Assessment of Uranium and Thorium Concentrations in Groundwater of Granite-Hosted Aquifers in Kano State and Radiological Risk Implications

Naturally occurring radioactive materials in granite terrain include uranium and thorium that can dissolve into groundwater from host rock minerals, creating potential radiological exposure risks for users of borehole and spring water. This study quantitatively assesses uranium and thorium concentrations in groundwater from granite-hosted aquifers in five local government areas of Kano State, North West Nigeria, and evaluates associated radiological and chemical risks. Water samples were collected from 40 boreholes in granitic terrain during the dry season and analysed for uranium by inductively coupled plasma mass spectrometry and thorium by radiochemical separation followed by alpha spectrometry. Complementary physicochemical parameters including pH, TDS, hardness, alkalinity, and sulphate were determined. Saturation indices for uranium-bearing minerals were calculated using the PHREEQC geochemical modelling code to assess geochemical controls on uranium release. Results show that uranium concentrations range from 1.2 to 89.4 micrograms per litre, with 28 percent of samples exceeding the WHO drinking water guideline of 30 micrograms per litre. Thorium was below detection in most samples, consistent with its low solubility under prevailing pH conditions. PHREEQC modelling indicates that uraninite dissolution and oxidising conditions are the dominant controls on uranium enrichment. Annual effective dose estimates for uranium ingestion via drinking water from the highest-contaminated boreholes were 0.12 millisieverts, approaching the WHO reference level of 0.1 millisieverts. Blending of high-uranium boreholes with low-uranium sources is recommended as an immediate intervention. Keywords: uranium, thorium, groundwater, granite aquifers, Kano State