Agricultural intensification accelerates soil erosion and eutrophication, yet its role in regulating toxic metal accumulation and chemical speciation in lake sediments remains poorly understood. This study deciphers the linkages between the spatiotemporal trajectories of As, Cd, Cr, Cu, Ni, Pb and Zn in Qilu Lake, a eutrophic plateau lake in southwest China, by analyzing six sediment cores and fourteen surface sediments. Results indicate a clear metal regime shift since the 1980s, characterized by decreasing total metal concentrations. This trend was attributed to sediment coarsening, primarily resulting from lake drainage for cropland expansion. However, enrichment factor analysis identified enhanced As, Cd, Pb and Zn contamination since the 1980s, peaking in the 2000s with Cd and Pb reaching heavy contamination levels primarily tied to agricultural non-point sources. Spatially, the western lake areas adjacent to intensive agricultural watersheds exhibited amplified contamination. Critically, chemical speciation analysis revealed that 81% of Cd in surface sediments occurred in bioavailable forms, followed by Zn (58%) and Cu (45%). The percentages of all metals in oxidizable form were markedly elevated compared to background levels, indicating enhanced metal-organic matter associations under eutrophic conditions. Integrating sediment quality guidelines, potential ecological risk index and chemical speciation strengthened the rigor of the risk assessment, identifying Cd as the sole high-risk metal. Other metals, while currently low-risk, may pose elevated mobilization risks under physical disturbances. This work advances the understanding of agricultural intensification as a dual geochemical engine reshaping both the abundance and biogeochemical behavior of metals in addition to driving eutrophication.
Li et al. (Thu,) studied this question.