Geological constraints on the distribution of naturally occurring uranium and thorium in soils of southwestern Spain

Abstract

Naturally occurring uranium (U) and thorium (Th) concentrations in soils of southwestern Spain were explored to assess their spatial variability, establish regional geochemical baselines, and identify anomalies. A total of 505 soil and 95 bedrock samples were collected from distinct geological domains across the Iberian Massif, Betic Cordillera, and Guadalquivir Basin. Total U and Th concentrations (size fraction < 2 mm) were determined by ICP-OES, with median values of 2.30 mg kg−1 for U and 8.80 mg kg−1 for Th in topsoil, slightly exceeding those in subsoil and bedrock. Parent rock lithology was found to be the primary factor controlling U and Th concentrations in soils. The highest contents were observed in Cambisols developed on granitic rocks over the Central Iberian Zone and the Ossa-Morena Zone of the Iberian Massif. Geochemical baselines were established, with upper limits of 18.0 mg kg−1 for Th and 4.7 mg kg−1 for U in topsoil, and 16.1 mg kg−1 for Th and 4.2 mg kg−1 for U in subsoil. Anomalies exceeding these threshold values were mainly observed in samples from monzogranites of the Los Pedroches batholith and tonalites of the Santa Olalla stock. Statistical analysis revealed strong correlations between Th and light rare earth elements, suggesting that Th is primarily hosted by monazite. The variability in U and Th concentrations could have environmental monitoring implications, as elevated levels may influence radiation exposure. Future work should integrate high-resolution mapping, mineralogical analysis, and radiological assessments to refine anomaly detection and evaluate potential risks.