for naturally occurring radionuclides transported from geological sources and may concentrate radioactivity through particle settling and organic matter association. The Cross River Estuary in Cross River State receives sediment from a large catchment encompassing basement complex and sedimentary geological formations of South East and South South Nigeria. This study analyses natural radioactivity in sediment samples from 18 stations distributed along the estuarine salinity gradient from Opobo to the coastal delta front. Surface sediment samples and short gravity cores are collected by grab sampler and analysed by high-purity germanium gamma spectrometry for Ra-226, Th-232, K-40, and Cs-137. Sequential extraction of sediment phases is performed to partition radionuclide associations between carbonate, oxide, organic, and residual fractions. Results indicate Ra-226 activity concentrations of 12 to 58 Bq per kilogram, increasing toward the fluvial end of the estuary consistent with basement complex source geology. K-40 activities of 210 to 680 Bq per kilogram reflect variations in feldspar and mica mineral content. Cs-137 activity below 0.5 Bq per kilogram in most samples indicates limited fallout deposition history. Radionuclide distribution patterns track estuarine circulation and sedimentation dynamics. Keywords: natural radioactivity, sediment, Cross River Estuary, gamma spectrometry, radionuclide partitioning