Radiological assessment of both unperturbed and agricultural soils from southern Ecuador

Abstract

The radiological background in soils across various areas in Southern Ecuador, such as Vilcabamba, Loja, Palanda, Catamayo, and Taquil, was studied to understand the impact of agriculture and fertilizer usage on soil radiological composition. Areas impacted by agricultural activities alongside reference areas unaffected by human intervention were chosen, encompassing comprehensive analyses including gamma and alpha radiation measurements, isotopic ratio determinations, and assessments of radioactive decay chain disequilibrium. Key radiological parameters such as the radium equivalent index, the external gamma absorbed dose rate and the external hazard index were calculated. The external gamma absorbed dose is equal or lower than 57 nGy h−1 (mean value worldwide in the Earth's crust). The remaining indexes as well as the annual dose rate are far from the limits allowed for public. Fallout due to 137Cs is negligible and below the minimum detectable activity (MDA). Ratios 238U/232Th varies from 0.55 in Catamayo to 1.11 in Taquil. These values show disequilibrium between the two radioactive chains and it is possible to assume A (238U)∼0.64 A (232Th) (excluding Taquil, value 1.11). Furthermore, the radiological composition of commonly used fertilizers, including urea, NPK, NH4NO3, KNO3 and compost, was analyzed, identifying 40K and 210Pb as significant radionuclides. Interestingly, no correlation was found between the types of fertilizers used and the radiological composition of soils. The predominance of the radiological background provided by the parent rock remains the main factor influencing soil composition. The radiological background values for 226Ra, 228Ra, and 40K were established, with a mean value of 23 ± 5 Bq kg−1, 30 ± 4 Bq kg−1, and 450 ± 80 Bq kg−1, respectively. Finally, the distribution of 210Pb in unperturbed soils suggests a higher concentration in the top layer, which is indicative of natural deposition patterns, except in areas affected by erosion such as Vilcabamba, Loja, and Catamayo.