Soil-to-plant transfer and radiological risk assessment of naturally occurring radionuclides in vegetables from Ilorin, Nigeria

In this study, the activity concentration (AC) levels of 40K, 228Ra and 226Ra in irrigation water, soils, and the most widely consumed vegetables: spinach (Amaranthus hybridus) and cabbage (Brassica oleracea) in Nigeria, were assessed using a 3 x 3-inch lead-shielded NaI(TI) detector. The soil-to-crop Concentration Ratios of the radionuclides and the corresponding radiation doses were calculated. The average AC of ⁴⁰K, ²²⁶Ra, and ²²⁸Ra in the irrigation water were found to be 24 ± 5, 5 ± 2, and 4 ± 2 Bq l^-1, respectively. The mean ACs of ⁴⁰K, ²²⁸Ra, and ²²⁶Ra in the soil were 499 ± 182, 74 ± 29, and 78 ± 26 Bq kg^-1, respectively, exceeding the global average values for background radiation. In cabbage, the average ACs were recorded as 247 ± 93 Bq kg^-1 for ⁴⁰K, 7 ± 1 Bq kg^-1 for ²²⁸Ra, and 6 ± 3 Bq kg^-1 for ²²⁶Ra. In contrast, spinach exhibited higher average ACs, with values of 510 ± 161 Bq kg^-1 for ⁴⁰K, 9 ± 3 Bq kg^-1 for ²²⁸Ra, and 8 ± 3 Bq kg^-1 for ²²⁶Ra. Spinach exhibited higher CRs than cabbage, with ⁴⁰K reaching a CR of 1.03. The calculated annual effective doses (AED) were 0.04 mSv y^-1 for cabbage and 0.08 mSv y^-1 for spinach, both below the ICRP recommended dose limit of 1.0 mSv y^-1. However, the associated excess lifetime cancer risks (ELCRs) were estimated at 1.3 × 10^-4 for cabbage and 2.8 × 10^-4 for spinach, values that are close to or slightly above the USEPA's upper acceptable threshold (10^-4). These findings suggest that while the immediate radiological health impact is low, long-term spinach consumption could modestly increase cancer risk. The study underscores the need for regular monitoring, dietary risk communication, and mitigation measures to ensure food safety in Ilorin and similar agroecosystems.