Natural radioactivity and geophysical assessment of subsurface lithology: implications for groundwater vulnerability in the coastal Niger Delta region

This research investigates the potential radiological contamination of groundwater aquifers by evaluating the lithology-depth in the hydrocarbon-rich coastal area of the Niger Delta, Nigeria. Geophysical surveys were carried out using vertical electrical sounding (VES) and Sodium iodide NaI(Tl) detector analysis to ascertain the depth of the groundwater aquifer and the soil layers' radiological contamination level, respectively. The geophysical survey results revealed the different curve types of the lithology-depth as HAA, HKH, AAA, AAK, AKQ, KHA, and KQH. While the layers' resistivity and depth range from 6.6 to 1375.0 Ωm and 0.7 to 80.0 m across the different layers, respectively. The mean activity concentration of radionuclides analyzed are 1687.90 ± 35.33 Bq kg⁻ 1 for 40 K, 42.97 ± 11.96 Bq kg⁻ 1 for 238 U, and 44.42 ± 21.76 Bq kg⁻ 1 for 232 Th, respectively. The obtained mean potential for human radiological risk values for radium equivalent, Representation Index, external hazard index, internal hazard index, absorbed dose rate, annual gonadal equivalent dose, annual effective dose rate (AEDE outdoor and indoor) and excess lifetime cancer risk are 236.44 ± 45.80, 1.86, 0.64, 0.76, 117.43 nGy h⁻ 1 , 848.44 µSv y⁻ 1 , 144.01 µSv y⁻ 1 , 576.05 µSv y⁻ 1 , and 504.05 × 10⁻ 6 respectively. Both the obtained values and the computed radiological hazard index values exceeded the various world-recommended limits except for Raeq, Hex, and Hin which remained below their respective reference limits. Thus, the radioactivity results revealed that there is possibility of groundwater radiological vulnerability within the coastal Niger Delta area and if consumed, may lead to radiological health detriment of the residents of the area.