Quantification of trace metal contamination and ecological risks in river sediments of the Himalaya

Riverbed mining exerts significant pressure on Himalayan river systems, yet its influence on trace metal dynamics in sediments remains poorly understood across elevation gradients. This study quantifies the concentration and ecological risk of six trace metals Cr, Ni, Co, Cu, Zn, and Pb in surface sediments from the Chandra River (Upper Himalayas), Beas River (Middle Himalayas), and Chakki Khad (Lower Himalayas), Himachal Pradesh, India. Ten surface sediment samples were collected from each river and evaluated using pollution indices (Igeo, PI_Nemerow, PLI, PERI, MERMQ, and MEC), sediment quality guidelines (SQGs), Pearson's correlation, and hierarchical cluster analysis. Mean metal concentrations in Chandra and Beas Rivers followed Cr > Zn > Ni > Pb > Cu > Co, while Chakki Khad showed Cr > Ni > Zn > Pb > Cu > Co. Chromium exceeded probable effect concentration (PEC) values in 40%, 20%, and 40% of samples from Chandra, Beas, and Chakki Khad, respectively, while Ni surpassed PEC thresholds in 10%, 40%, and 70% of samples from the three rivers. PI_Nemerow indicated warning-level contamination in 70% of Chakki Khad samples. MERMQ values in Chakki Khad reached 1.4, with 50% of samples registering 49% toxicity probability. Igeo values were predominantly negative across all rivers, with an isolated maximum of 1.71 for Pb in the Chandra River. Statistical analyses indicated geogenic metal sources in the Chandra River, mixed influences in the Beas River, and pronounced anthropogenic inputs from sand mining and dyeing industry effluents in Chakki Khad. Overall, Chakki Khad exhibited the highest contamination, followed by the Beas and Chandra Rivers. This study delivers the first systematic trace metal baseline for these river sediments and underscores the need for basin-scale monitoring and regulation of mining activities across Himalayan fluvial systems.