Heavy Metal Source Apportionment, Environmental Capacity, and Health Risk Assessment in Agricultural Soils of a Rice-Growing Watershed in Eastern China

This study collected 427 cultivated topsoil samples from the Mohe watershed in Tangcheng County, eastern China. By integrating positive matrix factorization (PMF) for quantitative source apportionment with self-organizing maps (SOMs) for spatial clustering, we effectively identified pollution factors and conducted a systematic evaluation of pollution sources, environmental capacity, and health risks. The results show that: (1) the soils were slightly acidic and enriched in Cd, Cr, Cu, Hg, and Pb, with Cd and Hg showing high spatial variability linked to anthropogenic inputs. (2) Quantitative source apportionment indicated that 25.9% of heavy metals (As, Cr, Ni, Pb) originated mainly from natural pedogenic sources, while agricultural activities contributed 20.8% (Cd) and 42.8% (Cu, Zn). Hg (10.5%) enrichment was attributed to residential coal combustion and wind patterns, demonstrating source-specific anthropogenic influences. (3) The environmental capacity assessment indicated a moderate capacity level across the study area. However, the improved index (PImin) revealed overload phenomena at localized sites, and these overloaded areas exhibited high spatial consistency with the distributions of agricultural and mixed sources. (4) Health risk evaluation indicated that hand-to-mouth ingestion was the dominant exposure pathway, with children facing significantly higher risks than adults. Non-carcinogenic risks remained within safe limits, but carcinogenic risks were non-negligible, with 86.7% of sites exceeding the threshold for children, especially in cultivated lands and riverbank villages. Findings underscore the importance of addressing synergistic effects of natural and agricultural sources in watershed management and prioritizing children’s health protection.