Heavy metal accumulation in soil-wheat system of a semi-arid agricultural floodplain: quantitative analysis by...

Severe heavy metal (HM) contamination in soils poses a growing threat to the safety of crops and spatial heterogeneity in soil properties dives distinct patterns of HM enrichment. Based on 110 soil and matched wheat grain samples collected from Ningjin County-a typical area within the semi-arid agricultural floodplain of the Lower Yellow River-this study presents the first systematic assessment of HM accumulation and soil-to-wheat transfer in this understudied floodplain region. A systematic analysis was performed to characterize soil properties (pH, organic matter (OM), cation exchange capacity (CEC)) and total concentrations of eight HMs (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) in soil and crops. The geo-accumulation index ( $I_{\mathrm{geo}}$ ) and potential ecological risk index (RI) were applied to evaluated pollution levels. Multivariate statistical approaches, including PCA and partial least squares path modeling (PLS-PM), were further employed to reveal the factors in HM translocation from soil to wheat. The results revealed that the study area is generally safe, with slightly alkaline conditions, high OM and high CEC, and Cd was the sole element exceeding the local background value. In wheat grains, only As, Cd, and Pb were detected with all well below national food safety limits, yet Cd exhibited the highest enrichment. Statistical results revealed the association patterns of Cd enrichment. Soil Cd was positively associated with Cd accumulation in wheat grains (path coefficient = 0.45, standardized). Other coexisting HMs were also associated with Cd enrichment in wheat grains, with As, Cr, Cu, Hg, and Pb showing a positive combined association (path coefficient = 0.07, standardized) and Ni and Zn showing a negative combined association (path coefficient = -0.20, standardized). As and Pb within wheat grains showed a negative association with Cd uptake (path coefficient = -0.11, standardized). Based on these findings, we innovatively developed a three-tier pollution control zoning framework for the region, demonstrating the value of region-specific risk assessment and management-oriented zoning.