Source analysis and human health risk assessment of heavy metals in farmland soil-rice system in Nandu River Basin, Zhanjiang City, Guangdong Province, China.

Abstract

This study characterized the contamination, sources, and associated risks of heavy metals in the soil-rice system of the Nandu River Basin, a typical subtropical red soil region in China. Concentrations of eight heavy metals and metalloids (Cr, Ni, Cu, Zn, Pb, Cd, Hg, As) were determined in forty paired soil-rice samples. Pollution status and ecological risk were evaluated using the Nemerow integrated pollution index and Hakanson's potential ecological risk index, respectively. Human health risks were assessed via a multi-pathway exposure model, and source apportionment was performed by integrating correlation analysis, principal component analysis (PCA), and the absolute principal component score-multiple linear regression (APCS-MLR) receptor model. Results showed that although most soil metals did not exceed national risk screening thresholds, Hg was severely enriched (6.44 times the local background level), posing a high ecological risk. In rice grains, Cr exceeded the food safety standard in 12% of samples. Rice consumption posed unacceptable carcinogenic risks from As and Cd, and unacceptable non-carcinogenic risks from As and Cr. The high spatial variability of Hg and Pb in rice (coefficient of variation, CV > 140%) indicated strong anthropogenic influence. Source analysis identified four principal components. PC1 (47.3%) represented a mixed source of volcanic weathering and traffic/agricultural activities (Pb, Zn, Ni). PC2 (25.66%) reflected combined geogenic and agricultural inputs (As, Cu, Cr). PC3 (10.86%) was linked to electronics and traffic emissions (Hg). PC4 (7.32%) was attributed to fertilizer application (Cd). Health risk assessment showed children faced a 78% higher carcinogenic risk from oral soil As intake than adults. Dermal contact with soil Cr also posed a notable non-carcinogenic risk for adults. In conclusion, contamination in the basin originates from composite natural-anthropogenic sources. Hg is the primary driver of ecol