Geochemical Distribution, Integrated Contamination, and Human Health Risks of Potentially Toxic Elements in Najran Agricultural Soils, Saudi Arabia

This study investigates the geochemical distribution, integrated contamination status, and human health risks of potentially toxic elements (PTEs) in 31 agricultural topsoil samples collected from Najran, southwestern Saudi Arabia. As, Co, Cr, Cu, Ni, Pb, and Zn were measured and interpreted using spatial distribution maps, the pollution load index (PLI), Nemerow integrated pollution index (PN), chronic daily intake (CDI), hazard quotient (HQ), hazard index (HI), cancer risk (CR), and lifetime cancer risk (LCR) for both children and adults. The measured concentrations (mg/kg) varied from 2–8 for As, 2–14 for Co, 11–50 for Cr, 5–50 for Cu, 9–50 for Ni, 2–28 for Pb, and 10–249 for Zn. Based on mean values (mg/kg), the PTEs followed the descending sequence Zn (52.94) > Cr (26.68) > Ni (24.55) > Cu (16.68) > Co (6.81) > Pb (6.65) > As (4.45). PLI values ranged between 0.117 and 0.642, with an average of 0.355, suggesting generally low combined pollution by multiple elements across all sampling locations. However, PN values extended from 0.137 to 1.930, classifying 27 locations as safe, 2 as warning-level sites, and 2 as slightly polluted, mainly due to elevated Zn and Pb levels. The assessment of non-carcinogenic risk indicated no exceedance of the level of concern, as total HI values ranged from 0.156 to 0.678 for children and from 0.017 to 0.075 for adults. On average, children had HI values about 9.1 times higher than adults, with soil ingestion representing the principal exposure route. The element-specific mean HI results indicated that As and Cr were the dominant contributors to non-carcinogenic risk. Carcinogenic risk was assessed for As, Cr, and Pb. Total LCR values ranged from 1.005 × 10⁻⁵ to 4.387 × 10⁻⁵ for children and from 4.460 × 10⁻⁶ to 1.950 × 10⁻⁵ for adults, remaining within the widely accepted or tolerable risk range. Among the carcinogenic elements, Cr accounted for the highest proportion of total LCR, contributing 68.67%–69.66%, followed by As with 30.09%–31.07%, while Pb made a very small contribution of only 0.25%–0.26%. In spatial terms, the greatest HI and LCR values were concentrated mainly in the western to central-western agricultural areas, where several elements relevant to health risk were present together.