Unraveling the complex interplay between magnetic properties and toxic metals in fluvial sediments

The relationship between magnetic parameters and toxic metals is influenced by multi-sources of metal pollutants; however, the underlying mechanisms remain uncertain. In this study, 53 surface sediment samples were systematically collected from the Qingshui River (China) to investigate these relationships by integrating magnetic techniques with geochemical analyses. The results indicated that both the grain size and concentration of magnetic minerals in the sediments exhibited a gradual increase from upstream to downstream, reflecting variations in the input sources within the watershed. The magnetic susceptibility at low frequency (χ_LF) of surface sediments demonstrated weak correlations with the seven analyzed toxic metals, as indicated by correlation coefficients below 0.6 (P < 0.01). The spatial distributions of χ_LF and toxic metals along the flow direction displayed a discordant increasing relationship, which was attributed to source-dependent processes. The limited magnetic minerals from upstream natural and agricultural sources, along with midstream transportation activities, combined with surface processes that separate magnetic minerals from toxic metals, results in χ_LF being unable to sensitively indicate toxic metal pollution within the upper and middle reaches of the Qingshui River. In contrast, the downstream areas that included industrial zones generated a significant amount of magnetic minerals. Elevated χ_LF values at pollution hotspots may exceed thresholds for monitoring toxic metals, indicating high-intensity pollution events. Our results underscore the importance of evaluating both the applicability and prerequisites of environmental magnetism as a tool for monitoring toxic metals in fluvial sediments.