Spatial Distribution, Source Apportionment, and Source-Specific Ecological Risk Assessment of Heavy Metals in Farmland Soils of the Ningxia Yellow River Irrigation Area

To characterize heavy metal accumulation, potential sources, and source-specific ecological risks in farmland soils of the Ningxia Yellow River Irrigation Area, 537 topsoil samples were analyzed for As, Hg, Cd, Pb, and Cr. positive matrix factorization (PMF) was coupled with the potential ecological risk index to quantify source-specific mass and ecological risk contributions. Cd and Hg showed the strongest enrichment relative to regional background values, with mean concentrations of 2.38 and 2.02 times the respective background values. PMF resolved four factors interpreted as an agricultural input-related source, a parent material-dominated natural source, an urban industrial- and combustion-related atmospheric deposition source, and a Yellow River alluvial–hydrological natural background source. The mean potential ecological risk index calculated from PMF reconstructed concentrations was 168.45, closely matching the observed value of 168.77. Source-specific mass and ecological risk contributions were clearly decoupled: the two natural source factors contributed 71.19% of the modeled heavy metal mass but only 24.21% of the ecological risk, whereas the two anthropogenic source factors contributed 28.81% of the mass but 75.80% of the risk. The atmospheric deposition and agricultural input-related sources contributed 47.27% and 28.53% of the ecological risk, respectively. Site-level bootstrap resampling and alternative allocation procedures retained the source risk ranking. These findings indicate that risk-based management should prioritize Hg-related atmospheric deposition and Cd-related agricultural inputs rather than total heavy metal mass alone.