Trace metal signatures and geochemical behaviour in the Chandra River: a combined multivariate and speciation...

Mountain rivers in the Himalayas are highly susceptible to geogenic and anthropogenic influences. In this study, fourteen trace/heavy metals (Al, As, B, Ba, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sr, Zn) were evaluated in the Chandra River, Lahaul-Spiti Region, during June-October 2022 to understand the dynamics of trace metals in the Himalayan waters. Water samples collected along the downstream flow of the Chandra river (Batal to Sangam), a major tributary of the Indus River, were analysed by ICP-OES and evaluated using pollution indices, and PCA analysis combined with PHREEQC speciation modelling. Findings revealed that aluminium (39-44 μg L^-1), arsenic (8-10.1 μg L^-1), and cadmium (3.0-4.0 μg L^-1) often exceeded BIS limits (Al: 30, As: 10, Cd: 3 μg L^-1) in the downstream sites (Sissu and Sangam) under the influence of drainage, recreational, and constructional activities. Upstream sites showed low pollution with heavy metal pollution index (HPI: 4.5 to 7.0), heavy metal evaluation index (HEI: < 4.14), and Nemerow pollution index (NPI: < 1.3), while downstream areas exhibited higher pollution with HPI rising upto 13 to 16.5 and ecological risk index (ERI) increasing from upstream (10-15) to downstream (30-42). Total Hazard Index (THI) values (> 2) and ILCR (> 1.62 × 10^-4) at Sissu/Sangam exceed acceptable limits, indicating a potential public health risk. Three PCA factors (85.23% variance explained) separated heavy metals (HMs) sources into geogenic, agricultural, and other anthropogenic sources (tourism, traffic originated and erosion). Speciation modelling showed aluminium mainly as Al(OH)₄^-, iron as Fe(OH)₂⁺ or FeHCO₃⁺, along with zinc (Zn) and cadmium (Cd) as mobile sulfate and chloride complexes, while strontium was mostly SrSO₄ (> 85%). Saturation indices (SI) show a distinct hydrogeochemical shift along the Chandra River, moving from strong undersaturation at upstream (Batal) to conditions nearing equilibrium or exhibiting supersaturation at downstream (Sissu and Sangam). Strong metal-anion correlations indicate carbonate and sulfate weathering control metal mobility. The ongoing presence of mobile Cd and Zn in the downstream underscores the need for continued monitoring.