Improving the performance of contamination indices by accounting for local baselines in stream sediment: A sample catchment basin approach

Geochemical prospecting serves as a fundamental approach for examining the influence of geological settings on stream sediment composition and identifying natural or anthropogenic geochemical anomalies within a river catchment. Various indices have been developed to assess sediment quality and environmental status, typically based on ratios between observed element concentrations and reference values representing undisturbed conditions. However, these reference values often fail to account for the influence of river dynamics on sediment variability, potentially compromising the reliability of contamination assessments. Since the chemical composition of stream sediments reflects the cumulative influence of their upstream catchment basins, the dilution correction method has been extensively employed in previous studies to mitigate the dilution effects caused by fluvial processes. To address this, the present study evaluates whether local geochemical baselines can improve the performance of contamination indices. The Sarno River basin, heavily impacted by urban and industrial activities, was selected as a case study. Ninety-six sediment samples were analysed using geomorphological and hydrological parameters to define each Sample Catchment Basin (SCB). Baseline concentrations of Cd, Cr, Cu, Hg, Ni, Pb, Sb, and Zn were then estimated by calculating the weighted average element content based on lithological unit proportions within each SCB. Then, contamination indices were computed using both uncorrected (raw) and SCB-corrected baselines and spatially mapped. The use of SCB-based reference values led to a 23 % increase in the detection of high-impact sites using the Enrichment Factor and a 366 % increase using the Contamination Factor, revealing a substantial underestimation of contamination in the raw approach. Comparative analysis also showed a more balanced distribution of element contributions in the corrected indices, indicating improved representativeness of multi-element contamination. These results highlight the advantages of incorporating local baseline variability into geochemical evaluations, providing a more accurate and spatially consistent framework for environmental risk assessment in fluvial systems. • The Sarno River basin was divided into Sample Catchment Basins to assess contamination. • SCBs calculate local baseline values using weighted averages of lithological units. • Local baselines improve the contamination index accuracy over traditional methods. • Local baselines identify better anthropogenic sources, especially in downstream areas. • Sensitivity analysis shows local baseline method reduces bias in contamination assessment.