Pre-Mining Concentrations Of Heavy Metal Elements and Radioactivity Concentrations Of Primordial Radionuclides in Soils Around the Niobium Mining Site in Kwale County, Kenya

Abstract

An elemental analyses and radiological survey of soils around proposed Nb and rare earth mining site in Mrima Hill, Coastal Kenya, was conducted with an aim of providing the status of elemental and radionuclides concentrations. Elemental analysis was done using EDXRF spectroscopy, while HPGe gamma spectroscopy was used for determination of the activity. Most elements were determined at concentration levels above the recommended limits in agricultural soils. For instance, Fe, Mn and Ti were found to be the major constituent in the soil samples with mean concentration of 29%, 7.5% and 3.4% respectively. Other soil constituents include Zn (94 - 2549 μg g-1), Pb (205 – 2661 μg g-1) and Sr (551 – 6606 μg g-1). Concentrations higher than the normal range of 10 - 20 μg g-1 in soils were reported for Nb, at a range of 151 – 7511 μg g-1. Similar observations were made with respect to rare earth elements i.e. Y (100 – 2776 μg g-1) and Zr (142 – 542 μg g-1). These values can be associated with the huge mineral deposits, particularly Nb and rare earths, discovered in the region. These results are however comparable to a previous study conducted in the area. Activity levels above global average were recorded for 232Th and 238U, with mean concentrations of 1400 ± 795 Bq kg-1 for 232Th and 143 ± 11 Bq kg-1 for 238U. However, 40K was determined at levels below global average, with a mean of 200 ± 92 Bq kg-1. Using these measured activity and conversion factors, radiological risks were assessed. Radiation hazard indices that include; radium equivalent dose, representative gamma index, annual effective dose, external hazard index and absorbed dose rate were calculated. All the indices were found above the recommended limits. Therefore, Mrima Hill can be confirmed as a high background radiation area.