Secondary multifractal analysis for precise background value extraction and heavy metal pollution assessment in Shantou soils.

Determining reliable natural background values is critical for accurately assessing soil heavy metal contamination in human-impacted regions. However, conventional approaches based on fixed background values may incorporate anthropogenic signals and lead to uncertainties in pollution assessment. This study proposes a secondary multifractal analysis framework to refine soil heavy metal background values by separating natural geochemical signals from anthropogenic disturbances. The framework was applied to 511 surface soil samples (0-20 cm) and 139 deep soil samples (150-200 cm) from Shantou City, a coastal region affected by intensive human activities. Secondary multifractal analysis successfully distinguished natural background distributions from anthropogenic overlays and refined background values, with As decreasing from 1.39 to 0.812 mg/kg and Cd from 0.06 to 0.0277 mg/kg. Elevated concentrations of As, Cd, Pb, and Bi were mainly observed in industrial and agricultural regions, particularly in Chenghai, Haojiang, and Chaonan districts, whereas Ti and V exhibited relatively stable spatial distributions and were predominantly controlled by geogenic processes. The mean concentrations of As and Cd exceeded their refined background values by 8.77 and 5.05 times, respectively. Contamination factor (CF) assessment based on refined background values further confirmed the anthropogenic enrichment patterns identified by secondary multifractal analysis. By integrating multifractal analysis with GIS-based spatial mapping, this study provides a reliable and transferable framework for improving background value estimation and distinguishing natural and anthropogenic contributions in complex coastal environments.