Magnetic susceptibility measurements remain one of the most reliable methods used to investigate the pollution of both surface and subsurface soil from metallic anthropogenic sources. Most of the geological rocks within the study area increase the susceptibility of the soil; however, knowing the natural background susceptibility of the parent rocks will give an idea of the anthropogenic influence on the susceptibility of the soil. This study was carried out in Kogi State, North Central Nigeria, with the aim of determining the depth of the boundary between the anthropogenic influences on soil magnetic susceptibility from those of a lithogenic origin. Magnetic susceptibility measurements were carried out on 1,760 soil samples, collected from 220 soil profiles at a depth of 80.0 cm and at intervals of 10.0 cm. From the spatial distribution of magnetic susceptibility maps at different depths, the boundary between the basement complex and the sedimentary basin was clearly demarcated. The result further shows the highest magnetic susceptibility values of 350–650 × 10−5 SI, which dominates the surface soil to a depth of 40.0 cm. At the depth of 40–50 cm, the result indicates the combination of a natural anthropogenic influence on soil magnetic susceptibility with an average of 250 × 10−5 SI. Furthermore, no evidence of layering along the depth sections was observed, suggesting that the soil profiles indicate areas mainly covered by anthropogenically influenced susceptibility, which were localized and restricted to commercial places within the state. This study reveals that the average depth of soil affected by anthropogenic pollutants is between 40–50 cm in commercial places and 20–30 cm in other places with less commercial activities.

The gamma dose rates and the activity concentrations of Potassium-40, Uranium-238, and Thorium-232 in the study area were carried out using calibrated hand-held gamma detector, (RS-125 Gamma-Spectrometer) and NaI (Tl) gamma spectroscopy. The in-situ measurements result of dose rate indicates the hotspot at location 4 with a value of 100 nGyh-1, almost twice higher than the recommended limits. The results from NaI (Tl) gamma detector revealed the highest activity concentrations of Potassium-40, Uranium-238, and Thorium-232 to be 288.09, 96.49, 136.12 Bqkg-1 for sediments and 257.31, 66.93, 96.57 Bqkg for water, respectively. The highest mean activity concentration of Potassium-40 and Uranium-238 was observed in Catfish with values of 151.87 and 38.00 Bqkg-1, whereas the highest value for the activity of Thorium-232 was observed in Tilo Fish with a value of 89.02 Bqkg-1. In comparison, all the observed values are higher than the population-weighted average of 420.00, 32.00, and 45.00 Bqkg-1 for Potassium-40, Uranium-238, and Thorium-232 by UNSCEAR. Geologically, this may be attributed to the marine incursion of regional tectonic subsidence during transgression. Statistically, the correlation results confirmed that the enhanced outdoor dose rates at the coastlines environment were caused mainly by Uranium-238, followed by Thorium-232 and then Potassium-40 in magnitude. The mean hazard indices for the samples were observed to be within the world average values recommended by ICRP. The accumulation of radionuclides by fishes may be via ingestion and adsorption to surfaces, which culminate in speciation and mobility of radionuclides, alongside the feeding habits of fishes and other aquatic animals.